Labeled bile acids VI: [22,23,24‐ 13 C 3 ]‐methyl 3α,7α‐diacetoxy 5β‐cholan‐24‐oate, [22,23,24‐ 13 c 3 ]‐methyl 3β‐acetoxychol‐5‐en‐24‐oate, and [16,17,22,23‐ 3 h 4 ]‐methyl 3β‐acetoxychol‐5‐en‐24‐oate (1)

A general short procedure for the introduction of 13 C 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‐ 13 C 3 ]‐methyl propiolate. For the labeling with tritium, the unlabeled product of the ene synthesis, a Δ 5,16,22 ‐triene was saturated selectively at 16,17 and 22,23 with tritium gas.