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DOI: 10.1055/a-2523-4859
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Emerging Trends in Organic Chemistry: A Focus on India

Synthesis of Branched Sugars and Chiral Aromatic Building Blocks Derived from Glycals

Mittali Maheshwari
,

Science and Engineering Research Board, India (EEQ/2021/000553)
 


Abstract

The transformation of sugar enolic ethers into branched sugars through C–H activation or cross-coupling reaction represents a compelling strategy in carbohydrate chemistry. These branched sugars serve as versatile intermediates for the synthesis of annulated sugars, which are key components in numerous biologically active molecules. This Account highlights various methodologies for the development of branched sugars, focusing on approaches utilizing glycals and alkenes through C–H activation and cross-coupling reactions. Additionally, it explores literature reports on the synthesis of chiral fused ring systems derived from functionalized glycals, offering insights into their significance in synthetic chemistry.

1 Introduction

2 Synthesis of Branched Sugars from Glycals

3 Transformation of Branched Sugars to Annulated Sugars

4 Conclusions


# 1

Introduction

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Mittali Maheshwari is a research scholar under the supervision of Dr. Nazar Hussain at Banaras Hindu University (BHU). She completed her Master’s degree in 2019 at Subodh College, University of Rajasthan, India. She cleared CSIR-NET with AIR-50 in 2019 and also GATE in 2019 with AIR-1163, Her research focuses on the study of C-glycosides that hold significant medicinal potential. Dr. Nazar Hussain earned his Ph.D. in chemical sciences in 2019 under the mentorship of Dr. Debaraj Mukherjee at the CSIR-Indian Institute of Integrative Medicine (IIIM), Jammu, India. His doctoral research centered on the synthesis of 2-C-branched sugars and their subsequent transformation into chiral aromatic building blocks, which hold significant potential in synthetic organic chemistry and medicinal applications. Following the completion of his Ph.D., Dr. Hussain joined the Armand Frappier Research Institute-INRS, Canada, as a postdoctoral research associate under the guidance of Professor Charles Gauthier. During this tenure, he expanded his expertise in carbohydrate and medicinal chemistry, contributing to the development of innovative methodologies and compounds. Currently, Dr. Hussain is an assistant professor in the Department of Medicinal Chemistry at the Institute of Medical Sciences, Banaras Hindu University (BHU), Varanasi, India. His research interests include the synthesis of C-glycosides, palladium-catalyzed reactions, the total synthesis of natural products, and the design of novel compounds for medicinal applications. Through his work, he aims to advance the field of synthetic organic chemistry and explore its intersection with drug discovery and development.

The functionalization[1] of glycals utilizing nonactivated sp2 or activated sp2 carbon centers followed by C–C bond formation at the C-2 position has emerged as a dynamic and rapidly evolving field, offering powerful strategies for constructing complex organic molecules.[2] C-Branched sugars serve as key structural elements in various naturally occurring compounds, including antibiotics, macrolides, and polysaccharides, as exemplified in Figure [1].[3] [4] [5] Although numerous methods exist for introducing carbon units at the anomeric position through C/O/N/S-glycosylation,[6–9] the development of efficient and selective strategies for this purpose continues to be a key area of interest in glycoscience,[10] with significant implications for medicinal chemistry and drug discovery.[11] The methods available for attaching carbon chains to the carbon atoms of sugar units are quite limited.[12] Two main strategies are used to synthesize 2-C-branched sugars: the first requires the preactivation of glycals,[13] whereas the other utilizes activated alkenes.[14] Nevertheless, both methods exclusively lead to sp2–sp2 bond formation. The first methodology involves the synthesis of synthons for the cross-coupling reaction, such as 2-formyl glycals,[15] 2-nitro glycals,[16] 1,2-cyclopropanated glycals,[17] 2-halo glycals,[18] and many more. The second approach requires a stoichiometric amount of a Lewis acid, which can be problematic for acid-sensitive functional groups.

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Figure 1 Natural products derived from glycals
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Figure 2 Biologically active drugs containing a chiral fused aromatic system

C-Branched sugars have been used to develop fused aromatic systems. Fused aromatic ring systems are present in a variety of biologically active natural products and π-conjugated functional materials.[19] Such drug molecules possess a core structure that includes naphthalene[20] or phenanthrene moieties.[21] They include chloroquinocin, which acts as an antibacterial drug;[22] benzosimuline, a natural product that exhibits anti-platelet-aggregation activity;[23] mansonone F, which has antibacterial activity, particularly against Staphylococcus aureus;[24] trigonostemone, an extract of the Trigonostemon lii species that is traditionally used in medicine as an expectorant, a laxative, and for treating certain skin diseases;[25] and denbinobin, a phenanthrenone compound, recognized for its antitumor and antiinflammatory properties (Figure [2]).[26]

This review covers the literature on the synthesis of branched sugars from glycals and their further utilization to afford a chiral fused-ring system.

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Scheme 1 C–H activation of glycals with nonactivated cycloalkenes, affording C-branched sugars

# 2

Synthesis of Branched Sugars from Glycals

In 2016, Hussain and co-workers introduced an efficient method for coupling glycals 1 with cycloalkenes 2 at the C2 position by using a palladium catalyst to achieve allylic substitution with remarkable yields (Scheme [1]).[27] This approach employed 10 mol% of palladium acetate as the catalyst and silver acetate as the oxidant in a 20:1 DMF–DMSO solvent mixture at 80 °C for 20 hours. The reaction successfully produced C2-branched sugars 3 in high yields. The versatility of this protocol was demonstrated by its application to various cycloalkenes, including cyclopentene (3a), cycloheptene (3b), and cyclooctene (3c). Additionally, the reaction was extended to various protected glycals (3df), which also delivered the desired products in excellent yields.

In 2016, Lubin-Germain and co-workers reported an aminocarbonylation of 2-iodoglycals 4 with amines 5 using Mo(CO)6 and a palladium catalyst to access C-2 branched sugars 6 (Scheme [2]).[28] Under the optimized conditions, a variety of primary and secondary amines reacted successfully to give the carbonylative products 6af in moderate to good yields.

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Scheme 2 Cross-coupling of 2-iodoglycals with amines: access to C2-branched glycoconjugates

In 2018, the Hussain group devised an effective strategy for synthesizing C2-branched sugar dienes through the cross-dehydrogenative coupling of glycals 1 with terminal alkenes 7 (Scheme [3]).[29] This versatile method is suitable for both pyran- and furan-based enol ethers, facilitating smooth coupling with electron-rich and electron-deficient alkene sources. The reaction consistently produced sugar dienes 8 with complete E-stereoselectivity. The optimized reaction conditions were applied to various styrenes, including styrene (8a), 4-acetylstyrene (8b), and 4-fluorostyrene (8c), yielding the desired products in moderate to good yields. Furthermore, activated alkenes such as methyl acrylate (8d and 8e) and butyl acrylate (8f) reacted efficiently with glycals 1, producing C2-branched sugars in excellent yields while maintaining complete E-stereoselectivity.

In 2019, the Hussain group developed a carbonylative coupling reaction between glycals 1 and arylboronic acids 9, using palladium acetate as a catalyst, to produce sugar-based enones 10 in good yields (Scheme [4]).[15] This reaction proceeded efficiently with various substituted phenylboronic acids, including the 4-methoxy (10a), 3-methyl (10b), and 2-bromo (10c) compounds, yielding the desired carbonylative products in moderate to good yields. Subsequently, various glycals were subjected to this reaction, leading to the formation of sugar-based arylones 10df in good yields.

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Scheme 3 Synthesis of C-2 branched sugars by cross-dehydrogenative coupling of glycals with terminal alkenes
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Scheme 4 Synthesis of sugar-based enones in the presence of palladium acetate

The acetyl-protected coupling products were further transformed into substituted furan derivatives 11ac in the presence of a Lewis acid (Scheme [5]).

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Scheme 5 Synthesis of 3,5-disubstituted furans in the presence of a Lewis acid

In 2019, Ahmed and co-workers synthesized C-2 carboxylic acid derivatives by the carbonylative coupling of iodo glycals 4 with formic acid (12) as a carbonyl source by employing palladium acetate as a catalyst and iron complex 13 as the ligand (Scheme [6]).[30] The substrate scope was investigated using various glycal derivatives, including a benzylated glucal (14a), a benzylated galactal (14b), an ethyl-protected glucal (14c), an acetylated glucal (14d), a benzylated rhamnal (14e), and an acetylated xylal (14f), which all reacted with formic acid to produce the desired products in good yields.

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Scheme 6 Palladium-catalyzed carbonylation of 2-iodoglycals to give C-2 carboxylic acids derivatives

In 2019, Stefani and co-workers introduced an efficient method for synthesizing amidoglycals and glycosyl esters from 2-iodoglycals 4 by using Mo(CO)₆ as carbonyl source and PdCl₂ as a catalyst.[31] The optimized conditions were applied to various amines, including those with electron-donating, electron-withdrawing, or neutral substituents. This approach yielded 2-amidoglycals 16af with moderate to good efficiency, showcasing its versatility across various amine substrates (Scheme [7]).

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Scheme 7 Transformation of 2-iodoglycals into amidoglucals by carbonylative coupling

Additionally, iodoglycals 4 reacted with alcohols 17 to give the C-2 branched glycosyl esters 18af in good to excellent yields (Scheme [8]).

These results highlight the robustness and broad applicability of the method in preparing functionalized carbohydrate derivatives.

In 2020, Messaoudi and co-workers introduced a method for the efficient synthesis of C2-aryl glycosides in good yields (Scheme [9]).[32] The coupling reaction involved acetylated pseudoglycals 19 and aryl iodides 20 in the presence of 10 mol% palladium acetate with AsPh3 as a ligand and AgTFA as a base and it proceeded in 1,4-dioxane at 120 °C to give the desired products 21. A variety of aryl iodides bearing diverse functional groups, including COOMe, COMe, CHO, CF3, F, and NO2, reacted with pseudoglycals to afford the target products 21af in moderate to good yields.

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Scheme 8 Carbonylative coupling of 2-iodoglucals with alcohols providing access to glucal esters
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Scheme 9 Pd-catalyzed synthesis of C2-aryl glycosides

In 2020, the Ferry group reported a method for synthesizing 2-cyanoglycals 22 from 2-iodoglycals 4 under mild conditions (Scheme [10]).[33] The transformation uses a palladium complex as the catalyst and is conducted in a 1:1 mixture of t-BuOH and water at 75 °C under an argon atmosphere. This approach efficiently gave the desired branched sugars 22ah in good amounts.

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Scheme 10 Mild palladium-catalyzed cyanation of unprotected 2-iodoglycals

Following this, the resulting 2-cyanoglycals 22 were employed to synthesize several C-2 branched sugars (Scheme [11]). The reaction of 2-cyanoglycals with Pd/C as a catalyst under a hydrogen atmosphere led to the formation of the desired amino compounds 23 and 24. Subsequently, enamine 25 was obtained by reducing the CN group using sodium borohydride in the presence of a cobalt salt. Additionally, 2-cyanoglycals were also used to synthesize the 2-amido-based C-2 branched glycals analogues 26 and 27, and the tetrazolyl derivative 28.

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Scheme 11 Synthesis of C-2 branched glycol analogues from 2-cyanoglycals. (a) Reaction conditions: A: 22 (1 equiv), H2 (4 bar), Pd(OH)2/C (20 mol%), 1 M HCl, i-PrOH, r.t., 18–72 h. B: 22 (1 equiv), NaBH4 (4.1 equiv) CoCl2·6H2O (cat.), THF–H2O (2:1), 0 °C to r.t., 2–6 h. C: 22 (1 equiv), MeMgBr (6 equiv), THF, 65 °C, 18 h; then 1 M HCl, 65 °C, 30 min. D: 22 (1 equiv), NaN3 (1.1 equiv), CuI (0.2 equiv), DMF, 100 °C, 18 h.

In 2022, Hussain and co-workers developed a palladium-catalyzed method for the direct functionalization of glycals 1 with cycloalkenones 29 (Scheme [12]).[34] The reaction employed 10 mol% of palladium acetate as a catalyst, along with 2.5 equivalents of silver acetate as an oxidant, in a 20:1 DMF–DMSO solvent mixture. This protocol efficiently produced C-2 branched sugars 30 in good yields. The substrate scope included various glycosyl donors such as acetyl- (30a), benzyl- (30b), and benzoyl-protected glucal (30c); acetyl- (30d) and benzoyl-protected galactal (30e); and acetyl-protected rhamnal (30f), leading to the formation of 2C-branched alkenones in moderate to good yields. Cyclopentenone also reacted with glycosyl donors, yielding products 30g and 30h in moderate yields.

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Scheme 12 C–H activation of glycals with cycloalkenones in the presence of palladium acetate
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Scheme 13 Plausible mechanism for the coupling reaction between glycals and cycloalkenones

The proposed mechanism (Scheme [13]) begins with the complexation of Pd(OAc)₂ and DMSO with the glycal, followed by electrophilic C–H substitution at the C-2 position, forming palladium complex B. This intermediate then undergoes nucleophilic attack on the β-position of the cycloalkenone to generate complex C or D. Finally, syn-β-hydride elimination leads to the formation of the desired product.

In 2022, Zargar and his team introduced a method for C2 alkenylation of pseudoglycals 31 using palladium acetate as a catalyst and silver acetate as an oxidant in acetonitrile as the reaction medium (Scheme [14]).[35] The scope of this protocol was demonstrated through the C–H activation of pseudoglycals 31 with various substituted activated terminal alkenes such as 7 [R = Me (32a) or Et (32b)], yielding the desired products in good yields. Substituted styrenes were also tested under the optimized conditions, producing the intended products 32cf with moderate to good efficiency.

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Scheme 14 Synthesis of C-2 alkenyl products from pseudoglycals
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Scheme 15 Synthesis of 2-alkenyl-3-amino or thioaryl/alkyl sugars from pseudoglycals by using a benzylic amine (2.0 equiv) or a thiol (1.2 equiv) as the nucleophile, respectively.

Moreover, the resulting C-2 alkenylated products 32 were further transformed into 2,3-disubstituted 3-deoxyglycals 33 through a Michael-type addition at the C-3 position (Scheme [15]); this reaction was enabled by the π-extended conjugation with the nucleophile. Reactions of substituted benzylamines (33a and 33b) or various thiophenols (33c) with compound 32a successfully generated the target products, albeit in moderate yields.

In 2023, the Hussain group presented a Lewis acid-mediated method for synthesizing 2,3-difunctionalized compounds with high efficiency (Scheme [16]).[36] Their approach features a stereoselective azido group insertion at the C3 position of the C2-substituted glycal. The protocol uses acetyl-protected galactals 34 with various electron-withdrawing groups introduced at the C2 position. On treatment with trimethylsilyl azide and boron trifluoride etherate, an azide group adds selectively at the C3 position, yielding the desired products 35af with good efficiency. Additionally, the methodology was extended to various glycals, including d-glucal, d-xylal, and d-maltal, leading to the formation of the 2,3-functionalized compounds 35gi with excellent results.

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Scheme 16 Transformation of C-2 substituted glycals into chiral azides
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Scheme 17 Transformation of substituted glycals into chiral fused aromatic cores by an annulative π-extension reaction with an aryne

# 3

Transformation of Branched Sugars to Annulated Sugars

In 2018, the Hussain group reported the synthesis of naphthalene derivatives through a Diels–Alder reaction in between aryne derived from 2-(trimethylsilyl)phenyl triflate (36) and substituted glycals 8, followed by π-annulation (Scheme [17]).[37] This innovative approach effectively constructs naphthalene frameworks, offering a valuable method for synthesizing complex aromatic compounds. The Diels-Alder reaction was performed in the presence of KF as a base, with 18-crown-6 as an additive, in acetonitrile at room temperature. The reaction proceeded efficiently, completing within ten hours, and achieving a complete conversion of the starting materials, resulting in a good yield of the desired product. The reaction showed broad substrate scope giving the naphthalene products 37ah from the corresponding substituted glycals 8 in moderate to good yields.

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Scheme 18 Possible mechanism for the annulative π-extension reaction

The mechanism of the reaction (Scheme [18]) involves an initial cycloaddition between the benzyne source 36 and the diene 8g, resulting in the formation of an allylic carbanion intermediate 36b. This intermediate subsequently eliminates an alkoxide group, followed by protonation. The process concludes with annulation and π-extension, ultimately forming the desired aromatic product 37a as the system stabilizes to achieve aromaticity.

In 2019, the Sun group developed a versatile approach for synthesizing oxadecalins and C2-branched glycoconjugates (Scheme [19]).[38] The method uses 2-iodo glycals 4 and N-tosylhydrazones 40 in a palladium acetate-mediated Diels–Alder reaction. This strategy provides an efficient route to complex glycoconjugates with significant synthetic utility. A diverse range of annulated products were synthesized with various substituents on the phenyl ring, including methyl (42a), methoxy (42b), chloro (42c), and phenyl (42d), all of which were obtained in good yields. Furthermore, benzoylated glycals reacted successfully with substituted acceptors, yielding the desired products 42e and 42f with high efficiency, demonstrating the versatility of this method.

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Scheme 19 Cross-coupling reaction of 2-iodoglycals with N-tosylhydrazones

In 2019, Vankar and co-workers introduced a one-pot synthesis of 1,2-annulated compounds derived from 2-formyl galactal.[39] The method employed t-BuOK and Zn/HCl–AcOH as reagents, followed by acetylation as outlined in Scheme [20], offering an efficient and streamlined approach for synthesizing the desired compounds. The scope of this protocol was examined by using various alcohols 44 derived from methyl acrylates and aryl aldehydes. Various substituents on the aryl group, such as chloro (45a), methoxy (45b), and methyl (45c), were successfully accommodated in the reaction and the corresponding 1,2-annulated moieties were obtained in good yields. An alcohol derived from 1-naphthaldehyde, a sterically bulky group, also participated in the reaction, yielding the corresponding product 45d in 53% yield.

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Scheme 20 Stereoselective synthesis of substituted 1,2-annulated sugars by domino double-Michael addition reaction

The proposed mechanism (Scheme [21]) begins with the formation of intermediate 43A through an intermolecular Michael attack of a methylene nucleophile on a dienone. Subsequently, intermediate 43B undergoes an intramolecular Michael addition reaction from the α-face leading to the formation of intermediate 43C. This process ultimately yields the desired 1,2-annulated sugars 45.

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Scheme 21 Proposed mechanism for the double-Michael addition reaction of 2-nitrogalactal

In 2022, the Hussain group reported a synthesis of the chiral phenanthrenones 46ae by a Diels–Alder cycloaddition reaction of specifically C-2 branched sugars 30 with the aryne precursor 36 in the presence of CsF in acetonitrile at 40 °C (Scheme [22]).[34]

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Scheme 22 Synthesis of chiral phenanthrenones

Phenanthrenones are notable for their diverse biological activities, including antitumor and antiinflammatory properties. Extracts from the Trigonostemon lii species are traditionally used as expectorants, laxatives, and treatments for certain skin conditions.[40] Among phenanthrenones, denbinobin stands out as a promising antitumor and antiinflammatory agent.[26]

In 2024, Hussain and co-workers disclosed a synthesis of naphthalene-fused derivatives from branched sugars through a 4+2 cycloaddition reaction (Scheme [23]).[41] The substrate scope was investigated by using substituted glycal dienes 47, which reacted with 4,5-dimethoxy-2-(trimethylsilyl)phenyl triflate (36′) in the presence of CsF in acetonitrile at 60 °C for ten hours to give the naphthalene-fused compounds 48ag in moderate to good yields.

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Scheme 23 Synthesis of naphthalene-fused pyran derivatives from glycal dienes

# 4

Conclusion

This Account presents various strategies for transforming and functionalizing glycals into branched sugars through C–H activation and cross-coupling reactions. The methodologies predominantly involve metal-catalyzed reactions, with palladium salts being the most frequently employed catalysts to facilitate these transformations. By delving into the intricacies of these reactions, the Account provides valuable insights into the mechanisms driving the synthesis of complex sugar structures with distinctive branching patterns. Additionally, it discusses protocols for synthesizing aromatic fused systems from branched sugars, including core structures such as naphthalene and phenanthrene, which are integral to many medicinally active drug molecules.


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Conflict of Interest

The authors declare no conflict of interest.


Corresponding Author

Nazar Hussain
Department of Medicinal Chemistry, Institute of Medical Sciences, Banaras Hindu University
Varanasi-221005
India   
Email: nazar10@bhu.ac.in   

Publication History

Received: 19 December 2024

Accepted after revision: 23 January 2025

Accepted Manuscript online:
23 January 2025

Article published online:
08 April 2025

© 2025. Thieme. All rights reserved

Georg Thieme Verlag KG
Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany


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Mittali Maheshwari is a research scholar under the supervision of Dr. Nazar Hussain at Banaras Hindu University (BHU). She completed her Master’s degree in 2019 at Subodh College, University of Rajasthan, India. She cleared CSIR-NET with AIR-50 in 2019 and also GATE in 2019 with AIR-1163, Her research focuses on the study of C-glycosides that hold significant medicinal potential. Dr. Nazar Hussain earned his Ph.D. in chemical sciences in 2019 under the mentorship of Dr. Debaraj Mukherjee at the CSIR-Indian Institute of Integrative Medicine (IIIM), Jammu, India. His doctoral research centered on the synthesis of 2-C-branched sugars and their subsequent transformation into chiral aromatic building blocks, which hold significant potential in synthetic organic chemistry and medicinal applications. Following the completion of his Ph.D., Dr. Hussain joined the Armand Frappier Research Institute-INRS, Canada, as a postdoctoral research associate under the guidance of Professor Charles Gauthier. During this tenure, he expanded his expertise in carbohydrate and medicinal chemistry, contributing to the development of innovative methodologies and compounds. Currently, Dr. Hussain is an assistant professor in the Department of Medicinal Chemistry at the Institute of Medical Sciences, Banaras Hindu University (BHU), Varanasi, India. His research interests include the synthesis of C-glycosides, palladium-catalyzed reactions, the total synthesis of natural products, and the design of novel compounds for medicinal applications. Through his work, he aims to advance the field of synthetic organic chemistry and explore its intersection with drug discovery and development.
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Figure 1 Natural products derived from glycals
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Figure 2 Biologically active drugs containing a chiral fused aromatic system
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Scheme 1 C–H activation of glycals with nonactivated cycloalkenes, affording C-branched sugars
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Scheme 2 Cross-coupling of 2-iodoglycals with amines: access to C2-branched glycoconjugates
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Scheme 3 Synthesis of C-2 branched sugars by cross-dehydrogenative coupling of glycals with terminal alkenes
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Scheme 4 Synthesis of sugar-based enones in the presence of palladium acetate
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Scheme 5 Synthesis of 3,5-disubstituted furans in the presence of a Lewis acid
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Scheme 6 Palladium-catalyzed carbonylation of 2-iodoglycals to give C-2 carboxylic acids derivatives
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Scheme 7 Transformation of 2-iodoglycals into amidoglucals by carbonylative coupling
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Scheme 8 Carbonylative coupling of 2-iodoglucals with alcohols providing access to glucal esters
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Scheme 9 Pd-catalyzed synthesis of C2-aryl glycosides
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Scheme 10 Mild palladium-catalyzed cyanation of unprotected 2-iodoglycals
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Scheme 11 Synthesis of C-2 branched glycol analogues from 2-cyanoglycals. (a) Reaction conditions: A: 22 (1 equiv), H2 (4 bar), Pd(OH)2/C (20 mol%), 1 M HCl, i-PrOH, r.t., 18–72 h. B: 22 (1 equiv), NaBH4 (4.1 equiv) CoCl2·6H2O (cat.), THF–H2O (2:1), 0 °C to r.t., 2–6 h. C: 22 (1 equiv), MeMgBr (6 equiv), THF, 65 °C, 18 h; then 1 M HCl, 65 °C, 30 min. D: 22 (1 equiv), NaN3 (1.1 equiv), CuI (0.2 equiv), DMF, 100 °C, 18 h.
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Scheme 12 C–H activation of glycals with cycloalkenones in the presence of palladium acetate
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Scheme 13 Plausible mechanism for the coupling reaction between glycals and cycloalkenones
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Scheme 14 Synthesis of C-2 alkenyl products from pseudoglycals
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Scheme 15 Synthesis of 2-alkenyl-3-amino or thioaryl/alkyl sugars from pseudoglycals by using a benzylic amine (2.0 equiv) or a thiol (1.2 equiv) as the nucleophile, respectively.
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Scheme 16 Transformation of C-2 substituted glycals into chiral azides
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Scheme 17 Transformation of substituted glycals into chiral fused aromatic cores by an annulative π-extension reaction with an aryne
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Scheme 18 Possible mechanism for the annulative π-extension reaction
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Scheme 19 Cross-coupling reaction of 2-iodoglycals with N-tosylhydrazones
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Scheme 20 Stereoselective synthesis of substituted 1,2-annulated sugars by domino double-Michael addition reaction
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Scheme 21 Proposed mechanism for the double-Michael addition reaction of 2-nitrogalactal
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Scheme 22 Synthesis of chiral phenanthrenones
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Scheme 23 Synthesis of naphthalene-fused pyran derivatives from glycal dienes