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 3β-acetoxy-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.
Financial support by the Investigation Programs of the Ministerio de Economía y Competitividad (CTQ2010-18244) is acknowledged. D.A.-D. thanks the Ministerio de Economía y Competitividad for a fellowship. The authors acknowledge support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI).