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 degrees 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 degrees C followed by hydrolysis of the intermediate iminium salts 64 with aqueous KHCO(3) 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.
Overman
Larry E
LE
Department of Chemistry, 516 Rowland Hall, University of California, Irvine, California 92697-2025, USA. leoverma@uci.edu
Wolfe
John P
JP
D000475:Alkenes / Q000138:chemical synthesis*; D002384:Catalysis; D002625:Chemistry, Organic / Q000379:methods*; D003500:Cyclization; D003517:Cyclopentanes / Q000032:analysis / Q000138:chemical synthesis / Q000737:chemistry; D006868:Hydrolysis; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D013237:Stereoisomerism; D013329:Structure-Activity Relationship; D013696:Temperature
D000475:Alkenes; D003517:Cyclopentanes