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DOI: 10.1055/a-1894-8417
Synthesis of Heterocyclic Scaffolds via Prins, Oxonium-Ene and Related Cyclization Reactions
I am grateful to the Council of Scientific and Industrial Research (CSIR), New Delhi, (Grant Nos. 01(1809)/02/EMR-II, 01(2332)/09/EMR-II, 02/0159/13/EMR-II, 02/(0364)/19/EMR-II and 02(0364)19/EMR-II), the Department of Science and Technology (DST), New Delhi, (Grant No. SR/S1/OC-33/2007), the Science and Engineering Research Board (SERB), New Delhi, (Grant No. EMR/2016/006411) for financial support, and the Centre of Excellence-Frontier Areas of Science and Technology (COE-FAST) Program of the Ministry of Human Resources Development (MHRD), Ministry of Education (Grant No. 5-5/2014-TS VII) and the North East Centre for Biological Sciences and Healthcare Engineering (NECBH), Indian Institute of Technology Guwahati (Grant No. BT/COE/34/SP28408/2018) for NMR and XRD facilities.
Abstract
A variety of oxygen, nitrogen and sulfur heterocyclic compounds are synthesized via one-pot multicomponent Prins, aza-Prins, thia-Prins, oxonium-ene, iminium-ene and thionium-ene cyclization reactions. The reactions proceeds with high diastereo- and regioselectivity. Importantly, C–C, C–N, C–O and C–S bonds are formed in a singsle step. These procedures are extended for the synthesis of biologically active molecules and natural products.
1 Introduction
2 Prins Cyclization Reactions
3 Oxonium-Ene Cyclization Reactions
4 Conclusion
#
Biographical Sketch
Anil K. Saikia received his Ph.D. from the CSIR-North East Institute of Science and Technology (CSIR-NEIST), Jorhat, India under the guidance of former director Dr. Anil C. Ghosh. He subsequently worked at the Indian Institute of Chemical Biology, Kolkata and at Chembiotek Research International, Kolkata, India for a short period of time. He then undertook postdoctoral research at Okayama University, Japan, with Prof. Sadao Tsuboi, and at Florida State University, USA, with Prof. Robert A. Holton. In 2001, he joined the Chemistry Department of the Indian Institute of Technology Guwahati, India as an assistant professor, and became a full professor in 2011. His research interests include synthetic methodology and asymmetric and natural product synthesis. He is the recipient of the Chemical Research Society of India (CRSI) Bronze Medal and is an elected Fellow of the Royal Society of Chemistry (FRSC), London. Currently, Prof. Saikia is the Subject Editor of the Indian Journal of Chemistry.
Introduction
Heterocycles are the basic core structure of many biologically active molecules and natural products.[1] They are also used as synthetic intermediates, chiral auxiliaries, and metal ligands in asymmetric synthesis,[2] and have found diverse applications in cosmetics and agrochemicals.[3] Prins and oxonium-ene cyclization reactions are versatile tools for the construction of various heterocyclic scaffolds in organic synthesis. The major advantages of these two reactions are the ease of formation of C–C and carbon–heteroatom bonds in a single step with high diastereoselectivity. The Prins reaction, first reported by H. J. Prins in 1919, involves the addition of formaldehyde (1) to styrene (2) in aqueous acidic medium, resulting in a mixture of three different products, i.e., 1,3-diol 3, allylic alcohol 4 and 1,3-dioxane 5 (Scheme [1]).[4] In 1955, Hanschke modified the Prins reaction for the selective synthesis of a tetrahydropyran (THP) ring 8 by reacting the homoallylic alcohol 3-buten-1-ol (6) with aldehydes or ketones 7 in the presence of acid; this procedure subsequently became known as the Prins cyclization (Scheme [1]).[5] In most cases, Prins cyclizations under thermodynamically controlled conditions generate 2,4,6-cis-tetrahydropyrans,[6] except for a few substrate-specific cases in which the Prins cyclization produces 2,4,6-trans-tetrahydropyrans.[7]
The diastereoselectivity of the Prins cyclization is attributed to the formation of an oxocarbenium intermediate 2A, which undergoes 6-endo-trig cyclization to give the more stable tetrahydropyranyl cation 2B. DFT calculations confirm that carbocation 2B, in its chair conformation, is stabilized by stereoelectronic effects.[8] The C2–C3 and C5–C6 σ* and σ orbitals overlap both the equatorial lone pair of the oxygen atom and the vacant p orbital at C4. Optimal overlap is reached when the hydrogen atom at C4 is pseudoaxial. This stabilization favors equatorial attack by the nucleophile to give 2,4,6-cis-tetrahydropyran 11 (Scheme [2]).
Over the years a variety of complex natural products and biologically active molecules have been synthesized using this protocol.[9] For example, phomactin A, a structurally complex molecule was synthesized by Lee’s group using a Prins/Conia-ene reaction pathway.[10] Similarly, the macrolides clavosolide A[11] and mandelalide A[12] were synthesized by the Peh and Yamini groups, respectively. Meanwhile, Rychnovsky and Bahnck synthesized the macrolide (–)-kendomycin[13] by using a Prins cyclization reaction. Bryostatins 1 and 8, which are naturally occurring complex molecular structures, were synthesized by the Keck[14] and Zhang[15] groups. Zhang’s group have described a vinylogous Mannich and an intramolecular Heathcock/aza-Prins cyclization strategy for the total synthesis of the structurally complex alkaloid (–)-vindoroisine.[16] Furthermore, using this chemistry, Yadav’s group reported the syntheses of several natural products, such as (–)-galantinic acid and 1-deoxy-5-hydroxysphingolipids,[17a] diosniponols A and B,[17b] rhoiptelol B,[17c] attenols A and B,[17d] (–)-colletol,[17e] aculeatins A and B,[17f] (+)-pseudohygroline,[17g] and diospongin A.[17h] Overman’s group utilized the Prins cyclization for the first total syntheses of the briarellin diterpenes E and F[18a] and for the enantioselective total synthesis of shahamin K,[18b] whilst Willis employed the Prins methodology for the synthesis of natural products such as the marine metabolite (–)-clavosolide D,[19a] a diarylheptanoid isolated from Zingiber officinale,[19b] the marine natural product clavosolide A,[19c] and (–)-blepharocalyxin D.[19d] Additionally, Loh’s group synthesized (–)-centrolobine[20a] and (±)-moluccanic acid methyl ester by using the Prins cyclization.[20b] Several other groups have developed methodologies for the construction of various heterocyclic compounds using the Prins cyclization reaction. In this context the contributions of Reddy,[21] Frontier[22] and Dobbs[23] are notable. Recently, List’s group demonstrated an asymmetric organocatalytic version of the Prins cyclization.[24]
The ene reaction was discovered by Alder in 1943. It was defined as a six-electron pericyclic process in which electrophilic carbon–carbon and carbon–heteroatom multiple bonds add to the alkene from which an allylic hydrogen migrates to the electrophile and thereby generates two new σ-bonds and a π-bond.[25] The alkene is termed as the ‘ene’ and the electrophile is called the ‘enophile’. The ‘ene’ cyclization is an efficient method for a stereocontrolled cyclization with carbon–carbon and carbon–heteroatom bond formation, and therefore it is an efficient and versatile protocol for the synthesis of heterocycles. There are six different types of ene cyclization reactions depending on the nature of the connectivities between the ene and enophile, and among them the (1,5), (2,5) and (3,5) types have been mostly used (Figure [1]).[26]
The oxonium-ene cyclization is a special type of ene-reaction where the enophile is an oxonium ion (X=O+). Various heterocyclic compounds have been synthesized using the oxonium-ene cyclization reaction.[27] The contributions of Overman,[27`] [b] [c] Mikami,[26,28] Loh,[29] Jacobsen,[30] List,[31] and Trost[32] in this field are noteworthy.
There is a similarity between the Prins cyclization and the oxonium-ene cyclization. In both cases the oxocarbenium ion is the intermediate (Schemes 2 and 3). In the Prins cyclization, nucleophiles attack the carbocation 3B to give saturated cyclic compounds, whereas in the oxonium-ene cyclization one proton is eliminated to provide olefins (Scheme [3]).
Aside from the Prins and oxonium-ene cyclization reactions, one-pot multicomponent reactions are also gaining interest in organic synthesis due to their ability to form multiple bonds in a single step, which is important for the synthesis of pharmaceutically active and natural products.[33] In this account we present some of our research which is in principle based on Prins, oxonium-ene and one-pot multicomponent reactions.
# 2
Prins Cyclization Reactions
2.1C–O Bond Formation (Prins Reaction)
Inspired by its diastereoselectivity and ability to form C–C and carbon–heteroatom bonds in a single step, we started our journey with the synthesis of 4-amidotetrahydropyrans 14 from allylsilane 12, aldehydes 13 and nitriles via a one-pot, three-component Prins cyclization using a stoichiometric amount of boron trifluoride diethyl etherate (BF3·OEt2) at room temperature (Scheme [4]).[34] The reaction was generalized with different aldehydes and nitriles. Both aliphatic and aromatic aldehydes gave good yields with high diastereoselectivities. It was observed that the presence of electron-donating groups on the aromatic ring of the aldehyde led to lower yields compared to their counterparts with electron-withdrawing groups on the aromatic ring. On the other hand, aliphatic aldehydes were found to be better substrates for the reaction. Acetonitrile, dichloroacetonitrile and benzonitrile were used in the reaction. Similarly, when an arene was used instead of a nitrile, 4-aryltetrahydropyran derivatives 15 were obtained in very good yields (Scheme [4]).[35] Both aliphatic and aromatic aldehydes were effective and gave excellent product yields. Benzene, toluene, o-, m- and p-xylene and anisole were screened as nucleophiles. Except for p-xylene, all the other nucleophiles gave regioisomeric mixtures with different ratios. For example, anisole gave a product with an o/p ratio of 2:1.
The mechanism involved three steps. The first step was the reaction of the aldehyde with allylsilane under Lewis acidic conditions to form a homoallylic alcohol, a process known as the Sakuri–Hosomi reaction. This was followed by addition of a second molecule of the aldehyde to give tetrahydropyranyl cation intermediate 5A via an oxocarbenium ion (Prins cyclization). The intermediate 5A, after reaction with the nitrile and subsequent hydrolysis, gave 4-amidotetrahydropyrans (Ritter reaction). Alternatively, it may react with an arene under Friedel–Crafts conditions to give 4-aryltetrahydropyrans (Scheme [5]).
In the above reaction we were able to synthesize symmetric 2,6-disubstituted-4-amido-/4-aryltetrahydropyrans via Sakurai–Hosomi–Prins–Ritter and Sakurai–Hosomi–Prins–Friedel–Crafts reaction sequences, respectively, in a single pot with excellent diastereoselectivity, however, they failed to afford unsymmetrical 2,4,6-trisubstituted tetrahydropyrans. In order to synthesize unsymmetrical 2,4,6-trisubstituted tetrahydropyrans 18, substituted homoallylic alcohols 16 and aldehydes 17 in the presence of arenes were used.[36] As depicted in Scheme [6], aromatic, heteroaromatic and aliphatic aldehydes provided very good product yields and high diastereoselectivities. The substituents on the aromatic ring had promising effects on the reaction. Aromatic aldehydes having electron-withdrawing groups on the ring gave better yields compared to aliphatic aldehydes with electron-donating groups. This might be due to the increased electrophilicity of the aldehydes (Scheme [6]).
Different arenes such as benzene, toluene, o/m/p-xylenes, anisole and 1-methoxy-4-methylbenzene were used as nucleophiles. In the case of ketones 20, the reaction was heated to 40 °C and took a longer time. Thus the reaction of cyclohexanone and 1,4-cyclohexadione with but-3-en-1-ol (19) at 40 °C afforded spirocyclic compounds 21a,b in 56% and 40% yields, respectively, whereas symmetrical dichloroacetone gave 2,2-bis-chloromethyl-4-phenyltetrahydropyran (21c) in 50% yield (Scheme [7]). The low yields were attributed to the lower reactivity of ketones compared to aldehydes.
In a similar fashion, 4-fluorotetrahydropyrans 22 were synthesized from substituted homoallylic alcohols 16, aldehydes/ketones 20 and titanium tetrafluoride as the nucleophile source and Lewis acid (Scheme [8]).[37] Both aliphatic and aromatic aldehydes efficiently provided the desired products in very good yields and high diastereoselectivities. The reactions with ketones were sluggish and gave low yields.
Epoxides are considered as carbonyl equivalents as they can rearrange into carbonyl compounds under acidic conditions and act as carbon electrophiles. Our next strategy was to use epoxides as aldehyde equivalents for the synthesis of the tetrahydropyran moiety. Differently substituted epoxides 23 were reacted with homoallylic alcohols 16 and arenes to give tetrahydropyrans 24 in excellent yields and high diastereoselectivities (Scheme [9]).[38]
The mechanism involves initial ring opening of the epoxide by the Lewis acid to generate carbocation 10A, which after rearrangement gives stable oxocarbenium ion 10B. Intermediate 10B then reacts with the homoallylic alcohol to give acetal 10C. Acetal 10C decomposes to oxocarbenium ion 10D, which undergoes Prins cyclization to give tetrahydropyranyl cation 10E. Finally, a Friedel–Crafts reaction of 10E with the arene gives the desired 4-aryltetrahydropyran derivatives (Scheme [10]).
Next, we used homopropargyl alcohols for the synthesis of dihydropyrans (DHP). Thus, the reaction of homopropargyl alcohols 25 with aldehydes 20 and arenes in the presence of BF3·OEt2 afforded 4-aryl-5,6-dihydropyrans 26 in good yields (Scheme [11]).[39] The reaction holds good for simple aromatic aldehydes and aromatic aldehydes having electron-withdrawing groups on the aromatic ring. In contrast, aromatic aldehydes having electron-donating groups such as anisaldehyde failed to give the product. This was attributed to the stability imparted by the benzylic carbocation due to the presence of a methoxy group on the aromatic ring, which leads to oxina[3,3]-sigmatropic rearrangement.[40] On the other hand, aliphatic aldehydes gave very good yields. The utility of the method was extended to various arenes such as toluene, p-xylene, mesitylene, anisole and 1,3,5-trimethoxybenzene. The reaction with ketones such as acetone and cyclohexanone gave only 15% and 28% yields, respectively.
The same reaction in the presence of triflic acid provided the corresponding 3,6-dihydropyran-4-yltrifluoromethanesulfonates 28 in moderate to good yields (Scheme [12]).[41] Both aromatic and aliphatic aldehydes worked well under the reaction conditions. The triflate group at the vinylic position of the dihydropyran ring had not been reported previously, hence its use in different coupling reactions was investigated. First, DHPs 28 were converted into 4-aryldihydropyrans 29 using the Suzuki coupling reaction. In general 4-aryldihydropyrans are prepared by Prins–Friedel–Crafts reactions, as described above in Scheme [11]. The major disadvantage of our previous method was that electron-deficient aromatic rings did not take part in the Friedel–Crafts reaction. Therefore, the use of triflate-substituted dihydropyrans as arylating units via Suzuki couplings would be a better alternative for the introduction of various aryl groups possessing electron-donating and electron-withdrawing functionalities (Scheme [13]). Similarly, Heck, Stille and Sonogashira coupling products 30–32 were obtained, as shown in Schemes 14–16, respectively.
Acrylyl enol ethers were engaged for the synthesis of dihydropyrans via the Prins cyclization reaction, where the acrylyl enol ethers served as homoallylic alcohols and aldehydes in a single molecule.[42] The reaction of enol ether 33 with trimethylsilyl trifluoromethanesulfonate (TMSOTf) in CH2Cl2 at 0 °C to room temperature over 1 hour afforded dihydropyrans 34 in good yields with an ester group present on the side chain (Scheme [17]). The reaction was highly regio- and diastereoselective. The substrate scope of the reaction was studied with different substituents on the homoallylic alcohol component, and it was found that the reaction generated only one diastereomer with 2,6-cis-configuration. The process was also regioselective as only 5,6-dihydropyrans were isolated from the reaction. Both aliphatic and aromatic substituents gave good yields. The reaction with a substrate possessing an electron-donating substituent on the aromatic group gave only a 30% yield. A crossover product was observed (30% yield), which might be due to a retro-Prins cyclization reaction. On the other hand, a highly electron-withdrawing nitro group on the aromatic ring resulted in a decomposition product.
The mechanism of the reaction involves initial activation of the carbonyl group of the ester functionality of the enol ether by TMSOTf to generate oxocarbenium ion 18A, which after Prins cyclization forms carbocation 18B (Scheme [18]). The side-chain enolate abstracts a proton from C-3 of the tetrahydropyranyl cation 18B via a six-membered cyclic transition state to form the C3–C4 double bond in the dihydropyran ring. In another pathway, the steric hindrance between the equatorial bulky silyl enol ether and the C-3 hydrogen forces elimination of the C-3 proton to release the steric constraint. This is responsible for the formation a single regioisomer.
This strategy was utilized for the asymmetric synthesis of cis-6-methyltetrahydropyran-2-yl acetic acid (39), also known as civet cat compound, which was isolated from the glandular secretions of civet cats by Maurer in 1979 and used as an additive by the perfume industry.[43] The synthesis started with chiral (S)-pent-4-en-2-ol (35) (Scheme [19]). The (S)-alcohol 35 was O-alkylated with ethyl propiolate to give enol ether 36, which was then cyclized using TMSOTf under Prins cyclization conditions to give dihydropyran 37. Reduction of the double bond followed by hydrolysis of the ester group gave chiral civet cat compound 39 in 85% yield and 17% overall yield in four steps.
When enol ether 33 was subjected to the reaction with p-toluenesulfonic acid, 4-tosylated tetrahydropyrans 40 were obtained in moderate to good yields. Both aromatic and aliphatic groups in the homopropargyl alcohol units were tolerated and provided moderate to good product yields and high diastereoselectivities (Scheme [20]).[44] The mechanism involved the formation of oxocarbenium ion 21A in an acidic medium, which after cyclization gave carbocation 21B. The tosyl group then attacked the carbocation 21B via two different pathways to give two diastereomeric compounds. Equatorial attack of the tosyl group gave the more stable major product 40, whereas attack from the axial position gave the less stable isomer 41, due to strong steric repulsion between the C-2H hydrogen and the C-4-tosyl group (Scheme [21]). This methodology was further extended to the synthesis of 4-iodotetrahydropyrans 42, tetrahydropyrans 43 and 4-azidotetrahydropyrans 44, by the reactions of cerium(III)chloride heptahydrate/NaI/CH3CN, NaBH4/DMSO and NaN3/DMF, respectively, as shown in Schemes 22–24.
We have extended the Prins cyclization reaction to acrylyl enol ethers having an alkyne side chain. It was previously observed that acrylyl enol ethers having an alkene side chain produced dihydro- and tetrahydropyrans. In contrast, enol ethers having an alkyne side chain gave tetrahydrofurans under Prins cyclization conditions (Scheme [25]).[45] Thus the reaction of enol ether 45 with In(OTf)3 in CH2Cl2 gave tetrahydrofurans 46 in good yields and high diastereoselectivities. The scope of the reaction was screened with different substrates. It was observed that enol ethers having aromatic-substituted alkyne side chains gave better yields compared to aliphatic-substituted alkynes. An enol having a strong electron-withdrawing NO2 group on the aromatic ring decomposed under the reaction conditions. On the other hand, when the R group was an aromatic or an aliphatic substituent, very good product yields were obtained. In addition, a thiophene-substituted enol ether gave a moderate yield of the expected product.
The mechanism of the reaction involves a 5-endo-trig cyclization of an oxocarbenium ion intermediate 26A to give vinylic carbocation 26B, which after addition of water followed by rearrangement gives the tetrahydrofuran ring in product 46 (Scheme [26]). The formation of a five-membered ring is in contrast to Baldwin’s rules,[46] but in this situation the formation of a five-membered ring is possible because of proper alignment of the molecular orbital of the alkyne group with the sp2-hybridized carbonyl group.[47] The synthetic application of the reaction was investigated by conducting Beckmann rearrangements of products 46a,b, which resulted in inseparable mixtures of products 47a,b in ratios of 9:1 and 10:0 and overall yields of 50% and 42%, respectively (Scheme [27]).
We have developed a methodology for the synthesis of chiral hexahydro[3,4-b]furan-4-ol 49 and its dimer 50 via tandem Prins reaction and pinacol rearrangement starting from the chiral alcohol ((4S,5R)-2,2-dimethyl-5-vinyl-1,3-dioxolan-4-yl)methanol (48) and aldehydes 27 in the presence of BF3·OEt2 in dichloroethane at 0 °C to room temperature (Scheme [28]).[48] The dimer formed was converted into its monomer by treatment with zinc in aqueous THF. Both aliphatic and aromatic aldehydes gave moderate product yields. Aromatic aldehydes produced single diastereomers, whereas aliphatic aldehydes such as butanal and heptanal generated two anomers in ratios of 85:15 and 83:17, respectively. The formation of an anomeric mixture was attributed to the steric effect of the alkyl chains. As both the alkyl chain and lactol ring were cis to each other, the lactol ring experienced a repulsive force from the alkyl chain for leading to opening of the lactol ring to generate an anomeric mixture.
The mechanism involved initial deprotection of protected triol 48, followed by formation of oxocarbenium ion 29A, which after Prins cyclization and pinacol rearrangement gave intermediate 29C. Cyclization of 29C formed hexahydro[3,4-b]furan-4-ol 49. Compound 49 under Lewis acidic conditions generated oxocarbenium ion 29D, which after reaction with another molecule of 49 gave dimer 50 (Scheme [29]).
During the entire process the stereogenic center at Cγ remained unchanged and the six-membered transition state 30A was responsible for the configurations at other stereogenic centers because the CH2OH and R groups would adopt equatorial positions in the six-membered transition state 30A (Scheme [30]). Suprafacial pinacol rearrangement produced intermediate 30C, which formed a second furan ring. Transition state 30A′ was not a preferred conformation due to the 1,3-diaxial interactions experienced by both the primary and secondary alcohol groups; it would produce intermediate 30C′, which cannot form a second furan ring as both the ring-forming groups are in trans arrangement.
# 2.2
C–N Bond Formation (Aza-Prins Reaction)
The synthesis of aza-bi- and aza-tricyclic compounds containing amido and phenyl groups has been accomplished using aza-Prins–Ritter and aza-Prins–Friedel–Crafts reaction sequences, via acyl-iminium ions, respectively (Scheme [31]).[49] Thus the reaction of N-alkylamido alcohols 51 with nitriles produced aza-bi- and aza-tricyclic compounds 52 in good yields and high diastereoselectivities. Various nitriles such as acetonitrile, benzonitrile, allylnitrile, and dichloroacetonitrile were screened as nucleophiles and all of them gave good product yields (Scheme [31]). Similarly, the corresponding phenyl-substituted aza-bi- and aza-tricyclic compounds 53 were synthesized in good yields and excellent diastereoselectivities. The stereochemistry of the compounds was determined with the help of NOE and X-ray crystallographic analysis.
The mechanism of the reaction was explained as follows. Under Lewis acidic conditions, the reduced N-homoallyl imide gave the corresponding N-acyliminium ion intermediate 32A, which underwent 6-endo-trig cyclization to give intermediate 32B having the ‘R’ group in an axial position due to strong 1,3-strain and less steric hindrance between the ‘R’ substituent and the carbonyl group of the N-acyliminium ion intermediate (Scheme [32]). The carbocation 32B thus formed was trapped by a nitrile from an equatorial position to generate intermediate 32C, which after hydrolysis (Ritter reaction) gave the corresponding amido aza-bi- and aza-tricyclic compounds 52. Similarly, after a Friedel–Crafts reaction with benzene, the carbocation 32B gave phenyl-substituted aza-bi- and aza-tricyclic compounds 53. DFT calculations at the B3LYP level were carried out to address the diastereoselectivity of the reaction. The B3LYP/6-31G (d,p)-optimized carbocation intermediates 32B and 32B′ showed a preference for formation of 32B over 32B′ by an energy of 46.304 kJ/mol.
The importance of the methodology was shown by synthesizing an unnatural alkaloid (Scheme [33]).[49] Thus the reduction of compound 53a with LiAlH4 gave (2R*,4S*,9aS*)-4-isobutyl-2-phenyloctahydro-1H-quinolizine (54) in 68% yield.
When the same reaction was performed in the presence of p-toluenesulfonic acid (p-TSA), it provided the corresponding tosylated aza-bi- and aza-tricyclic compounds 55 in good yields and high diastereoselectivities (Scheme [34]).[50] The tosyl group can be reduced selectively with Mg in methanol in good yield.
This strategy was implemented for the total synthesis of the natural epi-indolizidine products 62a and 62b (Scheme [35]).[51] The synthesis started with secondary homoallylic alcohols 16a,b, which on treatment with commercially available succinimide (56) under Mitsunobu reaction conditions gave the corresponding homoallyl imides 57a,b. Selective reduction of amides 57a,b with NaBH4/MeOH followed by an aza-Prins cyclization using p-TSA gave exclusively tosylated aza-bicyclic compounds 59a,b. Compound 59b was treated with LiAlH4 in order to reduce both the tosyl and amide groups and obtain the target product in one step, but to our dismay, this process gave the two ring-opening products 60a and 60b in yields of 37% and 28%, respectively. Therefore, the tosyl group was first reduced using NaBH4 in DMSO at 85 °C to give lactams 61a,b, which were then reduced with LiAlH4 under reflux to give the target (+)-epi-indolizidine alkaloids 62a and 62b in 64% and 66% yields, respectively.
We also developed a diastereoselective method for the synthesis of pyrroloisoindolones 64 and pyridoisoindolones 65 via aza-Prins cyclizations of endocyclic N-acyliminium ions, which were derived in excellent yields by treatment of N-alkynyl amido alcohols 63 with triflic acid (Scheme [36]).[52] The reaction was generalized with differently substituted aromatic groups on the alkyne side chain and it was observed that both electron-donating and electron-withdrawing groups on the aromatic ring gave good yields. This process was also extended to the heteroaromatic ring thiophene, which gave a 60% product yield. The reaction failed to give products with alkyl-substituted alkyne side chains. This might be due to the reduced stabilization of vinylic carbocation 37B by alkyl groups compared to aryl groups (see Scheme [37]). Furthermore, the reaction was extended to the synthesis of pyrimidoisoindolone 67 using N-alkylnitrile-substituted amido alcohol 66 (Scheme [36]).
The mechanism was similar to that previously reported for the synthesis of aza-bi- and aza-tricyclic compounds. The N-acyliminium ion 37A on aza-Prins cyclization reaction formed vinylic carbocation 37B, which was then trapped by water to generate enols 37C and 37C′ (Scheme [37]). The enols 37C and 37C′ underwent tautomerization to give thermodynamically more stable tricyclic compounds 64 and 65 having the benzoyl group exo to the pyrroloisoindolone and pyridoisoindolone ring systems. On the other hand, the opposite isomers 64′ and 65′ having the benzoyl group endo to the pyrroloisoindolone and pyridoisoindolone ring systems would be unstable due to repulsion between the benzoyl group and the isoindolinone moiety. Another possibility involves formation of the most stable isomers 64 and 65 by equilibration of the initially formed compounds 64 and 65 and 64′ and 65′ via enolization under acidic conditions.
In order to prove the formation of carbocation intermediate 37B, intermediate 37B′ was trapped with MeOH under anhydrous conditions to give methoxylated compound 38C in 62% yield (Scheme [38]). Further, to confirm that the product was derived from a thermodynamically controlled reaction, compound 64a was treated with KOH in dry MeOH and it was found that the ratio of diastereomers 64a and 64a′ was 77:23 (Scheme [39], eq. 1). Interestingly, when both the diastereomers 64a and 64a′ were subjected to a reaction with triflic acid under the standard conditions, there was no change in the ratio of diastereomers 64a and 64a′ (Scheme [39], eq. 2). These observations indicated that the final products 64 and 65 were formed due to the stability of diastereomers 64 and 65 and not via enolization of the initially formed products 64/65 and 64′/64′ under acidic conditions.
An intramolecular aza-Prins-type cyclization reaction was utilized for the synthesis of pyrimido[2,1-a]isoindolones 69 and isoindolo[2,1-a]quinazolinones 70 in excellent yields and high diastereoselectivities (Scheme [40]). In this reaction amido alcohol 68 having an N-alkyl side chain possessing an amide group was treated with boron trifluoride in dichloromethane at 0 °C to room temperature. The reaction was generalized with a range of substrates and it was observed that aryl-, benzyl- and alkyl-substituted amides all underwent this reaction and provided products with very similar yields. Electron-withdrawing and electron-donating groups on the aromatic ring had an equal effect on the rate of the reaction. Substrates having isoindolone, succinimide and saturated isoindolone moieties were also studied during the reaction (Scheme [40]).[53] Importantly, a cyclopropyl moiety was tolerated under these reaction conditions.
Post-synthetic applications of the above reaction were demonstrated by converting compound 69a into triazole derivative 71 via click chemistry, tetracyclic compound 72 via double protonation and double Friedel–Crafts reactions, and ketone 73 via hydration of the alkyne moiety (Scheme [41]).
# 2.3
C–S Bond Formation (Thia-Prins Reaction)
We also studied the reaction of thioacrylates 74 (similar to acrylyl enol ethers) under the same reaction conditions and observed the formation of tetrahydrothiophenes 75 and tetrahydrothiopyrans 76 in moderate to good yields and high diastereoselectivities (Scheme [42]).[54] Both aromatic and aliphatic substituents on the alkyne side chain and branched chain gave good yields. Electron-withdrawing groups on the aromatic ring gave better yields compared to electron-donating groups. The mechanism is similar to that previously shown in Scheme [26].
#
# 3
Oxonium-Ene Cyclization Reactions
3.1C–O Bond Formation (Oxonium-Ene Reaction)
We have utilized oxonium-ene cyclization reactions for the synthesis of oxabicyclic compounds, namely oxabicyclo[3.3.1]nonanones 78, in moderate to good yields and high diastereoselectivities by reacting aldehydes 27 with the commercially available terpenoid trans-p-menth-6-ene-2,8-diol (77) in the presence of BF3·OEt2 in dry toluene (Scheme [43]). Both aromatic and aliphatic aldehydes were screened and it was found that aliphatic aldehydes gave better yields compared to their aromatic counterparts (Scheme [43]).[55] The same oxabicyclic compounds were also prepared from epoxides 79 as the aldehyde equivalent. However, a monosubstituted terminal epoxide failed to give the desired product. Interestingly, compound PS-203 was found to have antileishmanial activity with an IC50 value 4.9 + 0.4 μm.[56]
Mechanistically, under Lewis acidic conditions, aldehydes 27 reacted with diol 77 to form oxocarbenium ion 44A, which after a (3,5)-oxonium-ene cyclization reaction gave enol 44B. The enol 44B then underwent tautomerization to produce oxabicyclic compounds 78 (Scheme [44]).
In our efforts to synthesize oxabicyclo[3.3.1]nonenes 81 from geraniol (80), a naturally occurring terpenoid, reactions with aldehydes 27 in the presence of BF3·OEt2 afforded oxabicyclo[3.3.1]nonenes 81 along with the tetrasubstituted isomeric tetrahydropyrans 82 and 83 with varied compositions (Table [1]).[57] It was observed that aromatic aldehydes gave oxabicyclic compounds as the major products, whereas aliphatic aldehydes provided tetrahydropyrans as the major products. Surprisingly, substituted aromatic aldehydes gave only oxabicyclo products. This might be due to the steric effect of the bulky aldehydes, which cannot approach the carbocation intermediate 45A (Scheme [45]). Importantly, one of the oxabicyclo[3.3.1]nonenes (Figure [2], A) is known to behave as an estrogen receptor ligand,[58] whilst an alkyl-substituted tetrahydropyran (Figure [2], B) is used as an aroma and flavoring substance in pharmaceuticals, cosmetics and foodstuffs.[2b] A plausible mechanism for the formation of the three products has been proposed. Under Lewis acidic conditions geraniol forms carbocation 45A, which after cyclization gives carbocation 45B. Carbocation 45B, after nucleophilic attack by aldehydes 27, gives oxocarbenium ion 45C. The intermediate 45C then undergoes an oxonium-ene cyclization to form oxobicyclo[3.3.1]nonenes 81 (Scheme [45]). In the case of sterically less hindered aldehydes such as aliphatic aldehydes, the carbocation 46A is attacked by aldehydes 27 to form the more favored transition state 46B (Scheme [46]). The oxocarbenium ions 46B and 46C, after oxonium-ene cyclizations, provided major products 82 and minor products 83, respectively. Furthermore, the reaction of geraniol with salicylaldehyde gave the tetracyclic compound 84 in 40% yield (Scheme [47]).
The reaction was extended to epoxides and it was found that tetrahydropyrans 86 and 87 were formed exclusively as isomeric mixtures, except for 2,2-dimethyloxirane, which gave oxabicyclic compound 85, albeit in a low yield (Table [2]).
In a similar fashion, oxabicyclo[3.2.1]octenes 89 were prepared in very good yields from the reaction of the naturally occurring terpenoid (–)-terpinen-4-ol (88) with aldehydes 27 in the presence of 10 mol% of indium(III) triflate (Scheme [48]).[59]
Both aliphatic and aromatic aldehydes were found to be good substrates for the reaction giving good yields of the desired products. On the other hand, electron-donating groups on the aromatic ring provided better yields compared to when electron-withdrawing groups were present on the aromatic ring. This was due to the better stability provided to oxocarbenium ion 49A by the electron-donating groups compared with electron-withdrawing groups (Scheme [49]). The situation was the same with aliphatic aldehydes. The mechanism can be explained as follows. Indium(III) triflate activates the aldehyde for nucleophilic attack by the alcohol to give oxocarbenium ion 49A, which after oxonium-ene cyclization provides the oxabicyclo[3.2.1]octene product 89.
Similarly, oxabicyclo[3.2.1]octanes 89 could be synthesized from 88 and epoxides 79, albeit with lower yields compared to those obtained from aldehydes (Table [3]).
 |
|||
|
Entry |
Epoxide |
Product |
Yield (%) |
|
1 |
 |
 |
68 |
|
2 |
 |
 |
60 |
|
3 |
 |
 |
62 |
|
4 |
 |
 |
58 |
In an effort to synthesize four-membered oxygen heterocycles, homoallylic alcohols 90 were reacted with different aldehydes 27 using BF3·OEt2, and three different products, i.e., regioisomeric 3,6-dihydrotetrahydropyrans 91–93 were obtained in different compositions. The scope of the reaction was investigated with different types of aldehydes and alcohols. In this reaction aromatic aldehydes gave tetrahydropyrans 91 and 93 in different ratios (Table [4]).[60] Reactions of aldehydes with methyl (R1 = Me) substituted alcohols produced endocyclic compounds the major isomers and exocyclic compounds as minor isomers. On the other hand, aliphatic aldehydes produced two regioisomeric endocyclic products. Importantly, reactions of phenyl-substituted alcohols with aromatic aldehydes gave only single dihydropyrans. Similarly, reactions of methyl (R1 = Me) and aryl (R2 = aryl) substituted alcohols with aromatic aldehydes gave only exocyclic tetrahydropyrans. Importantly, Friedel–Crafts products with benzene were not observed during the reaction, which might be due to steric hindrance between the ‘R1’ group (Me or Ph) of compound 90 and the benzene nucleophile (Scheme [50]). The reaction mechanism followed typical oxocarbenium ion formation 50A followed by cyclization to form carbocation 50B, which after elimination of a proton via three possible pathways gave the three different products 91–93.
Similar reactions of alcohols 94 in the presence of nitriles gave 4-amidotetrahydropyrans 95 and 96 via a Prins cyclization reaction. Both aliphatic and aromatic aldehydes worked well, giving moderate to good yields of diastereomeric mixtures, except in the case of m-nitrobenzaldehyde, which produced only one isomer (Table [5]).
The scope of the reaction was further extended to a chiral cyclic alcohol for the synthesis of chromenes 98. The reaction of chiral 2-isopropenyl-5-methylcyclohexanol (97) with p-bromobenzaldehyde or butyraldehyde under standard conditions gave (2R,7R,8aR)-2-(4-bromophenyl)-4,7-dimethyl-3,5,6,7,8,8a-hexahydro-2H-chromene (98a) and (2S,7R,8aR)-4,7-dimethyl-2-propyl-3,5,6,7,8,8a-hexahydro-2H-chromene (98b) in 60% and 64% yields, respectively (Scheme [51]).
After utilizing homoallylic and homopropargylic alcohols in synthesizing various oxygen-containing heterocyclic compounds, we investigated the use of substituted 5-methylhex-4-en-1-ol, a homologue of homoallylic alcohol. Initially, a diastereomeric mixture of alcohol 99 (ratio of 1:1) was treated with different aliphatic and aromatic aldehydes to give the corresponding tetrahydropyrans 100 and 101 in good yields and 1:1 diastereomeric ratios (Scheme [52]).[61]
This reaction was also performed with anti-alcohol 102 and syn-alcohol 103, and the desired products 104 and 105, respectively, were obtained as single diastereomers (Schemes 53 and 54). Mechanistically, two isomeric alcohols under Lewis acidic conditions gave two oxocarbenium ions 55A and 55B, which after cyclization followed by proton elimination produced the corresponding diastereomers 100/104 and 101/105 (Scheme [55]).
The reaction was further exemplified by converting anti-alcohol 102 and syn-alcohol 103 into the corresponding tricyclic chromenes 106 and 107 in moderate yields via reactions with salicylaldehyde (Scheme [56]).
The mechanism involved intramolecular attack of the phenolic group on carbocation 57B instead of elimination (Scheme [57]).
The use of tertiary homoallylic alcohols 108 was also studied for the synthesis of dihydropyrans 109a via oxonium-ene cyclizations.[62] Here, tertiary homoallylic alcohols 108 were treated with aldehydes 27 in the presence of a catalytic amount of TMSOTf in CH2Cl2 at 0 °C. Both aromatic and aliphatic aldehydes were effective in providing good yields with high diastereo- and regioselectivities (Scheme [58]). However, aromatic aldehydes having electron-withdrawing groups on gave lower yields compared to those with electron-donating groups. This is in accordance with the normal trends of the stabilizing effect of electron-donating groups on oxocarbenium ion 59A (Scheme [59]). Similarly, epoxides 79 gave dihydropyrans 109b in moderate yields (Scheme [58]).
The advantage of this methodology over the previous methods for the synthesis of dihydropyrans is that it produced single regioisomers by using a catalytic amount of TMSOTf. The formation of single regioisomers can be explained by considering a concerted mechanism for the reaction, as shown in Scheme [59].
As part of our continuing efforts to explore the diastereo- and regioselective synthesis of oxacycles, we introduced another method for the synthesis of 2,6-dihydropyrans 111 from silyl-homoallylic alcohols 110 and aldehydes 27 using BF3·OEt2 as a Lewis acid in CH2Cl2 at –45 °C. Substrates having primary and secondary alcohols as well as aliphatic and aromatic aldehydes participated efficiently under the reaction conditions to give good yields with high diastereo- and regioselectivities. Aromatic aldehydes having electron-withdrawing and electron-donating groups gave good yields (Scheme [60]).[63] However, reactions with aliphatic aldehydes provided regioisomeric mixtures of two products in ratios of 62:38 and 63:37, respectively. Alkyl-substituted secondary alcohols gave good yields with high diastereoselectivity but low regioselectivity, whereas an aryl-substituted secondary alcohol gave a mixture of regioisomers in a ratio of 80:20. The remarkable feature of the reaction was the configurational control of the vinylsilane for exclusive formation of 5,6-dihydro-2H-pyrans in most of the cases.
The mechanism of the reaction is believed to take place via an oxonium-ene cyclization. Under Lewis acidic conditions, oxocarbenium ion 61A is formed, which after oxonium-ene cyclization followed by proton elimination gives silylated dihydropyrans 61C. Finally, removal of the silyl group by in situ generated HF acid gives the dihydropyrans (Scheme [61]). The formation of regioisomeric mixtures (111/111′) in some cases could be due to the oxonia-Cope rearrangement of 61A.
Further efforts were devoted to the regioselective synthesis of dihydropyrans and β-allenols were examined in this connection. Thus, when β-allenols 112 were reacted with aldehydes 27 in the presence of Bi(OTf)3 dihydropyrans 113 and 114 were obtained in moderate to good yields (Scheme [62]).[64] Both aromatic and aliphatic aldehydes worked well under the reaction conditions. However, aromatic aldehydes with electron-donating or electron-withdrawing groups on the aromatic ring gave better yields compared to aliphatic aldehydes. Interestingly, β-allenols with cyclopentyl, cyclohexyl, cycloheptyl and cyclooctyl ring systems also reacted well under the reported conditions to give moderate yields of the expected products.
This methodology was also extended to the synthesis of pyranopyran skeleton 115 using TMSOTf as a Lewis acid at –45 °C (Scheme [63]). Both electron-donating and electron-withdrawing groups on the aromatic ring resulted in moderate yields. However, aliphatic aldehydes failed to provide the desired products. As far as the mechanism is concerned, it is believed that under the Lewis acidic conditions, as is usual, oxocarbenium ion of type 64A is formed, which after cyclization produces carbocation 64B. Intermediate 64B, after rearrangement, generates carbocation 64C via path a. Elimination of a proton from 64C gives the final product 113 at room temperature. On the other hand, carbocation 64B, after elimination of a proton via pathway b, gives compound 114. At a lower temperature (–45 °C), compound 113 is in equilibrium with its s-cis product 64D, which after reaction with another molecule of aldehyde 27 in a hetero-Diels–Alder fashion gives pyranopyran 115 (Scheme [64]).
The hetero-Diels–Alder reaction is highly regio- and diastereoselective. The regioselectivity can be explained by the fact that the frontier orbitals of the dienes are polarized due to the presence of the electron-donating CH3 group or the CH2 moiety, as shown in Figure [3].
The observed diastereoselectivity of the reaction was due to the formation of the more stable transition state 65A in which there is less steric repulsion between the cyclohexene group of the dihydropyran and the hydrogen of the aldehyde group compared to transition state 65B, which experiences strong steric repulsion between the cyclohexene ring and the aryl group. As a result, the desired products under these circumstances might be 115 or 115′ with the protons at C-9H, C-10H and C-17H in an all-cis-configuration. The diastereomer 115′ is not stable due to the presence of steric repulsion between the cyclohexyl ring and the aryl group at the C-20 position. Therefore, the obtained product is 115 with two phenyl groups in trans configuration (Scheme [65]).
# 3.2
C–N Bond Formation (Iminium-Ene Reaction)
The synthesis of tetrahydroquinolines 117 was achieved by employing allylic anilines 116. Thus, reactions of anilines 116 with aromatic aldehydes in the presence of BF3·OEt2 afforded tetrahydroquinolines 117 in moderate to good yields and high diastereoselectivities (Scheme [66]).[65] Both electron-donating and electron-withdrawing groups on the aromatic ring of the aldehydes produced good yields. Although an electron-donating group on the aniline ring gave a very good yield, in contrast, the presence of an electron-withdrawing group did not lead to the desired product. This was due to deactivation of the aniline by the electron-withdrawing group. The mechanism involves the initial formation of an imine from the aldehyde and amine, which is then activated by the Lewis acid toward nucleophilic attack by the alkene to generate carbocation intermediate 67C via iminium ion intermediate 67B. In the transition state 67B, the aromatic ring is trans to the isopropenyl and other aromatic group, thus making the system more stable. Finally, elimination of a proton from intermediate 67C results in formation of the tetrahydroquinoline 117 (Scheme [67]).
# 3.3
C–S Bond Formation (Thionium-Ene Reaction)
Tetrahydrothiophenes 119, the sulfur analogues of tetrahydropyrans, were synthesized using substituted 5-methylhex-4-ene-1-thiols 118. Thus the reactions of thiols 118 with aromatic and aliphatic aldehydes in the presence of BF3·OEt2 gave 2,3,6-trisubstituted tetrahydrothiophenes in moderate to good yields and high diastereoselectivities (Scheme [68]).[66] Primary and secondary thiols as well as aliphatic and aromatic aldehydes gave good yields. The electron-rich aromatic aldehyde 3,4,5-trimethoxybenzaldehyde and phenyl acetaldehyde produced tricyclic compounds. These two reactions proved that the reaction proceeded through a stepwise mechanism via a carbocation intermediate similar to the reaction mechanism shown in Scheme [55].
#
# 4
Conclusion
This account has highlighted the synthesis of oxygen, nitrogen and sulfur heterocyclic compounds via one-pot multicomponent, tandem, Prins, aza-Prins, thia-Prins, oxonium-ene and iminium-ene cyclization reactions. Various heterocyclic compounds such as tetrahydrofurans, tetrahydrothiophenes, tetrahydrothiopyrans, hexahydrofurofurans, tetrahydropyrans, dihydropyrans, pyranopyrans, hexahydroindolizinones, hexahydroquinolizinones, tetrahydropyridoisoindolones, pyrroloisoindolones, pyrridoisoindolones, pyrimidoisoindolones, isoindoloquinazolinones, oxabicyclo[3.3.1]nonanones, oxabicyclo[3.3.1]nonenes, 6-oxabicyclo[3.2.1]octanes and tetrahydroquinolines can be efficiently prepared using these techniques. Some of the methodologies have been used for the synthesis of biologically active molecules and natural products. In addition, several of the synthesized products have been utilized for cross-coupling reactions via Suzuki, Stille, Heck and Sonogashira reactions.
#
#
Conflict of Interest
The author declares no conflict of interest.
Acknowledgment
I am grateful to the Indian Institute of Technology Guwahati for providing facilities for pursuing this research work. I am also indebted to my co-authors who contributed to the work included in this account. They are as follows: Drs Udagandla C. Reddy, Pipas Saha, Somasekhar Bondalapati, Kiran Indukuri, Paramartha Gogoi, Madhurjya Borah, Manash J. Deka, Sabera Sultana, Bijoy K. Das, Priya Ghosh, Ramanjaneyulu Unnava, Ngangbam Renubala Devi, Upasana Borthakur, Namita Devi, Sujit Sarkar, Mr. Malay Das, Mr. Sudip Shit, Mr. Subhamoy Biswas, Miss Bikoshita Porashar and Mr. Pallav J. Arandhara.
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- 64 Devi NR, Sultana S, Borah M, Saikia AK. J. Org. Chem. 2018; 83: 14987
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- 66 Bondalapati S, Gogoi P, Indukuri K, Saikia AK. J. Org. Chem. 2012; 77: 2508
Corresponding Author
Publication History
Received: 29 May 2022
Accepted after revision: 08 July 2022
Accepted Manuscript online:
08 July 2022
Article published online:
19 August 2022
© 2022. Thieme. All rights reserved
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