Synthesis of mycarose and epi-axenose from non-carbohydrate precursors.

A six-step synthesis of racemic mycarose from allylacetylene is described. Key transformations include the threo-selective epoxidation of (E)-4-methyl-1,4-heptadien-6-ol and the alpha-opening of xylo-4,5-epoxy-4-methylhept-1-en-6-ol (7), which was accomplished via a neighboring group-assisted reaction of xylo-4,5-epoxy-4-methyl-6-(N-phenylcarbamoyloxy)hept-1-ene (12). The latter conversion proceeded wih lower selectivity (3:1) than observed with disubstituted epoxyurethanes because of the greater tendency of trisubstituted epoxides to undergo substitutions with SN1 character at the tertiary center. Methanolysis of ribo-4-methylhept-1-ene-4,5,6-triol 5,6-carbonate, obtained from 12 in up to 61% yield, afforded ribo-4-methylhept-1-ene-4,5,6-triol, which was converted into mycarose by ozonolysis. Similarly, ozonolysis of lyxo-4-methylhept-1-ene-4,5,6-triol, which was prepared (64%) by hydrolysis of 7, afforded racemic 3-epi-axenose.