Hemiacetal collapse

Me2S attacks one of the O atoms involved in the 0-0 bond, displacing 0. Hemiacetal collapse to the carbonyl compounds then occurs. 

A chiral oxaziridine could have been used but the company prefers a catalytic method the asymmetric dihydroxylation (chapter 25) of the silyl enol ether 308. Presumably the diol 309 is formed but the hemiacetal collapses on workup and the mesylate of (R)-306 allows displacement with -BuNH2 and the synthesis of buprion 307. 

In examining the mechanism leading to the nucleophile-mediated conversion of an ester to a ketone, initial addition of a nucleophile to the carbonyl results in formation of a hemi-acetal intermediate. Subsequent collapse of the hemiacetal intermediate liberates a ketone and an alkoxide leaving group. This mechanistic sequence, illustrated in Scheme 7.19... 

The base is important because it removes the proton from the alcohol as it attacks the carbonyl group. A base commonly used for this is pyridine. If tbe electrophile had been an aldehyde or a ketone, we would have got an unstable hemiacetal, which would collapse back to starting materials by eliminating the alcohol. With an acyl chloride, the alkoxide intermediate we get is also unstable. It collapses again by an elimination reaction, this time losing chloride ion, and forming the ester. 

J=4 Hz, H-20), after D2O exchange, it collapsed to a singlet, showing that the hemiacetal proton had no vicinal proton. 

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