Labeled bile acids VI: [22,23,24-13C3]-methyl 3α,7α-diacetoxy 5β-cholan-24-oate, [22,23,24-13c3]-methyl 3β-acetoxychol-5-en-24-oate, and [16,17,22,23-3h4]-methyl 3β-acetoxychol-5-en-24-oate (1)

A general short procedure for the introduction of 13C to the side chain of bile acids is described. Suitable (Z)-pregn-17(20)-enes are key intermediates, while the isotope is introduced by an ene reaction with [1,2,3-13C3]-methyl propiolate. For the labeling with tritium, the unlabeled product of the ene synthesis, a Δ5,16,22-triene was saturated selectively at 16,17 and 22,23 with tritium gas.