Dirhodium(II)-catalysed intramolecular carbon-hydrogen insertion reaction of α-diazo ketones: Stereoselective synthesis of 2,3-cis-2-alkyl-5-oxo-3-silyloxycyclopentanecarboxylates

Methyl 5-(tert-butyldimethylsilyloxy)-2-diazo-3-oxoheptanoate 2a, upon treatment with 1 mol% of dirhodium(II) tetraacetate in dichloromethane under reflux, gives a mixture of methyl (1R*,2R*,3S*)-3-(tert-butyldimethylsilyloxy)-2-methyl-5- oxocyclopentanecarboxylate 3a and its (1S*,2S*,3S*) isomer 4a in a ratio of 79:21 and 87% combined yield. Similarly, 5-(tert-butyldimethylsilyloxy)-2-diazo-3-oxoalkanoates 2c-g give 2,3-cis-2-alkyl- (or 2-phenyl)-3-(tert-butyldimethylsilyloxy)cyclopentanecarboxylates 3c-g as the major products. The presence of both the keto and ester groups in the precursors was found to be essential for this site- and stereo-selective C-H insertion to take place. A possible interpretation for the observed stereoselectivity is presented.