Enantioselective Norrish–Yang Cyclization Reactions of N-(ω-Oxo- ω-phenylalkyl)-Substituted Imidazolidinones in Solution and in the Solid State

Abstract

The four N‐(ω‐oxo‐ω‐phenylalkyl)‐substituted imidazolidinones 5–8 were prepared from N‐acetylimidazolidinone (4). Upon irradiation, these substrates underwent Norrish–Yang cyclization to the racemic products rac‐9–rac‐12 (51–75 %). The reactions of the N‐2‐oxoethylimidazolidinones 5 and 6 were conducted in tBuOH, and yielded 1:1 mixtures of exo/endo diastereoisomers rac‐9 a/rac‐9 b and rac‐10 a/rac‐10 b, accompanied by Norrish type II cleavage products. The reactions of the N‐3‐oxopropylimidazolidinones 7 and 8 were performed in toluene. The exo diastereoisomers rac‐11 a and rac‐12 a were the major diastereoisomers (d.r. ≅4:1). In the presence of the chiral compounds 1–3, the photocyclization of substrate 8 proceeded with significant enantiomeric excess (5–60 % ee). The more sophisticated complexing agents 3 and ent‐3 provided better enantiofacial differentiation (up to 60 % ee) than the lactams 1 and 2 (up to 26 % ee). Low temperatures and an excess of the complexing agent helped to increase the enantioselectivity. The absolute configuration of the major exo product 12 a obtained from compound 8 in the presence of complexing agent 3 was unambiguously established by single‐crystal X‐ray crystallography of its chiral N‐methoxyphenylacetyl derivative 15 a. In a similar fashion, the absolute configurations of the endo products 12 b and ent‐12 b were established. The N‐2‐oxoethylimidazolidinone 5, which crystallized in a chiral space group, was irradiated in the solid state. At low levels of conversion, the product 9 a/ent‐9 a was formed with high enantiomeric excess (78 % ee). The enantioselectivity deteriorated at higher levels of conversion.