0-benzylidene derivatives of d-arabinose diethyl and dipropyl dithioacetal

Article abstract of DOI:10.1071/CH9960273

Benzylidenation of D-arabinose diethyl and dipropyl dithioacetals with α,α-diinethoxytoluene in the presence of p-toluenesulforiic acid has been studied in detail. Under kinetic control the two terminal dioxolan-type 4,5-O-(R)- and 4,5-O-(S)-benzylidene diastereomers are formed first which are in equilibrium with each other. In the thermodynamic phase of the reaction the corresponding dioxan-type 3.5-O-(R)-benzylidene isomer is formed too. but all three monobenzylidene isomers are 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 hydrolysis 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. spectrosropy. A mechanism suggested for the reaction was partially supported by equilibration studies.