TANDEM [4+2]CYCLOADDITION [3+2]CYCLOADDITION WITH NITROETHYLENE/

The simplest nitroalkene, nitroethylene, undergoes Lewis acid-promoted [4 + 2] cycloaddition with chiral vinyl ethers to afford cyclic nitro nates with high diastereoselectivity. The resulting cyclic nitronates react with electron deficient alkenes to effect a face selective [3 2] cycloaddition. The origin of stereocontrol in the [3 + 2] cycloaddi tion is due to the single ring substituent, a remote acetal center. Th e scope and limitations of the use of nitroethylene as a 4 pi componen t in Lewis acid-promoted cycloadditions are documented and discussed. Additionally, concise syntheses of the pyrrolizidine alkaloids (+)-mac ronecine and (+)-petasinecine are presented.