Microbial Formation of 17α‐C₂₁ Steroids: Stereochemistry of Saturation of the Δ¹⁶‐Double Bond

After anaerobic incubation in deuterated water of 3β‐hydroxy‐5,16‐pregnadien‐20‐one with caecal contents from rats, 3β‐hydroxy‐17α‐pregn‐5‐en‐20‐one with deuterium label in ring D was isolated. Enolization of this compound with alkali yielded 3β‐hydroxy‐17β‐pregn‐5‐en‐20‐one with about 0.4 atoms of deuterim in ring D. Incubation of this compound with rat liver microsomes fortified with NADPH yielded 3β,16α‐dihydroxy‐17β‐pregn‐5‐en‐20‐one devoid of deuterium in ring D. The results show that the saturation of the Δ¹⁶‐double bond, catalyzed by intestinal microorganisms, involves a trans addition of hydrogen to the 16α‐ and 17β‐positions.