Memory effect of chirality in the photocyclization of modified dipeptides

Tetrahedron Letters

Volumn 39, Issue 22, 1998, Pages 3685-3688

  Sauer, Stefan ^a   Schumacher, Andreas ^a   Barbosa, Frédérique ^a   Giese, Bernd ^a  

^a UNIVERSITY OF BASEL   (Switzerland)

Author keywords

[No Author keywords available]

Indexed keywords

ARTICLE; CHIRALITY; CONFORMATIONAL TRANSITION; CYCLIZATION; PHOTOLYSIS; STRUCTURE ANALYSIS;

EID: 0032575148     PISSN: 00404039     EISSN: None     Source Type: Journal    
DOI: 10.1016/S0040-4039(98)00621-2     Document Type: Article

References (15)

1. 1. C. Wyss, R. Batra, C. Lehmann, S. Sauer, B. Giese, Angew. Chem. Int. Ed. Engl. 1996, 35, 2529.
2. 2. Synthesis and photocyclization of the modified dipeptides 5, 8, 11, 14, and 16 followed the conditions described in ref. 1.
3. 2 which inverted the alcoholic carbon center. This cis carbamate was a diastereomer of the cis carbamate formed directly from 7 by deprotection of the Boc group and cyclization with N,N'-carbodiimide.
4. 4. The photocyclizations of dipeptides 5 and 8 occur also into the isopropyl group of valine. This leads to pyrrolidine derivatives in 11% (from 5) and 36% (from 8), respectively. Diastereomers of 6 and 7, in which the configuration of the alcoholic carbon center is inverted, are formed only in traces.
5. 5. For a discussion of the term "memory effect" see: J. C. Scaiano, Tetrahedron 1982, 38, 819; A. G. Griesbeck, H. Mauder, S. Stadtmüller, Acc. Chem. Res. 1994, 27, 70; K. Fuji, T. Kawabata, Chem. Eur. J. 1998, 4, 373.
6. 5. For a discussion of the term "memory effect" see: J. C. Scaiano, Tetrahedron 1982, 38, 819; A. G. Griesbeck, H. Mauder, S. Stadtmüller, Acc. Chem. Res. 1994, 27, 70; K. Fuji, T. Kawabata, Chem. Eur. J. 1998, 4, 373.
7. 5. For a discussion of the term "memory effect" see: J. C. Scaiano, Tetrahedron 1982, 38, 819; A. G. Griesbeck, H. Mauder, S. Stadtmüller, Acc. Chem. Res. 1994, 27, 70; K. Fuji, T. Kawabata, Chem. Eur. J. 1998, 4, 373.
8. 6. To our knowledge only two other examples of retention of the stereochemistry in type-II photocyclization reactions of acyclic systems are known: S. Kohmoto, T. Kreher, Y. Miyaji, M. Yamamoto, K. Yamada, J. Org. Chem. 1992, 57, 3490; W. Weigel, S. Schiller, G. Reck, H. G. Henning, Tetrahedron Lett. 1993, 34, 6737.
9. 6. To our knowledge only two other examples of retention of the stereochemistry in type-II photocyclization reactions of acyclic systems are known: S. Kohmoto, T. Kreher, Y. Miyaji, M. Yamamoto, K. Yamada, J. Org. Chem. 1992, 57, 3490; W. Weigel, S. Schiller, G. Reck, H. G. Henning, Tetrahedron Lett. 1993, 34, 6737.
10. 7. After conformational analysis (MacroModel software, Amber force field, Monte Carlo search method) one finds conformations for 5 and 8 in which the distance between the benzoyl oxygen and the hydrogen at the chiral center of alanine are close enough for H-abstraction (2.6-2.9 Å). These calculated conformations are used in Scheme 3 for the photoexcited molecules 5* and 8*, see also ref. 1.
11. 8. The photocyclization could be completely prevented by 2,5-dimethylhexadiene as triplet quencher.
12. 9. L. S. Hegedus, E. Lastra, Y. Narukawa, D. C. Snustad, J. Am. Chem. Soc. 1992, 114, 2991.
13. 10. The trans products were formed only in minute amounts.
14. 11. The main cyclization product 17 (from 16) corresponds to the main cyclization product 6 (from 5). However, the minor products 18 and 7 differ from each other by different configurations at the alcohol center. In order to understand these differences, work with other amino acids in different solvents will be carried out.
15. 12. The structure was proven by the X-ray crystal structure of the trifluoroacetate salt of the protonated trans product 17. Crystallographic data have been deposited at the Cambridge Crystallographic Data Centre.