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

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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.

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