Catalytic enantioselective C-H functionalization of indoles with α-diazopropionates using chiral dirhodium(II) carboxylates: asymmetric synthesis of the (+)-α-methyl-3-indolylacetic acid fragment of acremoauxin A

An enantioselective C-H functionalization of N-methoxymethyl (MOM)-protected 2,3-unsubstituted indoles with alpha-diazopropionates has been effected via catalysis by dirhodium(II) tetrakis[N-phthaloyl-(S)-triethylalaninate], Rh-2((S)-PTTEA)(4), providing alpha-methyl-3-indolylacetates in high yields and with enantioselectivities of up to 86% ee. The effectiveness of this protocol was demonstrated by the first catalytic asymmetric synthesis of the (+)-alpha-methyl-3-indolylacetic acid fragment of acremoauxin A, a potent plant-growth inhibitor. Furthermore, the Fujioka protocol using a combination of TMSOTf and 2,2'-bipyridyl was shown to be superior for the removal of the N-MOM group. (C) 2011 Elsevier Ltd. All rights reserved.