Pyranoid glycal

An interesting application of acetal elimination from sugars that is promoted by base is the formation of furanoid and pyranoid glycals from isopropylidene acetals of ftiranosyl and pyranosyl halides.285-287 Treatment of 2,3 5,6-di-0-isopropylidene-a-D-mannofuranosyl bromide (289) with a base gave the fiiranoid glycal 290. The yield was... 

Other examples of furanoid and pyranoid glycals were independently prepared287 by using lithium in liquid ammonia, a procedure claimed to be more efficient and general. For instance, 2,3-O-isopro-pylidene-4-O-methyl-a-L-rhamnopyranosyl chloride (291) and 6-... 

Similar chemistry involving glycosyl-Cr(III)-linked intermediates, produced from 0-acetylated glycosyl bromides or chlorides by treatment with [Cr(OAc)2 - H20]2 in the presence of EDTA, also results in the formation of 0-acetylated pyranoid glycals in high yields.14... 

L. Somsak and I. Nemeth, A simple method for the synthesis of acetylated pyranoid glycals under aprotic conditions, J. Carbohydr. Chem., 12 (1993) 679-684. 

G. Kovacs, J. Gyarmati, L. Somsak, and K. Micskei, Long-lived glycosyl-chromium (III) complex intermediates in aqueous medium. Preparation of pyranoid glycals, Tetrahedron Lett., 37 (1996) 1293-1296. 

M. Casillas, A. M. Gomez, J. C. Lopez, and S. Valverde, A simple entry to pyranoid glycals reaction of anomeric glycosyl sulfoxides with organolithiums, Synlett, (1996) 628-630. 

R. E. Ireland, S. Thaisrivongs, N. Vanier, and C. S. Wilcox, Enolate Claisen rearrangements from furanoid and pyranoid glycals, J. Org. Chem., 45 (1980) 48-61. 

G. Casiraghi, M. Cornia, G. Rassu, L. Zetta, G. G. Fava, and M. F. Belicchi, Stereoselective arylation of pyranoid glycals using bromomagnesium phenolates an entry to 2,3-unsaturated C-a-glycopyranosylarenes, Carbohydr. Res., 191 (1989) 243-251. 

M. Tingoli, B. Panunzi, and F. Santacroce, The reaction of furanoid and pyranoid glycals towards aromatic Grignard reagents and a nickel catalyst, Tetrahedron Lett., 40 (1999)... 

Amino glycosides.1 Furanoid or pyranoid glycals (2) when irradiated with this azodicarboxylate undergo a [4 + 2]cycloaddition to form a dihydrooxadiazine (3) in 70-80% yield. The adducts on treatment with methanol (TsOH) open with inversion at C, to give hydrazines (4), which furnish 2-amino glycosides (5) on hydrogenolysis and deprotection. 

By the cycloaddition of dibenzyl diazenedicarboxylate and furanoid and pyranoid glycals it is possible to diastereoselectively introduce an amino function at the C-2 position of a carbohydrate29-34. A single diastereomer is produced from furanoid glycals, where the cycloaddition takes place trans to the C-3 hydroxy substituent protected as the silyl ether, e.g., formation of 14-1630. The more reactive bis(2,2,2-trichloroethyl) diazenedicarboxylate can be employed even with less reactive 3-acetyl glycals31. 

Cheng, J C Y, Daves, G D Jr, C-glycosides from palladium-mediated reactions of pyranoid glycals. Stereochemistry of formation of intermediate organopalladium adducts and factors affecting their stability and decomposition, J. Org. Chem., 52, 3083-3090, 1987. 

The potential of the ester enolate Claisen rearrangement for the stereocontrolled synthesis of highly functionalized, complex systems has been demonstrated in numerous applications in natural product synthesis. Utilizing the 1,3-chirality transfer Ireland has synthesized oxygen heterocycles with chiral side chains, such as are found as units in polyether antibiotics and macrolides, starting from enantiomerically pure furanoid or pyranoid glycal systems of type (41), which are easily accessible from carbohydrates (Scheme 65). ... 

The results obtained in the cyclohexenyl series are contrasted by the rearrangement of the corresponding pyranoid derivatives. Thus, for both the ( )- as well as the (Z)-silyl ketenc acetals of pyranoid glycal 14 a strong preference for the boatlike transition state is delineated from the product stereochemistry515. 

The rearrangement of pyranoid glycal 18 parallels the findings obtained for 14MS. 

A useful transformation of pyranoid glycals by Danishefsky et leads to 1,2-trans 2-deoxy-2-sulfonylaminoglycopyranosyl azides. In this two-stage procedure, the glycal 162 first undergoes iodosulfonamidation to give the... 

Furanoid and pyranoid glycals. The classical method for preparation of glycals is reduction of pyranosyl halides with zinc and acetic acid. A more recent method is lithium-ammonia reduction of furanosyl and pyranosyl halides with a blocking acetonide group. ... 

Good yields of pyranoid glycals were obtained from acetal or ether protected D-gluco-, T)-galacto- or p-mamo- configured glycosyl sulfoxides by reaction with aryl- or alkyl-lithiums. It was determined that the nature of the lithium reagents... 

Ireland and his co-workers have amplified their studies on the synthesis of furanoid and pyranoid glycals (Scheme 1) and their use in the synthesis of 2,3-unsaturated C-glycosides (see Chapter 3). ... 

Unusual types of pyranoid glycal derivatives with electronegative substituents at C-5 have been encountered during studies of 5-bromo-D-xylopyranose tetraacetate (Scheme 2). In the course of related work a 5-bromo-D-glucopyranosyl bromide ester was obtained which gave a diene having a glycal structure with an additional exocylic double bond (see Chapter 7). 

The [4+2] cycloaddition reaction of furanoid and pyranoid glycals with azodicarboxylates allows the introduction of a nitrogen atom at C-2 of a carbohydrate, and the adducts thus obtained can be converted to structurally complex 2-amino saccharides under mild conditions. Towards this end, dibenzyl and bis(2,2,2-trichloroethyl) azodicarboxylates were used as dienes since they allow the free amines to be generated in a single operation under mild conditions later in the synthetic sequence. 

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