Synthesis of Spiro[2.2]pentanes and Spiro[2.3]hexanes Employing the Me 3 Al/CH 2 I 2 Reagent

Substituted alkylidenecyclopropanes reacted with 5 equivalents each of Me 3 Al and CH 2 I 2 at room temperature in hexane to give 1‐mono‐ and 1,1‐disubstituted spiro[2.2]pentanes in high yields. Surprisingly, the same reaction with substituted alkylidenecyclopropanes in CH 2 Cl 2 afforded exclusively 1,1‐disubstituted spiro[2.3]hexanes. The transformation of 1,1‐diphenylspiro[2.2]pentane into 1,1‐diphenylspiro[2.3]hexane was studied with the use of CD 2 I 2 and a plausible mechanism was suggested. The reaction of substituted alkylidenecyclobutanes with the Me 3 Al/CH 2 I 2 reagent in CH 2 Cl 2 gave only 1,1‐disubstituted spiro[2.3]hexanes.