Glycosyl lithium

Glycosyl lithium compounds (c.f. Schemes 9 and 10) can also be prepared from benzylated glycal derivatives and hence converted selectively into a- and 3-C-glycosides (Scheme 16). Reaction... 

The use of nucleophilic carbohydrates for the synthesis of C -pyranosides has been extended to include glycosyl-lithium (36 M = Li) which is generated under standard conditions from the corresponding stannane (36 M = SnBu )- The B-anomer of (37 M = SnBu ) is also available and both lithiated derivatives have been shown to be configurationally stable. 

An extensive range of methods involving anomerlc substitution reactions have been applied, several utilising glycosyl lithium Intermediates, and Sinay and his coworkers have described fully... 

Glycosyl-linkages were determined by GC-EIMS of the partially methylated alditol acetates. RG-II samples (2 mg) were methylated using sodium methyl sulfmyl carbanion and methyl iodide in dimethyl sulfoxide [24] followed by reduction of the uronosyl groups with lithium triethylborodeuteride (Superdeuteride , Aldrich) [23,25]. Methylated and carboxyl-reduced samples were then submitted to acid hydrolysis, NaBIlt reduction and acetylation, partially methylated alditol acetates being analysed by EIMS on two fused-silica capillary columns (DB-1 and DB-225) [20]. 

A significant role of lithium in controlling stereochemistry in glycosylation reactions has been reported. In the presence of lithium perchlorate, glycosylation with alcohols proceeds with excellent a-selectivity (Scheme 3).15... 

Starting from 3,4,6-tri-O-benzyl-D-glucal epoxide 93 (Scheme 31), a-C-glycosyl compounds 94 are obtained in good stereoselectivity by treatment with a lithium acetylide and zinc chloride.103... 

D-Ribonolactone is a convenient source of chiral cyclopentenones, acyclic structures, and oxacyclic systems, useful intermediates for the synthesis of biologically important molecules. Cyclopentenones derived from ribono-lactone have been employed for the synthesis of prostanoids and carbocyclic nucleosides. The cyclopentenone 280 was synthesized (265) from 2,3-0-cyclohexylidene-D-ribono-1,4-lactone (16b) by a threestep synthesis that involves successive periodate oxidation, glycosylation of the lactol with 2-propanol to give 279, and treatment of 279 with lithium dimethyl methyl-phosphonate. The enantiomer of 280 was prepared from D-mannose by converting it to the corresponding lactone, which was selectively protected at HO-2, HO-3 by acetalization. Likewise, the isopropylidene derivative 282 was obtained (266) via the intermediate unsaturated lactone 281, prepared from 16a. Reduction of 281 with di-tert-butoxy lithium aluminum hydride, followed by mesylation, gave 282. 

Scheme 3. General scheme of C-glycosylation procedures (LiN = lithium naphthalide)...

A set of 12 C-glycosyl asparagines has been synthesized from lithium C-glycoside acetylides and the Gamer aldehyde 169.[529]... 

Enantiomerically pure glycosyl stannanes gave, after tin-lithium exchange and reaction with oxiranes in the presence of boron trifluoride diethyl ether complex, alkylation products as diastereomers (1 1 -2 1 d.r.) with complete retention at the stereogenic center a to the oxygen atom41. 

Three further interesting examples, which delineate the utility of [3-lactams in synthesis, have been described recently. In an approach to C-linked glycosyl amino acids, Scheme 37, addition of the lithium dianion 112 to the corresponding N-Boc (3-lactam 111 provides the (3-amino ketone 113 [104],... 

Furthermore, this phosphorylation reaction could be applied not only to the glycosyl position but also to other hydroxyl groups. When compound 38 (Figure 11) was treated with butyl-lithium and diphenyl phosphorochloridate as above, a syrupy 4-phosphate 39 was obtained whose structure was unequivocally confirmed by 3oOMHz -NMR. A quartet signal of H-4 (6 4.70ppm,... 

By Ferrier glycosylation of di-O-acetyl-L-rhamnal (28) with the epoxide derivative 117 the disaccharide 118 is obtained. This is trans-opened by lithium iodide according to Fiirst-Plattner s rule. Upon different ways, two interesting products are obtained, either the original C-B disaccharide of aclacinomycin A 120 or its C-5 epimer 119 which contains one sugar in the D-configuration [69]. 

Mithramycin shows a completely P-linked chain of D-conflgurated saccharides. This requires a totally different approach for the synthesis which is also done by application of the DBE method. The previously obtained disaccharide 180 is P-glycosylated with the monosaccharide precursor 176 to give the trisaccharide 185. After reductive debromination (Bu3SnH), an acid deformylation deblocked the C-3" position which is oxidized with pyridinium dichromate. Nucleophilic attack at the carbonyl group by methyl lithium affords a 1 1.2 mixture of 186 and 187 none of which is the desired compound [93]. Obviously, the methyl branch is formed exclusively in the axial way. 

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