Selective carbon-carbon bond formation at the oxazole ring. Cycloadditions and Michael-type additions of ketenes to 1,3-oxazoles

1,3-Oxazole (1a) and 4-methyl-1, 3-oxazole (1b) react with dichloroketene (DCK) affording the 2-(dichloroacetyl) oxazoles 3a and 3b, respectively, very likely via the corresponding N-acyloxazolium ylides as intermediates. 2-(N-Benzyl-N-methylamino)-1,3-oxazole (1c) and terf-butylcyanoketene (TBCK) give the 5-acyl derivative 4c and a mixture of the diastereomeric 2:1 cycloadducts 5 and 6 constituted by a δ-lactone ring condensed across the former C4-C5 bond of 1c. The 4-methyl derivative Id affords the ketone 4d and the enol ester 7, whereas the 4,5-dimethyl derivative le undergoes the addition of 2 mol of TBCK and ring fission producing a 2,5-dihydrofuran derivative 8. Other ketenes, namely diphenyl-(DPK), chlorocyano-(CCK), and dichloroketene (DCK), react with 1c and Id to give the corresponding 5-acyl derivatives 4, and in the case of DCK only, also the enol ester 11. Plausible schemes accounting for the formation of products 4-11 involve as a common step, the initial attack of the k...