Tandem [4 + 2]/[3 + 2] Cycloadditions with Nitroethylene

The simplest nitroalkene, nitroethylene, undergoes Lewis acid-promoted [4 + 2] cycloaddition with chiral vinyl ethers to afford cyclic nitronates 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] cycloaddition is due to the single ring substituent, a remote acetal center. The scope and limitations of the use of nitroethylene as a 4π component in Lewis acid-promoted cycloadditions are documented and discussed. Additionally, concise syntheses of the pyrrolizidine alkaloids (+)-macronecine and (+)-petasinecine are presented.