TANDEM INTER [4 + 2] INTRA [3 + 2] NITROALKENE CYCLOADDITIONS .4. CYCLOADDITIONS WITH (E)-1-PROPENYL ETHERS AND (Z)-1-PROPENYL ETHERS

The tandem [4 + 2]/[3 + 2] cycloaddition of nitroalkene 1 was initiated efficiently by 1-propenyl ethers as the dienophile. Ethyl(E)-1-propenyl ether reacted highly selectively to give a single nitro so acetal cycloadduct 6. Hydrogenolysis of 6 afforded a single isomer of alpha-hydroxy lactam 8 wherein the newly installed methyl group occupied the beta-configuration. Similarly, ethyl(Z)-1-propenyl ether produced an anomeric mixture of cycloadducts 7a and 7b which afforded a single alpha-hydroxy lactam 9 upon hydrogenolysis. The methyl group in 9 occupied the alpha-orientation. The high and complementary selectivity for production of 8 and 9 was explained on the basis of an extreme endo preference for the ethoxy group of the 1-propenyl ether in the [4 + 2] cycloaddition.