Synthesis of an ecdysteroid inhibitor of ecdysone biosynthesis

Article abstract of DOI:10.1016/S0040-4039(00)93458-0

The hydroboration-oxidation of a Δ^5,7 pregnadiene leads to a Δ⁷6α-hydroxyderivative. This is converted to an acetylenic analogue of ecdysteroids, which is an inhibitor of their biosynthesis in vitro.

Full text of DOI:10.1016/S0040-4039(00)93458-0

Tetrahedron Letters, Vol.32, No.38, pp 5171-5174, 1991
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Pergamon Press plc
Synthesis of an Ecdysteroid Inhibitor
of Ecdysone Biosynthesis
A n t o n y M a u v a i s 1, Charles H e t r u2, and Bang Luu 1 .
1Laboratoire de Chimie Organique des Substances Naturelles, associ6 au CNRS,
Universit~ Louis Pasteur, 5 rue Blaise Pascal, 67084 Strasbourg, France,
2Laboratoire de Biologie G6n6rale, associ6 au CNRS, Universit6 Louis Pasteur,
12 rue de l'Universit~, 67000 Strasbourg, France.
Key words :Ecdysone; Ecdysteroids; Ecdysteroidinhibitor; Hydroboration; Acetylenic derivative.
Abstract : The hydroboration-oxidation of a A5.7 pregnadiene leads to a A7 6a-hydroxyderivative.
This is converted to an acetylenic analogue of ecdysteroids, which is an inhibitor of their biosynthesis
in vitro.
Several acetylenic cholesteryl derivatives have been reported by our group to irreversibly inhibit ecdysone
biosynthesis 1, 2 It was reasonable to assume that the cholesteryl nucleus was transformed into an ecdysteroid
precursor by the biosynthetic enzymes before reaching its target, the C-22 hydroxylase, which was inactivated by
the acetylenic function (Scheme 1).
1
....
2
....
Ecdysone
OH
/ \
FI ~
C-22 hydroxylase
Scheme 1
In an attempt to check this hypothesis, we needed to establish a short and efficient synthesis, enabling us
to obtain sufficient amounts of products for biological tests. The usual method for synthesizing these molecules
consists in either a strong oxidation of a A5,7 diene3, 4, or a bromination in the ct position to a C-6 ketone5, 6;
5171

5172
however, both methods gave moderate yields of the desired product. Therefore, we have investigated the
hydroboration of a A5,7 diene as an alternate reaction7, starting from pregnenolone. For this purpose, we have
introduced this diene by an allylic bromination followed by the elimination of the corresponding sulfoxide
(Scheme 2).
, ~. o
, ~ orr
w'~OH
3
4
5
,
Pregnenolone
, ~ OH
- ~ OH
, - ~ OH
TBDMSO~" ~ "
- < / ~ TBDMSO~" "-~ -../ f ~ "
TBDMSO~ "-1"
~ O H
_ ~ 1 ~ O
_ ~ , ~ O
TBDMS
O
OH
OH
~
TMS
OH
~
TMS
14
OH
H
12
13
TBDMSO~
~
~ 11
HO"
~
~
- - 0
HO ~
~
- 11
0
H
I~I
H ~
1u~OII
-a- TBDMSC1 (1.2 eq.), iPr2EtN, CH2C12/DMF, rt, 25h, quant.; -b- NaBH4 (5.0 eq.), MeOH, 0°C, 15 mn, 85%; ic-
Dibromodimethylhydantoin (1.0 eq.), hexane, reflux, 20 ran; -d- LiBr (2.0 eq.), toluene/acetone, 0°(7, 2h; -e- Thiophenol (1.3 eq.),
Et3N, rt, 30 ran; -f- mCPBA (1.1 eq.), AcOEt, 0°C, 45 ran; -g- Et3N, toluene, 700C, 30h, 56% starting from 4; -h- BH3:THF (5.0
eq.), THF, 0°C, 15 ran, NaOH/H202, 86%; -i- PCC (4.0 eq.), CH2C12, it, 20 mn, 81%; -j- SeO2 (10 eq.), dioxane, 80°C, 5 mn,
82%; -k- Mg, EtBr, TMSacetylene (50 eq.), THF, 0°C, 30 ran, 83%; -I- HFx:Py, PyfFHF, rt, lh, quant.; -m- nBu4NF (1.5 eq.),
THF, rt, 15 ran, quant.
Scheme 2
We have achieved the synthesis of the intermediate 88, possessing a A5,7 diene, by reducing TBDMS
protected pregnenolone with NaBH4, to give the corresponding 20(R) alcohol 4 as the major diastereomer.
Further conversion to the A5,7 diene was realized according to a selective procedure described by Confalone et

5173
al.9. After the classical brornination at C-7 with 2,3-dibromo-5,5-dimethylhydantoin and equilibration with LiBr
(or nBu4NBr), the mixture of bromides, where the 7~ epimer 5 predominates, was treated with thiophenol to
form the 713-allylic sulfide as the major product. Oxidation with mCPBA then gave the corresponding sulfoxide
7. The overall conversion (56% yield) was conducted without chromatography from compound 4 up to the
required diene 8, which was free of the A4,6 isomer. Hydroboration of 8 with BH3:THF followed by oxidation
with alkaline hydrogen peroxide yielded the desired allylic alcohol 910. All attempts to oxidize the crude
organoborane in a one pot procedure with PCCTM 12 were unsuccessful, however, oxidation of alcohol 9 with
PCC gave the corresponding diketone 1013. Subsequent treatment with SeO214 furnished the 14~-hydroxy
derivative I1 (30% from pregnenolone), already described by Kametani et al.6 (10% yield with the same starting
material), in the course of a 2-deoxyecdysone synthesis. Interestingly, condensation of the acetylenic side chain
required 50 equivalents of the acetylenie Grignard to reach completion, allowing us to isolate compound 12 in
83% yield15. We failed to achieve the deprotection of both TBDMS and TMS in one step with nBu4NF, but
treatment with the HFx:Py complex gave the TBDMS deprotected molecule 13 as the sole product16. TMS was
then removed with nBu4NF, giving the target molecule 1417 quantitatively.
Thus in conclusion, we have carried out a versatile method for synthesizing a series of potential inhibitors;
we are presently involved in the synthesis of a 2-deoxyecdysteroid acetylenic derivative (513-H, possessing a A/B
cis junction).
ACKNOWLEDGEMENTS
Financial support of this work by a grant from Ciba Geigy is gratefully acknowledged.We express our
gratitude to Professor Guy Ourisson for critical reading of this manuscript.
REFERENCES AND NOTES
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Caglioti L., Cainelli G., and Maina G., Tetrahedron, 1963, 19, 1057.
8 : 1H NMR 200 MHz (CDC13), 8 : 0.09 (s, 6H, SiMe2), 0.71 (s, 3H, H-18), 0.90 (s, 9H, H-SiCMe3),
0.95 (s, 3H, H-19), 1.17 (d, 3H, J=6.1, H-21), 3.58 (m, 1H, Wl/2=23, H-3), 3.75 (qd, 1H, Jl=6.1,
J2=3.4, H-20), 5.39 (m, 1H, wl/2=24, H-7), 5.56 (m, 1H, Wl/2=14, H-6). MS 70 eV, m/e (%) : 430
(M+, 24), 357 (7), 298 (28), 297 (23), 283 (84), 257 (100), 253 (16), 339 (10).
9.
Confalone P. N., Kulesha I. D., and Uskokovic M. R., J. Org. Chem., 1981, 46, 1030.
9 : 1H NMR 200 MHz (CDC13), 8 : 0.07 (s, 6H, H-SiMe2), 0.64 (s, 3H, H-18), 0,89 (s, 9H, H-
10.

5174
SiCMe3), 0.92 (s, 3H, H-19), 1.16 (d, 3H, J=6.1, H-21), 3.72 (qd, 1H, Jl=6.2, J2=3.4, H-20), 4.0
(m, 1H, Wl/,2=24, H-3), 5.07 (m, 1H, Wl/2=10, H-6), 6.47 (s, 1H, H-7). MS 70 eV, m/e (%) : 448
(M+, 20), 430 (58), 415 (10), 391 (70), 389 (10), 385 (8), 375 (23), 373 (40), 3~7 (5), 355 (3), 329
(I0), 315 (I00).
1I.
Ramana Rao V. V., Devaprabhakara D., and Chandrasckaran S., J. Organomet. Chem., 1978, C9,
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12.
13.
Brown H. C., Rothbcrg I., and Van der Jagt D. L., J. Org. Chem., 1972, 37, 4098.
10 : IH N M R 200 M H z (CDCI3), ~ : 0.07 (s, 6H, H-SiMe2), 0.57 (s, 3H, H-19), 0.89 (s, 9H, H-
SiCMe3), 0.97 (s, 3H, H-19), 2.16 (s, 3H, H-21), 2.23 (dd, IH, Ji=12, J2=3.5, H-5), 2.69 (t, IH,
J=9.0, H-9), 3.57 (m, wi/2=25, H-3), 5.75 (t, IH, J=2.2, H-7). M S 70 eV, m/e (%) : 44 (M+, I), 429
(2), 387 (100), 385 (7).
14.
15.
Fieser L. F., and Ourisson G., J. Am. Chem. Soc., 1953, 75, 4404.
In a dry three-necked flask fitted with a reflux condenser, Mg (50 eq., 264 mg, 10.9 retool.) was covered
with dry THF (3.0 ml) and EtBr(50 eq., 0.8 ml, 10.9 mmol.) was introduced dropwise under At, in
order to maintain a moderate reflux. When Mg was consumed, the reaction mixture was cooled to 0°C and
TMSacetylene (60 eq., 1.7 ml, 12.0 mmol) was cautiously added. After 30 ran, compound 11 (100 mg,
0.2 mmol.) in THF (5.0 ml) was transferred on the Grignard via a canula, and the mixture was allowed to
stir for another 30 mn. Reaction mixture was then treated with 10% aq. NH4C1 and extracted with Et20.
Combined extracts were dried (Na2SO4) and evaporated to dryness. Medium pressure chromatography
over silica gel, with elution with hexane/AcOEt 90/10, yielded 12 (101 rag, 83%).
16.
17.
This reaction was conducted in a polyethylene flask. Compound 12 (25 mg, 45 I.tmol.) in 1 ml of a
HFx:Py solution (prepared from 6.5 ml of the HFx:Py complex, 15.7 ml of dry pyridine and 50 ml of dry
THF) was stirred for lh at rt. The reaction mixture was then poured into 50 ml of a saturated aq.
NaHCO3 solution. Extraction with Et20, followed by drying over MgSO4 and evaporation to dryness
gave compound 13 (20 mg, quant.).
14 : IH NMR 200 MHz (MeOD), 8 : 0.85 (s, 3H, H-18), 0.97 (s, 3H, H-19), 1.48 (s, 3H, H-21), 2.74
(ddd, 1H, Ji=7.5, J2=3.0, J3=2.6, H-9), 2.83 (s, 1H, H-23), 3.54 (m, 1H wi/2=25, H-3), 5.82 (d,
1H, 1=2.7, H-7). MS 70eV, m/e (%): 372 (M+, 100), 354 (40), 339 (47), 321 (48), 311 (33),
303 (14).
(Received in UK 8 July 1991)