References Edmonds, J. W. & Hamilton, W. C. (1972). Acta Cryst. B28, 1362B28, -1366B28, . Elo, H. (1989a. Spectrosc. Lett. 22, 123-160. © 1996 International Union of Crystallography Printed in Great Britain -all rights reserved Comment As a part of our research directed towards the synthesis of 3-aryl-4(3H)-isoquinolinone derivatives, we have Acta Crystallographica Section C ISSN 0108-2701 © 1996 3170 CI3HI7NO4 designed a synthetic route in which the initial substrate is the simple a-amino acid DL-2-phenylglycine. The protection of its primary amino group with tert-butoxycarbonyl gave the title compound, (1).The I H-NMR spectrum of (1) in CDC13 indicates the presence of two conformers, Z and E, in the ratio 1:3. The stereochemical assignments were made on , the basis of their HN proton chemical shifts: at 5.52 p.p.m, for the Z isomer versus 8.04 p.p.m, for the E isomer in which the downfield proton is deshielded by the anisotropy cone of the carbonyl group (Silverstein, Bassler & Morril, 1981). This type of conformational equilibrium in solution is similar to that observed for amides in which the rotation around the acyl-to-nitrogen bond is restricted because of the partial double-bond character (Lewin, Frucht, Chen, Benedetti & Di Blasio, 1975). Consequently, we conducted a crystallographic study of N-(tert-butoxycarbonyl)-2-phenylglycine, (1), to determine the nature of the carbamate bond.
02~ O4 ClO~~ Fig. 1. ORTEPII (Johnson, 1976) drawing of (I) showing the atomic numbering scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms have been assigned arbitrary radii.In the solid-state, (1) (Carpenter & Donohue, 1950), this was calculated to be 33%. A strong hydrogen bond involving two carboxyl groups [O1...O2 i 2.644(4)A; (i) = -x, 2-y, -z] can be postulated from the observation of a broad band in the IR spectrum (KBr) at 3125-2800cm -~, attributable to the associated stretching vibration of the OH bond. Nevertheless, there are no hydrogen bonds involving the NH H atom which is consistent with the sharp band in the IR at 3380 cm -l, similar to the free NH stretching vibration of secondary amides (Silverstein, Bassler & Morril, 1981),
ExperimentalThe N-Boc a-amino acid, N-(tert-butoxycarbonyl)-2-phenylglycine, (1), was prepared by reaction of DL-2-phenylglycine and di-tert-butyl dicarbonate under basic conditions (Keller, Keller, van Look & Wersin, 1985), and was fully characterized by NMR and IR spectroscopy. Crystals suitable for X-ray analysis were obtained by recrystallization from hexane/ethyl acetate, 9.0:1.0. Financial support from the Basque Country University (project No. UPV 170.310-EA 167/95) is gratefully acknowledged.
Crystal dataLists of structure factors, anisotropic displacement parameters, Hatom coordinates and complete geometry have been deposited with the IUCr (Reference: TAI093). Copies may be obtained through The Managing Editor, International Union of Crystallography, 5 Abbey Square, Chester CHI 2HU, England.References Enraf-Nonius (1989 B. (1975). Tetrahe...