Synthesis of 3β-Hydroxy[21-14C]-5β-pregn-8(14)-en-20-one from Chenodeoxycholic Acid

3β-Hydroxy[21-¹⁴C]5β-pregn-8(14)-en-20-one ( 17 ) was prepared from chenodeoxycholic acid ( 1a ). The synthetic sequence involved: (i) degradation of the bile-acid side chain to an etianic acid; (ii) formation of the 8(14)-double bond; (iii) inversion of the configuration at C(3); (iv) construction of the acetyl side chain at C(17) with the required isotopic label at C(21). Structures of all described products were confirmed by chemical and spectroscopic (IR, ¹H-NMR, ¹³C-NMR, MS) methods.