En route to thromboxane compounds from carbohydrates, I. Synthesis of the unsaturated sugar precursors

Allyl 4‐ C ‐[(carboxamido)‐ and (ethoxycarbonyl)methyl]hex‐2‐enopyranosides 18a – c , representing chiral precursors for the carbohydrate‐based construction of the ring systems of thromboxane A 2 and B 2 were prepared by means of the Claisen‐type rearrangement of the suitably substituted hex‐3‐enopyranosides 7a, b . The latter sugars were synthesized from allyl α‐D‐glucopyraoside ( 9 ) by modification of known procedures, including selective O ‐benzoylation with 1‐(benzoyloxy)‐1 H ‐benzotriazole ( 11 ), and Tipson‐Cohen sulfonyl ester elimination of the 3,4‐di‐ O ‐mesylates 16 and 17 . Studies on the corresponding methyl glycosides 8a – c showed a reduced stability of the 6‐ O ‐( tert ‐butyldiphenylsilyl) ether group under the Tipson‐Cohen reaction conditions.