Stereoselective synthesis of cyclopentanones using dirhodium(II)-catalyzed intramolecular C-H insertion reaction

This review summarizes novel stereoselective syntheses of 3,4-cis- and 3,4-trans-3-alkyl-4-silyloxycyclopentanones using a dirhodium(II)-catalyzed intramolecular C-H insertion reaction as a key reaction. Treatment of diazoketoesters (20a-e) with 1 mol % of dirhodium(II) tetraacetate gave 2,3-trans-3,4-cis-cyclopentanones (21a-e) as major products. The presence of both the keto and ester groups in the precursors was found to be essential for this chemo- and stereoselective intramolecular C-H insertion reaction to take place. A possible interpretation for the observed stereoselectivity is presented. Optically active Corey lactone (38) was synthesized using a similar reaction of chiral α-diazo-β-ketoester (33). Next, reactions of 5,6-bisoxygenated diazoketones with Rh₂(OAc)₄ were investigated. An acetonide derivative (39), upon treatment with Rh₂(OAc)₄, gave 3,8-dioxabicyclo[3.2.1]octane (42) via oxonium ylide formation/1,2-shift. On the other hand, similar treatment of 5,6-bis(tert-butyldimethylsilyloxy) derivative (47) gave a C-H insertion product (51) which was purified by silica gel column chromatography to give 4-silyloxycyclopentenone (48). Direct reduction of 51 with lithium aluminum hydride gave stereoselectively diol (52) in 52% yield from 47. Reaction of 2-methoxycarbonylcyclopentenone (48) and a 2-benzenesulfonyl congener (50) with R₂CuLi or RMgBr-CuI stereoselectively gave 2,3-trans-3,4-trans-cyclopentanones (22, 57) as a major diastereoisomer. On the other hand, reaction with R₃ Al in toluene exclusively gave the corresponding 3,4-cis-adducts (21 and 58).