Indole-3-carboxylic acid branched

The Chi group further developed a highly enantioselective formal [4 + 2] cycloaddition of a-branched indole 3-carboxaldehydes with trifluoromethyl ketones or isatins to give polyeyelic and spiro lactones, respectively. The reaction is postulated to undergo a catalytic cycle of NHC-eatalyzed aetiva-tion of the C(sp )—H bond of 2-methyl indole 3-carboxaldehydes to produee ort/ o-quinodimethane intermediates. Notably, 2-methyl benzofuran and ben-zothiophene aldehydes are also suitable substrates for this reaction, while 2-methylbenzaldehyde gives the partially oxidized carboxylic acid under these conditions without the observation of the desired lactone product (Scheme 7.115). 

Zhang et al. effected palladium-catalyzed oxidative C3-vinylations of indol-izine-l-carboxylates 136 with various styrenes (Scheme 10.45)."° Use of the bidentate nitrogen ligand bipy was key to formation of the branched a-product regioisomer 137 over the linear P-product. The authors later demonstrated that the same indolizine-l-carboxylates could be C3-acylated with a variety of a,P-unsaturated carboxylic acids via C—H and C—C double bond cleavage under oxidative conditions (138). "... 

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