Enantioselective Norrish-Yang cyclization reactions of N-(ω-oxo-ω-phenylalkyl)-substituted imidazolidinones in solution and in the solid state

Thorsten Bach, Tobias Aechtner, Bernhard Neumüller

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61 Scopus citations

Abstract

The four N-(ω-oxo-ω-phenyl-alkyl)-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 exolendo diastereoisomers rac-9a/rac-9b and rac-10a/rac-10b, 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-11a and rac-12a 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 12a 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 15a. In a similar fashion, the absolute configurations of the endo products 12b and ent-12b 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 9a/ent-9a was formed with high enantiomeric excess (78% ee). The enantioselectivity deteriorated at higher levels of conversion.

Original languageEnglish
Pages (from-to)2464-2475
Number of pages12
JournalChemistry - A European Journal
Volume8
Issue number11
DOIs
StatePublished - 3 Jun 2002

Keywords

  • Asymmetric synthesis
  • Cyclization
  • Hydrogen bonds
  • Photochemistry
  • Radical reactions

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