Photochemical Synthesis of Highly Functionalized Cyclopropyl Ketones

A series of di‐ and trisubstituted cyclopropyl ketones 11 were prepared by irradiation of ketones 3 and 5 , which bear a leaving group adjacent to the carbonyl C‐atom. The required ketones 3 could be easily synthesized either by functionalization of ketones 1 with a hypervalent iodine reagent, 2 , or by O‐sulfonylation of α‐hydroxy ketones 7 . The nitrates 5 were obtained by treatment of the corresponding α‐bromo ketones with AgNO₃. The irradiation of 3 and 5 must be performed in the presence of an acid scavenger (1‐methyl‐1H‐imidazole) to obtain the cyclopropanes 11 in good yields. The synthetic efficiency of the method was, among other things, demonstrated by the preparation of a highly strained bicyclo[2.1.0]pentane 11i in good yield. The mechanism of the photochemical cyclization was investigated by means of photokinetic measurements, as well as by quantum‐chemical calculations. It was shown that the presence of the leaving group substantially influences all steps of the photochemical reaction cascade. The X‐ray crystal structures of 11j and exo‐ 11k were also determined.