Enantioselective [6π]-photocyclization reaction of an acrylanilide mediated by a chiral host.: Interplay between enantioselective ring closure and enantioselective protonation

被引:69
作者
Bach, T [1 ]
Grosch, B
Strassner, T
Herdtweck, E
机构
[1] Tech Univ Munich, Lehrstuhl Organ Chem 1, D-85747 Garching, Germany
[2] Tech Univ Munich, Lehrstuhl Anorgan Chem, D-85747 Garching, Germany
关键词
D O I
10.1021/jo026602d
中图分类号
O62 [有机化学];
学科分类号
070303 ; 081704 ;
摘要
The [6pi]-photocyclization of the anilides la and 5 was studied in the absence and in the presence of the enantiomerically pure chiral lactam 4. The relative configuration of the products was unambiguously established by single-crystal X-ray crystallography and by NMR spectroscopy. A significant enantiomeric excess was observed upon reaction of compound la to its photocyclization products at -55 degreesC employing lactam 4 as a chiral complexing agent in toluene as the solvent (66% yield). The trans product ent-3a was obtained in 57% ee, and the minor diastereoisomer (trans/cis = 73/27), cis product ent-2a, was obtained in 30% ee. DFT calculations were conducted modeling the complexation of intermediates 8 and ent-8 to host 4. In agreement with steric arguments concerning the conrotatory ring closure of la, the formation of ent-8 is favored leading to the more stable complex 4.ent-8 as compared to 4.8. Whereas the enantioselectivity in the photocyclization to trans compound ent-3a increased upon reduction in the reaction temperature, the enantiomeric excess in the formation of cis compound ent-2a went through a maximum at -15 degreesC (45% ee) and decreased at lower temperatures. Deuteration experiments conducted with the pentadeuterated analogue of la, d(5)-1a, revealed that the protonation of the intermediates 8 and ent-8 is influenced by chiral amide 4. In the formation of ent-3a/3a, both the enantioselective ring closure and the enantioselective protonation by amide 4 favor the observed (6aS,10aS)-configuration of the major enantiomer ent-3a. In the formation of ent-2a/2a, the enantioselective ring closure (and the subsequent diastereoselective protonation) favors the (6aR,10aS)-configuration that is found in compound 2a. Contrary to that, the enantioselective protonation by amide 4 shows a preference for ent-2a with the (6aS,10aR)-configuration.
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页码:1107 / 1116
页数:10
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