A short stereoselective preparation of dienamides from cyclobutene compounds. Application in the synthesis of a new cyclohexene nucleoside

A short stereoselective synthesis of N-acylamino-1,3-dienes was developed starting from the cyclobutene lactam 8, which was obtained from 2-hydroxypyridine by a photochemical electrocyclic reaction. The tert-butoxycarbonyl derivative 17 was prepared to facilitate nucleophilic attacks to the carbonyl group, and the subsequent thermal ring opening provided dienes 18-21. One of these (20) was used in the synthesis of the cyclohexene nucleoside 30. A Diels-Alder reaction between diene 20 and maleic anhydride provided the endo-cycloadduct 22a. Three additional steps yielded amine 26. Construction of the uracil moiety afforded intermediate 29. Cyclization and removal of the protecting groups occurred in one step in the presence of ammonia, giving the target molecule 30. Diene 20 also underwent [4 + 2] cycloaddition with methyl acrylate to provide predominantly the endo-product 23a, regioselectively.