%0 Journal Article %A Overman, Larry E. %A Wolfe, John P. %D 2002 %T New Cationic Olefin Cyclization−Pinacol Reactions. Ring-Expanding Cyclopentane Annulations That Directly Install Useful Functionality in the Cyclopentane Ring %U https://acs.figshare.com/articles/journal_contribution/New_Cationic_Olefin_Cyclization_Pinacol_Reactions_Ring-Expanding_Cyclopentane_Annulations_That_Directly_Install_Useful_Functionality_in_the_Cyclopentane_Ring/3695445 %R 10.1021/jo025927r.s003 %2 https://acs.figshare.com/ndownloader/files/5785824 %K cycloalkanone %K iminium salts 64 %K KHCO %K ci %K cyclic transition states %K Cyclopentane %K carbon side chain %K triflic anhydride %K cyclopentane C 2 position %K diastereoselectivitie %K DTBMP %X 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 2124 with triflic anhydride at −78 °C affords cycloalkanones 3134 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 1517 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 4648 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. %I ACS Publications