A simple and efficient synthesis of [3α-3H]5α- androst-16-en-3β-ol

Article abstract of DOI:10.1002/jlcr.1117

An efficient one step synthesis of [3α-³H]5α- androst-16-en -3β-ol by NaBT₄ reduction of a ketone precursor is described. The specific activity of the product was 21.6 Ci/mmol with a radiochemical purity >99%. Synthesis of the precursor, 5α-androst-16- en-3-one, from commercially available 5α-androst-16-en-3α-ol is also presented. Copyright

Full text of DOI:10.1002/jlcr.1117

JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS
J Label Compd. Radiopharm 2006; 49: 959–963
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/jlcr.1117
Research Article
A simple and efficient synthesis of [3a-3H]5a-androst-
16-en-3b-ol
S. Ren*, D. Hesk, P. McNamara and D. Koharski
Schering-Plough Research Institute, 2015 Galloping Hill Road, Kenilworth, NJ 07033,
USA
Summary
An efficient one step synthesis of [3a-3H]5a-androst-16-en-3b-ol by NaBT4 reduction
of a ketone precursor is described. The specific activity of the product was 21.6 Ci/
mmol with a radiochemical purity >99%. Synthesis of the precursor, 5a-androst-
16-en-3-one, from commercially available 5a-androst-16-en-3a-ol is also presented.
Copyright # 2006 John Wiley & Sons, Ltd.
Received 26 June 2006; Accepted 29 June 2006
Key Words: tritium labeled steroids; NaBT4
Introduction
We required [3H]5a-androst-16-en-3b-ol with a specific activity >10 Ci/mmol.
Romer and Wagner
¨
1
reported a synthesis of [5a, 6a-3H2]5a-androst-
16-en-3b-ol, 3, at 50 Ci/mmol, as an intermediate in the synthesis of
steroid 4 (Scheme 1). This synthesis had two disadvantages for our purposes:
(1) forcing conditions were needed to reduce the 5, 6 double bond
and (2) products 2 and 3 were difficult to separate from unsaturated
compounds in the reaction mixtures. Our attempt to prepare compound 2
by reduction of compound 1 was not satisfactory, so we abandoned
this route.
We subsequently developed an efficient, one step synthesis of [3a-3H]5a-
androst-16-en-3b-ol that gave an acceptable specific activity.
*Correspondence to: S. Ren, Schering Plough Research Institute, 2015 Galloping Hill Road, K-15-4545,
Kenilworth, NJ 07033, USA. E-mail: sumei.ren@spcorp.com
Copyright # 2006 John Wiley & Sons, Ltd.

960
S. REN ET AL.
I
I
T2/PtO2/HClO4/EtOAc
AcO
AcO
T
T
1
2
Jones reagent
EtOH/Na
O
HO
T
T
T
T
3
4
Scheme 1. Synthesis of [5a, 6a-3H2]5a-androst-16-en-3-one, 4
Results and discussion
Our initial attempt to synthesize [5a, 6a-3H2]5a-androst-16-en-3b-ol, 3, is
shown in Scheme 2.
Compound 1 was prepared in moderate yield from commercially available
dehydroisoandrosterone-3-acetate, 5, based on literature procedures.2,3
Compound 1 (2.5 mg, 5.7 mmmol) was reduced with 800 mCi (13.9 mmmol)
of tritium gas, using Romer and Wagner’s conditions,1 to give 4.8 mCi of
¨
crude product 2 at only 33% radiochemical purity. This route was not pursued
further.
Tritium labeled alcohols are often prepared by reduction of an appropriate
ketone with a complex tritide. NaBT4 is commercially available at high
specific activity, relatively stable and, therefore, the tritide of choice for
this transformation. Ketone 8, 5a-androst-16-en-3-one, was prepared by
Ley-Griffith oxidation4 of the inexpensive, commercially available alcohol
5a-androst-16-en-3a-ol, 7.
Reduction of 3-keto steroids with hydrides, such as NaBH4 and
LiAlH4, generally favor equatorial over axial alcohols in approxi-
mately a 10:1 ratio.5 Reduction of ketone 8 with NaBT4 in IPA:EtOH
(1 ꢀ 2) gave the expected equatorial alcohol as the major product.
The equatorial: axial (3b:3a) ratio was 85:15, comparable with the
result from a trial reaction with NaBH4. After purification by chromato-
graphy on silica gel, [3a-3H]5a-androst-16-en-3b-ol, 9, was obtained in
86% yield. The specific activity was 21.6 Ci/mmol. The synthesis is shown in
Scheme 3.
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 959–963
DOI: 10.1002/jlcr

SYNTHESIS OF [3a-3H]5a-ANDROST-16-EN-3b-ol
961
N
NH2
O
NH2NH2.XH2O, Et3N, EtOH, Reflux, 2h
AcO
96.1%
6
AcO
5
I
I
I2/THF,
1,1,3,3-Tetramethylguanidine
T2/PtO2/HClO4/EtOAc
"Trace"
34.5%
AcO
AcO
T
1
T
2
EtOH/Na
HO
T
T
3
Scheme 2. Synthesis of [5a, 6a-3H2]5a-Androst-16-en-3b-ol 3
H
H
TPAP/NMO/Molecular Sieves /CH2Cl2
79.8%
O
HO
H
H
8
7
H
NaBT4/ IPA/EtOH
86%
HO
T
H
9
Scheme 3. Synthesis of [3a-3H]5a-androst-16-en-3b-ol
Experimental
General
NaBT4 (80 Ci/mmol) was purchased from American Radiolabeled Chemicals,
Inc. All remaining reagents, authentic standards and solvents were purchased
from Aldrich and used as received. Radioactivity was measured on a Packard
2200CA liquid scintillation analyzer using Scintiverse BD liquid scintillation
cocktail. TLC plates were scanned on a Bioscan 1000 linear analyzer.
LC-MS: Waters Micromass with Waters 2695 Separation Module operating
in the ES+ ionization mode. Supelcosil LC-CN, 150 mm ꢁ 4.6 mm, 215 nm,
isocratic, 0.1% HCO2H in H2O: 0.1% HCO2H in CH3CN (65:35), 1.0 ml/min.
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 959–963
DOI: 10.1002/jlcr

962
S. REN ET AL.
RadioHPLC: Waters 600 Multisolvent Delivery System with Waters 2487
Absorbance Detector and Radiomatic 525TR Radioflow Detector. YMC-
Pack PVA-Sil, 150 mm ꢁ 4.6 mm, 215 nm. Hexanes:EtOH (98:2) for 15 min
followed by a step gradient to EtOH, 0.5 ml/min., HPLC elute: Packard Flo-
Scint III liquid scintillation cocktail (1:3).
1H-NMR (400 MHz) spectra was obtained on a Varian spectrometer.
Synthesis of 5a-androst-16-en-3-one (8)
To a solution of 5a-androst-16-en-3a-ol (7, 95 mg, 0.346 mmol) in anhydrous
CH2Cl2 (4.0 ml) at 08C, 5A molecular sieves (5 seeds), 4-methyl-morpholine N-
oxide (NMO, 70 mg, 0.60 mmol), and tetrapropylammonium perruthenate
(TPAP, 10 mg, 0.028 mmol) were added. The reaction was stirred for 1 h at
room temperature and was complete by TLC (Hexane:EtOAc ¼ 80 : 20, Rf:
3a-ol 7: 0.53; 3b-ol: 0.40; ketone 8: 0.76). The mixture was filtered through a
celite pad and concentrated to dryness. The crude product was purified
by flash chromatography (10g Sep-Pak, eluted with 1–5% EtOAc/Hexanes)
1
to give 75 mg (79.8% yield) 5a-androst-16-en-3-one, 8. H NMR (CDCl3),
d (ppm): 5.84 (m, 1H, C17 olefinic proton), 5.70 (m, 1H, H16 olefinic proton),
2.25–0.80 (m, 26H).
Synthesis of [3a-3H]5a-androst-16-en-3b-ol (9)
NaBT4 (300 mCi, 3.75 mmol, 80 Ci/mmol) was added to a suspension of 5a-
androst-16-en-3-one, 8, (4.0 mg, 14.7 mmol) in IPA (1 ml) and EtOH (2 ml) in a
reaction vial. The reaction was stirred at room temperature for 2 h, and then
solvent was removed under N2. The residue was dissolved in CH2Cl2 (5 ml),
washed with saturated NH4Cl (5 ml), dried over Na2SO4, filtered, and
concentrated. The crude product was purified by flash chromatography on
silica gel (elution with 1–10% EtOAc/Hexanes) to give 120 mCi of [3a-3H]
5a-androst-16-en-3-beta-ol, 9. Radiochemical purity (radioHPLC) was 99.5%.
The [3a-3H]5a-androst-16-en-3b-ol 9 co-eluted with an authentic standard
by radioHPLC ðtR ¼ 9:2 minÞ. The specific activity was 21.6 Ci/mmol. LC-MS:
tR ¼ 7:1 min, m/z: 259 [100, (M-H2O+H)+], unlabeled standard m/z:
257 [100, (M-H2O+H)+]. An additional 140 mCi of 9 (radiochemical
purity ¼ 97:5%)
containing
2.2%
[3b-3H]5a-androst-16-en-3-alpha-ol
(tR ¼ 7:5 min by radioHPLC) was also collected. In total, 260 mCi (86.7%)
of 9 was obtained.
Acknowledgements
The authors wish to thank Dr Tze-Ming Chan from SPRI Chemical Research
for the NMR analysis.
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 959–963
DOI: 10.1002/jlcr

SYNTHESIS OF [3a-3H]5a-ANDROST-16-EN-3b-ol
963
References
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´
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Company: Amsterdam, 1968; 135–143.
Copyright # 2006 John Wiley & Sons, Ltd.
J Label Compd Radiopharm 2006; 49: 959–963
DOI: 10.1002/jlcr