Stereoselective epoxidation of divinylmethanol: A synthetic approach to the pentitols

Peroxy-acid epoxidation of divinylmethanol (9), followed by acetylation afforded the acetylated monoepoxides 1 and 2 having the erythro (53%) and threo (47%) configurations. Peroxy-acid epoxidation of 1 and 2 yielded the acetylated diepoxides 3 (erythro-erythro, 36%) and 4 (erythro-threo, 64%) (from 1), and 4 (erythro-threo, 47%) and 5 (threo-threo, 53%) (from 2). Relative configurational assignments were made to 1-5 on the basis of (a) ¹H-n.m.r. chemical-shift and coupling-constant data, (b) the observation that 1 gave only 3 and 4, and that 2 gave only 4 and 5 on epoxidation, and (c) the fact that 3-5 separately undergo epoxide-ring opening preferentially at their primary carbon atoms with acetate ion in acetic anhydride, to afford the penta-acetates of ribitol, dl-arabinitol, and xylitol, respectively, as major products. Epoxide-ring formation favours the erythro configuration when either peroxy acids or tert-butyl hydroperoxide with catalytically active Ti⁴⁺, V⁵⁺, or Mo⁶⁺ complexes are employed as epoxidation reagents. However, the diastereoselectivities characterising the epoxidations and the regioselectivities governing the epoxide-ring openings are not sufficiently high to constitute an attractive synthesis of either ribitol, dl-arabinitol, or xylitol from divinylmethanol. {A figure is presented}.