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 H(2)O (1 equiv) at -35 degrees C for 48 h]. 1,3-Dithiane itself gave monoxide (30% ee) and the trans bis-sulfoxide (59% ee) but with low enantioselectivity. In contrast, ester derivatives (Me, Et, t-Bu, Ph) of 1,3-dithiane-2-carboxylates gave monoxides (80-95% ee) 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.

An enantioselective synthesis of (R)-salbutamol has been carried out using the chiral, C2 symmetric acyl anion equivalent, (1R,3R)-1,3-dithiane 1,3-dioxide, which undergoes addition to an aromatic aldehyde with very high stereocontrol at 0 degrees C. Pummerer reaction and work-up with lithium ethanethiolate generated the alpha-hydroxy thiolester in high yield and further transformations led to the target compound with high enantiomeric excess.

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