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Acta Cryst. (2009). E65, o587    [ doi:10.1107/S1600536809005741 ]

N-tert-Butyl-5[alpha]-androstane-17[beta]-carboxamide

X. Yan, S. Xu, J. Wang, Y. Chen and P. Xia

Abstract top

The title compound, C24H41NO, is a new derivative of the anti-HIV steroid 17[beta]-(N-tert-butylaminocarbonyl)androst-4-en-3-one. There are four rings in the structure and these are trans-fused. The three six-membered rings exhibit chair conformations, while the five-membered ring adopts an envelope conformation.

Comment top

17β-(N-tert-Butylcarboxamide)-androst-4-ene-3-one was reported to exhibit potent anti-HIV activity in acutely infected H9 lymphocytes with EC50 and therapeutic index values of 0.8 and 300 µM, respectively (Xia et al., 1999). During our work of structural modification, which is motivated by the search for more potent anti-HIV agents, we found that 17β-(N-tert-butylcarboxamide)-5α-androstane (I) could be obtained through Pd/C catalytic hydrogenation of 17β-(N-tert-butylcarboxamide)-androst-4-ene-3-ol in excellent yield (90%); full structural details of (I) are reported herein.

The molecular structure of (I), Fig.1, shows the A/B, B/C and C/D ring junctions to be all trans. The cyclohexane rings adopt chair conformations, and the cyclopentane ring adopts an envelope conformation. Based on the known configurations of the C10, C13-methyl groups, see Experimental, 5-H is assigned an α-configuration. The 17-N-tert-butylcarboxamide group is in a β-configuration. The stereogenic sites of (I) exhibit the following chirality: C5 = R, C8 = R, C9 = S, C10 = S, C13 = S, C14 = S and C17 = S.

Related literature top

For the anti-HIV activity of 17β-(N-tert-butylaminocarbonyl)-androst-4-en-3-one, see: Xia et al. (1999). For discussion of absolute configuration, see: Marker et al. (1940); Fieser & Fieser (1959); Throop & Tokes (1967); House (1972); Castro-Méndez et al. (2002);

Experimental top

Compound (I) was prepared from the corresponding 4-ene-3-ol by catalytic hydrogenation with 5% palladium-on-charcoal in EtOH for 1 day. After filtration and removal of the solvent, the residue was crystallized from acetone to give colourless crystals.

The starting material, 17β-(N-tert-butylcarboxamide)-androst-4-ene-3-ol, was obtained from the reduction of 17β-(N-tert-butylcarboxamide)-androst-4-ene-3-one with NaBH4. It is an intermediate in the synthesis of Finasteride and derived initially from diosgenin, for which the absolute configurations of all chiral centers of the steroid skeleton have been determined (Fieser & Fieser, 1959; Marker et al., 1940). Recently, the absolute configurations of the chiral centres were confirmed by the X-ray crystal structure determination of a 3-Br substituted steroid substrate synthesized from diosgenin (Castro-Méndez et al., 2002). The hydrogenation of 4-en-3-one moiety did not cause inversion of the configurations at C8, C9, C10, C13 and C14 (Throop & Tokes, 1967; House, 1972). Thus, by comparing the orientation of 5-H to that of methyl groups at C10 and C13, the absolute configuration of (I) could be determined.

Refinement top

All H atoms were placed in the idealized positions with N—H = 0.86 Å, methine C—H = 0.98 Å, methylene C—H = 0.97 Å and methyl C—H = 0.96 Å, and treated as riding with Uiso(H) = 1.2 Ueq(N-H, CH2 and CH) and 1.5Ueq(CH3). In the absence of significant anomalous scattering effects, 1971 Friedel pairs were averaged in the final refinement.

Computing details top

Data collection: SMART (Bruker, 2000); cell refinement: SMART (Bruker, 2000); data reduction: SHELXTL (Sheldrick, 2008); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures top
[Figure 1] Fig. 1. The molecular structure of (I), with atom labels and 30% probability displacement ellipsoids for non-H atoms.
N-tert-Butyl-5α-androstane-17β-carboxamide top
Crystal data top
C24H41NODx = 1.093 Mg m−3
Mr = 359.58Melting point: 451.5 K
Orthorhombic, P212121Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2abCell parameters from 962 reflections
a = 6.373 (2) Åθ = 2.8–19.0°
b = 12.802 (4) ŵ = 0.07 mm−1
c = 26.775 (9) ÅT = 294 K
V = 2184.3 (12) Å3Parallelepiped, colourless
Z = 40.15 × 0.08 × 0.06 mm
F(000) = 800
Data collection top
Bruker SMART CCD area-detector
diffractometer		2757 independent reflections
Radiation source: sealed tube1433 reflections with I > 2σ(I)
graphiteRint = 0.063
φ and ω scansθmax = 27.1°, θmin = 1.5°
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)		h = −8→8
Tmin = 0.990, Tmax = 0.996k = −16→16
10583 measured reflectionsl = −34→22
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.037Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.068H-atom parameters constrained
S = 0.75 w = 1/[σ2(Fo2) + (0.0299P)2]
where P = (Fo2 + 2Fc2)/3
2757 reflections(Δ/σ)max < 0.001
240 parametersΔρmax = 0.10 e Å−3
0 restraintsΔρmin = −0.12 e Å−3
Crystal data top
C24H41NOV = 2184.3 (12) Å3
Mr = 359.58Z = 4
Orthorhombic, P212121Mo Kα radiation
a = 6.373 (2) ŵ = 0.07 mm−1
b = 12.802 (4) ÅT = 294 K
c = 26.775 (9) Å0.15 × 0.08 × 0.06 mm
Data collection top
Bruker SMART CCD area-detector
diffractometer		2757 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 1996)		1433 reflections with I > 2σ(I)
Tmin = 0.990, Tmax = 0.996Rint = 0.063
10583 measured reflectionsθmax = 27.1°
Refinement top
R[F2 > 2σ(F2)] = 0.037H-atom parameters constrained
wR(F2) = 0.068Δρmax = 0.10 e Å−3
S = 0.75Δρmin = −0.12 e Å−3
2757 reflectionsAbsolute structure: ?
240 parametersFlack parameter: ?
0 restraintsRogers parameter: ?
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
N10.8035 (3)0.65842 (13)0.56653 (6)0.0540 (5)
H10.92470.68200.57510.065*
O10.4554 (3)0.67334 (13)0.58044 (5)0.0666 (5)
C11.1936 (4)0.62769 (17)0.84810 (7)0.0534 (6)
H1A1.21840.56670.82740.064*
H1B1.29160.68150.83770.064*
C21.2381 (4)0.59935 (18)0.90230 (7)0.0614 (7)
H2A1.15480.53880.91140.074*
H2B1.38490.58060.90570.074*
C31.1892 (4)0.6872 (2)0.93761 (8)0.0682 (7)
H3A1.29280.74200.93340.082*
H3B1.19930.66180.97170.082*
C40.9727 (4)0.73240 (19)0.92929 (7)0.0635 (7)
H4A0.95680.79490.94940.076*
H4B0.86810.68220.94020.076*
C50.9344 (4)0.75945 (17)0.87460 (7)0.0522 (6)
H51.04300.81060.86600.063*
C60.7298 (4)0.81485 (17)0.86688 (7)0.0610 (7)
H6A0.72390.87520.88870.073*
H6B0.61570.76850.87600.073*
C70.7000 (4)0.85044 (16)0.81299 (7)0.0614 (7)
H7A0.56010.87920.80910.074*
H7B0.80020.90530.80550.074*
C80.7295 (3)0.76086 (15)0.77615 (7)0.0426 (5)
H80.61960.70910.78260.051*
C90.9436 (3)0.70738 (15)0.78447 (6)0.0397 (5)
H91.04960.76210.78020.048*
C100.9693 (3)0.66681 (15)0.83881 (7)0.0400 (5)
C110.9900 (4)0.62511 (16)0.74440 (7)0.0508 (6)
H11A0.90220.56460.75050.061*
H11B1.13500.60310.74770.061*
C120.9537 (3)0.66238 (17)0.69070 (7)0.0499 (6)
H12A1.05650.71540.68230.060*
H12B0.97220.60420.66790.060*
C130.7344 (3)0.70711 (14)0.68449 (7)0.0382 (5)
C140.7092 (3)0.79588 (15)0.72231 (7)0.0436 (5)
H140.82500.84460.71620.052*
C150.5092 (4)0.85175 (17)0.70618 (7)0.0567 (6)
H15A0.51170.92450.71630.068*
H15B0.38630.81850.72050.068*
C160.5093 (4)0.84178 (16)0.64878 (7)0.0599 (7)
H16A0.53230.90950.63340.072*
H16B0.37600.81450.63720.072*
C170.6889 (4)0.76619 (14)0.63516 (6)0.0442 (6)
H170.81300.80770.62660.053*
C180.5691 (4)0.62057 (15)0.69106 (7)0.0539 (6)
H18A0.59900.56420.66850.081*
H18B0.43230.64830.68400.081*
H18C0.57300.59520.72480.081*
C190.8159 (4)0.57727 (15)0.84878 (8)0.0584 (7)
H19A0.85460.51780.82900.088*
H19B0.67640.59890.84010.088*
H19C0.82040.55890.88350.088*
C200.6366 (4)0.69668 (16)0.59120 (8)0.0487 (6)
C210.7988 (4)0.58025 (17)0.52642 (8)0.0598 (7)
C220.6959 (5)0.48089 (17)0.54580 (9)0.0912 (9)
H22A0.77370.45500.57390.137*
H22B0.69470.42920.51980.137*
H22C0.55450.49590.55590.137*
C231.0254 (5)0.5606 (2)0.51203 (10)0.1023 (10)
H23A1.08530.62360.49880.153*
H23B1.03140.50660.48720.153*
H23C1.10310.53920.54100.153*
C240.6785 (5)0.6221 (2)0.48153 (8)0.0816 (8)
H24A0.53530.63480.49080.122*
H24B0.68280.57170.45500.122*
H24C0.74150.68610.47050.122*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
N10.0549 (14)0.0628 (12)0.0443 (11)−0.0034 (11)0.0022 (10)−0.0105 (10)
O10.0572 (12)0.0831 (11)0.0595 (10)−0.0006 (11)−0.0099 (9)−0.0130 (9)
C10.0520 (16)0.0652 (14)0.0430 (13)0.0011 (13)−0.0016 (12)−0.0034 (11)
C20.0565 (18)0.0790 (15)0.0486 (14)0.0058 (14)−0.0050 (12)0.0054 (13)
C30.075 (2)0.0871 (18)0.0425 (14)0.0031 (17)−0.0089 (14)0.0036 (13)
C40.072 (2)0.0781 (17)0.0407 (14)0.0077 (16)0.0017 (13)−0.0055 (12)
C50.0539 (17)0.0644 (15)0.0382 (13)0.0022 (14)0.0015 (12)−0.0031 (11)
C60.069 (2)0.0696 (15)0.0443 (14)0.0139 (14)−0.0002 (12)−0.0153 (12)
C70.0739 (18)0.0619 (14)0.0483 (14)0.0217 (14)0.0024 (13)−0.0083 (12)
C80.0440 (15)0.0451 (12)0.0386 (12)0.0060 (11)0.0060 (10)−0.0030 (10)
C90.0386 (13)0.0454 (12)0.0351 (12)−0.0033 (11)0.0036 (10)−0.0027 (10)
C100.0367 (14)0.0471 (12)0.0363 (12)−0.0011 (11)0.0051 (10)0.0010 (10)
C110.0466 (15)0.0642 (14)0.0415 (12)0.0151 (12)−0.0001 (11)−0.0012 (11)
C120.0482 (15)0.0620 (14)0.0395 (12)0.0051 (13)0.0042 (11)−0.0075 (11)
C130.0392 (14)0.0419 (11)0.0336 (11)0.0017 (11)0.0008 (10)−0.0009 (10)
C140.0486 (15)0.0413 (11)0.0409 (12)0.0021 (12)0.0023 (11)0.0032 (10)
C150.0699 (19)0.0552 (14)0.0451 (13)0.0215 (14)−0.0044 (12)−0.0012 (11)
C160.0740 (19)0.0529 (13)0.0527 (14)0.0134 (14)−0.0027 (13)0.0028 (12)
C170.0508 (15)0.0464 (12)0.0355 (12)−0.0004 (12)−0.0007 (11)0.0017 (10)
C180.0607 (16)0.0527 (13)0.0485 (14)−0.0046 (13)0.0004 (12)0.0035 (11)
C190.0584 (18)0.0646 (15)0.0522 (14)−0.0089 (14)−0.0024 (13)0.0098 (12)
C200.0608 (18)0.0484 (13)0.0370 (13)0.0006 (14)0.0000 (13)0.0067 (11)
C210.073 (2)0.0573 (14)0.0492 (14)0.0029 (14)0.0018 (13)−0.0104 (13)
C220.135 (3)0.0530 (15)0.0855 (19)−0.0078 (19)0.0009 (19)−0.0047 (14)
C230.085 (2)0.124 (2)0.098 (2)0.017 (2)0.016 (2)−0.0481 (19)
C240.113 (2)0.0870 (17)0.0447 (14)−0.0078 (18)−0.0044 (16)−0.0127 (14)
Geometric parameters (Å, °) top
N1—C201.345 (3)C11—H11B0.9700
N1—C211.468 (3)C12—C131.519 (3)
N1—H10.8600C12—H12A0.9700
O1—C201.227 (3)C12—H12B0.9700
C1—C21.522 (3)C13—C141.531 (3)
C1—C101.535 (3)C13—C181.539 (3)
C1—H1A0.9700C13—C171.549 (3)
C1—H1B0.9700C14—C151.524 (3)
C2—C31.502 (3)C14—H140.9800
C2—H2A0.9700C15—C161.542 (2)
C2—H2B0.9700C15—H15A0.9700
C3—C41.513 (3)C15—H15B0.9700
C3—H3A0.9700C16—C171.542 (3)
C3—H3B0.9700C16—H16A0.9700
C4—C51.524 (3)C16—H16B0.9700
C4—H4A0.9700C17—C201.513 (3)
C4—H4B0.9700C17—H170.9800
C5—C61.498 (3)C18—H18A0.9599
C5—C101.541 (3)C18—H18B0.9599
C5—H50.9800C18—H18C0.9599
C6—C71.525 (2)C19—H19A0.9599
C6—H6A0.9700C19—H19B0.9599
C6—H6B0.9700C19—H19C0.9599
C7—C81.524 (2)C21—C231.516 (4)
C7—H7A0.9700C21—C241.523 (3)
C7—H7B0.9700C21—C221.522 (3)
C8—C141.515 (2)C22—H22A0.9599
C8—C91.542 (3)C22—H22B0.9599
C8—H80.9800C22—H22C0.9599
C9—C111.532 (2)C23—H23A0.9599
C9—C101.554 (2)C23—H23B0.9599
C9—H90.9800C23—H23C0.9599
C10—C191.530 (3)C24—H24A0.9599
C11—C121.532 (3)C24—H24B0.9599
C11—H11A0.9700C24—H24C0.9599
C20—N1—C21126.2 (2)C13—C12—H12B109.4
C20—N1—H1116.9C11—C12—H12B109.4
C21—N1—H1116.9H12A—C12—H12B108.0
C2—C1—C10113.91 (17)C12—C13—C14107.69 (17)
C2—C1—H1A108.8C12—C13—C18110.23 (17)
C10—C1—H1A108.8C14—C13—C18112.79 (16)
C2—C1—H1B108.8C12—C13—C17116.70 (16)
C10—C1—H1B108.8C14—C13—C17100.48 (14)
H1A—C1—H1B107.7C18—C13—C17108.70 (17)
C3—C2—C1112.51 (19)C8—C14—C15118.68 (17)
C3—C2—H2A109.1C8—C14—C13113.62 (16)
C1—C2—H2A109.1C15—C14—C13104.39 (16)
C3—C2—H2B109.1C8—C14—H14106.5
C1—C2—H2B109.1C15—C14—H14106.5
H2A—C2—H2B107.8C13—C14—H14106.5
C2—C3—C4112.5 (2)C14—C15—C16104.07 (17)
C2—C3—H3A109.1C14—C15—H15A110.9
C4—C3—H3A109.1C16—C15—H15A110.9
C2—C3—H3B109.1C14—C15—H15B110.9
C4—C3—H3B109.1C16—C15—H15B110.9
H3A—C3—H3B107.8H15A—C15—H15B109.0
C3—C4—C5111.98 (19)C15—C16—C17106.73 (16)
C3—C4—H4A109.2C15—C16—H16A110.4
C5—C4—H4A109.2C17—C16—H16A110.4
C3—C4—H4B109.2C15—C16—H16B110.4
C5—C4—H4B109.2C17—C16—H16B110.4
H4A—C4—H4B107.9H16A—C16—H16B108.6
C6—C5—C4112.29 (18)C20—C17—C16112.92 (19)
C6—C5—C10113.84 (17)C20—C17—C13114.68 (16)
C4—C5—C10113.54 (18)C16—C17—C13104.10 (15)
C6—C5—H5105.4C20—C17—H17108.3
C4—C5—H5105.4C16—C17—H17108.3
C10—C5—H5105.4C13—C17—H17108.3
C5—C6—C7112.36 (18)C13—C18—H18A109.5
C5—C6—H6A109.1C13—C18—H18B109.5
C7—C6—H6A109.1H18A—C18—H18B109.5
C5—C6—H6B109.1C13—C18—H18C109.5
C7—C6—H6B109.1H18A—C18—H18C109.5
H6A—C6—H6B107.9H18B—C18—H18C109.5
C8—C7—C6111.84 (16)C10—C19—H19A109.5
C8—C7—H7A109.2C10—C19—H19B109.5
C6—C7—H7A109.2H19A—C19—H19B109.5
C8—C7—H7B109.2C10—C19—H19C109.5
C6—C7—H7B109.2H19A—C19—H19C109.5
H7A—C7—H7B107.9H19B—C19—H19C109.5
C14—C8—C7112.48 (16)O1—C20—N1122.7 (2)
C14—C8—C9110.13 (16)O1—C20—C17122.2 (2)
C7—C8—C9110.47 (17)N1—C20—C17115.0 (2)
C14—C8—H8107.9N1—C21—C23106.2 (2)
C7—C8—H8107.9N1—C21—C24110.37 (18)
C9—C8—H8107.9C23—C21—C24109.7 (2)
C11—C9—C8112.02 (16)N1—C21—C22109.20 (18)
C11—C9—C10113.93 (16)C23—C21—C22111.0 (2)
C8—C9—C10112.15 (15)C24—C21—C22110.2 (2)
C11—C9—H9106.0C21—C22—H22A109.5
C8—C9—H9106.0C21—C22—H22B109.5
C10—C9—H9106.0H22A—C22—H22B109.5
C19—C10—C1108.81 (17)C21—C22—H22C109.5
C19—C10—C5112.08 (16)H22A—C22—H22C109.5
C1—C10—C5106.56 (16)H22B—C22—H22C109.5
C19—C10—C9110.26 (16)C21—C23—H23A109.5
C1—C10—C9111.04 (15)C21—C23—H23B109.5
C5—C10—C9108.05 (15)H23A—C23—H23B109.5
C9—C11—C12114.43 (17)C21—C23—H23C109.5
C9—C11—H11A108.7H23A—C23—H23C109.5
C12—C11—H11A108.7H23B—C23—H23C109.5
C9—C11—H11B108.7C21—C24—H24A109.5
C12—C11—H11B108.7C21—C24—H24B109.5
H11A—C11—H11B107.6H24A—C24—H24B109.5
C13—C12—C11111.04 (16)C21—C24—H24C109.5
C13—C12—H12A109.4H24A—C24—H24C109.5
C11—C12—H12A109.4H24B—C24—H24C109.5
C10—C1—C2—C3−54.5 (3)C11—C12—C13—C18−66.3 (2)
C1—C2—C3—C450.1 (3)C11—C12—C13—C17169.09 (17)
C2—C3—C4—C5−50.5 (3)C7—C8—C14—C15−55.7 (3)
C3—C4—C5—C6−173.8 (2)C9—C8—C14—C15−179.44 (17)
C3—C4—C5—C1055.3 (3)C7—C8—C14—C13−179.00 (18)
C4—C5—C6—C7175.0 (2)C9—C8—C14—C1357.3 (2)
C10—C5—C6—C7−54.3 (2)C12—C13—C14—C8−61.5 (2)
C5—C6—C7—C853.1 (3)C18—C13—C14—C860.3 (2)
C6—C7—C8—C14−177.64 (18)C17—C13—C14—C8175.87 (17)
C6—C7—C8—C9−54.1 (2)C12—C13—C14—C15167.69 (17)
C14—C8—C9—C11−48.6 (2)C18—C13—C14—C15−70.46 (19)
C7—C8—C9—C11−173.49 (17)C17—C13—C14—C1545.1 (2)
C14—C8—C9—C10−178.19 (16)C8—C14—C15—C16−161.48 (18)
C7—C8—C9—C1056.9 (2)C13—C14—C15—C16−33.8 (2)
C2—C1—C10—C19−65.7 (2)C14—C15—C16—C178.8 (2)
C2—C1—C10—C555.4 (2)C15—C16—C17—C20143.81 (18)
C2—C1—C10—C9172.80 (17)C15—C16—C17—C1318.8 (2)
C6—C5—C10—C19−67.1 (2)C12—C13—C17—C2081.4 (2)
C4—C5—C10—C1963.0 (2)C14—C13—C17—C20−162.6 (2)
C6—C5—C10—C1173.94 (18)C18—C13—C17—C20−44.0 (2)
C4—C5—C10—C1−55.9 (2)C12—C13—C17—C16−154.74 (18)
C6—C5—C10—C954.5 (2)C14—C13—C17—C16−38.70 (19)
C4—C5—C10—C9−175.31 (19)C18—C13—C17—C1679.89 (18)
C11—C9—C10—C19−61.5 (2)C21—N1—C20—O1−4.2 (3)
C8—C9—C10—C1967.1 (2)C21—N1—C20—C17172.60 (17)
C11—C9—C10—C159.2 (2)C16—C17—C20—O1−26.4 (3)
C8—C9—C10—C1−172.21 (16)C13—C17—C20—O192.6 (3)
C11—C9—C10—C5175.74 (18)C16—C17—C20—N1156.77 (17)
C8—C9—C10—C5−55.7 (2)C13—C17—C20—N1−84.2 (2)
C8—C9—C11—C1248.2 (2)C20—N1—C21—C23−178.2 (2)
C10—C9—C11—C12176.84 (18)C20—N1—C21—C2462.9 (3)
C9—C11—C12—C13−53.2 (2)C20—N1—C21—C22−58.5 (3)
C11—C12—C13—C1457.1 (2)
Acknowledgements top

The authors acknowledge the Center of Analysis and Measurement, Fudan University, for providing research facilities.

references
References top

Bruker (2000). SMART. Bruker AXS Inc., Madison, Wisconsin, USA.

Castro-Méndez, A., Sandoval-Ramírez, J. & Bernès, S. (2002). Acta Cryst. E58, o606–o608.

Fieser, L. F. & Fieser, M. (1959). Steroids, pp. 331–340. New York: Reinhold Publising Corporation.

House, H. O. (1972). Modern Synthetic Reactions, 2nd ed, p. 13. London: Benjamin WA.

Marker, R. E., Tsukamoto, T. & Turner, D. L. (1940). J. Am. Chem. Soc. 62, 2525–2532.

Sheldrick, G. M. (1996). SADABS. University of Göttingen, Germany.

Sheldrick, G. M. (2008). Acta Cryst. A64, 112–122.

Throop, L. J. & Tokes, L. (1967). J. Am. Chem. Soc. 89, 4789–4790.

Xia, P., Yang, Z. Y., Xia, Y., Zheng, Y. Q., Cosentino, L. M. & Lee, K. H. (1999). Bioorg. Med. Chem. 7, 1907–1911.