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The small synthetic peptide, benzyl 2-(tert-but­oxy­carbonyl-amino)­isobutyrate, C16H23NO4, has the α-helical conformation [|φ| = 55.8 (2)° and |ψ| = 37.9 (2)°] observed in peptide fragments of peptaibols containing the α-amino­isobutyric acid (Aib) residue. The structure shows no intramolecular hydrogen bonding, which would disrupt the limited conformational freedom associated with this amino acid. Two weak intermolecular hydrogen contacts are observed.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S010827010100110X/bk1584sup1.cif
Contains datablocks I, BK1584_TableS4, 1

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S010827010100110X/bk1584Isup2.hkl
Contains datablock I

CCDC reference: 163945

Computing details top

Data collection: Diffractometer Control Software (Enraf-Nonius, 1988); cell refinement: Diffractometer Control Software; data reduction: SHELXTL (Bruker, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.

tert butyloxycarbonyl-α-aminoisobutyrate top
Crystal data top
C16H23NO4F(000) = 316
Mr = 293.35Dx = 1.180 Mg m3
Triclinic, P1Melting point = 349–350 K
a = 9.273 (3) ÅMo Kα radiation, λ = 0.71073 Å
b = 9.333 (3) ÅCell parameters from 25 reflections
c = 10.372 (4) Åθ = 15.0–18.0°
α = 94.16 (2)°µ = 0.08 mm1
β = 112.17 (2)°T = 293 K
γ = 93.09 (2)°Plate, colorless
V = 825.9 (4) Å30.49 × 0.33 × 0.17 mm
Z = 2
Data collection top
CAD4
diffractometer
2312 reflections with I > 2σ(I)
Radiation source: normal focus Philips Mo tubeRint = 0.007
Equatorial mounted graphite monochromatorθmax = 25.0°, θmin = 2.1°
data from ω/2θ_scansh = 011
Absorption correction: empirical (using intensity measurements), ψ scan
(North et al., 1968)
k = 1111
Tmin = 0.943, Tmax = 0.986l = 1212
3043 measured reflections3 standard reflections every 60 min
2846 independent reflections intensity decay: not applied (less than 0.5)
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: inferred from neighbouring sites
R[F2 > 2σ(F2)] = 0.048H atoms treated by a mixture of independent and constrained refinement
wR(F2) = 0.116 w = 1/[σ2(Fo2) + (0.034P)2 + 0.4831P]
where P = (Fo2 + 2Fc2)/3
S = 1.01(Δ/σ)max < 0.001
2846 reflectionsΔρmax = 0.22 e Å3
195 parametersΔρmin = 0.19 e Å3
0 restraintsExtinction correction: SHELXL97 (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
Primary atom site location: structure-invariant direct methodsExtinction coefficient: 0.249 (9)
Special details top

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 (Å2) top
xyzUiso*/Ueq
N10.1726 (2)0.91661 (17)1.14755 (18)0.0450 (4)
H1N0.093 (3)0.940 (2)1.150 (2)0.059 (7)*
O10.16959 (16)0.94942 (14)0.88954 (15)0.0505 (4)
C10.1747 (3)1.1380 (2)0.7463 (2)0.0540 (5)
O20.2947 (2)0.75086 (17)0.8951 (2)0.0824 (6)
C20.0357 (3)1.1711 (3)0.6479 (3)0.0698 (7)
H2A0.04041.09770.59620.084*
O30.42378 (17)0.99491 (16)1.20007 (18)0.0646 (5)
C30.0089 (4)1.3137 (4)0.6256 (3)0.0881 (9)
H3A0.08491.33550.55850.106*
O40.27024 (16)1.11710 (14)1.28722 (15)0.0546 (4)
C40.1189 (5)1.4214 (3)0.7011 (4)0.0908 (10)
H4A0.10011.51680.68560.109*
C50.2563 (4)1.3908 (3)0.7994 (4)0.0835 (9)
H5A0.33111.46520.85140.100*
C60.2848 (3)1.2499 (3)0.8218 (3)0.0670 (7)
H6A0.37941.22960.88880.080*
C70.2067 (3)0.9854 (3)0.7703 (3)0.0729 (7)
H7A0.14320.92190.68770.088*
H7B0.31590.97340.78930.088*
C80.2225 (2)0.8268 (2)0.9414 (2)0.0483 (5)
C90.1714 (2)0.78836 (19)1.0587 (2)0.0428 (5)
C100.0028 (2)0.7213 (2)0.9897 (2)0.0548 (5)
H10A0.06210.79000.93620.082*
H10B0.03370.69481.06050.082*
H10C0.00180.63710.92890.082*
C110.2745 (3)0.6788 (2)1.1428 (3)0.0602 (6)
H11A0.38050.72071.18600.090*
H11B0.26940.59481.08160.090*
H11C0.23830.65201.21380.090*
C120.3008 (2)1.0093 (2)1.2112 (2)0.0452 (5)
C130.3860 (3)1.2415 (2)1.3573 (2)0.0572 (6)
C140.4277 (3)1.3174 (2)1.2519 (3)0.0650 (6)
H14A0.48591.25691.21430.097*
H14C0.49011.40601.29630.097*
H14B0.33381.33801.17760.097*
C150.2952 (4)1.3352 (3)1.4188 (3)0.0945 (10)
H15A0.20661.36471.34460.142*
H15C0.36141.41881.47220.142*
H15B0.26011.28151.47860.142*
C160.5275 (4)1.1923 (3)1.4696 (3)0.0987 (11)
H16A0.58251.13371.42700.148*
H16B0.49471.13721.52990.148*
H16C0.59521.27501.52350.148*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
N10.0408 (9)0.0415 (9)0.0544 (11)0.0010 (7)0.0220 (8)0.0028 (7)
O10.0592 (9)0.0484 (8)0.0563 (9)0.0122 (6)0.0338 (7)0.0120 (6)
C10.0664 (14)0.0560 (12)0.0548 (13)0.0094 (10)0.0388 (11)0.0119 (10)
O20.1197 (15)0.0545 (10)0.1135 (15)0.0291 (10)0.0860 (13)0.0162 (9)
C20.0708 (16)0.0865 (18)0.0573 (15)0.0013 (13)0.0311 (13)0.0077 (13)
O30.0413 (8)0.0562 (9)0.0922 (12)0.0007 (6)0.0243 (8)0.0076 (8)
C30.089 (2)0.117 (3)0.0771 (19)0.0443 (19)0.0433 (17)0.0433 (19)
O40.0583 (9)0.0465 (8)0.0587 (9)0.0080 (6)0.0269 (7)0.0109 (7)
C40.131 (3)0.0698 (18)0.111 (3)0.0330 (19)0.083 (2)0.0346 (18)
C50.105 (2)0.0640 (16)0.105 (2)0.0095 (16)0.070 (2)0.0006 (15)
C60.0624 (14)0.0782 (17)0.0705 (16)0.0043 (12)0.0364 (13)0.0103 (13)
C70.109 (2)0.0633 (15)0.0740 (17)0.0187 (14)0.0625 (16)0.0154 (12)
C80.0527 (11)0.0377 (10)0.0596 (13)0.0029 (9)0.0282 (10)0.0006 (9)
C90.0428 (10)0.0365 (9)0.0491 (11)0.0002 (8)0.0184 (9)0.0013 (8)
C100.0494 (12)0.0471 (12)0.0648 (14)0.0064 (9)0.0208 (10)0.0004 (10)
C110.0645 (14)0.0486 (12)0.0653 (14)0.0116 (10)0.0205 (11)0.0111 (10)
C120.0468 (11)0.0401 (10)0.0466 (11)0.0010 (8)0.0160 (9)0.0039 (8)
C130.0650 (14)0.0449 (11)0.0523 (13)0.0097 (10)0.0161 (11)0.0068 (9)
C140.0674 (15)0.0500 (12)0.0749 (16)0.0044 (11)0.0259 (12)0.0052 (11)
C150.117 (2)0.0716 (17)0.103 (2)0.0237 (16)0.063 (2)0.0384 (16)
C160.106 (2)0.0754 (18)0.0682 (18)0.0176 (16)0.0151 (16)0.0042 (14)
Geometric parameters (Å, º) top
N1—C121.346 (2)C7—H7B0.9700
N1—C91.454 (2)C8—C91.521 (3)
N1—H1N0.80 (2)C9—C111.525 (3)
O1—C81.338 (2)C9—C101.528 (3)
O1—C71.459 (3)C10—H10A0.9600
C1—C21.377 (3)C10—H10B0.9600
C1—C61.382 (3)C10—H10C0.9600
C1—C71.489 (3)C11—H11A0.9600
O2—C81.194 (2)C11—H11B0.9600
C2—C31.387 (4)C11—H11C0.9600
C2—H2A0.9300C13—C141.500 (3)
O3—C121.202 (2)C13—C161.509 (4)
C3—C41.355 (4)C13—C151.511 (3)
C3—H3A0.9300C14—H14A0.9600
O4—C121.341 (2)C14—H14C0.9600
O4—C131.477 (2)C14—H14B0.9600
C4—C51.359 (4)C15—H15A0.9600
C4—H4A0.9300C15—H15C0.9600
C5—C61.374 (4)C15—H15B0.9600
C5—H5A0.9300C16—H16A0.9600
C6—H6A0.9300C16—H16B0.9600
C7—H7A0.9700C16—H16C0.9600
C12—N1—C9122.40 (17)C9—C10—H10B109.5
C12—N1—H1N117.6 (17)H10A—C10—H10B109.5
C9—N1—H1N119.4 (17)C9—C10—H10C109.5
C8—O1—C7115.00 (16)H10A—C10—H10C109.5
C2—C1—C6118.3 (2)H10B—C10—H10C109.5
C2—C1—C7121.0 (2)C9—C11—H11A109.5
C6—C1—C7120.6 (2)C9—C11—H11B109.5
C1—C2—C3120.2 (3)H11A—C11—H11B109.5
C1—C2—H2A119.9C9—C11—H11C109.5
C3—C2—H2A119.9H11A—C11—H11C109.5
C4—C3—C2120.3 (3)H11B—C11—H11C109.5
C4—C3—H3A119.9O3—C12—O4126.05 (18)
C2—C3—H3A119.9O3—C12—N1123.80 (19)
C12—O4—C13121.49 (16)O4—C12—N1110.15 (17)
C3—C4—C5120.4 (3)O4—C13—C14110.34 (17)
C3—C4—H4A119.8O4—C13—C16109.96 (18)
C5—C4—H4A119.8C14—C13—C16112.3 (2)
C4—C5—C6119.9 (3)O4—C13—C15101.63 (18)
C4—C5—H5A120.0C14—C13—C15110.5 (2)
C6—C5—H5A120.0C16—C13—C15111.7 (2)
C5—C6—C1120.9 (3)C13—C14—H14A109.5
C5—C6—H6A119.6C13—C14—H14C109.5
C1—C6—H6A119.6H14A—C14—H14C109.5
O1—C7—C1108.56 (17)C13—C14—H14B109.5
O1—C7—H7A110.0H14A—C14—H14B109.5
C1—C7—H7A110.0H14C—C14—H14B109.5
O1—C7—H7B110.0C13—C15—H15A109.5
C1—C7—H7B110.0C13—C15—H15C109.5
H7A—C7—H7B108.4H15A—C15—H15C109.5
O2—C8—O1123.5 (2)C13—C15—H15B109.5
O2—C8—C9123.25 (19)H15A—C15—H15B109.5
O1—C8—C9113.11 (16)H15C—C15—H15B109.5
N1—C9—C8111.10 (15)C13—C16—H16A109.5
N1—C9—C11111.53 (17)C13—C16—H16B109.5
C8—C9—C11110.15 (17)H16A—C16—H16B109.5
N1—C9—C10107.66 (16)C13—C16—H16C109.5
C8—C9—C10106.85 (17)H16A—C16—H16C109.5
C11—C9—C10109.39 (16)H16B—C16—H16C109.5
C9—C10—H10A109.5
 

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