Oxidation of furost-20(22)-enes with 3-chloroperoxybenzoic acid and osmium tetroxide

Article abstract of DOI:10.1135/cccc20011746

6β-Methoxy-3α,5-cyclo-5α-furost-20(22)-ene and its 17α-hydroxy derivative were oxidized with 3-chloroperoxybenzoic acid and OsO₄. The former compound afforded hydroxylated products, whereas the latter underwent degradation to C₂₁ or C₁₉ steroids.

Full text of DOI:10.1135/cccc20011746

1746
Morzycki, Jastrzębska, Katryński:
OXIDATION OF FUROST-20(22)-ENES WITH
3-CHLOROPEROXYBENZOIC ACID AND OSMIUM TETROXIDE
1,
2
3
Jacek W. MORZYCKI *, Izabella JASTRZĘBSKA and Krzysztof S. KATRYŃSKI
Institute of Chemistry, University of Bialystok, al. Pilsudskiego 11/4, 15-443 Bialystok, Poland;
1
2
3
e-mail: morzycki@uwb.edu.pl, gierdas@uwb.edu.pl, katryn@wp.pl
Received July 11, 2001
Accepted Novem ber 18, 2001
Dedicated to the memory of Professor Václav Černý.
6β-Meth oxy-3α,5-cyclo-5α-furost-20(22)-en e an d its 17α-h ydroxy derivative were oxidized
with 3-ch loroperoxyben zoic acid an d OsO4. Th e form er com poun d afforded h ydroxylated
products, wh ereas th e latter un derwen t degradation to C21 or C19 steroids.
Keyw ord s: Pseudosapogen in s; Furostan es; Oxidation s; Steroids; Steroidal sapon in s.
Steroidal sapon in s com prise a diverse class of plan t glycosides wh ich pos-
sess a broad ran ge of in terestin g biological activities1,2. Th e m ost com m on
are sapon in s with th e steroid aglycon e (sapogen in ), such as spirostan e or
furostan e derivatives3. A n um ber of polyh ydroxy ch olestan e sapon in s with
cytostatic activity was recen tly reported4–7
.
Wh en treated with carboxylic acid an h ydrides, steroid sapogen in s
undergo cleavage of ring F (refs8,9). The products are furost-20-enes called pseudo-
sapogen in s. Th ese com poun ds are in term ediates in th e tran sform ation of
spirostanes to medicinally useful pregnane derivatives. Oxidation of pseudo-
sapogen in s with various reagen ts, such as CrO3/AcOH, KMn O4/NaIO4 or
H2O2/AcOH, h as been in ten sively studied10,11. Th ese reaction s led to cleav-
age of rin g E. In terestin gly, a sim ilar process also takes place in n ature an d
th e correspon din g sapon in s were recen tly foun d in som e Ch in ese m edici-
n al plan ts12. It is postulated th at 20,22-epoxide is an in term ediate in th is
en zym atic oxidation . In th e ch em ical oxidation of pseudosapogen in s vari-
ous com poun ds, such as diols, epoxides or allylic alcoh ols, were proposed
as in term ediates3. However, th e fin al reaction products were 16β-h ydroxy-
pregn an -20-on e esters or pregn -16-en -20-on e derivatives. In th e oxidation
of pseudosapogen in s con tain in g a free 26-h ydroxy group with 3-ch loro-
peroxyben zoic acid (MCPBA), th e products were 20-h ydroxyspirostan es13.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)
doi:10.1135/cccc20011746

Oxidation of Furost-20(22)-enes
1747
RESULTS AND DISSCUSSION
Th e aim of th is study was to explore th e possibility of application of
furost-20(22)-en es oxidation un der m ild con dition s to th e syn th esis of
polyh ydroxy ch olestan e derivatives. Th e startin g m aterials for th is study
were 6β-m eth oxy-3α,5-cyclo-5α-furost-20(22)-en e (1) an d its 17α-h ydroxy
derivative, i.e. 6β-m eth oxy-3α,5-cyclo-5α-furost-20(22)-en -17α-ol (2). Th e
latter com poun d is available by epoxide rin g cleavage in th e 16α,17α-epoxy-
20-oxo precursor as recen tly described14. Com poun d 1 was prepared from
diosgen in e (Sch em e 1). Buffered m eth an olysis of pseudodiosgen in e
ditosylate followed by lith ium alum in um h ydride reduction afforded com -
poun d 1.
CH2OTs
O
O
O
MeOH, AcOK
HO
TsO
diosgenine
CH2OTs
O
O
LiAlH4
OCH3
1
OCH3
SCHEME 1
Both furost-20(22)-en es were subjected to oxidation with MCPBA in di-
ch lorom eth an e or osm ium tetroxide in pyridin e. Th e results of th is study
are sh own in Sch em es 2 an d 3.
Th e reaction of com poun d 1 with MCPBA yielded th e allylic alcoh ol 3,
23-keton e 4, an d several m in or products. TLC m on itorin g of th e reaction
m ixture proved fast form ation of th e allylic alcoh ol 3 (a sin gle stereoisom er
was obtain ed but con figuration at th e C22–C23 double bon d could n ot be
elucidated from its spectra). Th e presum ed in itial reaction product,
20,22-epoxide, readily un dergoes rearran gem en t to th e allylic alcoh ol 3 via
th e stabilized carbocation . Furth er oxidation of 3 with MCPBA gives th e
α,β-un saturated keton e 4. Slow disappearan ce of both products 3 an d 4 on
prolon gin g th e reaction tim e was observed in favor to th e form ation of th e
α-h ydroxylacton e 5.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

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Morzycki, Jastrzębska, Katryński:
Th e reaction of com poun d 2 with th e sam e reagen t resulted in degrada-
tion to 6β-m eth oxy-3α,5-cyclo-5α-pregn -16-en -20-on e15,16 (6). A ten tative
m ech an ism of th e reaction is sh own in Sch em e 2. Th e drivin g force for
cleavage of th e C20–C22 sin gle bon d is presum ably th e form ation of a
h igh ly stabilized oxocarben ium ion . Th e low-m olecular-weigh t reaction
product was probably 4-m eth ylpen tan oic acid.
OH
O
O
MCPBA
H
O
O
OCH3
1
O
OH
O
OH
O
O
MCPBA
- H
MCPBA
O
- C5 fragment
3
4
5
O
O
O
HO
MCPBA
H
O
O
HO
- H2O
OCH3
2
O
O
H2O
- H
O
+
HOOC
6
SCHEME 2
Oxidation of 6β-m eth oxy-3α,5-cyclo-5α-furost-20(22)-en e (1) with osm ium
tetroxide (Sch em e 3) afforded th e correspon din g osm ate wh ich was foun d
to be extraordin arily stable. Th e osm ium derivative can be reduced with so-
dium h ydrogen sulfite/pyridin e but th is takes 2 days to com plete th e reac-
tion . Th e fin al product was com poun d 7 as expected. Th e com poun d exists
in its open -ch ain form probably due to an in tram olecular h ydrogen bon d.
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Oxidation of Furost-20(22)-enes
1749
O
O
Os
O
OH
O
O
O
OsO4
O
OH
OCH3
7
1
O
O
Os
O
O
O
O
HO
OsO4
O
OH
HO
+
C8 fragment
8
OCH3
SCHEME 3
2
Osm ylation of com poun d 2 led to an un usual fragm en tation in to th e C19
steroidal α-h ydroxy keton e 8 an d th e C8 product. Th e m ech an ism of th is
reaction is n ot clear but m ost likely cleavage of th e C17–C20 sin gle bon d is
assisted by form ation of 17-oxocarben ium ion . It seem s th at th e m ech a-
n ism of th e rearran gem en t is sim ilar to th at of a fragm en tation of com -
poun d
2 prom oted by MCPBA. Com poun d 8 proved resistan t to
autoxidation in con trast to th e 16α-h ydroxy-17-keton es17.
Of th e four reaction s studied on ly osm ylation of 6β-m eth oxy-3α,5-cyclo-
5α-furost-20(22)-en e (1) proceeded as expected an d m ay be useful for th e
syn th esis of som e n atural products.
EXPERIMENTAL
Meltin g poin ts were determ in ed on a Kofler apparatus of th e Boetius type. NMR spectra were
recorded with a Bruker AC 200F spectrom eter usin g CDCl3 solution s with TMS as th e in ter-
n al stan dard (on ly selected sign als in th e 1H NMR spectra are reported). Ch em ical sh ifts are
given in ppm (δ-scale), couplin g con stan ts (J) in Hz. In frared spectra (waven um bers in cm –1
)
were recorded on a Nicolet series II Magn a-IR 550 FT-IR spectrom eter as ch loroform solu-
tion s. Mass spectra were obtain ed at 70 eV with an AMD-604 spectrom eter. Th e reaction
products were isolated by colum n ch rom atograph y perform ed on 70–230 m esh silica gel
(Baker). Com poun d 2 was prepared from th e com m ercially available 3β-h ydroxypregn -
5-en -20-on e via 3β-h ydroxych olesta-5,16-dien -22-on e18 accordin g to th e procedure previ-
ously described14
.
Syn th esis of 6β-Meth oxy-3α,5-cyclo-5α-furost-20(22)-en e (1)
Th e title com poun d was prepared from diosgen in e as sh own in Sch em e 1. Rin g E of
diosgen in e was open ed with alum in um ch loride in acetic an h ydride accordin g to th e de-
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

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Morzycki, Jastrzębska, Katryński:
scribed procedure19. Th e resultin g 3β,26-diacetate (500 m g, 1 m m ol) was h ydrolyzed with
potassium h ydroxide (280 m g, 5 m m ol) in m eth an ol (100 m l) an d th e diol was treated with
TsCl (1 020 m g, 15.3 m m ol) in pyridin e (30 m l). Buffered m eth an olysis20 (0.31 g of potas-
sium acetate in 70 m l of MeOH) of th e crude pseudodiosgen in e ditosylate followed by lith -
ium alum in um h ydride (380 m g, 10 m m ol) reduction in refluxin g THF (100 m l) an d silica
gel colum n ch rom atograph y afforded com poun d 1 (282 m g, 68%; eluted with 5% eth yl
acetate–h exan e) as an oil. 1H NMR: 4.73 m , 1 H (H-16α); 3.33 s, 3 H (OCH3); 2.78 m , 1 H
(H-6α); 2.48 d, 1 H, J = 5.7 (H-17α); 1.59 s, 3 H (3 × H-21); 1.04 s, 3 H (3 × H-19); 0.89 d, 6 H,
J = 6.5 (3 × H-26 an d 3 × H-27); 0.74 s, 3 H (3 × H-18); 0.65 m , 1 H (H-4β); 0.44 dd, 1 H, J1
=
7.9, J2 = 5.1 (H-4α). For C28H44O2 (412.7) calculated: 81.50% C, 10.75% H; foun d: 81.31% C,
10.66% H.
Oxidation with 3-Ch loroperoxyben zoic Acid
To a stirred solution of a steroid (0.22 m m ol) in dich lorom eth an e (5 m l) 3-ch loroperoxy-
ben zoic acid (43 m g of a com m ercial 50–60% MCPBA; 0.14 m m ol) was added. Th e reaction
m ixture was stirred at room tem perature for 1 h , quen ch ed by addition of aqueous solution
of sodium sulfide an d extracted with ch loroform . Th e extract was dried over an h ydrous
m agn esium sulfate an d th e solven t was evaporated in vacuo. Th e reaction products were sep-
arated by silica gel colum n ch rom atograph y.
6β-Methoxy-3α,5-cyclo-5α-furost-22-en-20-ol (3). Am orph ous solid eluted with h exan e–eth yl
acetate (10%), yield 30%. IR: 3 589, 3 437, 1 690, 1 096. 1H NMR: 4.91 m , 1 H (H-16α);
4.33 dd, 1 H, J1 = 8.1, J2 = 6.6 (23-H); 3.32 s, 3 H (OCH3); 2.78 m , 1 H (H-6α); 1.51 s, 3 H
(3 × H-21); 1.02 s, 3 H (3 × 19-H); 0.88 an d 0.86 2 × d, 6 H, J(25,26) = J(25,27) = 6.5 (3 × H-26
an d 3 × H-27); 0.85 s, 3 H (3 × 18-H); 0.65 t, 1 H, J = 4.9 (H-4β); 0.44 dd, 1 H, J1 = 8.0, J2
=
5.1 (H-4α). 13C NMR: 161.9 (C), 93.2 (CH), 83.9 (CH), 82.1 (CH), 77.6 (C), 66.3 (CH), 56.6
(CH3), 56.6 (CH), 47.8 (CH), 43.4 (C), 40.6 (C), 39.5 (CH2), 35.2 (C), 35.0 (CH2), 34.0 (CH2),
33.3 (CH2), 33.1 (CH2), 29.6 (CH), 28.8 (CH), 24.9 (CH2), 22.5 (CH3), 22.3 (CH3), 21.9
(CH2), 21.5 (CH), 21.2 (CH3), 19.3 (CH3), 13.5 (CH3), 13.1 (CH2). EI MS, m/z (rel.%): 428
(27) [M+], 385 (70) [M+ – C3H7], 335 (12), 253 (37), 121 (100). HR-MS (m/z) for C28H44O3,
foun d: 428.3294; calculated: 428.3291.
6β-Methoxy-3α,5-cyclo-5α-furost-20(22)-en-23-one (4). Eluted with h exan e–eth yl acetate
(4%) as an oil, yield 17%. IR: 1 683, 1 615, 1 091. 1H NMR: 4.83 m , 1 H (H-16α); 3.33 s, 3 H
(OCH3); 2.79 m , 1 H (H-6α); 2.62 dd, 1 H, J1 = 10.3, J2 = 1.1 (H-17α); 2.43 dd, 2 H, J1 = 7.0,
J2 = 5.4 (24-H); 2.02 d, 3 H, J = 1.1 (3 × H-21); 1.04 s, 3 H (3 × H-19); 0.93 an d 0.92 2 × d,
6 H, J(25,26) = J(25,27) = 6.6 (3 × H-26 an d 3 × H-27); 0.77 s, 3 H (3 × H-18); 0.67 t, 1 H, J =
3.9 (H-4α); 0.45 dd, J1 = 8.0, J2 = 5.2, 1 H (H-4β). 13C NMR: 196.6 (C), 148.5 (C), 123.1 (C),
84.4 (CH), 82.1 (CH), 65.6 (CH), 56.6 (CH3), 54.8 (CH), 49.3 (CH2), 47.9 (CH), 44.7 (C), 43.4
(C), 39.9 (CH2), 35.4 (CH2), 35.2 (C), 34.3 (CH2), 33.3 (CH2), 29.9 (CH), 24.9 (CH2), 24.3
(CH), 22.7 (CH3), 22.6 (CH2), 22.6 (CH3), 21.4 (CH), 19.3 (CH3), 14.3 (CH3 × 2), 13.2 (CH2).
EI MS, m/z (rel.%): 426 (32) [M+], 394 (18) [M+ – MeOH], 180 (100). HR-MS (m/z) for
C
28H42O3, foun d: 426.3138; calculated: 426.3134.
20α-Hydroxy-6β-methoxy-3α,5-cyclo-5α-pregnane-20,16β-carbolactone (5). Am orph ous solid
eluted with h exan e–eth yl acetate (25%), yield 22% (ach ieved wh en th e reaction tim e was ex-
ten ded to 2 days). IR: 3 575, 3 347, 1 766, 1 097. 1H NMR: 5.09 m , 1 H (H-16α); 3.34 s, 3 H
(OCH3); 2.79 m , 1 H (H-6α); 2.27 m , 1 H; 2.11 d, 1 H, J = 8.6 (H-17α); 1.59 s, 3 H (3 ×
H-21); 1.03 s, 3 H (3 × H-19); 0.87 s, 3 H (3 × H-18); 0.67 t, 1 H, J = 3.9 (H-4α); 0.47 dd, J1
=
Collect. Czech. Chem. Commun. (Vol. 66) (2001)

Oxidation of Furost-20(22)-enes
1751
8.0, J2 = 5.2, 1 H (H-4β). EI MS, m/z (rel.%): 374 (18) [M+], 359 (21) [M+ – CH3], 342 (47)
[M+ – MeOH], 319 (100). HR-MS (m/z) for C23H34O4, foun d: 374.2464; calculated: 374.2457.
6β-Methoxy-3α,5-cyclo-5α-pregn-16-en-20-one (6). Eluted with h exan e–eth yl acetate (11%) as
an oil, yield 32%. IR: 1 707, 1 662, 1 587, 1 090. 1H NMR: 6.69 m , 1 H (H-16); 3.35 s, 3 H
(OCH3); 2.80 m , 1 H (H-6β); 2.25 s, 3 H (3 × H-21); 1.05 s, 3 H (3 × H-19); 0.94 s, 3 H (3 ×
H-18); 0.67 t, 1 H, J = 4.9 (H-4α); 0.45 dd, 1 H, J1 = 8.0, J2 = 5.1 (4β-H). 13C NMR: 196.7 (C),
155.5 (C), 144.4 (CH), 82.1 (CH), 56.6 (CH), 56.5 (CH3), 48.4 (CH), 46.4 (C), 43.5 (C), 35.3
(CH2), 35.1 (CH2), 35.0 (C), 33.0 (CH2), 32.2 (CH2), 28.8 (CH), 27.1 (CH3), 24.8 (CH2), 22.3
(CH2), 21.2 (CH), 19.1 (CH3), 16.1 (CH3), 13.1 (CH2).
Oxidation of 1 with Osm ium Tetroxide
To a stirred solution of com poun d 1 (58 m g, 0.14 m m ol) in pyridin e (3 m l), a solution of
osm ium tetroxide (38 m g, 0,15 m m ol) in pyridin e (2 m l) was added. Th e reaction m ixture
was stirred at room tem perature for 18 h , addition al 6 m l of pyridin e, 8 m l of water, an d
aqueous solution of sodium h ydrogen sulfite (2 m l of 40% NaHSO3) were added in order to
reduce osm ium com plex. Th e reaction m ixture was stirred for 2 days, poured in to water an d
extracted with eth er. Th e extract was wash ed with water, solven t was rem oved in vacuo from
th e extract, an d crude product was purified by silica gel ch rom atograph y.
16β,20α-Dihydroxy-6β-methoxy-3α,5-cyclo-5α-cholestan-22-one (7). Eluted with h exan e–eth yl
acetate (25%), yield 49%, m .p. 179–182 °C (dich lorom eth an e–h exan e). IR: 3 689, 3 446, 1 702,
1 093. 1H NMR: 4.71 m , 1 H (H-16α); 4.43 s, 1 H (OH); 3.87 brs, 1 H (OH); 3.32 s, 3 H
(OCH3); 2.77 m , 1 H (H-6α); 2.65 t, J = 7.5, 2 H (2 × H-23); 2.30 m , 1 H; 1.56 s, 3 H (3 × H-21);
1.05 s, 3 H (3 × H-19); 1.02 s, 3 H (3 × H-18); 0.93 d, J = 6.1, 6 H (3 × H-26 an d 3 × H-27);
0.65 t, J = 4.4, 1 H (H-4β); 0.44 dd, J1 = 8.0, J2 = 5.1, 1 H (H-4α). 13C NMR: 214.5 (C), 82.1
(CH), 81.5 (C), 73.5 (CH), 59.6 (CH), 56.5 (CH3), 54.4 (CH), 48.0 (CH), 43.4 (C), 42.6 (C),
39.2 (CH2), 37.1 (CH2), 35.3 (C), 35.2 (CH2), 34.8 (CH2), 33.3 (CH2), 32.4 (CH2), 29.4 (CH),
27.7 (CH), 26.7 (CH3), 24.9 (CH2), 22.4 (CH3 × 2), 22.1 (CH2), 21.5 (CH), 19.3 (CH3), 15.2
(CH3), 13.0 (CH2). EI MS, m/z (rel.%): 428 (12) [M+ – H2O], 385 (30) [M+ – C3H7], 347 (100).
HR-MS (m/z) (ESI) for C28H46O4Na, foun d: 469.3288; calculated: 469.3294.
Oxidation of 2 with Osm ium Tetroxide
To a stirred solution of com poun d 2 (120 m g, 0.28 m m ol) in pyridin e (3 m l), a solution of
osm ium tetroxide (71 m g, 0.28 m m ol) in pyridin e (5 m l) was added. Th e reaction m ixture
was stirred at room tem perature for 3 days, 20 m l of pyridin e–water (1 : 1 v/v) was added,
th en osm ium com plex was reduced with 3 m l of 40% aqueous solution of sodium h ydrogen
sulfite for 1 h . Th e reaction m ixture was extracted with eth er, solven t was rem oved in vacuo,
an d product was purified by silica gel ch rom atograph y.
16β-Hydroxy-6β-methoxy-3α,5-cyclo-5α-androstan-17-one (8). Eluted with h exan e–eth yl acetate
(28%) as an oil, yield 20%. IR: 3 558, 3 463, 1 744, 1 090, 1 008. 1H NMR: 4.38 d, 1 H, J =
7.6 (H-16α); 3.37 s, 3 H (OCH3); 2.84 m , 1 H (H-6α); 1.05 s, 3 H (3 × H-19); 1.02 s, 3 H (3 ×
H-18); 0.68 t, 1 H, J = 3.9 (H-4β); 0.47 dd, 1 H, J1 = 8.0, J2 = 5.1 (H-4α). 13C NMR: 219.5 (C),
81.8 (CH), 71.4 (CH), 56.7 (CH3), 48.3 (CH), 47.8 (C), 43.5 (C), 34.9 (CH2), 33.9 (CH2), 33.3
(C), 31.6 (CH2), 30.5 (CH2), 30.2 (CH), 24.8 (CH2), 21.6 (CH2), 21.2 (CH), 19.2 (CH3), 14.3
(CH3), 13.2 (CH2). For C20H30O3 (318.5) calculated: 75.43% C, 9.50% H; foun d: 75.21% C,
9.41% H.
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Morzycki, Jastrzębska, Katryński:
Financial support from the Polish Ministry of Education is gratefully acknowledged. We also thank
Mrs J. Maj for help in preparation of the manuscript.
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