Syntheses of labelled versions of 5-HT3 receptor antagonists, Alosetro
n and Lurosetron, are described, [C-14]Alosetron was prepared by route
s utilizing either Fischer indolisation of an amidohydrazine or pallad
ium-mediated cyclisation of an aryl enaminone as key steps. H-2 and C-
13 versions of Alosetron were prepared from suitably labelled imidazol
es. Lurosetron was labelled in either the methylene bridge carbon or c
arbonyl carbon, using C-14-labelled paraformaldehyde or phosgene, resp
ectively.