P. Ferraboschi et al. / Tetrahedron: Asymmetry 13 (2002) 2583–2586
Table 1. CAL B-catalyzed transesterification of 2/3 and alcoholysis of 4/5
2585
Solvent
Irreversible transesterification
Alcoholysis
Reaction
time (h)
Conversion
% Ratio of unreacted
substrates 2/3
Reaction
time (h)
Conversion
% Ratio of unreacted
substrates 4/5
of 2 (%)a
of 4 (%)a
Acetonitrile
2.5
3
5
1
27
100
100
100
100
0
0/100
0/100
0/100
0/100
7/3
72
72
24
24
48
100
93
88
100
33
0/100
11/89
20/80
0/100
55/45
Tetrahydrofuran
tert-Butylmethylether
Toluene
Chloroform
a Determined by GLC.
1
firmed by our results: in fact in the case of the isomeric
mixture of 2 and 3, having a ‘quasi-chair’ A ring
conformation, CAL-B is able to transform only the
equatorial 3a- and not the axial 3b-isomer.
3b-diol TR 12.56 min. In the H NMR spectrum two
multiplets at 3.94 and 4.16 ppm were present, in a 7/3
ratio, due to the H-3 of 3a- and 3b-diol, respectively.5
3.2. General procedure for CAL-B-catalyzed
irreversible transesterification
In conclusion, the good diastereoselectivity showed by
CAL-B provides a facile method for preparation of
both tibolone metabolites 2 and 3 in pure form, in the
amounts required (gram-scale) for biological evalua-
tion, avoiding careful and tedious chromatography and
crystallization exercises.
In a typical experiment the 7/3 mixture of 2 and 3 (1 g,
3.18 mmol) was dissolved in the required solvent (200
mL). Vinyl acetate (1.25 mL, 13.5 mmol) and CAL-B
(Novozym 435, 3.5 g) were added; the suspension was
kept at 30°C under stirring for the required time (see
Table 1). The reaction progress was monitored by
GLC. The enzyme was removed by filtration and the
residue obtained after evaporation at reduced pressure
was purified by silica gel column chromatography; elu-
tion with hexane/ethyl acetate (8/2) afforded pure 3a-
acetate 4 (0.85 g); endothermic peak fusion (DSC) at
3. Experimental
All solvents and reagents were purchased from Sigma-
Aldrich. All reactions were monitored by TLC on silica
gel 60 F254 plates (Merck) with detection by spraying
with 10% phosphomolybdic acid in ethanol solution
and heating at 110°C. Column chromatography was
performed on silica gel 60 (0.063–0.200 mm) (Merck).
Differential scanning calorimetry (DSC) were per-
formed on a Perkin Elmer DSC-7 instrument. GLC
analysis were performed on a Hewlett Packard HP5890
instrument at 260°C oven temperature, with an HP5-
WB capillary column (25 m×0.32 mm i.d., 0.52 mm film
thickness). Optical rotations were determined on a
Perkin Elmer model 241 polarimeter in a 1 dm cell at
25°C. All NMR spectra were recorded in CDCl3 solu-
tions with a Bruker AM-500 spectrometer. Chemicals
shifts are reported on the l (ppm) scale and are relative
to TMS as internal reference.
1
70°C (acetone/water); [h]D +118 (c 1, chloroform); H
NMR l 0.74 (d, 3H, 7a-CH3), 0.84 (s, 3H, CH3-18),
2.02 (s, 3H, CH3CO), 2.55 (s, 1H, CH-21), 4.83 (m, 1H,
CH-O). Pure 3b-diol 3 (0.24 g) was recovered with
hexane/ethyl acetate (7/3) as eluant; its chemico-physi-
cal data are in agreement with those reported in Ref. 5.
3.3. 17a-Ethynyl-7a-methyl-5(10)-estren-3a,17b-diol, 2
Acetate 4 (0.8 g, 2.2 mmol) in methanol (28 mL) was
treated with a solution of K2CO3 (1.1 g, 8.0 mmol) in
water (3 mL) at room temperature (3 h). After addition
of water (15 mL) the precipitate was recovered by
suction affording pure 3a-diol 2 (0.65 g, 95%); its
chemico-physical data are in agreement with those
reported in Ref. 5.
3.1. 17a-Ethynyl-7a-methyl-5(10)-estren-3j,17b-diols, 2
and 3
3.4. 17a-Ethynyl-7a-methyl-5(10)-estren-3j,17b-diol, 3-
To a solution of 1 (1 g, 3.2 mmol) in toluene (60 mL)
at 0°C, was added DIBAL (1 M in hexane, 6.9 mL).
After stirring at 0°C for 3 h, the reaction mixture was
treated with a saturated aqueous NH4Cl solution (20
mL) and filtered through a Celite pad; the organic
phase was separated and the aqueous phase extracted
with ethyl acetate (3×20 mL). The collected organic
phases were dried over Na2SO4 and evaporated at
reduced pressure. The residue (1 g) was purified on
column chromatography (silica gel 1/20) by elution
with hexane/ethyl acetate (7/3), a mixture of 2 and 3
(0.8 g, 80%) was obtained. GLC: 3a-diol TR 12.75 min,
acetates, 4 and 5
The 7/3 mixture of 2 and 3 (0.312 g, 1 mmol) was
treated with acetic anydride (0.5 mL, 5.3 mmol) in
pyridine (1 mL) at 0°C for 3 h. The reaction mixture
was poured into water (3 mL) and extracted with
dichloromethane (3×3 mL). The collected organic
phases were washed with water (2×4 mL), and dried
over sodium sulfate. Evaporation of the solvents at
reduced pressure afforded a mixture of acetates 4 and 5
(0.32 g, 90%). GLC: 3b-acetate TR 15.73 min, 3a-ace-
tate TR 16.95 min. In the 1H NMR spectrum two