Studies on the asymmetric oxidation of ester derivatives of 1,3-dithiane-2-carboxylates. Asymmetric synthesis of trans-1,3-dithiane 1,3-dioxide

Asymmetric oxidation of a range of 1,3-dithianes was studied using the Kagan protocol [CHP (4 equiv), (+)-DET (2 equiv), Ti(OiPr)(4) (1 equiv), and H2O (1 equiv) at -35 degrees C for 48 h]. 1,3-Dithiane itself gave monoxide (30% eel and the trans bis-sulfoxide (59% eel but with low enantioselectivity. In contrast, ester derivatives (Me, Et, t-Bu, Ph) of 1,3-dithiane-2-carboxylates gave monoxides (80-95% eel and trans bis-sulfoxides (> 97% ee) in high enantioselectivity. Optimum conditions for the oxidation of ethyl 1,3-dithiane-2-carboxylate required the Modena protocol [CHP (4 equiv), (+)DET (2 equiv), and Ti(OiPr)(4) (0.5 equiv) at -22 degrees C for 24 h], and this gave the trans bis-sulfoxide in 60% yield and high enantioselectivity. The bis-sulfoxides were found to be acid sensitive and required rapid workup and purification for optimum yields. The differences between the Modena and Kagan oxidants are discussed together with a discussion on the origin of the high enantio- and diastereoselectivity of the reaction. Finally, hydrolysis and decarboxylation furnished trans-1,3-dithiane 1,3-dioxide.