An unexpected rearrangement during Mitsunobu epimerization reaction of sugar derivatives

Mitsunobu reaction on the glucose derivative (3S,4R,5R,6R)-3,4,5,7- tetrabenzyloxy-6-hydroxy-1-heptene yielded an unexpected rearrangement major product. Its structure was determined as (3R,4R,5R,6S)-4,5,6,7- tetrabenzyloxy-3-hydroxy-1-heptene. The suggested rearrangement mechanism involves an initial intramolecular cyclization, followed by ring opening by the nucleophile p-nitrobenzoate. Product distribution of the Mitsunobu reaction was substrate-dependent, with the corresponding mannose derivative (the 3R epimer) giving less of the initial intramolecular reaction products and the corresponding galactose derivative (the 5S epimer) yielding almost exclusively the expected epimerization product. Varying the Mitsunobu reaction conditions (addition of base and using nonpolar solvent) led to the expected epimerization product of the glucose derivative.