Ketenes, 3-keto aminals

The title phosphonate and related substances undergo thermal decomposition to B-acyl ketenes at temperatures in excess of 50°C. Thus thermolysis in the presence of alcohols, amines, a-hydroxy esters, and a-amino esters affords the corresponding g-keto esters and amides the latter two classes can be cyclized upon subsequent base treatment to unsaturated tetronic and tetramic acids and the related phosphonate reagents. ... 

Martin et developed a biogenetic entry into Ala-methylvellosimine (168) which employed an iminium ion-mediated cyclization similar to van Tamelen s original proposal. As shown in Scheme 4, dihydrocar-boline 169, which was readily obtained from commercially available D-tryptophan, was allowed to react with the vinyl ketene acetal 170 to afford a single product which was directly converted into the i-butyl ester 171. The Nb acylation of amine 171 with diketene furnished an intermediate P-keto amide that underwent facile cyclization via an intramolecular... 

Similarly, the acyl transfer from the resin /3-keto ester (9) (R = CH2C02Et and R = CH2COCH3) was supposed to take place via the intermediate formation of a ketene. Although the formation of ketene could not be demonstrated in this reaction, it could still be trapped on the amine-resin. The formation of a ketene as an acylating species was assumed by analogy to Bruice s EjcB reaction (Holmquist and Bruice, 1969). 

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