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Enantioselective Norrish–Yang Cyclization Reactions of N ‐( ω ‐Oxo‐ ω ‐phenylalkyl)‐Substituted Imidazolidinones in Solution and in the Solid State
Author(s) -
Bach Thorsten,
Aechtner Tobias,
Neumüller Bernhard
Publication year - 2002
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/1521-3765(20020603)8:11<2464::aid-chem2464>3.0.co;2-s
Subject(s) - diastereomer , chemistry , enantioselective synthesis , toluene , enantiomer , enantiomeric excess , stereochemistry , medicinal chemistry , substrate (aquarium) , organic chemistry , catalysis , oceanography , geology
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 t BuOH, 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.