Tandem Inter [4 + 2]/Intra [3 + 2] Nitroalkene Cycloadditions. 4. Cycloadditions with (E)- 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 nitroso acetal cycloadduct 6. Hydrogenolysis of 6 afforded a single isomer of α-hydroxy lactam 8 wherein the newly installed methyl group occupied the β-configuration. Similarly, ethyl (Z)-1-propenyl ether produced an anomeric mixture of cycloadducts 7a and 7b which afforded a single α-hydroxy lactam 9 upon hydrogenolysis. The methyl group in 9 occupied the α-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.