Preliminary experiments have been carried out on the oxidation of the double bonds of 2.3-dideoxy-a-D-hex-2-enopyranosides with a view to developing glycoside syntheses. Hydroxylation of 4,6-di-0-acetyI-2,3-dideoxya-eryrhro-hex -2en osi de com po u n d s with n eu t ra I perm an g a n a te affords p red o m i n a n t I y mannoadd u cts as was exemplified by the synthesis of 6-0-a-D-mannopyranosyl-D-galactose from an unsaturated disaccharide derivative.Epoxidation of methyl 4,6-di-0-acetyl-2,3-dideoxy-a-~-erythro-hex-2-enopyranoside gave the rnanno-and allo-adducts in the ratio 3 : 2. This ratio could be controlled to some extent by varying the allylic substituents ; the analogous t-butyl glycoside gave a ratio 4:1, and for the deacetylated methyl compound the ratio was 1 :3. Similar epoxidations of methyl 4,6-di-0-acetyl-2,3-dideoxy-a-~-~~~eo-hex-2-enopyranos~de and its deacetylated derivative were studied. The four methyl 4.6-di-O-acetyl-2,3-anhydro-a-D-hexopyranosides prepared by these means were isolated by preparative g.1.c. and characterised by n.m.r. spectroscopy.galactopyranose gave a crystalline 1 : 1 mixture of the isomeric epoxides in high yield. Controlled hydrolysis of the product afforded 6-O-a-D-altropyranosyl-D-galactose and 6 -0 -(3.6-anhydro-a-D-glucopyranosyl) -D-galactose which were isolated in pure form. Hydrolyses of the monosaccharide epoxides were examined briefly, crystalline a-D-altopyranosyl derivatives were isolated during the study, and the problem of synthesising glucosides by this means was considered. 1 (a) Part IX, R. J. Ferrier and N. Prasad, preceding paper;(0) portions of Parts I X and X have been reported in a pre-
On heating in inert solvents 1 -deoxyald-1 -enopyranose esters rearrange completely and stereospecifically to isomeric 3-deoxyald-2-enopyranose esters which have the same configuration at C-1 as was present at C-3 in the original compounds. An SNiJ mechanism is envisaged, and the reaction affords excellent means for obtaining thermodynamically unstable anomers in some instances. Alternatively, the isomerisations can be brought about at room temperature by the use of small amounts of boron trifluoride, but under these conditions anomerisation rates exceed those of the allylic rearrangements and equilibrated mixtures of a-and P-products are obtained.IT is now well established that lJ2-dideoxyald-l-enose f (glycal) esters in the absence of protonic acids can undergo nucleophilic attack at C-1 and afford 2,3-unsaturated
CH2 OACCH2 OAC X = e.g., OH, OMe, SAC, OPh, or F3 SCHEME glycosyl derivatives as is exemplified for tri-O-acetyl-D-glucal in the Scheme. The only analogous reaction which has been reported for 1-deoxyald-l-enose 7 (2j-Although no conventions hzve bet: formally adopted for naming unsaturated sugars, the enose method is herein used to accord with anticipated nomenclature rules. This represents a departure from the " dehydrodeoxy " method used hitherto in this series.
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