COMMUNICATIONS
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effects were in agreement with those reported by Gawronski
and co-workers for several N-phthaloyl amino acids.[16] Due to
the photocyclization, the long-wavelength absorption max-
imum of the chromophore is shifted hypsochromic (from 290
to <250 nm)[7] and consequently the maxima in the CD
spectra were also shifted to 233 nm (4 f-H, De 88.9) and
234 nm (4g-H, De 51.9). The absolute configuration of the
products were determined through the exciton rule:[17] Com-
pound 4g-H shows positive exciton chirality (and thus a
negative Cotton effect in the short-wavelength region). The
mirror-image behavior of the CD spectra of substrates and
products when comparing proline and 2-azabicyclo[3.3.0]oc-
tanoic acid derivatives clearly proved that inversion of
configuration also occurs for the photocyclization of the
proline substrate 2 f-H (Figure 1).
[13] X-ray crystal structure analyses of 4 f-H and 4g-H. Data collection:
Enraf-Nonius-CAD4 diffractometer, MoKa, graphite monochromator,
Wyckoff-Scan, q-range [8]: 1.75 ± 27.5. Structure analysis und refine-
ment: Solution by direct phase determination, refinement by full
matrix least square fit techniques against F 2 with Siemens SHELXL-
93. Crystal data: 4 f-H (C19H16N2O3, from acetone): M 320.3,
orthorhombic, a 8.987(1), b 11.098(1), c 15.338(1) , space
group P212121, 5037 reflections measured, 2093 reflections with F >
2s(F), R 0.069, Rw 0.041. 4g-H (C22H20N2O3xC3H6O, from ace-
tone): M 418.48, orthorhombic, a 10.043(1), b 11.512(1), c
18.511(1) , space group P212121, 12515 reflections measured, 3038
reflections with F > 2s(F), R 0.065, Rw 0.040. Crystallographic
data (excluding structure factors) for the structures reported in this
paper have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC-150112 (4 f-H) and
-150113 (4g-H). Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
(44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
As an explanation for the high degree of memory of
chirality, we assume that the intermediary (atropisomeric) 1,7-
triplet diradicals exhibit high activation barriers for rotation
about the central C N single bond and thus preserve their
absolute axial chirality during the course of the reaction.
Force-field calculations for the proline substrate 2 f-H result-
1 [18]
ed in an activation barrier of approximately 55 kJmol .
This classifies these substrates as non-diaryl atropisomers with
restricted single bond rotations in the arylimide, as well as the
amide moiety.[19] The kinetically controlled simple diastereo-
selectivity (see above) most probably results from diradical
conformations determined by spin-orbit coupling that are
capable of rapid spin inversion. This model was already
successfully applied to cyclization reactions involving 1,4-[20]
and 1,6-diradicals.[21]
[14] HPLC analyses of 4 f-H and 4 f-Cl: CHIRACEL OD, hexane/2-
propanol/HNEt2 (80/20/0.1), DtR 11.0 and 10.1 min, respectively.
The HPLC measurements were performed by Dr. H.-J. Hamann,
Humboldt-Universität, Berlin.
Experimental Section
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Waluk, J. Marczyk, Tetrahedron 1996, 52, 13201 ± 13214; b) J. Ga-
A solution of carboxylic acid 2 (3.00 mmol) in acetone (10 mL) was treated
with a solution of potassium carbonate (1.5 mmol) in water (15 mL) and
heated to 508C for 2 min. After dilution with acetone (90 mL) and water
(90 mL), the solution is irradiated for 12 h under a nitrogen atmosphere in a
Rayonet photoreactor (l 300 Æ 10 nm, ca. 800 W) at 158C. After treat-
ment with saturated sodium bicarbonate solution (10 mL) and extraction
with ethyl acetate (3 Â 40 mL), the organic phase was separated, washed
with diluted aqueous sodium bicarbonate, and dried. After evaporation of
the solvent, 4 was recrystallized or purified by column chromatography.
Â
wronski, F. Kazmierczak, K. Gawronska, U. Rychlewska, B. Norden,
Â
A. Holmen, J. Am. Chem. Soc. 1998, 120, 12083 ± 12091; c) P.
Skowronek, J. Gawronski, Tetrahedron: Asymmetry 1999, 10, 4585 ±
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[17] N. Berova, B. Borhan, J. G. Dong, J. Guo, X. Huang, E. Karnaukhova,
A. Kawamura, J. Lou, S. Matile, K. Nakanishi, B. Rickman, J. Su, Q.
Tan, I. Zanze, Pure Appl. Chem. 1998, 70, 377 ± 383.
Received: August 10, 2000 [Z15619]
[18] Calculations were performed on an SGI O2 workstation with the
Amber force field, implemented in Macromodel 6.0 (F. Mohamadi,
N. G. J. Richards, W. C. Guida, R. Liskamp, M. Lipton, C. Caufield, G.
Chang, T. Hendrickson, W. C. Still, J. Comput. Chem. 1990, 11, 440 ±
467).
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[5] This is indeed the case for cyclization reactions involving triplet
diradicals under sensitization conditions in ref. [4b], where the ee
values drop to zero.
Angew. Chem. Int. Ed. 2001, 40, No. 3
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