Inter- and Intramolecular [4+2] Cycloaddition of Nitroalkenes with Allenylsilanes. A Case of Unexpected Regioselectivity

A conjugated nitroalkene tethered to an allenylsilane undergoes formal [4+2] cycloaddition in the presence of tin tetrachloride to afford a bicyclic nitronate.The stereo-and regiochemical course of the reaction was established by a subsequent dipolar cycloaddition with 4-bromophenyl acrylate to generate a crystalline nitroso acetal.Single crystal X-Ray crystallographic analysis of the nitroso acetal revealed an unexpected stereostructure which arose from attack of the nitroalkene on the central carbon of the allenylsilane and closure of the oxygen atom on the terminal carbon.This outcome is interpreted as arising from a stepwise reaction via a silicon-stabilized carbocation which can undergo rotation to an allylic cation prior to ring closure.An intermolecular example of this same process was also documented.