Hetero-Diels-Alder reaction with thiazolyl oxabutadiene-model studies toward the synthesis of directly linked C-disaccharides

A method for the construction of substituted pyranoses by means of the hetero-Diels Alder (HDA) reaction of ethyl vinyl either with 1-oxabuta-1,3-dienes bearing a thiazolyl ring at C-2 is described. The cycloaddition with 1-(thiazol-2-yl)-2-penten-1-one (2) occurred with good endo/exo selectivity to give cis- and trans-3,4-dihydro-2H-pyrans 3a and 3b in a ca. 9:1 ratio and 91% overall yield. The elaboration of 3a through the conversion of the thiazole ring into the formyl group and reduction of the latter to alcohol, followed by hydroxylation of the double bond through hydroboration oxidation led to the ethyl 2,3-dideoxypyranoside 8. The asymmetric version of this synthetic sequence started from the HDA cycloaddition of the same alkene with the chiral oxabutadiene 10 bearing the D-galacto-pentopyranosid-5-yl moiety at C-3. This reaction afforded a mixture of the four diastereomeric cycloadducts 3,4-dihydro-2H-pyrans 11 a,b and 12 a,b in 97% overall yield. The reaction was moderately en...