Asymmetric synthesis in organic photochemistry

Selective formation of an optically active cyclobutanol has been achieved in type II cyclization of salts of aminoketone 18 (Scheme 7). While irradiation of the free aminoketone in benzene gives c-19 (c ) = 0.105), t-19 (0.048), and 20 (0.120), little or none of t-19 is produced when the salts formed between 18 and enantiomerically pure acids are photolysed in the crystalline state (Table 55.6). Enantiomeric excesses obtained for c-19 are fairly good except for the iV-Cbz-L-phenylalanine salt of 18. The decrease in enantioselectivity with increasing conversion is due to the breakdown of the ordered crystal lattice as photoproducts replace starting material. The solid-state ionic chiral auxiliary approach may become one of general techniques for asymmetric synthesis in organic photochemistry. 

---