A NOVEL PENTOSE SYNTHESIS VIA PALLADIUM(II)-CATALYZED CYCLIZATION OF AN UNSTABLE HEMIACETAL

摘要

PdCl2(PhCN)2 (5 mol%)-catalyzed cyclization of a hemiacetal derived from (E,2S,3R)-2,3-isopropylidenedioxy-6-(tetrahydro-2H-pyran-2-yl)-4-hexenal and methanol gave substituted furanoside in moderate yield, exclusively via 5-exo-mode cyclization, without the need for a reoxidant. New stereogenic centers at C1 and C4 on the tetrahydrofuran ring showed preferential 1R and 4R stereochemistry due to anomeric effect (n_o- σ*_(c-o)) and A~(1,2) strain, respectively. This methodology was applied to stereocontrolled synthesis of pentoses: D-ribose and L-lyxose.