Regio- and stereo-selective bromo(alkoxylation)s of (E)-alpha-(2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyloxymethylene) carbonyl compounds. A route to near-stereopure alpha-bromo alpha-dioxymethyl carbonyl compounds

(E)-4-Methoxymethoxy-3-methylbut-3-en-2-one 17b reacts with NBS in propan-1 -ol in a highly regio- and anti-stereo-selective manner to give (3R*,4R*)-3 -bromo-4-methoxymethoxy-3-methyl-4-propoxybutan-2-one 18. Compound 10, a re lative of the butenone 17b in which the methoxymethyl group is replaced by the 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl unit, undergoes an analogo us bromo(propoxylation) reaction with reasonable facial selectivity to give an 86:14 mixture of (3R,4R)-3-bromo-3-methyl-4-propoxy-4-(2',3',4',6'-tetr a-O-acetyl-beta-D-glucopyranosyloxy)butan-2-one 11c and its (3S,4R)-diaster eomer 12c. The major bromo(propoxy) derivative, isolable in 57% yield by fr actional crystallisation, is assigned the stereostructure 11c by single-cry stal X-ray crystallographic analysis. Other primary alcohols and methanol p articipate in the reaction of compound 10 with NBS, leading predominantly ( with selectivities ranging from 75:25 to 89:11) to bromo(alkoxy) products o f type 11 which are usually separable from their diastereomers of type 12 b y fractional crystallisation (in yields ranging from 41 to 64%). A model to account for the role of the 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl u nit in the stereoinduction process is proposed. Related bromo(propoxylation)s are observed with the vinylogous esters 24, 2 5a and 25b, leading to the isolation of the major products, 28, 30a and 30b (in yields ranging from 39 to 55%), and with the vinylogous carbonates 32a , 32c, 37a and 37b, providing access to the major products 33a, 33c, 38a an d 38b (in yields ranging from 52 to 73%). In the presence of trifluoroacetic acid and ethane-1,2-diol, the bromo(prop oxy) derivatives 11c, 28, 30b and 33c undergo transacetalisation to give th e ethylene glycol acetals 40a, 40b, 41 and 40c with ees of 94-98%, in yield s ranging from 56 to 67%.