Reactive intermediates in peptide synthesis: the N-oxy­succinimido ester of N^α-para-toluene­sulfonyl-α-amino­isobutyric acid

The preparation, characterization, and molecular and crystal structures of succinimido 2-(tosyl­amino)­isobutyrate, C₁₅H₁₈N₂O₆S, are described. The succinimido ring is nearly orthogonal to the ester group.

S1. Comment

X-Ray diffraction has been shown to represent an excellent technique to contribute to our understanding of α-amino acid chemistry (Toniolo et al., 1996). However, reactive intermediates in peptide synthesis are commonly characterized by low melting points and poor crystallinities. Using an appropriate combination of a high molecular weight aromatic N^α-protecting group [e.g. the para-toluenesulfonyl (Tos) group] and a conformationally restricted α-amino acid (e.g. a C^α,^α-disubstituted glycine, such as Aib, α-aminoisobutyric acid), known to exhibit a higher tendency to crystallize than protein residues, this drawback may be easily overcome.

We describe here the synthesis, characterization, and results of a crystallographic analysis of Tos-Aib-OSu (OSu is oxysuccinimide), or succinimido 2-(tosylamino)isobutyrate, (I). Among the variety of N^α-protected α-amino acid hydroxylamine derivatives reported in the literature, one of the most popular methods for activation of the COOH function in peptide synthesis, the formation of N-oxysuccinimido esters (Anderson et al., 1964), has been extensively exploited because: (i) the water-soluble by-product N-hydroxysuccinimide can be easily removed by extraction and (ii) they allow amide bond formation with no racemization (epimerization), even if a peptide segment is activated (Benoiton et al., 1995). The only X-ray diffraction structure published for an N-oxysuccinimido ester of an amino acid or peptide is Boc-L-Val-OSu (Boc is tert-butyloxycarbonyl) (Sukumar et al., 1993).

In Tos-Aib-OSu (Fig. 1), the C11═O3 bond length (Table 1) is close to that typical of carboxylic esters (Allen et al., 1987). Atoms C8, C11, O3, and O4 of the ester group are coplanar within 0.030 (2) Å, while atom N2 is displaced from the ester plane by 0.189 (2) Å.

The pentaatomic succinimido ring is slightly puckered towards the ⁴T₃ (twist) disposition, as indicated by the values of the puckering parameters (relative to the N2—C12—C13—C14—C15 atom sequence) of q₂ = 0.095 (2) Å and ϕ₂ = -94.2 (13)° (Cremer & Pople, 1975). Such ring conformation is characterized by a local pseudo-twofold axis along N2 and the midpoint of the C13—C14 bond. The characteristic O4—N2 bond is 1.391 (2) Å. The value of the ratio Δ/σ between the two N—Csp² bond lengths, 4.01, is just significant. The internal bond angles of the succinimido moiety have values in the range 104.9 (2)–106.2 (1)°, as expected for a pentagonal ring, with the exception of the bond angle at nitrogen (Table 1). The exocyclic bond angles involving the two carbonyl O atoms are remarkably expanded, and lie in the range 124.1 (2)–130.8 (2)°, and the C11—O4—N2 bond angle is wide (Table 1). The dihedral angle between the average –COO ester and succinimido planes is nearly orthogonal, 105.2 (1)°, presumably to reduce potential lone-pair repulsion between O3 and the O5 and O6 carbonyl O atoms. The O3···O5 and O3···O6 distances are 3.255 (3) and 3.834 (3) Å, respectively. A similar succinimido geometry has been reported for Boc-L-Val-OSu (Sukumar et al., 1993).

The S1—N1—C8—C11 and N1—C8—C11—O4 amino acid backbone torsion angles are 65.5 (2) and 33.2 (2)°, respectively. These values confirm the propensity of Aib for the helical region of the conformational map (Karle & Balaram, 1990; Toniolo & Benedetti, 1991). The conformationally sensitive Aib τ(N1—C8—C11) bond angle (Paterson et al., 1981) is close to the regular tetrahedral value (Table 1). The C5—S1—N1—C8 torsion angle is close to 90°, its value being 97.64 (18)°. Significantly short intramolecular distances between non-bonded atoms are O2···C11 of 2.938 (3) Å and O2···O4 of 3.140 (3) Å. The S1 atom is 2.746 (2) Å from C8, 3.291 (4) Å from C10, and 3.325 (2) Å from C11. The dihedral angle between the average planes of the aromatic ring of the Tos group and the ester –COO group is 149.1 (1)°, while that involving the Tos aromatic ring and the succinimido moiety is 128.4 (1)°.

In the crystal, the molecules form dimers stabilized by double intermolecular hydrogen bonds between (sulfonamido) N1—H1 groups and (succinimido) O6═ C15 groups of symmetry-related molecules. The N1···O6ⁱ distance and N1—H···O6ⁱ angle [symmetry code: (i) 1 - x, 1 - y, 2 - z; Table 2] are within the accepted range for such an interaction (Görbitz, 1989).

S2. Experimental

For the synthesis of Tos-Aib-OSu, Tos-Aib-OH (Leplawy et al., 1960) (386 mg, 1.5 mmol) and N-ethyl-N'-[(3-dimethylamino)propyl]carbodiimide hydrochloride (288 mg, 1.5 mmol) were dissolved at 273 K in acetonitrile (5 ml) in the presence of N-hydroxysuccinimide (173 mg, 1.5 mmol) and N-methylmorpholine (0.165 ml, 1.5 mmol). The reaction mixture was stirred for 1 h at 273 K and 3 h at room temperature. The solvent was evaporated, and the residue was taken up in ethyl acetate and washed with 0.5 M citric acid, water, 5% NaHCO₃, and water. The organic layer was dried over anhydrous Na₂SO₄ and evaporated to dryness. The title compound was obtained in 73% yield from ethyl acetate–petroleum ether (1:3) as a colourless solid with a melting point of 448–449 K. TLC: R_F (CHCl₃/ethanol, 9:1) 0.85; IR (KBr): 3309, 1811, 1778, 1731 cm^-1; ¹H NMR (200 MHz, CDCl₃): δ 7.83–7.26 (4H, 4 aromatic CH), 5.27 (1H, Aib NH), 2.84 (4H, OSu 2 CH₂), 2.42 (3H, Tos CH₃), 1.64 (6H, 2 Aib CH₃). Single crystals were obtained from ethyl acetate/petroleum ether by vapour diffusion.

S3. Refinement

All H atoms were placed at idealized positions and refined as riding (N—H = 0.86 Å and C—H = 0.93–0.97 Å), with U_iso values 1.2 (or 1.5 for methyl H atoms) times the U_eq of the parent atom.

Computing details

Data collection: FEBO (Belletti, 1993); cell refinement: FEBO; data reduction: FEBO; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: PARST96 (Nardelli, 1995).

Figures

Fig. 1. A view of the molecule of the title compound with the atom-numbering scheme. Displacement ellipsoids are shown at the 30% probability level and H atoms are represented by spheres of arbitrary radii.

N^α-para-toluenesulfonyl-α-aminoisobutyric acid N-oxysuccinimido ester

Crystal data

C15H18N2O6S	F(000) = 372
Mr = 354.37	Dx = 1.378 Mg m^−3
Triclinic, P1	Melting point = 448–449 K
a = 9.094 (2) Å	Mo Kα radiation, λ = 0.71073 Å
b = 12.575 (2) Å	Cell parameters from 48 reflections
c = 8.352 (1) Å	θ = 7–12°
α = 93.03 (3)°	µ = 0.22 mm^−1
β = 115.84 (3)°	T = 293 K
γ = 93.97 (3)°	Prism, colourless
V = 853.9 (3) Å^3	0.40 × 0.40 × 0.40 mm
Z = 2

Data collection

Philips PW1100 diffractometer	Rint = 0.033
Radiation source: fine-focus sealed tube	θmax = 28.0°, θmin = 2.5°
Graphite monochromator	h = −12→10
θ–2θ scans	k = −16→16
4129 measured reflections	l = 0→11
4111 independent reflections	3 standard reflections every 50 reflections
2635 reflections with I > 2σ(I)	intensity decay: none

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.044	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.142	H-atom parameters constrained
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0774P)^2] where P = (Fo^2 + 2Fc^2)/3
4111 reflections	(Δ/σ)max = 0.003
217 parameters	Δρmax = 0.33 e Å^−3
0 restraints	Δρmin = −0.47 e Å^−3

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq
S1	0.50577 (6)	0.25466 (5)	0.69163 (7)	0.04807 (18)
O1	0.4914 (2)	0.33587 (14)	0.5727 (2)	0.0693 (5)
O2	0.65872 (18)	0.21134 (15)	0.7838 (2)	0.0686 (5)
O3	0.81110 (17)	0.27878 (13)	1.25327 (19)	0.0552 (4)
O4	0.75233 (15)	0.41003 (11)	1.06611 (18)	0.0456 (4)
O5	1.03799 (19)	0.34401 (13)	1.0541 (2)	0.0611 (4)
O6	0.84233 (19)	0.58495 (14)	1.3197 (2)	0.0650 (5)
N1	0.4605 (2)	0.30570 (14)	0.8442 (2)	0.0471 (4)
H1	0.3899	0.3519	0.8131	0.057*
N2	0.91272 (18)	0.45475 (13)	1.1719 (2)	0.0442 (4)
C1	−0.0124 (4)	−0.1061 (3)	0.2386 (5)	0.1038 (12)
H1A	0.0378	−0.1555	0.1912	0.156*
H1B	−0.0995	−0.0774	0.1421	0.156*
H1C	−0.0563	−0.1428	0.3088	0.156*
C2	0.1145 (3)	−0.0161 (2)	0.3547 (4)	0.0665 (7)
C3	0.2591 (3)	−0.0373 (2)	0.4922 (4)	0.0770 (8)
H3	0.2773	−0.1080	0.5155	0.092*
C4	0.3784 (3)	0.0442 (2)	0.5970 (3)	0.0692 (7)
H4	0.4756	0.0280	0.6894	0.083*
C5	0.3531 (2)	0.14900 (17)	0.5645 (3)	0.0471 (5)
C6	0.2085 (3)	0.1720 (2)	0.4286 (4)	0.0803 (9)
H6	0.1894	0.2427	0.4063	0.096*
C7	0.0913 (3)	0.0890 (2)	0.3251 (4)	0.0932 (11)
H7	−0.0060	0.1051	0.2328	0.112*
C8	0.5286 (2)	0.28062 (17)	1.0306 (3)	0.0451 (5)
C9	0.4491 (3)	0.3489 (3)	1.1229 (3)	0.0717 (8)
H9A	0.4705	0.4232	1.1113	0.108*
H9B	0.4943	0.3371	1.2472	0.108*
H9C	0.3326	0.3291	1.0679	0.108*
C10	0.5018 (3)	0.1621 (2)	1.0512 (4)	0.0803 (9)
H10A	0.3874	0.1373	0.9843	0.120*
H10B	0.5361	0.1522	1.1751	0.120*
H10C	0.5650	0.1222	1.0072	0.120*
C11	0.7134 (2)	0.31702 (15)	1.1302 (2)	0.0392 (4)
C12	1.0473 (2)	0.41760 (16)	1.1557 (3)	0.0427 (4)
C13	1.1938 (2)	0.49006 (17)	1.2884 (3)	0.0476 (5)
H13A	1.2585	0.5192	1.2313	0.057*
H13B	1.2631	0.4509	1.3860	0.057*
C14	1.1254 (2)	0.57926 (18)	1.3570 (3)	0.0500 (5)
H14A	1.1781	0.5880	1.4866	0.060*
H14B	1.1430	0.6465	1.3131	0.060*
C15	0.9448 (3)	0.54536 (17)	1.2873 (3)	0.0450 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
S1	0.0387 (3)	0.0546 (3)	0.0435 (3)	−0.0056 (2)	0.0140 (2)	−0.0062 (2)
O1	0.0780 (12)	0.0715 (12)	0.0530 (9)	−0.0247 (9)	0.0287 (8)	0.0048 (8)
O2	0.0384 (8)	0.0789 (12)	0.0736 (11)	0.0059 (7)	0.0138 (8)	−0.0203 (9)
O3	0.0449 (8)	0.0605 (10)	0.0459 (8)	0.0031 (7)	0.0062 (7)	0.0149 (7)
O4	0.0342 (7)	0.0462 (8)	0.0468 (8)	−0.0025 (6)	0.0095 (6)	0.0084 (6)
O5	0.0555 (9)	0.0626 (10)	0.0641 (10)	−0.0021 (7)	0.0291 (8)	−0.0166 (8)
O6	0.0552 (9)	0.0723 (11)	0.0685 (11)	0.0184 (8)	0.0274 (8)	−0.0037 (9)
N1	0.0389 (9)	0.0566 (11)	0.0378 (9)	0.0113 (8)	0.0089 (7)	0.0015 (8)
N2	0.0318 (8)	0.0437 (9)	0.0513 (10)	−0.0030 (7)	0.0150 (7)	−0.0035 (7)
C1	0.0661 (18)	0.079 (2)	0.129 (3)	−0.0194 (15)	0.0190 (19)	−0.042 (2)
C2	0.0461 (13)	0.0599 (15)	0.0770 (17)	−0.0051 (11)	0.0165 (12)	−0.0207 (13)
C3	0.0780 (18)	0.0475 (14)	0.0815 (19)	0.0006 (12)	0.0155 (15)	−0.0083 (13)
C4	0.0598 (15)	0.0540 (15)	0.0650 (15)	0.0062 (11)	0.0023 (12)	−0.0066 (12)
C5	0.0403 (10)	0.0501 (12)	0.0430 (11)	−0.0016 (9)	0.0128 (9)	−0.0045 (9)
C6	0.0652 (16)	0.0538 (15)	0.0767 (18)	−0.0001 (12)	−0.0085 (14)	−0.0016 (13)
C7	0.0567 (16)	0.0727 (19)	0.093 (2)	0.0022 (14)	−0.0167 (15)	−0.0132 (16)
C8	0.0362 (10)	0.0541 (12)	0.0369 (10)	−0.0030 (9)	0.0098 (8)	0.0030 (9)
C9	0.0490 (13)	0.115 (2)	0.0560 (14)	0.0071 (14)	0.0282 (11)	0.0010 (14)
C10	0.0671 (16)	0.0705 (18)	0.0736 (17)	−0.0250 (13)	0.0064 (14)	0.0238 (14)
C11	0.0396 (10)	0.0403 (10)	0.0363 (9)	0.0021 (8)	0.0160 (8)	0.0009 (8)
C12	0.0410 (10)	0.0457 (11)	0.0412 (10)	0.0010 (8)	0.0182 (8)	0.0053 (9)
C13	0.0374 (10)	0.0526 (12)	0.0474 (11)	−0.0011 (9)	0.0148 (9)	0.0012 (9)
C14	0.0469 (11)	0.0504 (12)	0.0450 (11)	−0.0057 (9)	0.0157 (9)	−0.0046 (9)
C15	0.0453 (11)	0.0446 (11)	0.0429 (11)	0.0073 (9)	0.0167 (9)	0.0063 (9)

Geometric parameters (Å, º)

S1—O2	1.4282 (17)	C4—H4	0.9300
S1—O1	1.4352 (18)	C5—C6	1.374 (3)
S1—N1	1.617 (2)	C6—C7	1.386 (4)
S1—C5	1.769 (2)	C6—H6	0.9300
O3—C11	1.180 (2)	C7—H7	0.9300
O4—N2	1.391 (2)	C8—C10	1.524 (3)
O4—C11	1.398 (2)	C8—C9	1.536 (3)
O5—C12	1.194 (2)	C8—C11	1.536 (3)
O6—C15	1.208 (3)	C9—H9A	0.9600
N1—C8	1.464 (3)	C9—H9B	0.9600
N1—H1	0.8600	C9—H9C	0.9600
N2—C15	1.380 (3)	C10—H10A	0.9600
N2—C12	1.397 (3)	C10—H10B	0.9600
C1—C2	1.510 (3)	C10—H10C	0.9600
C1—H1A	0.9600	C12—C13	1.508 (3)
C1—H1B	0.9600	C13—C14	1.521 (3)
C1—H1C	0.9600	C13—H13A	0.9700
C2—C7	1.372 (4)	C13—H13B	0.9700
C2—C3	1.372 (3)	C14—C15	1.504 (3)
C3—C4	1.385 (3)	C14—H14A	0.9700
C3—H3	0.9300	C14—H14B	0.9700
C4—C5	1.374 (3)
O2—S1—O1	120.19 (12)	N1—C8—C9	106.89 (17)
O2—S1—N1	106.15 (10)	C10—C8—C9	111.0 (2)
O1—S1—N1	106.59 (10)	N1—C8—C11	111.4 (2)
O2—S1—C5	108.84 (11)	C10—C8—C11	108.53 (18)
O1—S1—C5	106.20 (10)	C9—C8—C11	105.17 (17)
N1—S1—C5	108.44 (10)	C8—C9—H9A	109.5
N2—O4—C11	112.2 (2)	C8—C9—H9B	109.5
C8—N1—S1	126.06 (14)	H9A—C9—H9B	109.5
C8—N1—H1	117.0	C8—C9—H9C	109.5
S1—N1—H1	117.0	H9A—C9—H9C	109.5
C15—N2—O4	120.85 (16)	H9B—C9—H9C	109.5
C15—N2—C12	116.4 (2)	C8—C10—H10A	109.5
O4—N2—C12	122.45 (16)	C8—C10—H10B	109.5
C2—C1—H1A	109.5	H10A—C10—H10B	109.5
C2—C1—H1B	109.5	C8—C10—H10C	109.5
H1A—C1—H1B	109.5	H10A—C10—H10C	109.5
C2—C1—H1C	109.5	H10B—C10—H10C	109.5
H1A—C1—H1C	109.5	O3—C11—O4	122.5 (2)
H1B—C1—H1C	109.5	O3—C11—C8	126.4 (2)
C7—C2—C3	117.7 (2)	O4—C11—C8	110.8 (2)
C7—C2—C1	121.6 (2)	O5—C12—N2	124.3 (2)
C3—C2—C1	120.7 (3)	O5—C12—C13	130.8 (2)
C2—C3—C4	121.4 (3)	N2—C12—C13	104.9 (2)
C2—C3—H3	119.3	C12—C13—C14	106.1 (2)
C4—C3—H3	119.3	C12—C13—H13A	110.5
C5—C4—C3	120.0 (2)	C14—C13—H13A	110.5
C5—C4—H4	120.0	C12—C13—H13B	110.5
C3—C4—H4	120.0	C14—C13—H13B	110.5
C6—C5—C4	119.5 (2)	H13A—C13—H13B	108.7
C6—C5—S1	119.41 (18)	C15—C14—C13	105.7 (2)
C4—C5—S1	121.05 (17)	C15—C14—H14A	110.6
C5—C6—C7	119.4 (3)	C13—C14—H14A	110.6
C5—C6—H6	120.3	C15—C14—H14B	110.6
C7—C6—H6	120.3	C13—C14—H14B	110.6
C2—C7—C6	122.0 (2)	H14A—C14—H14B	108.7
C2—C7—H7	119.0	O6—C15—N2	124.1 (2)
C6—C7—H7	119.0	O6—C15—C14	130.0 (2)
N1—C8—C10	113.52 (19)	N2—C15—C14	105.9 (2)
O2—S1—N1—C8	−19.2 (2)	S1—N1—C8—C11	65.5 (2)
O1—S1—N1—C8	−148.40 (17)	N2—O4—C11—O3	−4.6 (3)
C5—S1—N1—C8	97.64 (18)	N2—O4—C11—C8	170.28 (15)
C11—O4—N2—C15	−104.6 (2)	N1—C8—C11—O3	−152.2 (2)
C11—O4—N2—C12	81.6 (2)	C10—C8—C11—O3	−26.5 (3)
C7—C2—C3—C4	−0.5 (5)	C9—C8—C11—O3	92.4 (3)
C1—C2—C3—C4	178.4 (3)	N1—C8—C11—O4	33.2 (2)
C2—C3—C4—C5	0.1 (5)	C10—C8—C11—O4	158.87 (19)
C3—C4—C5—C6	0.5 (4)	C9—C8—C11—O4	−82.2 (2)
C3—C4—C5—S1	−178.0 (2)	C15—N2—C12—O5	−175.6 (2)
O2—S1—C5—C6	−162.4 (2)	O4—N2—C12—O5	−1.6 (3)
O1—S1—C5—C6	−31.7 (2)	C15—N2—C12—C13	3.9 (2)
N1—S1—C5—C6	82.5 (2)	O4—N2—C12—C13	177.95 (16)
O2—S1—C5—C4	16.2 (2)	O5—C12—C13—C14	171.1 (2)
O1—S1—C5—C4	146.9 (2)	N2—C12—C13—C14	−8.4 (2)
N1—S1—C5—C4	−98.9 (2)	C12—C13—C14—C15	9.8 (2)
C4—C5—C6—C7	−0.8 (5)	O4—N2—C15—O6	8.3 (3)
S1—C5—C6—C7	177.8 (3)	C12—N2—C15—O6	−177.6 (2)
C3—C2—C7—C6	0.2 (5)	O4—N2—C15—C14	−171.79 (16)
C1—C2—C7—C6	−178.7 (3)	C12—N2—C15—C14	2.4 (2)
C5—C6—C7—C2	0.4 (5)	C13—C14—C15—O6	172.4 (2)
S1—N1—C8—C10	−57.4 (3)	C13—C14—C15—N2	−7.5 (2)
S1—N1—C8—C9	179.86 (16)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O6^i	0.86	2.15	2.964 (2)	159

Symmetry code: (i) −x+1, −y+1, −z+2.