CLAISEN REARRANGEMENT OF N-SILYL KETENE N,O-ACETALS GENERATED FROM ALLYL N-PHENYLIMIDATES

Deprotonation and subsequent N-silylation of allyl N-phenylimidates 1 lead to N-silyl ketene N,O-acetals 5 which undergo a Claisen rearrangement to yield gamma,delta-unsaturated anilides 3 after hydrolytic work-up. The temperature for the rearrangement step is dependent on the nature of the substituent R2 in S. Ketene acetals S with R2 = H rearrange readily at room temperature, while heating at 130-degrees-C is required if R2 not-equal H. The degree of simple diastereoselection attainable for the conversion of 1 to 3 is strongly affected by the size of the substituent R4. With R4 > H excellent anti/syn selectivity is caused by efficient suppression of the rearrangement pathway via a boat transition state. This rationale is supported by NOE difference data obtained for several allyl N-phenylimidates and N-silyl ketene N,O-acetals.