- Polarized Ketene Dithioacetals. 28. A New General Highly Stereoselective and Regiospecific Method for Homologation of Ketones to α,β-Unsaturated Esters via α-Oxoketene Dithioacetals
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A new highly stereoselective and regiospecific general method for the conversion of active methylene ketones to α,β-unsaturated O-methyl esters, S-methyl esters, and aldehydes via the corresponding oxoketene dithioacetals has been developed.Thus the carbinols 7a-g obtained by sodium borohydride reduction of the corresponding oxoketene dithioacetals 6a-g, derived from acetophenones and their higher homologues, have been shown to undergo boron trifluoride etherate assisted methanolysis to give the corresponding (E)-methylcinnamates 3a-d and their α-alkyl derivatives 3e-g in high yields.Also the acetals 6h-k derived from alkylmethyl ketones gave the corresponding (E)-methylcrotonates 3h-k.Similarly, the acetals 6l-p derived from alicyclic ketones gave the corresponding cyclic ene esters 3l-p under identical conditions.A few carbinols, 7a-d, were shown to undergo partial hydrolysis in the presence of boron trifluoride etherate/water to give the corresponding S-methyl thiocinnamates 8a-d.The cyclic acetals 6l-n were also similarly converted to the corresponding cyclic S-methylene thio esters 8e-g.The (methylthio)alkenyl ketones 20a-d and 18 after borohydride reduction and acidic hydrolysis gave the corresponding (E)-α,β-unsaturated aldehydes 22a-d and 19.The mechanism governing these transformations has been proposed.
- Myrboh, Bekington,Ila, Hiriyakkanavar,Junjappa, Hiriyakkanavar
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p. 5327 - 5332
(2007/10/02)
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- α-OXOKETENE DITHIOACETALS: VERSATILE SUBSTRATES FOR 1,3-CARBONYL TRANSPOSITIONS
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A two step procedure for the conversion of α-oxoketene dithioacetals to β-substituted α,β-unsaturated thioesters or acids is described.The overall transformation represents a 1,3-carbonyl transposition in which the original ketone carbonyl emerges as the carbonyl of an acid or thioester.The resulting thioesters and acids constitute versatile acylating reagents in accord with established procedures.
- Dieter, R. Karl,Jenkitkasemwong, Yawares
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p. 3747 - 3750
(2007/10/02)
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