Kinetic benzylidenation. Part I. The selective formation of five-membered ring benzylidene acetals from aldose diethyl dithioacetals
Reaction of D-arabinose diethyl dithioacetal with one equivalent of benzaldehyde dimethyl acetal in the presence of p-toluenesulfonic acid at −40 °C gave a mixture of the two epimers of 5-O-methoxyphenylmethyl-D-arabinose diethyl dithioacetal initially. After 14 h at −20 °C, the major products were R- and S-4,5-O-benzylidene-D-arabinose diethyl dithioacetal. The structure of the S isomer was determined by X-ray crystallography. The crystal was orthorhombic, with space group P212121, cell dimensions a = 5.179(4), b = 12.469(3), c = 27.150(4) Å, and Z = 4. The crystal structure was solved using the SHELX (76) system and refined to R = 0.060 for 714 reflections. The sugar chain was in a zigzag conformation, the 4,5-O-benzylidene ring in a 0−5Tc conformation, and the plane of the phenyl ring was nearly perpendicular to the plane of the five-membered ring (88° angle). There were two short OH—S hydrogen bonds, one intramolecular and one intermolecular. Reaction of the diethyl dithioacetals of D-glucose, D-galactose, D-mannose, and D-ribose at −20 °C as above also gave mixtures of the terminal five-membered ring O-benzylidene diastereomers.