Paterno-Biichi reaction 2,3-dihydrofuran

Solvent polarity can trigger a photoinduced electron-transfer (PET) (Section 5.2) step in the Paterno Biichi reaction of benzaldehyde to dihydrofuran, thus affecting the reaction regio- and diastereoselectivity (Scheme 6.106).899 Whereas the reaction in benzene proceeds via a triplet biradical intermediate, a radical ion pair is formed in acetonitrile. The electron transfer alters the charge distribution in the reactants, which promotes the formation of two different regioisomers with inverted diastereoselectivity. 

The second report concerns the factors controlling the stereoselectivity of Paterno-Biichi reactions between aromatic aldehydes and 2,3-dihydrofuran32-34, Whereas furan yields exclusively 6-phenyl-2,7-dioxabicyclo[3.2.0]hept-3-ene (9) with the aryl substituent in the ecto-posi-tion 10,32-36, 2,3-dihydrofuran forms the eWo-compound 11, interestingly this occurs even with sterically demanding substituents. 

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