New Cationic Olefin Cyclization−Pinacol Reactions. Ring-Expanding Cyclopentane Annulations That Directly Install Useful Functionality in the Cyclopentane Ring

Two new tandem cationic olefin cyclization−pinacol reactions that provide cyclopentane-fused cycloalkanone products are described. Treatment of cis-1-[2-alkenyl-2-(triethylsiloxy)cycloalkyl]but-3-en-2-ol derivatives 21−24 with triflic anhydride at −78 °C affords cycloalkanones 31−34 in 54−90% yields with diastereoselectivities of typically >20:1. In this unusual transformation, the starting cycloalkanone is ring-expanded and fused to a 2-alkenylcyclopentane fragment. Reaction of cis-(2-siloxy-2-alkenylcycloalkyl)pyrrolidin-1-ylethanones 15−17 with triflic anhydride and 2,6-di-tert-butyl-4-methylpyridine (DTBMP) at −20 to +65 °C followed by hydrolysis of the intermediate iminium salts 64 with aqueous KHCO₃ affords cycloalkanediones 46−48 in moderate yield and high diastereoselectivity (>20:1). These are the first examples of ring-expanding cyclopentane annulations that directly introduce a carbon side chain or carbonyl functionality at the cyclopentane C2 position. The high diastereoselectivities observed in these reactions are believed to arise from reaction through highly organized cyclic transition states.