Synthesis and Rearrangement of Epoxy Sugars
Abstract: Ring-Contraction of sugar-derived epoxy alcohols, functionalised in position 1 and 6, by treatment with lithium bromide and N, N, N', N'-tetramethylurea in refluxing toluene gave enantiomerically pure a,b-unsaturated aldehydes of furanosides. O-Glycosidic epoxy alcohols gave only one, easily purified, aldehyde in approximately 30-60% yield depending on the substitution pattern. C-Glycosidic epoxy alcohols gave an isomeric mixture, in ratio 1:4, of two aldehydes which could not be separated. The major isomer confirmed that two aldehydes were formed also from O-glycosides but that one isomer is destroyed during the reaction thereby accounting for the low yield of the isolated isomer. Alkylation with methyl 2,3-anhydro-6-deoxy-6-bromo-a-D-allopyranoside of various allyl-sulfides and -sulfones under hexabutyldistannane-induced radical conditions gave the C-6-allylated sugar epoxides in good yields. Acetobromogalactose alkylated the allyl-sulfides and -sulfones under the same conditions to give C-a-galactosides in good yields. Alkylation of methyl 2,3-anhydro-6-deoxy-6-bromo-a-D-allopyranoside by treatment with furfuryl- and decylmercaptan in dimethylformamide, with potassium carbonate as base, gave the corresponding sulfides in position 6 in good yields. Oxidation of sulfur with 4-chloroperbenzoic acid gave the corresponding sulfones. Epoxidation of partly protected 2,2,2-trichloroethyl, and furfuryl glucosides by treatment with triphenylphosphine and diethyl azodicarboxylate in refluxing toluene gave epoxide mixtures, mainly of the corresponding 2,3-anhydro-allopyranoside. C-Glycosidic epoxy alcohols were prepared from C-propyl tetra-O-acetyl-a-D-glucoside by treatment of a mixture of monotosylates with sodium methoxide in methanol. Intramolecular Diels-Alder reaction of an unsaturated aldehyde carrying a suitable linked furfurylsulfide gave one single polycyclic oxanorbornane in good yield. The Diels-Alder reaction was affected by slowly eluting the unsaturated aldehyde on a silica column. Reduction or oxidation of the aldehyde adduct followed by a number of selective transformations gave a series of polycyclic oxanorbornanes. Oxanorbornanes, where the sulfide has been oxidised to a sulfone carries three oxygens geometrically well arranged for coordination of lithium or magnesium ion. One synthetic example of a reaction where the oxanorbornane might act as a crown ether was found.
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