Un nouvel exemple de transposition: l'épimérisation en C(3) d'hydroxy‐3‐δ 1 ‐pyrazolines dérivées de sucres

A Novel Example of Reversible Ring Opening: The Epimerization at C(3) of Sugar 3‐Hydroxy‐Δ 1 ‐pyrazolines Reaction of 1 (either geometrical isomer) with hydrazine followed by in situ Ag 2 O oxidation led to two pairs of interconverting isomers 4 ⇄ 5 and 6 ⇄ 7 . By the same treatment, (Z)‐ 10 and (or) ( E )‐ 10 gave the pair 11 ⇄ 12 . Acetylation of 4 ⇄ 5 led to a non interconverting mixture of 8 and 9 . This fact, and the lack of incorporation of 18 O when the epimerization took place in the presence of H 2 18 O indicated that the most probable mechanism consisted in a reversible ring opening ( D ⇄ E ⇄ F ). The kinetic parameters of these reactions are given and structural assignments proposed for the new compounds.