Regio- and stereoselectivity of the Norrish−Yang photocyclization of Dialkyl 1,2-Diketones: solution versus solid state photochemistry of two polymorphS

As shown by X-ray crystallography, crystals of 3βacetoxy-16,17-seco-17,20-dioxopregn-5-ene-16-nitrile are dimorphic. The regioselectivity of the Norrish−Yang type II photocyclization under visible light of this steroidal 1,2 diketone, which bears primary, secondary, and tertiary nonequivalent abstractable γ-hydrogens, dramatically increases in the crystalline state of both polymorphs. X-ray crystallography and molecular mechanics calculations reveal crystal structure−solid state photochemistry relationships