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Abstract
The stereochemistry of the deprotonation of β -unsaturated dithioesters with LDA has been examined : a stereochemically pure dienethiolate is formed from methyl buten-3-dithioate. Protonation of lithium dienethiolates mostly affords conjugated unsaturated dithioesters. S-alkylation of dienethiolates with allylic bromides furnishes quantitatively S-allyl ketene dithioacetals. The thermal rearrangement of these dithioacetals leads to diethylenic dithioesters under mild conditions (from 20°C to 100°C). The thio-Claisen transposition is thus confirmed as a facile sigmatropic shift. It is also demonstrated that the reverse reaction (retro thio-Claisen) proceeds simultaneously. The equilibrium mixture of ketene dithioacetal and dithioester ie usually in the range of 70 : 30 to 95 : 5. Thiophilic addition to these diunsaturated dithioesters followed by treatment with alkyl halides affords diunsaturated dithioacetals. These masked ketones may be viewed as arising from alpha regioselective allylation of β -unsaturated ketones.
Published Version
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