jo025927r_si_003.pdf (863.48 kB)
New Cationic Olefin Cyclization−Pinacol Reactions. Ring-Expanding Cyclopentane Annulations That Directly Install Useful Functionality in the Cyclopentane Ring
journal contribution
posted on 2002-08-08, 00:00 authored by Larry E. Overman, John P. WolfeTwo 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.