HETERO-DIELS-ALDER REACTION WITH THIAZOLYL OXABUTADIENE-MODEL STUDIESTOWARD 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 ether with 1-oxabuta -1,3-dienes bearing a thiazolyl ring at C-2 is described. The cycloadd ition 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 3 b in a ca. 9:1 ratio and 91% overall yield. The elaboration of 3a thro ugh the conversion of the thiazole ring into the formyl group and redu ction of the latter to alcohol, followed by hydroxylation of the doubl e bond through hydroboration-oxidation led to the ethyl 2,3-dideoxypyr anoside 8. The asymmetric version of this synthetic sequence started f rom the HDA cycloaddition of the same alkene with the chiral oxabutadi ene 10 bearing the D-galacto-pentopyranosid-5-yl moiety at C-3. This r eaction afforded a mixture of the four diastereomeric cycloadducts-3,3 -dihydro-2H-pyrans 11a,b and 12a,b- in 97% overall yield, The reaction was moderately endo and face selective. A high level of endo selectiv ity (96%) was obtained by the use of catalytic Eu(fod)(3). The elabora tion of the endo cycloadducts 11a and 12a by the same synthetic sequen ce as that developed for 3a (i.e. thiazolyl-to-formyl conversion and h ydroxylation of the double bond) gave the uncommon C-disaccharides 15 and 16 featuring two directly linked pyranose rings.