Regio‐ und stereoselektive Photocycloadditionen aromatischer Aldehyde an Furan und 2,3‐Dihydrofuran

Regio‐ and Stereoselective Photocycloaddition Reactions of Aromatic Aldehydes to Furan and 2.3‐Dihydrofuran The Paterno‐Büchi reaction between benzaldehyde (1a), tolualdehyde (1b), mesitylaldehyde (1c), and 2,4‐di‐tert‐butyl‐4‐methylbenzaldehyde (1d) and the electron‐rich alkenes furan and 2,3‐dihydrofuran has been investigated. The reaction with furan leads exclusively to exo‐6‐aryl‐2,7‐dioxabicyclo[3.2.0]‐hept‐3‐enes 2a to 2d in yields of 78% to 97% (2a, 2b). In the photoreaction of 2a and 2b with 2,3‐dihydrofuran, however, mixtures of exo‐ and endo‐7‐aryl‐2,6‐dioxabicyclo[3.2.0]heptanes 3a and 4a (7:1) as well as 3b and 4b (15:1) could be isolated. The sterically more hindered aldehydes 1c and 1d give rise to exclusive formation of the endo adducts 3c and 3d. Irradiation of 1d in the presence of 2,3‐dihydrofuran leads to the benzocyclobutenol 5 (Norrish‐type II; up to 45%) as a side product. The regioselectivity observed in these reactions may be explained by comparing the stability of the possible 1,4‐diradical intermediates. The observed stereoselectivity could be rationalized by the geometry of the triplet 1,4‐diradicals during the ISC process and by the life times of the resulting singlet 1,4‐diradicals.