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DOI: 10.1055/s-0037-1609660
Synthesis of Chlorinated Arenes and Hetarenes by One-Pot Cyclizations of 1,3-Bis-silyl Enol Ethers
Financial support by the Higher Education Commission of Pakistan (HEC) and DAAD is gratefully acknowledged.
Publication History
Received: 30 August 2018
Accepted after revision: 05 May 2018
Publication Date:
04 January 2019 (online)
In memory of our colleague and friend Muhammad Zeeshan.
Abstract
This account describes our recent findings and progress in synthesizing chlorinated arenes and hetarenes by one-pot cyclizations of 1,3-bis-silyl enol ether derivatives. These reactions allow for the preparation of highly functionalized products with a high level of regioselectivity. The synthetic routes are cost-effective avoiding additional functionalization steps. The products are difficult to be accessed by other methods. The chlorine atom is of relevance in medicinal and agriculture chemistry. In addition, it allows further functionalizations by transition-metal-catalyzed cross-coupling reactions.
1 Introduction
2 Cyclizations of 2-Chloro-1,3-bis(silyloxy)-1,3-butadienes
2.1 3,5-Dihydroxychlorophthalates
2.2 2,4-Dihydroxy-homochlorophthalates
2.3 2-(Arylsulfonyl)chloropyridines
2.4 1-Azaxanthones
3 Cyclizations of 4-Chloro-1,3-bis(trimethylsilyloxy)-1,3-butadienes
3.1 3-Chlorosalicylates
3.2 Functionalized Chlorobiaryls
3.3 3-Chloro-5-(2-chloroethyl)-salicylates
3.4 2,4-Dihydroxychlorobenzophenones
4 Cyclizations of 2-Chloro-3-(silyloxy)-2-en-1-ones
4.1 Functionalized Chlorophenols
4.2 Functionalized Chlorinated Biaryls and Chlorofluorenones
4.3 Functionalized Chlorochromenones
4.4 Functionalized 3-(Methylthio)chlorophenols
4.5 Functionalized 3-Chloromethylphenols
5 Conclusions
6 List of Abbreviations
#
Key words
silyl enol ethers - one-pot cyclizations - chlorine - arenes - heterocycles - regioselectivityBiographical Sketches
Zahid Hassan was born and grew up in Parachinar, a valley in the northwestern part of Pakistan. He studied at HEJ Research Institute of Chemistry, University of Karachi, at Institute of Organic Chemistry, Leibniz University of Hannover, and at Institute of Chemistry, University of Rostock, Leibniz-Institute for Catalysis e. V. an der University of Rostock, Germany. He obtained his Dr. rer. nat. (with Prof. Dr. multi. Peter Langer, 2011) for his research work on C–C, C–SON selective cross-coupling reactions for the formation of targeted heterocycles. During his postdoctoral fellowship at the Center for Self-assembly and Complexity, IBS, POSTECH Pohang (with Prof. Kimoon Kim) he has been working on applications of MOFs and heterogeneous catalysis. After postdoctoral work, in 2014, he started his independent career at the University of Nizwa, Oman. He is currently based at the Material Research Center for Energy Systems (MZE), Institute of Organic Chemistry (with Prof. Stefan Bräse), Karlsruhe Institute for Technology, Germany. His current research focuses on applying homogeneous and heterogeneous catalysis in synthesis, and mechanism investigations.
Peter Langer was born in Hannover (Germany) in 1969. He studied chemistry at the University of Hannover and at the Massachusetts Institute of Technology (MIT) and received his Diploma under the guidance of Prof. Dietmar Seyferth in March 1994. In February 1997, he obtained his Dr. rer. nat. for a synthetic work on cinchona alkaloids under the supervision of Prof. H. Martin R. Hoffmann at the University of Hannover. During a postdoctoral period with Prof. Steven V. Ley, FRS (Cambridge, UK) he worked on the synthesis of oligosaccharides. In 1998, he moved to the University of Göttingen where he started his independent research (related to cyclization reactions of dianions) associated to Prof. Armin de Meijere. He completed his habilitation in July 2001. In April 2002, he took a permanent position as a full professor (C4) at the University of Greifswald. In December 2004, Peter Langer moved to a new position as a full professor (C4) at the University of Rostock which is located in the North-East of Germany at the Baltic Sea. Since July 2005, he is also affiliated to the Leibniz-Institute of Catalysis e.V. at the University of Rostock (LIKAT). Prof. Langer is co-author of more than 650 research papers and reviews. His research is focused on the development of new synthetic methods and their application to the synthesis of biologically relevant ring systems and natural products. This includes one-pot cyclizations, domino reactions, palladium-catalyzed reactions, azide and isothiocyanate chemistry, arene and heterocyclic chemistry, carbohydrate chemistry, organofluorine chemistry, medicinal chemistry, natural products, and new materials. Awards and scholarships: Studienstiftung des deutschen Volkes (1992-94), Fonds der Chemischen Industrie (1995-96), Feodor-Lynen-scholarship (1997-98), Liebig-scholarship (1999-2001), Heisenberg-scholarship (2001). Several honorary doctorates, honorary professorships, and medals of various universities. Elected member of the Academy of Sciences of the Republic of Armenia and of the Academy of Sciences of Pakistan. Civil award “Sitara-i-Quaid-i-Azam” by the President of the Islamic Republic of Pakistan (2015).
Introduction
Chlorine-containing arenes and hetarenes represent a rapidly expanding group of molecules that play an important role as pharmaceuticals,[1] agrochemicals, and synthetic materials.[2] Such compounds are present in a variety of biologically relevant molecules, such as antibacterial,[3] antifungal,[4] antiviral,[5] and cytotoxic molecules,[6] and in ecologically relevant compounds, such as ichthyotoxic[7] and insecticidal molecules.[8] They have been used as pharmaceuticals and large-scale commercialized crop protection agents.[9] The presence of chlorine atoms in organic molecules profoundly influences and alters their physical, chemical, and biological properties, which is due to their unique electronic characteristics. Because of the metabolic stability, lipophilicity, and electronic character, halogen substituents are often essential for the significant biological activities of natural products.[10] Among chlorine-functionalized natural products of therapeutic interest is vancomycin, an important antibiotic for the treatment of multiple-drug resistant Staphylococcus aureus infections.[11] The two chlorine substituents located at specific positions on aromatic rings within vancomycin are required to achieve the clinically active conformation of the antibiotic that inhibits a bacterial enzyme through the control of atropisomer distribution.[12] In contrast to the active substance, its dechlorinated analogue lacks antimicrobial activity.[13] Likewise, chlorinated arenes occur in many other antibiotics, such as clindamycin, chloramphenicol, griseofulvin,[14] and antitumor compounds, such as cryptophycin, rebeccamycin,[15] neopyrolomycin,[16] and in a significant number of other promising drug candidates in clinical development.
The synthetic use of chlorinated arenes as substrates in transition-metal-catalyzed cross-coupling reactions has gained considerable attention in recent years.[17] They have been used in a wide range of transformations to synthesize other complex organic structures.[18] New methods, reagents, and procedure-wise simple routes for the selective carbon–halogen bond formation and investigation of their properties have been reported recently and highlight the importance of this transformation in chemical sciences.[19]
The direct insertion of chlorine in organic molecules suffers from limitations or drawbacks, such as low regioselectivity or multiple functionalization. Therefore, an important strategy to synthesize chlorinated molecules, especially ring systems, relies on the application of a building block approach. The [4+2] cycloaddition (Diels–Alder reaction) is among the most prominent and practical synthetic routes for the synthesis of carbocycles and heterocycles and usually proceeds with excellent regio-, diastereo-, and enantioselective control.[20] Schlosser et al.,[21] Portella et al.,[22] and Manzanares et al.[23] reported the synthesis of halogenated arenes based on [4+2] cycloadditions of chlorinated building blocks.
In recent years, we have reported the application of 1,3-bis(trimethylsilyloxy)-1,3-butadienes, such as Chan’s diene,[24] as building blocks in cyclization reactions, such as formal [3+2], [3+3], [4+2] and [4+3] cyclizations and other transformations.[25] These include one-pot cyclizations of 1,3-bis(silyloxy)-1,3-butadienes with oxalyl chloride,[26] 3-silyloxy- and 3-alkoxy-2-en-1-ones,[27] iminium salts,[28] benzopyrylium triflates,[29] and various other electrophiles.[30] [31] [32] In this context, we have also studied the use of chlorinated systems and the present article provides an account with respect to the synthesis of chlorinated arenes and hetarenes starting from 1,3-bis(trimethylsilyloxy)-1,3-butadienes.
This account highlights the synthesis of chlorine-containing arenes and hetarenes by one-pot cyclizations on the basis of the employment of three building blocks: (a) 2-chloro-1,3-bis(silyloxy)-1,3-butadienes; (b) 4-chloro-1,3-bis(trimethylsilyloxy)-1,3-butadienes, and (c) chlorinated enones. These transformation reactions are based on the concept of a ‘building block strategy’ that relies on incorporating appropriate chlorine-containing synthons with the aim of tuning the structures and their properties. These procedures have some practical advantages, such as (1) a high level of regioselectivity, (2) high yields, (3) functional group tolerance, and (4) inexpensive starting materials.
# 2
Cyclizations of 2-Chloro-1,3-bis(trimethylsilyloxy)-1,3-butadienes
Chlorinated silyl-enol ethers 2a–c were prepared in 65–90% yield by reactions of commercially available methyl and ethyl 2-chloroacetoacetates 1a–c with Me3SiCl and triethylamine. The corresponding 2-chloro-1-alkoxy-1,3-bis(silyl-oxy)-1,3-butadienes 3a–c were prepared by deprotonation (with use of lithium diisopropylamide, LDA) of 2a–c at –78 °C and slow addition of trimethylchlorosilane.[32] The chloride functionality proved to be compatible with the reaction conditions (Scheme [1]).
3,5-Dihydroxychlorophthalates
The [4+2] cycloaddition of 3b with dimethyl acetylenedicarboxylate (DMAD, 4) provided dihydroxyphthalate 5 in 50% yield (Scheme [2]).[33] The cyclization reaction was scaled-up from 1 mmol to 30 mmol and gave the desired product in good yields. The hydroxyl groups were then transformed into the corresponding triflates which were further functionalized by Suzuki–Miyaura cross-coupling reactions. These reactions proceeded regioselectively in favor of the sterically less hindered site of the molecule.
# 2.2
2,4-Dihydroxy-homophthalates
To obtain chlorinated 2,4-dihydroxyhomophthalates, [4+2] cycloadditions were carried out with various reagents in high regioselectivity (Scheme [3]). Reaction of dienes 3b,c with dimethyl allene-1,3-dicarboxylate (6) gave homophthalates 7a,b. Compounds 7a,b are not selectively available by direct halogenation and other synthetic procedures of the corresponding homophthalate, because of limitations such as the formation of regioisomeric mixtures and poor selectivities.[32] [33] [34]
# 2.3
2-(Arylsulfonyl)chloropyridines
Hetero-Diels–Alder reactions of 1,3-bis(silyloxy)-1,3-butadienes with phenyl sulfonyl cyanide 8 were developed. This reaction was applied to chlorinated diene 3b to afford chlorine-containing 2-(arylsulfonyl)pyridine 9 (Scheme [4]).[32] [33] [34]
# 2.4
1-Azaxanthones
The MeSiOTf-mediated condensation of chlorinated diene 3b with 3-cyanochromone 10 gave product 11. The reaction proceeds by a regioselective attack of the terminal carbon atom of the diene onto carbon position C-2 of the cyano-functionalized chromone.[32] [33] [34] Treatment of the crude product 11 with triethylamine furnished the chlorinated 1-azaxanthone 12 (Scheme [5]). The chlorine atom remained unattacked. The transformation of 11 to form large and structurally complex heteroarenes 12 can be explained by a domino reaction.[32] [33] [34]
#
# 3
Cyclizations of 4-Chloro-1,3-bis(trimethylsilyloxy)-1,3-butadienes
Extending our experience, we focused on the development of site-selective cyclization reactions through substrate control. 4-Chloro-1-methoxy-1,3-bis(silyloxy)-1,3-butadienes 15a,b were obtained by deprotonation of 3-silyloxy-2-en-1-ones 14a,b with LDA at –78 °C and subsequent addition of trimethylchlorosilane (Scheme [6]).[35] [36] The resulting chlorinated dienes 15a,b are stable enough to be stored for several weeks under inert atmosphere and can be used as building blocks.
3-Chlorosalicylates
The reaction of chlorinated 1,3-bis(silyloxy)-1,3-dienes 15a,b with various 1,3-dielectrophiles as reaction partners resulted in formation of various chlorosalicylate derivatives through one-pot cyclizations.[36] For example, the TiCl4-mediated [3+3] cyclization of 4-chloro-1,3-bis(silyloxy)-1,3-diene 15a with 3-silyloxy-2-en-1-ones 16a–g, prepared by silylation of the corresponding 1,3-diketones, afforded 3-chlorosalicylates 17a–g (Scheme [7]). The product formation can be rationalized by conjugate addition of the terminal carbon atom of 15 onto 16 and subsequent cyclization.[27] The TiCl4-mediated reaction of 1,3-bis(silyloxy)-1,3-dienes 15a,b with 1-methoxybut-1-en-3-ones 16a–c resulted in the regioselective formation of 3-chlorosalicylates 17a–c (25–40%) (Scheme [8]).[36]
The reaction of 4-chloro-1,3-bis(trimethylsilyloxy)buta-1,3-diene 15b with 3-oxoorthoesters 18a–j, following our previously established methodology,[37] furnished 3-chloro-4-methoxysalicylates 19a–j in 36–67% yield (Scheme [9]).[36] The desired products, containing a chlorine atom and a methoxy group located at the meta and para position of the ester group, respectively, were regioselectively formed.
# 3.2
Functionalized Chlorobiaryls
The TiCl4-mediated cyclization of 4-chloro-1,3-bis(silyloxy)-1,3-dienes 15a,b with 3-aryl-3-silyl-oxy-2-en-1-ones 20a–f resulted in a range of chlorinated biphenyls 21a–f (Scheme [10]).[36] This cyclization again proved to be regioselective and allowed the preparation of products containing the aryl moiety located at the ortho position to the ester group. Mechanistically the regioselectivity of the process can be interpreted, as suggested by Chan et al.,[38] namely by isomerization of the building blocks 20a–f mediated by TiCl4, conjugate addition by attack of carbon C-4 of the diene onto 20, and cyclization/aromatization. The cyclocondensation of silyl enol ethers 25a–d (available from the corresponding precursors 24a–d) with 1,3-bis(silyloxy)-1,3-diene 15a gave chlorinated biaryls 26a–d (Scheme [11]).[39]
# 3.3
3-Chloro-5-(2-chloroethyl)-salicylates
We also applied chlorinated 1,3-bis(silyloxy)-1,3-diene 15 in a reaction with 1,1-diacylcyclopropane 27. This reaction follows a domino [3+3]-cyclization–homo-Michael reaction.[40] The reaction of 15 with 27 afforded salicylate 28 containing two chlorine atoms (Scheme [12]).[36]
# 3.4
2,4-Dihydroxychlorobenzophenones
The Me3SiOTf-catalyzed reaction of 3-formylchromone 29 with chlorinated 1,3-bis(silyloxy)-1,3-diene 15a afforded the chlorinated 2,4-dihydroxybenzophenone 30 in 33% yield (Scheme [13]). The corresponding products displayed further a broad scope, permitting transformations by using transition-metal-catalyzed cross-coupling reactions.[35] [41]
Likewise, the reaction of dichloro-3-formylchromone 31 with 4-chloro-1,3-bis(silyloxy)-1,3-diene 15a afforded trichlorinated 2,4-dihydroxybenzophenone 32 (Scheme [14]).[36] A number or related chlorinated products were prepared. The product formations can be illustrated according to the mechanism outlined in Scheme [14], namely, by a domino “Michael/retro-Michael/Mukaiyama/aldol” process.[41]
The reaction of cyanochromone 33 with 1,3-bis(silyloxy)-1,3-diene 15b gave intermediate 34. The latter was further transformed into azaxanthone 35 by treatment with triethylamine (Scheme [15]). A number of related products were successfully prepared. The formation of the products can be explained, similar to the formation of product 12 (Scheme [5]), by a domino retro-Michael/nitrile addition/hetero-cyclization reaction.[29]
The reaction of 4-chloro-1,3-bis(silyloxy)-1,3-diene 15b with phthaloyl dichloride 36, following our previously reported protocol,[42] resulted in formation of product 37 in 51% yield (Scheme [16]). The reaction presumably proceeds via isophthaloyl chloride as an intermediate. It was surprising that the reaction proceeded through the central carbon atom of the diene. In our earlier investigations, we observed this unusual reaction pattern also for reactions of 36 with nonchlorinated 1,3-bis(silyloxy)-1,3-butadienes, but only for those derived from 1,3-diketones.[43] Following our recently developed synthetic procedure,[44] the TiCl4-mediated cyclization of 4-chloro-1,3-bis(trimethylsilyloxy)-1,3-butadiene 15b with phthalic aldehyde 38 afforded the chlorinated benzotropone 39 in 36% yield.[36]
#
# 4
Cyclizations of 2-Chloro-3-(silyloxy)-2-en-1-ones
4.1Functionalized Chlorophenols
The synthesis of chlorophenols has been reported previously by [4+2] cycloaddition using chlorinated thiophene with dimethyl acetylene dicarboxylate.[44a] [b] We have developed formal [3+3] cyclizations of 1,3-bis-silyl enol ethers 42a–j with 2-chloro-3-(silyloxy)alk-2-en-1-ones 41a,b, mediated by TiCl4, to achieve the desired 4-chlorosalicylates 43a–j in 26–67% (Scheme [17]).[45] [46] Different substitution patterns varying at R1, R2, and R3 were screened and provided similar results under our protocol.
# 4.2
Functionalized Chlorinated Biaryls and Chlorofluorenones
2-Chloro-3-(silyloxy)alk-2-en-1-one derivatives 48 were prepared as the active precursors towards the synthesis of different biaryls and chlorofluorenones. Various benzoyl chlorides 44 were employed to react with ketones 45. The 1,3-diketones 46, on treatment with N-chlorosuccinimide (NCS), gave the chlorinated 1,3-diketones 47, which were transformed by silylation into 48 (Scheme [18]). The cyclization of 2-chloro-3-(silyl-oxy)alk-2-en-1-ones 48 with 1,3-bis(silyl enol ethers) 42 provided chlorinated biphenyls 50 in 26–51% yield.[46] Biaryls 50 on treatment with concentrated sulphuric acid gave, by intramolecular Friedel–Crafts acylation, chlorinated fluorenones 51 (55–86%).[46]
# 4.3
Functionalized Chlorochromenones
Chlorinated biaryls 50 containing an OCH3 group at the ortho position of the phenyl group were transformed into 10-chloro-7-hydroxy-6H-benzo[c]chromen-6-ones 52a–f (Scheme [19]) by demethylation using BBr3 and subsequent treatment with aqueous basic solution.[46]
# 4.4
Functionalized 3-(Methylthio)chlorophenols
A feasible strategy was developed by us to synthesize highly functionalized 3-(methylthio)phenols through [3+3] cyclocondensations of 1,3-bis(silyl enol ethers) with 1,1-bis(methylthio)-1-en-3-ones.[47] This method was successfully applied to chlorinated derivatives. Chlorinated 1,1-bis(methylthio)-1-en-3-ones 54 were prepared, as illustrated in Scheme [20] [48] by reaction of ketones 53 with CS2 and methyl iodide. Cyclocondensation of 54 with 1,3-bis(silyl enol ethers) 55 afforded the chlorinated 3-(methylthiophenols) 56 in 45–88% yield. We observed that the regioselectivity (initial 1,2-addition) is opposite to what is described for cyclocondensations of 1,3-bis(silyl enol ethers) with 3-alkoxy- and 3-silyloxy-2en-1-ones (initial 1,4-addition).[27]
# 4.5
Functionalized 3-Chloromethylphenols
The condensation of dichloroacetic anhydride 58 with alkenyl ethyl ether 57 afforded enone 59 (Scheme [21]). Reaction of 1,3-bis(trimethylsilyloxy)-1,3-butadiene 55 with 59 gave 3-dichloro-methyl-4-chlorophenol 60 with high regioselectivity.[49]
#
# 5
Conclusions
Summarizing the results illustrated in this account, we have demonstrated the utility of one-pot cyclization procedures of various chlorinated 1,3-bis(silyloxy)-1,3-butadienes and 3-alkoxy- and 3-silyloxy-2-en-1-ones to deliver highly functionalized valuable chlorinated arenes and hetarenes. These reactions allow for the synthesis of chlorine-containing benzoates, phthalates, homophthalates, benzophenones, phenols, thiophenols, biaryls, and heterocycles, such as pyridines, azaxanthones, fluorenones, and coumarins. This methodology is a synthetically feasible and attractive strategy to synthesize products which are difficult to access by other methods. Chlorine-containing arenes can be employed in other synthetic avenues, for example, transition-metal-mediated reactions. Considering the critical role of halogen motifs in many biologically active compounds, pharmaceuticals, and agrochemicals, we expect that the utility of such processes can be further exploited in the synthesis of pharmacologically active structures and valuable natural products. Future progress in developing new efficient synthetic strategies involving carbon–halogen bond formation with efficient control of regio- or stereospecificity and mechanistic investigations that deal with the biological activities of such organohalogens (ADMET) will have broad impact and provide an interesting opportunity for medicinal research.
# 6
List of Abbreviations
TMS trimethylsilyl
SMC Suzuki–Miyaura cross-coupling reaction
CX halogen bonding
hetarenes (SON) S-, O-, and N-containing heterocycles
ADMET adsorption, distribution, metabolism, excretion, toxicology
LDAxicology lithium diisopropylamide
NCS N-chlorosuccinimide
SON sulfur, oxygen, and nitrogen atom
#
#
Acknowledgment
The work on silyl enol ether chemistry was conducted in our laboratories at the University of Rostock. We gratefully acknowledge our students and co-workers, whose names are cited in the references, for their intellectual and experimental contributions. We would like to thank all our collaborators as well as visitors who contributed to our research projects in this chemistry. This research in particular was conducted by our former doctoral students Asad Ali, Constantin Mamat, Farooq Ibad, Ibrar Hussain, Ihsan Ullah, Renske Klasse, Muhammad Adeel, Mathias Lubbe, Muhammad Sher, Obaid-Ur-Rahman, Stefanie Reim, Verena Wolf, Arfan Yawer and Zafar Ahmed.
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