Glycosyl Imidates, 76. Synthesis of Aryl C-Glycosides of Glucosamine

Tunable Approach to C-Linked Analogs of Glycosamines

The synthesis of (1R)-2-amino-2-deoxy-β-l-gulopyranosyl benzene and the α and β forms of 2-amino-2-deoxy-l-idopyranosyl benzene derivatives was accomplished through stereospecific addition of tributylstannyllithium to readily available (S_R)- or (S_S)-N-tert-butanesulfinyl-arabinofuranosylamine building blocks, followed by stereoretentive Pd-catalyzed Migita-Kosugi-Stille cross-coupling, stereoselective reduction, and an activation-cyclization strategy. Application of this methodology paves the way to new three-dimensional chemical space and preparation of unknown (non-natural) and complex 2-amino-2-deoxy sugars of biological interest.

2-nitroglycals as powerful glycosyl donors: application in the synthesis of biologically important molecules

[Reaction: see text]. The biological significance of oligosaccharides and glycoconjugates is profound and wide-ranging. For example, the mucins have attracted attention because of their role in fundamental cellular processes such as fertilization, parasitic infection, inflammation, immune defense, cell growth, and cell-cell adhesion. Increased expression of mucins is implicated in malignant transformation of cells. Antifreeze glycoproteins also are of interest because they are important for the survival of many marine teleost fishes that live in polar and subpolar waters. The synthesis of glycoconjugates requires methods for glycoside bond formation, the most difficult aspect of which is the assembly of monosaccharide building blocks. This Account discusses a valuable addition to the repertoire of methods for glycoconjugate synthesis: an approach that involves 2-nitroglycal concatenation. For a long time, methods for glycosylation via glycosyl donor generation required either an anomeric oxygen exchange reaction or anomeric oxygen retention. In the case of an anomeric oxygen exchange reaction, activation of the glycosyl donors demands a promoter in at least equimolar amounts. However, anomeric oxygen retention, such as base-catalyzed formation of O-glycosyl trichloroacetimidates, can be activated by catalytic amounts of acid or Lewis acid. Alternatively, glycals, which are readily available from sugars, can be an attractive starting material for glycoside bond formation. Their nucleophilic character at C-2 permits reactions with oxygen, nitrogen, and sulfur electrophiles that under high substrate stereocontrol generally lead to three-membered rings; ring opening under acid catalysis furnishes the corresponding glycosides, whichdepending on the electrophile Xare also employed for 2-deoxyglycoside synthesis. Glycals also can be transformed into derivatives that have at C-2 an electron-withdrawing group and are amenable to Michael-type addition. A good example are 2-nitroglycals. In this case, glycoside bond formation is achieved under base catalysis and leads to 2-deoxy-2-nitroglycosides. These intermediates are readily converted into 2-amino-2-deoxyglycosides, which are constituents of almost all glycoconjugates. This 2-nitroglycal concatenation has been extensively investigated with 2-nitrogalactal derivatives. When alcohols are used as nucleophiles and strong bases used as catalysts, the result is primarily or exclusively the alpha-galacto-configured adducts. Some studies show that weaker bases may lead to preferential formation of the beta-galacto-configured products instead. The reaction was very successfully extended to other nucleophiles and also to other 2-nitroglycals that undergo base-catalyzed stereoselective Michael-type additions. Thus, 2-nitroglycals are versatile synthons in glycoconjugate and natural-products synthesis, and it is foreseeable that many more applications will be based on these readily available and highly functionalized skeletons.

Synthesis and induction of apoptosis in B cell chronic leukemia by diosgenyl 2-amino-2-deoxy-beta-D-glucopyranoside hydrochloride and its derivatives

2-Acetamido-2-deoxy-D-glucose hydrochloride (D-glucosamine hydrochloride) has been used for the preparation of 1,3,4,6-tetra-O-acetyl-2-deoxy-2-trifluoroacetamido-beta- (4) and 2-tetrachlorophthalimido-alpha,beta-D-glucopyranose (6), which have been transformed into the appropriate bromides and the chloride. Both bromo and chloro sugars were used as a glycosyl donors for the glycosylation of diosgenin [(25R)-spirost-5-en-3beta-ol]. These condensations were conducted under mild conditions, using silver triflate as a promoter, and gave diosgenyl glycosides 9 and 12. Each of them was converted into diosgenyl 2-amino-2-deoxy-beta-D-glucopyranoside hydrochloride (11) and N-acylamido derivatives. The structures of all new glycosides were established by 1H and 13C NMR spectroscopy. These diosgenyl glycosides are the first saponins containing the D-glucosamine residue that have been synthesized. These compounds show promising antitumor activities. The synthetic saponins increase the number of apoptotic B cells, in combination with cladribine (2-CdA), that are isolated from chronic lymphotic leukemia (B-CLL) patients.

Total synthesis of nucleobase-modified adenophostin A mimics

The adenophostins exhibit approximately 10-100 times higher receptor binding and Ca2+ mobilising potencies in comparison with the natural second messenger D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3]. Despite many synthetic attempts to determine the minimal structural requirement for this unusual behaviour of the adenophostins, few related simplified analogues displaying higher activity than that of Ins(1,4,5)P3 have been reported. However, biological evaluation of such analogues has revealed that one of the key factors for the enhanced biological activity is the adenine moiety. To further understand the effect that the adenine base has upon the activity of the adenophostins, congeners in which this functionality is replaced by uracil, benzimidazole, 2-methoxynaphthalene, 4-methylanisole and 4-methylnaphthalene using the common intermediate 1,2-di-O-acetyl-5-O-benzyl-3-O-(3,4-di-O-acetyl-2,6-di-O-benzyl-alpha-D-glucopyranosyl)-ribofuranose have been synthesised using a base replacement strategy. The synthesis of the uracil and benzimidazole analogues was achieved using the Vorbrüggen condensation procedure. The 1'-C-glycosidic analogues were prepared using Friedel-Crafts type C-aryl glycosidation reactions. Phosphate groups were introduced using the phosphoramidite method with subsequent removal of all-benzyl protecting groups by catalytic hydrogenation or catalytic hydrogen transfer. Apart from one analogue with an alpha-glycosidic linkage all compounds were more potent than Ins(1,4,5)P3 and most tended more towards adenophostin in activity. These analogues will be valuable tools to unravel the role that the adenine moiety plays in the potent activity of the adenophostins and demonstrate that this strategy is effective at producing highly potent ligands.

Synthesis of N4-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-L-asparagine analogues: succinamide, L-2-hydroxysuccinamide, and L-2-hydroxysuccinamic acid hydrazide analogues

The syntheses of three analogues of N4-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-L-asparagine are described. N-(2-Acetamido-2-deoxy-beta-D-glucopyranosyl)succinamide was synthesized by the reaction of pentafluorophenyl succinamate with 2-acetamido-2-deoxy-beta-D-glucopyranosylamine. 2-Acetamido-3,4,6-tri-O-acetyl-2-deoxy-beta-D-glucopyranosylamine was synthesized, and the complete assignment of the 1H NMR spectrum is given. Reaction of the protected beta-D-glycosylamine with L-malic acid chloralid in the presence of a coupling agent (EEDQ) gave N4-(2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-beta-D-glucopyranosyl)-L-malamic acid chloralid that was deprotected two ways: (1) using ammonia, which gave N4-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-L-2-hydroxysuccinamide, and (2) using hydrazine, which gave N4-(2-acetamido-2-deoxy-1-D-glucopyranosyl)-L-2-hydroxysuccinamic acid hydrazide.

The synthesis of diosgenyl 2-amino-2-deoxy-beta-D-glucopyranoside hydrochloride

The N-trifluoroacetyl- and N-tetrachlorophthaloyl-protected bromide of D-glucosamine has been used for the first time as a glycosyl donor for the glycosylation of diosgenin [(25R)-spirost-5-en-3beta-ol]. Both 1,3,4,6-tetra-O-acetyl-2-deoxy-2-trifluoroacetamido-beta-D-glucopy ranoside and 1,3,4,6-tetra-O-acetyl-2-deoxy-2-tetrachlorophthalimido-alpha,beta -D-glucopyranoside were transformed into the appropriate glycosyl bromides. These reacted with diosgenin under mild conditions, using silver triflate as a promoter, and gave the corresponding protected diosgenyl glycosides. Each was deprotected to give diosgenyl 2-amino-2-deoxy-beta-D-glucopyranoside hydrochloride. The structures of the new glycosides were established by 1H NMR spectroscopy.

Synthesis of N-acetylglucosamine containing Lewis A and Lewis X building blocks based on N-tetrachlorophthaloyl protection--synthesis of Lewis X pentasaccharide

Phenyl 6-O-benzyl-2-deoxy-2-tetrachlorophthalimido-1-thio-beta-D- glucopyranoside (5a) and thexyldimethylsilyl 6-O-benzyl-2-deoxy-2-tetrachlorophthalimido-beta-D- glucopyranoside (5b) gave with O-(2,3,4,6-tetra-O-acetyl-alpha-D-galactopyranosyl)trichloroacetimida te (8) in the presence of BF3.Et2O as catalyst exclusively lactosamine derivatives 7a and 7b, respectively, in high yields. Ensuing reaction with O-(3, 4-di-O-acetyl-2-O-benzyl-alpha-L-fucopyranosyl) trichloroacetimidate (9) in the presence of TMSOTf as catalyst afforded Le(x) trisaccharide intermediates 10a,b. With fucosyl donor 9 and 5a,b as acceptors in the presence of TMSOTf as catalyst glycosylation either at the 3-O or the 4-O was observed, thus leading to mixtures of disaccharides 11a/12a and 11b/12b, respectively; their reaction with 8 furnished Le(x) trisaccharide intermediates 10a,b and Le(a) trisaccharide intermediates 14a,b. Transformation of 10b into the corresponding trichloroacetimidate 17 and reaction with lactose acceptor 19 in the presence of Zn(OTf)2 as catalyst gave protected Le(x) pentasaccahride intermediate 21, which on deprotection led to unprotected Le(x) pentasaccharide 1.

N-trichloroethoxycarbonyl-glucosamine derivatives as glycosyl donors

D-Glucosamine can be readily transformed into 1,3,4,6-tetra-O-acetyl-2-deoxy-2-(2,2,2-trichloroethoxycarbonylamino+ ++) -D-glucopyranose (2). From this intermediate valuable glycosyl donors can be obtained; reaction with ethanethiol in the presence of boron trifluoride etherate afforded ethyl 3,4,6-tri-O-acetyl-2-deoxy-1-thio-2-(2,2, 2-trichloroethoxycarbonylamino)-beta-D-glucopyranoside (4) which gave, upon N-acetylation, the N-acetyl-N-trichloroethoxycarbonyl derivative (5). Selective removal of the 1-O-acetyl group in 2 followed by treatment with trichloroacetonitrile in the presence of base afforded 3,4,6-tri-O-acetyl-2-deoxy-2-(2,2,2-trichloroethoxycarbonylamino)- alpha -D-glucopyranosyl trichloroacetimidate (6). Reaction of 5 with five selectively protected glycosides as glycosyl acceptors in the presence of N-iodosuccinimide/trifluoromethanesulfonic acid as the promoter system furnished the corresponding beta-glycosides in good yields, thus exhibiting the valuable glycosyl donor properties of 5. Reductive removal of the trichloroethoxycarbonyl (Teoc) group afforded the corresponding N-acetyl-protected saccharides in high yields. The imidate 6 reacted with three of the above acceptors in the presence of catalytic amounts of trimethylsilyl trifluoromethanesulfonate to give the beta-linked disaccharides in even better yields. The direct replacement of the N-Teoc group by the N-acetyl group using zinc/acetic anhydride, via the free amines as transient intermediates, adds to the high efficiency and convenience of this methodology.