N-Carboxy-L-phenyl­alanine anhydride

The mol­ecules of the title compound, 4-benzyl-1,3-oxazolidene-2,5-dione, C₁₀H₉NO₃, are linked by intermolecular hydrogen bonds between the imino group of the five-membered ring and an adjacent carbonyl O-atom, along the c axis. The benzyl groups are stacked in a layer and the five-membered rings are arranged in another layer sandwiched by the benzyl group layer. This sandwich structure should explain the high polymerizability of the title compound in the solid state.

S1. Comment

N-Carboxy anhydrides (NCAs) of amino acids are crystalline compounds and are usually polymerized in solution to prepare polypeptides (Bamford et al., 1956). Several amino acid NCA crystals are known to be decomposed or polymerized by moisture. When butylamine is added to NCA crystals immersed in non-solvents, polymerization takes place in the solid state. The authors studied this solid-state polymerization and found the polymerizability of NCAs was extremely dependent on the kind of amino acid used. The crystal structures of amino acid NCAs have not been studied since a very early report by Lechus (1906).

The present authors determined the crystal structures of glycine NCA (Kanazawa et al. 1976a) and L-alanine NCA (Kanazawa et al. 1976b) and discussed their polymerizability with reference to the crystal structure (Kanazawa & Kawai, 1980). In addition, the crystal structures of γ-benzyl-L-glutamate NCA (Kanazawa et al., 1978a), L-leucine NCA (Kanazawa et al., 1978b), L-valine NCA (Kanazawa & Ohashi, 1984), DL-valine NCA (Takenaka et al., 1994) and DL-phenylalanine NCA (Kanazawa et al., 1997) have been determined. The polymerization of L-leucine NCA, which was the most reactive in the solid state among the examined NCAs, has been studied in detail (Kanazawa et al., 1982; Kanazawa, 1992a,b). Recently, the title compound, (I), was found to be still more reactive than any other NCAs in the solid state, although it gave almost no reactivity in solution (Kanazawa, 1997). As compound (I) is very sensitive to moisture in air, it often polymerizes spontaneously in the crystallization process. Thus, crystals suitable for the present X-ray work were obtained after many attempts. This paper describes the crystal structure of compound (I). \scheme

Intermolecular N1—H1···O1(5/2 - x, -y, 1/2 + z) hydrogen bonds are formed along the c axis. Hydrogen bond lengths and angles are N1···O1 2.912 (2) Å, H1···O1 2.03 (2) Å, and N1—H1···O1 162 (2)°. From Fig. 2, it can be seen that the benzyl groups are in a layer and the benzene rings seem almost parallel to each other. The five-membered NCA rings are packed in another layer and these two layers are aligned alternately. The five-membered rings could easily react with one another within the layer. In fact, electron microscopy suggests that the polymerization mainly proceeds along the c axis. This sandwich structure is one of the preferred requirements for high reactivity in the solid state (Kanazawa, 1992a). In the crystal of DL-phenylalanine NCA, a sandwich structure composed of D and L molecules was observed (Kanazawa et al., 1997), and the crystal was also reactive in the solid state.

S2. Experimental

Compound (I) was obtained from L-phenylalanine by phosgenation in tetrahydrofuran with trichloromethyl chloroformate, using a method similar to that used for the other NCAs (Kanazawa & Kawai, 1980). The reaction product was purified by recrystallization from a solution in a mixture of ethyl acetate and hexane. Crystals for X-ray analysis were obtained from a solution in ethyl acetate with hexane vapor at about 283 K, avoiding contamination by moisture.

S3. Refinement

As the absolute structure could not be determined reliably from the Flack parameter and since it is known from the synthesis, the Friedel pairs were merged. Extinction conditions indicated that the space group is P2₁2₁2₁. H atoms were located at geometrically calculated positions and were fixed in refinement, except for H1 attatched to N1, which was freely refined.

Computing details

Data collection: PROCESS (Rigaku, 1996); cell refinement: PROCESS; data reduction: TEXSAN (Molecular Structure Corporation, 1999); program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: TEXSAN; molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: TEXSAN.

Figures

	Fig. 1. The molecular structure of (I) showing 50% probability displacement ellipsoids and the atom-numbering scheme (ORTEPII; Johnson, 1976). H atoms are drawn as small circles of arbitrary radii and only H1 is numbered.
	Fig. 2. Stereo packing diagram for (I). Note the hydrogen bond is indicated by a solid line.

(I)

Crystal data

C10H9NO3	Dx = 1.353 Mg m^−3
Mr = 191.19	Mo Kα radiation, λ = 0.7107 Å
Orthorhombic, P212121	Cell parameters from 1440 reflections
a = 10.881 (2) Å	θ = 1.3–29.0°
b = 15.753 (2) Å	µ = 0.10 mm^−1
c = 5.4764 (5) Å	T = 293 K
V = 938.7 (2) Å^3	Needle, colourless
Z = 4	0.25 × 0.15 × 0.15 mm

Data collection

Rigaku RAXIS-IV Imaging Plate diffractometer	Rint = 0.059
Detector resolution: 10 pixels mm^-1	θmax = 29.0°
ω scans	h = −14→14
6823 measured reflections	k = −21→21
1427 independent reflections	l = −7→7
1032 reflections with F^2 > 2σ(F^2)

Refinement

Refinement on F^2	w = 1/[σ^2(Fo^2) + {0.067[Max(Fo^2,0) + 2Fc^2]/3}^2]
R[F^2 > 2σ(F^2)] = 0.055	(Δ/σ)max < 0.001
wR(F^2) = 0.158	Δρmax = 0.21 e Å^−3
S = 1.25	Δρmin = −0.17 e Å^−3
1427 reflections	Extinction correction: Zachariasen (1967) type 2 Gaussian isotropic
133 parameters	Extinction coefficient: 0.075 (9)
H atoms treated by a mixture of independent and constrained refinement

Special details

Refinement. Refinement using reflections with F² > -10.0 σ(F²). The weighted R-factor (wR) and goodness of fit (S) are based on F². R-factor (gt) are based on F. The threshold expression of F² > 2.0 σ(F²) is used only for calculating R-factor (gt).

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	1.1488 (2)	−0.0071 (1)	−0.0384 (3)	0.0855 (6)
O2	0.9531 (1)	0.0230 (1)	0.0607 (3)	0.0640 (4)
O3	0.7821 (2)	0.0600 (1)	0.2637 (4)	0.0870 (6)
N1	1.0946 (2)	0.0406 (1)	0.3457 (3)	0.0624 (5)
C1	1.0772 (2)	0.0163 (2)	0.1157 (4)	0.0593 (5)
C2	0.8905 (2)	0.0501 (1)	0.2637 (4)	0.0604 (5)
C3	0.9816 (3)	0.0642 (1)	0.4667 (4)	0.0641 (5)
C4	0.9762 (3)	0.1560 (2)	0.5641 (4)	0.0895 (8)
C5	0.9914 (3)	0.2227 (1)	0.3679 (5)	0.0809 (7)
C6	0.8899 (3)	0.2577 (2)	0.2521 (7)	0.1007 (10)
C7	0.9054 (4)	0.3163 (2)	0.0613 (8)	0.107 (1)
C8	1.0181 (4)	0.3393 (2)	−0.0105 (6)	0.1009 (9)
C9	1.1172 (5)	0.3065 (2)	0.1027 (9)	0.122 (1)
C10	1.1053 (4)	0.2482 (2)	0.2907 (8)	0.1054 (10)
H1	1.172 (2)	0.038 (2)	0.407 (5)	0.070 (7)*
H2	0.9656	0.0253	0.5979	0.0751*
H3	0.8986	0.1636	0.6421	0.1119*
H4	1.0397	0.1626	0.6828	0.1119*
H5	0.8089	0.2407	0.3070	0.1179*
H6	0.8317	0.3389	−0.0213	0.1275*
H7	1.0295	0.3782	−0.1436	0.1179*
H8	1.1965	0.3262	0.0544	0.1421*
H9	1.1794	0.2239	0.3640	0.1260*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0680 (10)	0.123 (1)	0.0659 (9)	0.011 (1)	0.0144 (8)	−0.015 (1)
O2	0.0559 (8)	0.095 (1)	0.0409 (6)	−0.0006 (8)	−0.0030 (6)	0.0010 (7)
O3	0.0644 (9)	0.102 (1)	0.095 (1)	0.0106 (9)	0.0174 (9)	0.020 (1)
N1	0.066 (1)	0.072 (1)	0.0493 (8)	0.0063 (10)	−0.0139 (8)	0.0002 (8)
C1	0.0579 (10)	0.074 (1)	0.0459 (8)	0.0028 (10)	0.0002 (8)	0.0008 (9)
C2	0.0652 (10)	0.064 (1)	0.0520 (9)	0.004 (1)	0.0117 (8)	0.0115 (10)
C3	0.085 (1)	0.065 (1)	0.0419 (8)	0.008 (1)	0.0078 (9)	0.0067 (9)
C4	0.151 (3)	0.069 (1)	0.0480 (9)	0.005 (2)	0.011 (2)	−0.0056 (9)
C5	0.128 (2)	0.056 (1)	0.059 (1)	−0.002 (1)	0.006 (1)	−0.0099 (9)
C6	0.122 (2)	0.074 (1)	0.106 (2)	0.031 (2)	0.028 (2)	0.014 (2)
C7	0.146 (2)	0.067 (1)	0.109 (2)	0.026 (2)	0.000 (3)	0.017 (2)
C8	0.166 (3)	0.056 (1)	0.080 (2)	−0.017 (2)	0.007 (2)	0.005 (1)
C9	0.142 (3)	0.094 (2)	0.130 (3)	−0.045 (2)	−0.011 (3)	0.031 (2)
C10	0.116 (2)	0.092 (2)	0.108 (2)	−0.030 (2)	−0.033 (2)	0.015 (2)

Geometric parameters (Å, º)

O1—C1	1.207 (3)	C4—C5	1.513 (4)
O2—C1	1.388 (3)	C5—C6	1.390 (5)
O2—C2	1.372 (3)	C5—C10	1.369 (5)
O3—C2	1.187 (4)	C6—C7	1.406 (6)
N1—C1	1.329 (3)	C7—C8	1.335 (7)
N1—C3	1.447 (4)	C8—C9	1.346 (7)
C2—C3	1.508 (4)	C9—C10	1.387 (7)
C3—C4	1.542 (4)
O1···N1^i	2.911 (3)	O2···C4^ii	3.441 (3)
O1···C3^ii	3.451 (4)	O3···O3^v	3.400 (3)
O1···N1^ii	3.504 (3)	O3···O3^iv	3.400 (3)
O1···O1^iii	3.522 (3)	O3···C9^vi	3.416 (5)
O1···O1^i	3.522 (3)	O3···C8^vi	3.565 (5)
O1···C1^i	3.535 (3)	N1···C8^vii	3.518 (4)
O2···O3^iv	3.303 (3)	N1···C7^vii	3.570 (4)
O2···C3^ii	3.331 (3)	C3···C8^vii	3.551 (4)
C1—O2—C2	109.4 (2)	C2—C3—C4	111.6 (2)
C1—N1—C3	112.7 (2)	C3—C4—C5	113.7 (2)
O1—C1—O2	119.9 (2)	C4—C5—C6	120.8 (3)
O1—C1—N1	131.2 (3)	C4—C5—C10	121.5 (3)
O2—C1—N1	108.9 (2)	C6—C5—C10	117.6 (3)
O2—C2—O3	122.3 (3)	C5—C6—C7	120.1 (4)
O2—C2—C3	108.4 (2)	C6—C7—C8	120.6 (4)
O3—C2—C3	129.3 (3)	C7—C8—C9	119.7 (3)
N1—C3—C2	100.6 (2)	C8—C9—C10	121.4 (4)
N1—C3—C4	115.6 (3)	C5—C10—C9	120.5 (4)
O1—C1—O2—C2	179.7 (3)	C1—N1—C3—C4	−120.7 (3)
O1—C1—N1—C3	179.7 (3)	C2—C3—C4—C5	−54.3 (4)
O2—C1—N1—C3	1.2 (3)	C3—C4—C5—C6	92.7 (4)
O2—C2—C3—N1	−0.7 (3)	C3—C4—C5—C10	−84.3 (4)
O2—C2—C3—C4	122.5 (2)	C4—C5—C6—C7	−176.7 (3)
O3—C2—O2—C1	−179.3 (3)	C4—C5—C10—C9	177.0 (4)
O3—C2—C3—N1	−179.9 (3)	C5—C6—C7—C8	0.0 (6)
O3—C2—C3—C4	−56.7 (4)	C5—C10—C9—C8	−0.7 (7)
N1—C1—O2—C2	−1.7 (3)	C6—C5—C10—C9	−0.1 (6)
N1—C3—C4—C5	59.9 (4)	C6—C7—C8—C9	−0.7 (7)
C1—O2—C2—C3	1.4 (3)	C7—C6—C5—C10	0.4 (5)
C1—N1—C3—C2	−0.3 (3)	C7—C8—C9—C10	1.1 (7)

Symmetry codes: (i) −x+5/2, −y, z−1/2; (ii) x, y, z−1; (iii) −x+5/2, −y, z+1/2; (iv) −x+3/2, −y, z−1/2; (v) −x+3/2, −y, z+1/2; (vi) x−1/2, −y+1/2, −z; (vii) −x+2, y−1/2, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O1	0.91 (2)	2.03 (2)	2.912 (2)	162 (2)