O-BENZYLIDENE DERIVATIVES OF D-ARABINOSE DIETHYL AND DIPROPYL DITHIOACETAL

Benzylidenation of D-arabinose diethyl and dipropyl dithioacetals with alpha,alpha-dimethoxytoluene in the presence of p-toluenesulfonic aci d has been studied in detail. Under kinetic control the two terminal d ioxolan-type 4,5-O-(R)- and 4,5-O-(S)-benzylidene diastereomers are fo rmed first which are in equilibrium with each other. In the thermodyna mic phase of the reaction the corresponding dioxan-type 3,5-O-(R)-benz ylidene isomer is formed too, but all three monobenzylidene isomers ar e gradually converted into the four possible dioxolan-type 2,3:4,5-di- O-benzylidene diastereomers. The dioxan-type 2,4: 3,5-di-O-benzylidene isomer was present only in trace amounts. When benzaldehyde was used as reagent in the presence of hydrochloric acid or zinc chloride only the 2,3:4,5-di-O-benzylidene diastereomers were formed. Partial hydrol ysis of the dibenzylidene derivatives yielded the corresponding 2,3-O- benzylidene diastereomers. Structures, including the chirality of the benzylidene groups, were determined by n.m.r. spectroscopy. A mechanis m suggested for the reaction was partially supported by equilibration studies.