Glycals pyranoid

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-... 

B. H. Lipshutz, S. L. Nguyen, and T. R. Elworthy, Preparation and Diels-Alder reactions of a pyranoid vinyl glycal Model studies for anthraquinone aglycone and carbohydrate syntheses, Tetrahedron 44 3355 (1988). 

Compounds of this category are vinyl ethers having double bonds between C-l and C-2 of pyranoid or furanoid aldose derivatives. Analogues with exocyclic C-l-C-2 double bonds in cyclic 2-ketoses are sometimes referred to as evo-glycals and are covered briefly in Section IV.3 isomers with C-2-C-3 unsaturation in 2-ketoses can be considered as C-l-substituted glycals and are referred to in Section II.3.a. Together the glycals and their derivatives constitute the most useful set of unsaturated carbohydrate derivatives for application in synthesis. 

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... 

Relevant pyranoid and furanoid compounds (with the exception of the furanoid 3-enes that, as enol ethers, are noted in Section IV. 1) have isolated double bonds in the sense that they are not components of vinyl ethers. In consequence their chemistry is distinct from that of the glycals and their derivatives in particular, their addition reactions are seldom regioselective as are those of the glycals. 

An entirely different approach to pyranoid 4-enes depends on eliminations from 5-bromo compounds obtained from pyranoid hexuronic acid derivatives by photobromination. Treated with zinc-acetic acid, the bromide 249 gives the glycal -like 250 (62%), while the 4-acetoxy compound 251 is formed when DBU is used to promote elimination. Similarly, base treatment of penta-0-acetyl-5-bromo-/J-D-glucose with DBU causes the analogous loss of hydrogen bromide and formation of the 4-acetoxy-4-ene, but use of zinc-acetic acid affords mainly the 5-exo-methylene alkene by the alternative available elimination process.237... 

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)... 

J. K. Hale, S. Manaviazar, and R. Tupprasoot, The synthesis and reactions of some novel pyranoid 5,6-glycals derived from D-fructose, Tetrahedron, 53 (1997) 16365-16376. 

F. W. Lichtenthaler, S. Hahn, and F.-J. Flath, Pyranoid exo- and endo-glycals from D-fructose and L-sorbose, Liebigs Ann. Chem., (1995) 2081-2088. 

This delightfully simple methodology is generally applicable to hydroxy-glycal esters and has provided acylated ketoximes of type 25 in large variety acylated pyranoid ketoximes, featuring such useful properties as high tendency for crystallization, ease of isolation and stability (29). 

The name glucal was proposed by Emil Fischer because his first impure preparations gave aldehyde reactions. When it was found later that this observation does not apply to pure glucal, the name had already become well established. Sugars epimeric at carbon 2 yield identical glycals, because the asymmetry at both carbons 1 and 2 is lost. In the older literature, the ring structures appear as the furanoid type, but this was later shown to be incorrect, and in the present article they will be changed to the correct pyranoid structures. 

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. 

The fluorination and stereochemistry of the reaction of pyranoid and furanoid glycals with acetyl hypofluorite are described41. In related work42 the synthesis of 2-deoxy-2-fluoro-D-galactapyranose by treatment of 2-deoxy-D-galactapyranose with acetyl hypofluorite is described and the 18F-labelled product was similarly prepared through the use of Ac018F. 

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. 

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