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■ A Novel Pentose Synthesis via Palladium(II)-Catalyzed Cyclization of an Unstable Hemiacetal

Ken-ichiro Awasaguchi, Masahiro Miyazawa,* Ikuyo Uoya, Koichi Inoue, Koji Nakamura, Hajime Yokoyama, and Yoshiro Hirai*

*Department of Chemistry, Graduate School of Science and Engineering, Toyama University, Gofuku 3190, Toyama 930-8555, Japan

Abstract

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 (no-σ*c-o) and A1,2 strain, respectively. This methodology was applied to stereocontrolled synthesis of pentoses: D-ribose and L-lyxose.