Synthesis of indoles and indolizines labelled specifically with 13C in the five-membered ring

The syntheses of 2-13C and 3-13C indoles and indolizines and 1-13C indolizine are described. They are obtained from 1-13C bromoacetaldehyde on (2-pyridyl)ethylacetate for 2-13C indolizine, and through cyclisations of suitably labelled intermediates:o-formotoluidide for 2-13C indole, o,β-dinitrostyrene for 3-13C indole, and 3-(2-pyridyl)-1 propanol for 3-13C and for 1-13C indolizines. Every position is more than 90% 13C-labelled. (author)