Abstract
Irradiation of Δ5,7‐9α, 10β‐steroids in solution with λ = 253,7 nm yielded no Δ5,7‐9β, 10α‐steroids andvice versa. No back reaction from precalciferol2to ergosterol could be detected by kinetic measurements. This leads to the conclusion that with the mercury resonance line only ring opening and 6,7‐cis/transisomerization but no ring closure occurs.
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