The hydroxylation and amidation of equilenin acetate catalyzed by chloro[5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]manganese(III)

Article abstract of DOI:10.1039/b000463o

The aromatic steroid equilenin acetate undergoes regioselective and stereoselective hydroxylation and amidation catalyzed by a manganese porphyrin using iodosobenzene (PhIO) and N-tosyliminophenyliodinane (PhINTs) as the oxygen and nitrogen donor, respectively.

Full text of DOI:10.1039/b000463o

The hydroxylation and amidation of equilenin acetate catalyzed by
chloro[5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato]manganese(III
)
Jerry Yang, Richard Weinberg and Ronald Breslow*
Department of Chemistry, Columbia University, New York, NY 10027, USA. E-mail: rb33@columbia.edu
Received (in Corvallis, OR, USA) 11th January 2000, Accepted 14th February 2000
Published on the Web 13th March 2000
The aromatic steroid equilenin acetate undergoes regiose-
lective and stereoselective hydroxylation and amidation
catalyzed by a manganese porphyrin using iodosobenzene
(PhIO) and N-tosyliminophenyliodinane (PhINTs) as the
oxygen and nitrogen donor, respectively.
It is important to develop chemical methods for the re-
giospecific oxidation of natural products such as steroids to
replace microbial fermentations that are currently used.1
Biochemically, such oxidations are normally performed by the
heme-containing cytochrome P-450 class of enzymes, and
metalloporphyrins have been studied as models for these
enzymes.2 We have recently reported the regiospecific catalytic
hydroxylation of steroids, using a water soluble porphyrin
carrying hydrophobic binding units, that performs efficient and
specific oxidations directed by the well defined geometry
between the catalyst and substrates.3–5 Herein, we report that
the aromatic steroid equilenin acetate 1 can be hydroxylated and
amidated at very specific positions with good catalytic turnover
by iodosobenzene (PhINO) and N-tosyliminophenyliodinane
(PhI = NTs),6 catalyzed by chloro[5,10,15,20-tetrakis(penta-
fluorophenyl)porphyrinato]manganese (III) 2.
The metalloporphyrin-catalyzed oxidation of substituted
arenes with several oxidants has been studied by the groups of
Baciocchi7 and Meunier.8 Both groups found that quinones
were the dominant products produced from these reactions,
especially in the case of substituted naphthalenes. However,
when we treated a 54 mM solution of equilenin acetate 1 with
270 mM PhINO in the presence of 54 mM chloro[5,10,15,20-
tetrakis(pentafluorophenyl)porphyrinato] manganese(III) 2, no
quinones were produced after 12 h. Instead, the major product
isolated was the 6-hydroxylated product 3, along with a smaller
amount of the 11b-OH product 4 (Scheme 1). No starting
material was recovered. Analytical HPLC studies using a
catalyst:substrate:oxidant mole ratio of 1:50:250 (CH2Cl2,
23 °C, 12 h) resulted in a 57% conversion of 1 to 42% of 3 and
15% of 4, indicating ca. 25 catalytic turnovers (Scheme 1).†
The products were characterized by MS and NMR spectra.
Scheme 1
solution of 1 was stirred for 12 h under argon in 1 ml of distilled
CH2Cl2 containing metalloporphyrin 2 (60 mM), PhINNTs (300
mM) and molecular sieves to produce the 11b-amidation
product 5 and trace amounts of 3 and 4, with complete
conversion of starting material (Scheme 2).† In contrast to
hydroxylation, the amidation reaction went completely to the
11b position of the steroid without any detectable amidation at
the 6 position. A mole ratio of 1+50+250 catalyst:substrate+
PhINNTs afforded an 82% conversion of 1 to 30% 3, 5% 4 and
47% 5, as determined by HPLC assay, so there are ca. 40
turnovers.
Compound 5 had the expected MS, and again a downfield
shifted C-11 proton at 5.43 ppm was coupled to the C-12
methylene and showed NOE coupling to the C-1 proton. Thus
the tosylamide group is attached at the 11-b position. The
1
Compound 3 had a H NMR spectrum with one less aromatic
proton than the starting material. COSY spectroscopy con-
firmed coupling between the 1, 2, and 4 protons as well as the
allylic coupling between the 7 and 14 protons, indicating that
hydroxylation occurred at C-6.
Compound 4 lacked the original C-11 benzylic methylene
group signal at 3.3–3.4 ppm, and had a new peak at 5.77 ppm as
expected for a downshifted C-11. The C-11 proton showed
coupling to a methylene group with no other neighbors (C-12),
and also showed NOE coupling to the C-1 proton. This coupling
indicates that the H on C-11 is equatorial, so the OH is on the b
face. This 11-b OH assignment is also consistent with an upfield
shift of the angular methyl group from 0.80 ppm in 1 to 0.72
ppm in 4. The remaining C-14, C-15, C-16 coupled protons and
the aromatic protons were still present.
We first reported the metalloporphyrin catalyzed amidation
of organic compounds in 1982.9 Most recently, Che and
coworkers reported the asymmetric amidation of substituted
naphthalenes using chiral ruthenium and manganese porphyr-
ins.10 We find that such an amidation can also be performed on
the equilenin steroid substrate 1 with good selectivity. A 60 mM
Scheme 2
DOI: 10.1039/b000463o
Chem. Commun., 2000, 531–532
This journal is © The Royal Society of Chemistry 2000
531

material. COSY spectroscopy confirmed coupling between the 1, 2 and 4
protons as well as the allylic coupling between the 7 and 14 protons. All
remaining aliphatic protons were identified by COSY, also confirming the
identity of the product as C6-hydroxylated equilenin acetate.
angular methyl group shifts strongly upfield from 0.80 ppm in
1 to 0.55 ppm in 5.
The hydroxylation products 3 and 4 reflect hydrolysis of the
MnNNTs intermediate by traces of water in the small scale
analytical runs, and they are minimally present in larger
preparative scales. In previous work, we had found that the
enzyme cytochrome P-450 could aminate cyclohexane with
PhINNTs, but that some hydroxylation also occurred in the
water solution.11 Since the relative amount of hydroxylation
depended on the particular isoform of the enzyme used,
hydrolysis of the metalloporphyrin intermediate was the most
likely explanation. The finding that benzylic substitution in
compounds 4 and 5 occurs on the beta face of the steroid must
reflect the stereoelectronic control of the flat conjugated
benzylic radical intermediate in these reactions.
The readily available manganese porphyrin 2 is able to
catalyze the hydroxylation and amidation of an aromatic
steroid. These reactions are especially interesting for their
apparent differences in regiospecific oxidations. Although not
explicitly stated in the literature, metalloporphyrin catalyzed
oxidations of substituted naphthalene compounds have been
reported to afford mostly quinones after aromatic ring hydrox-
ylation in the presence of oxygen donors, whereas they tend to
produce only amides at previously saturated carbon positions in
the presence of PhINTs as the nitrogen donor.7–10
Apparently the oxidations involve preferential oxygen atom
donation to the aromatic ring before hydrogen loss, while the
tosylamidations involve initial hydrogen removal from a
benzylic position. Perhaps aromatic ring addition is more
sterically demanding, and thus more available to a small oxygen
atom than to a large tosylamide group. We see that this
difference generally holds true in the case of equilenin acetate
and provides a method by which steroids of this class can be
functionalized at different positions. Furthermore, the amida-
tion of a steroid substrate can lead to the development of a novel
class of nitrogen containing steroids that may have useful
biological properties.
J. Y. acknowledges support from an NCERQA EPA
Graduate Fellowship and a Bristol-Myers Squibb Graduate
Fellowship. R. W. acknowledges support from a Goldwater
Scholarship, a Pfizer Undergraduate Fellowship, a Perkin-
Elmer Undergraduate Fellowship, and the Columbia University
Rabi Scholars Program. This work was supported by the NIH
and NSF.
4: 1H NMR (CDCl3, 500 MHz): d 8.38 (1H, d, C1-H), 7.83 (1H, d, C6-
H), 7.60 (1H, d, C4-H), 7.37–7.28 (2H, m, C7-H and C2-H), 5.77 (1H, br m,
C11a-H), 3.43 (1H, m, C14-H), 2.39 (3H, s, acetate Me), 0.72 (3H, s, C18-
Me), 2.83–1.91 (6H, steroid envelope). CI-MS: m/z 342 (M + 1 + NH3).
Product 4 COSY indicated coupling of both C15 hydrogens to the easily
identifiable C14 proton at 3.43 ppm. The C15 protons were coupled to the
C16 protons, indicating that oxidation must have occurred on the C11 or
C12 steroid position. The C11 protons, originally at 3.3-3.4 ppm in 2, were
not present in product 4 and a new CH–OH appeared at 5.7 ppm, consistent
with a benzylic oxidation. The relatively large upfield shift of the angular
C18 methyl is inconsistent with C12 oxidation and is furthermore consistent
only with oxidation occurring on the beta face of the last remaining steroid
carbon position at C11 (most C11 a hydroxlations occur with large
downfield shifts of the C18 methyl). Lastly, strong NOE coupling between
the C1-H and the C11a-H indicated that compound 4 was indeed the 11b
hydroxylated product.
In the analytical runs, the formation of products relative to starting
material were monitored with 2-methoxynaphthalene as an internal
standard, comparing NMR ratios and HPLC responses to calibrate the
relative absorption coefficients of the products and starting material in the
UV-VIS HPLC detector.
5: 1H NMR (CDCl3, 500 MHz): d 7.81 (1H, d, C1-H), 7.77 (1H, dd, C6-
H), 7.72 (2H, d, toluenesulfonamide), 7.52 (1H, d, C4-H), 7.31 (1H, dd, C7-
H), 7.29(2H, d, toluenesulfonamide), 7.09 (1H, dd, C2-H), 5.43 (1H, ddd,
C11a-H), 4.45 (1H, d, N–H), 3.36 (1H, m, C14-H), 2.45 (3H, s, toluene
Me), 2.36 (3H, s, acetate Me), 0.55 (3H, s, C18-Me), 2.66–1.96 (6H, steroid
envelope). CI-MS: m/z 495 (M + 1 + NH3). The same COSY pattern was
present as with product 4. The C14 hydrogen was coupled to both C15
hydrogens, which in turn were coupled to both C16 hydrogens indicating
oxidation at C11 or C12. Again, there was a large upfield shift of the angular
C18 methyl, and the new CH-NH at 5.43 ppm is consistent with a benzylic
oxidation. Also, strong NOE coupling between the C1-H and the C11a-H
confirmed 5 as the 11b amidated product.
1 H. L. Holland, Steroids, 1999, 64, 178.
2 M. J. Gunter and P. Turner, Coord. Chem. Rev., 1991, 108, 115; B.
Meunier, Chem. Rev., 1992, 92, 1411.
3 R. Breslow, X. Zhang and Y. Huang, J. Am. Chem. Soc., 1997, 119,
4535.
4 R. Breslow, Y. Huang, X. Zhang and J. Yang, Proc. Natl. Acad. Sci.
USA, 1997, 94, 11156.
5 R. Breslow, B. Gabriele and J. Yang, Tetrahedron Lett., 1998, 39,
2887.
6 Y. Yamada, T. Yamamoto and M. Okawara, Chem. Lett., 1975, 361.
7 E. Baciocchi, O. Lanzalunga and A. Lapi, Tetrahedron Lett., 1995, 36,
3547.
Notes and references
8 R. Song, A. Sorokin, J. Bernadou and B. Meunier, J. Org. Chem., 1997,
62, 673.
9 R. Breslow and S. H. Gellman, J. Chem. Soc., Chem. Commun., 1982,
1400.
10 X. G. Zhou, X. Q. Yu, J. S. Huang and C. M. Che, Chem. Commun.,
1999, 2377.
11 E. W. Svastits, J. H. Dawson, R. Breslow and S. Gellman, J. Am. Chem.
Soc., 1985, 107, 6427.
† Equilenin acetate was synthesized by acylation of equilenin (Steraloids,
Inc.) with acetic anhydride in pyridine using standard procedures. All
products were isolated by column chromatography and characterized by 1H-
NMR, COSY, NOESY and CI-MS.
3: 1H NMR (CDCl3, 500 MHz): d 7.86 (1H,d, C4-H), 7.82 (1H, d, C1-H),
7.41 (1H, dd, C2-H), 6.29 (1H, d, C7-H), 3.38 (1H, ddd, C14-H), 2.34 (3H,
s, acetate Me), 0.76 (3H, s, C18-Me), 2.8–1.9 (8H, steroid envelope). CI-
MS: m/z = 342 (M + 1 + NH3), 323 (negative, M 2 1). Product 3 in its 1H
NMR spectrum showed one less aromatic proton as compared to the starting
Communication b000463o
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Chem. Commun., 2000, 531–532