Acetal N,S-ketene

The procedure illustrates a general method for the preparation of ketene S,N-acetals via thioamides and their crystalline quaternary iodides some other examples are shown in Table I. 2,2-Dialkyl-substituted ketene S,N-acetals cannot be prepared by this method because the nature of the products makes rigorous purification impractical. Ketene S,N-acctals are useful starting materials for many syntheses.11,13 15... 

Thieno[3,2-l7]azocines can also be produced (01JCS(P1)2774) from arylthio-ketene S,N-acetals 143 and trimethylsilyl ethers of enoles (Scheme 39). 

All operations should be carried out in a well-ventilated hood, because of the stench evolved during working with P2S5 and the ketene-S,N- acetal.-... 

In a related manner, ketene S,N-acetals also react with alkali metal amides to give ynamines [19] (Eq. 30). Some examples are shown in Table V. 

Ynamines from Ketene S,N-Acetals as Described in Equation (30) [19]... 

These problems can be obviated by using alkylated hiolates-ketene-S,N-acetals ft 66 - 68>... 

The reaction of 1-chlorothioformimidinium chlorides with nucleophilic carbon atoms results in the formation of a carbon-carbon bond. For example, reaction of 1-chlorothioformimidinium chlorides with malonitrile yields the ketene S,N-acetals LIV C "). 

This facile and efficient thiophene formation can be rationahzed by primary S-alkylation of 33 to give the iminium cation 35 followed by deprotonation (HBr ehmination) to the ketene-S,N-acetal 36, which cyclizes by intramolecular enamine addition to the carbonyl group (36->-37) intermediate 37 aromatizes to the products 34 by H2O elimination. Thiomorpholides (33) are easily obtained by Willgerodt-Kindler reaction of acetophenones with sulfur/morphoHne [81]. 

Deprotonation of 4 gives rise to the ketene-S,N-acetal 6, which undergoes proton-catalyzed condensation with the orthoformate via the nucleophihc CH2-group (—>7). The intermediate 7 reacts analogously with the second 2-methylbenzothiazolium moiety to afford the cyanine system 5, in which finally (for preparative reasons) the methosulfate anion is exchanged by tetrafluoroborate. 

Keten S,N-acetals, e.g. CH2= (SR )NRL " and N-acyl analogues have been described, and their use in synthesis of 2-dialkylaminopyran-4-ones by reaction with diketen. ... 

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