Selective Carbon-Carbon Bond Formation at the Oxazole Ring. Cycloadditions and Michael-Type Additions of Ketenes to 1,3-Oxazoles

Article abstract of DOI:10.1021/jo00193a009

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 tert-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 1d affords the ketone 4d and the enol ester 7, whereas the 4,5-dimethyl derivative 1e undergoes the addition of 2 mol 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 1d 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 ketene at C₅ of the oxazole ring to give a zwitterion which undergoes different reactions depending on the substitution at C₄ and C₅ of the heterocycle.