Synthesis of the spirochroman core of dihypoestoxide and stereochemical proposal for the natural product

Article abstract of DOI:10.1021/ol102296t

The tricyclic spirochroman core of dihypoestoxide has been synthesized from geranoic acid in seven steps using a hetero-Diels-Alder cycloaddition as a key step, thus providing support for the proposed biosynthesis of the natural product. Furthermore, analysis of the ¹³C NMR data obtained for all four diastereoisomers of the synthetic spirochroman core has allowed us to propose a full stereochemical assignment for dihypoestoxide.

Full text of DOI:10.1021/ol102296t

ORGANIC
LETTERS
2010
Vol. 12, No. 22
5294-5297
Synthesis of the Spirochroman Core of
Dihypoestoxide and Stereochemical
Proposal for the Natural Product
Maliha Uroos and Christopher J. Hayes*
School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham,
NG7 2RD, U.K.
chris.hayes@nottingham.ac.uk
Received September 24, 2010
ABSTRACT
The tricyclic spirochroman core of dihypoestoxide has been synthesized from geranoic acid in seven steps using a hetero-Diels-Alder
cycloaddition as a key step, thus providing support for the proposed biosynthesis of the natural product. Furthermore, analysis of the 13C
NMR data obtained for all four diastereoisomers of the synthetic spirochroman core has allowed us to propose a full stereochemical assignment
for dihypoestoxide.
Hypoestes rosea (Acanthaceae) is an evergreen tropical
shrub indigenous to Nigeria, and extracts of the plant have
long been used as folk medicines to treat a wide variety
of conditions such as skin rashes and fungal infections.1
This ethnobotanical information has stimulated research
efforts to identify the biologically active components of
the extract, and the closely related diterpene-derived
natural products dihypoestoxide (1)2 and hypoestoxide (2)3
were among the first compounds to be isolated. Although
hypoestoxide displays significant biological activity (anti-
inflammatory,4a anticancer,4b antimalaria4c), similar studies
on dihypoestoxide (1) are yet to be reported, but the wide
range of therapeutic uses claimed for the plant extracts,
coupled with the close structural association of 1 and 2,
justifies further investigation of the potential biological
activity of 1. The gross structure of dihypoestoxide (1)
was determined using a combination of NMR spectroscopy
and mass spectrometry, and it was proposed that 1 is
formed in vivo via a hetero-Diels-Alder dimerization of
2. The isolation chemists showed that 1 is not an artifact
of the isolation of 2, as a deliberate attempt to dimerize
hypoestoxide under the conditions of isolation failed to
provide any dihypoestoxide (1). Unfortunately, a stereo-
chemical assignment was not achieved at the time of
isolation, but if 1 is formed via the hetero-Diels-Alder
dimerization of hypoestoxide (2) in vivo, then it is sensible
(4) (a) Ojo-Amaize, E. A.; Kapahi, P.; Kakkanaiah, V. N.; Takahashi,
T.; Shalom-Barak, T.; Cottam, H. B.; Adesomoju, A. A.; Nchekwube, E. J.;
Oyemade, O. A.; Karin, M.; Okogun, J. I. Cell. Immunol. 2001, 209, 149.
(b) Ojo-Amaize, E. A.; Nchekwube, E. J.; Cottam, H. B.; Bai, R.; Verdier-
Pinard, P.; Kakkanaiah, V. N.; Varner, J. A.; Leoni, L.; Okogun, J. I.;
Adesomoju, A. A.; Oyemade, O. A.; Hamel, E. Cancer Res. 2002, 62, 4007.
(c) Ojo-Amaize, E. A.; Nchekwube, E. J.; Cottam, H. B.; Oyemade, O. A.;
Adjesomoju, A. A.; Okogun, J. I. Exp. Parasitol. 2007, 117, 218.
(1) Gill, L. S. Hypoestes rosea: Ethnomedicinal Uses of Plants in
Nigeria; Uniben Press: Benin City, Nigeria, 1992.
(2) Adesomoju, A. A.; Okogun, J. I. J. Nat. Prod. 1984, 47, 308.
(3) Adesomoju, A. A.; Okogun, J. I.; Cava, M. P.; Carroll, P. J.
Heterocycles 1983, 20, 2125.
10.1021/ol102296t  2010 American Chemical Society
Published on Web 10/29/2010

to propose that 1 shares the same stereochemistry as 2 at
all common stereocenters5 (Figure 1). If this hypothesis
Scheme 1. Preparation of R-Methylene Cyclohexanone (()-12
of the epoxides 9 and 8, which were then rearranged using
Yamamoto’s conditions (TMP, n-BuLi, Et2AlCl)7 to afford
the corresponding allylic alcohols 10 and 11 in excellent
yield. Oxidation of the 4:1 mixture of 10 and 11 under Swern
conditions (DMSO, (COCl)2, Et3N)8 finally gave the desired
racemic enone 12.
Figure 1. Proposed biosynthesis of dihypoestoxide (1) via hetero-
Diels-Alder dimerization of hypoestoxide (2).
is true, then the only remaining stereocenter to be assigned
is that present in the spirochroman ring system formed
during the hetero-Diels-Alder dimerization.
A similar synthetic sequence was used to access the closely
related acetate-protected enone 16 (Scheme 2). Thus, cy-
As part of our studies directed toward a total synthesis of
1, we decided to try and assign the stereochemistry at the
spirochroman stereocenter by completing a synthesis of all
four possible diastereoisomers of a simplified tricyclic
spirochroman core structure 3 and then compare the data
obtained with that of the natural product 1. To access 3, we
planned to use the biomimetic hetero-Diels-Alder dimer-
ization of a suitably functionalized R-methylene cyclohex-
anone precursor as a key step, thus simultaneously testing
the validity of the biosynthetic proposal. In this letter, we
describe the successful completion of our studies in this area
that have resulted in the synthesis of the spirochroman core
(3) of dihypoestoxide and have allowed us to propose a
complete stereochemical assignment of the natural product
1 (Figure 1).
Scheme 2. Preparation of R-Methylene Cyclohexanone (()-16
Our synthetic efforts began by preparing the R-methylene
cyclohexanones 12 and 16, which were to be used in the
key hetero-Diels-Alder cycloaddition reaction. The racemic
enone 12 was readily prepared from geranoic acid 4 by first
treating with H3PO4 to provide R-cyclogeranoic acid 5 in
reasonable yield6 (Scheme 1).
The carboxylic acid 5 was then reduced with lithium
aluminum hydride to give cyclogeraniol 6, and the resulting
primary alcohol was protected to give the TBS-ether 7.
Epoxidation with m-CPBA gave a 4:1 diastereomeric mixture
clogeraniol (6) was epoxidized with m-CPBA to afford the
epoxy alcohol 13 as a single diastereoisomer,9 and this
material was then rearranged to provide the diol 14 using
Yamamoto’s conditions. Selective acetate protection of the
primary alcohol was achieved using acetyl chloride at low
temperature,10 and the secondary alcohol was oxidized under
Swern conditions to provide the desired racemic enone 16.
(7) Yasuda, A.; Yamamoto, H.; Nozaki, H. Bull. Chem. Soc. Jpn. 1979,
52, 1705.
(8) Mancuso, A. J.; Swern, D. Synthesis 1981, 165.
(9) Serra, S.; Gatti, F. G.; Fuganti, C. Tetrahedron: Asymmetry 2009,
20, 1319.
(5) The stereochemistry of 2 has previously been determined by X-ray
crystallography. See ref 3 for details.
(6) Gan, Y.; Li, A.; Pan, X.; Chan, A. S. C.; Yang, T.-K. Tetrahedron:
Asymmetry 2000, 11, 781.
(10) Hassner, A.; Maurya, R.; Friedman, O.; Gottlieb, H. E.; Padwa,
A.; Austin, D. J. Org. Chem. 1993, 58, 4539.
Org. Lett., Vol. 12, No. 22, 2010
5295

In addition to preparing the racemic enones 12 and 16,
we also needed access to enantiomerically enriched material
to minimize the number of possible diastereomeric outcomes
of the hetero-Diels-Alder dimerization. Fortunately, we were
able to easily obtain the enantiomerically enriched (>95%
ee) enones (+)-12 and (+)-16 via enzymatic resolution of
racemic cyclogeraniol (6) using commercially available lipase
PS11 (Scheme 3). Enantiomerically enriched cyclogeraniol
Scheme 4
.
Hetero-Diels-Alder Dimerizations of Racemic
Enones
Scheme 3. Lipase PS Resolution of Cyclogeraniol
(-)-6 (36% yield, 95% ee) was readily separated from the
acetate 17 via silica gel column chromatography, and this
gave access to (+)-12 and (+)-16 by following the routes
previously used to prepare the equivalent racemic material
(Schemes 1 and 2).
in the dieneophile component. In comparison, the less bulky
acetate-protected enone (()-16 seems equally likely to
undergo cycloaddition from either face when acting as the
1-oxa-diene component in the cycloaddition as (()-3 and
(()-20 are produced in a 1:1 ratio, and (()-21 and (()-22
are also produced in a 1:1 ratio. However, good levels of
diastereoselectivity (8:1) were still observed with respect to
the dienophile component, although there is a slight erosion
in selectivity when compared to the more hindered TBS
example discussed previously.
The analogous dimerization reactions were next performed
using the enantiomerically enriched enones (+)-12 and (+)-
16. As seen in the racemic series, the spirochroman (+)-18
was the major product of the dimerization of (+)-12, with
the previously unseen minor diastereoisomer (-)-23 being
produced in very small quantities (dr ) 80:1) (Scheme 5).
Similarly, dimerization of (+)-16 produced the spirochroman
(+)-3 as the major product, with (-)-21 being produced as
a minor product (dr ) 70:1).
As none of the spirochroman products (3, 18-23) were
crystalline, stereochemical assignment proved to be an
interesting challenge and was only possible because we had
access to both the racemic and enantiomerically enriched
enones 12 and 16. Since we knew that all four possible
diastereoisomers (8:8:1:1 ratio) were produced during the
hetero-Diels-Alder dimerization of (()-16 and that only two
products ((+)-3 and (-)-21) could possibly be (and indeed
were) produced during the dimerization of the single enan-
tiomer (+)-16, we could immediately identify compounds 3
and 21 within the four products produced from (()-16. As
this type of hetero-Diels-Alder cycloaddition prefers an
endo-pathway,12 we could confidently assign the major
With both the racemic and enantiomerically enriched
enones 12 and 16 in hand, we were able to begin our studies
on the hetero-Diels-Alder dimerization process. Thus, the
racemic TBS-protected enone (()-12 was heated at 80 °C
in the absence of solvent, and the diastereomeric spirochro-
mans (()-18 and (()-19 (4:1 ratio) were isolated as the only
products in quantitative yield (Scheme 4). Similarly, dimer-
ization of (()-16 was also achieved under the same condi-
tions to afford all four possible diastereoisomers of the
corresponding spirochroman products 3, 20, 21, and 22 (8:
8:1:1 ratio) in quantitative yield.
A detailed discussion regarding the stereochemical as-
signments of all spirochroman products is provided later, but
the selective formation of (()-18 and (()-19, in preference
to the other two possible diastereoisomers, shows that the
TBS-protected hydroxymethyl group plays a key role in
controlling which face of the “dienophile” is available for
cycloaddition. The spirochromans (()-18 and (()-19 are
produced via cycloaddition on the least hindered upper face
(as drawn) of the enone (()-12. The 4:1 diastereoselectivity
observed during the formation of (()-18 and (()-19 reflects
the facial selectivity expressed by the enone (()-12 when
acting as the 1-oxa-diene component in the cycloaddition.
Once again, the TBS-protected hydroxymethyl group directs
cycloaddition to the least hindered face, although its directing
influence is weaker in the 1-oxa-diene component than it is
(11) Luparia, M.; Boschetti, P.; Piccinini, F.; Porta, A.; Zanoni, G.;
Vidari, G. Chem. BiodiVersity 2008, 5, 1045.
5296
Org. Lett., Vol. 12, No. 22, 2010

18, 19, and 20 possess the stereochemistry represented by
structure A (Figure 2) as they all have δCdO ) 208 ( 1 ppm,
Scheme 5. Hetero-Diels-Alder Dimerizations of
Enantiomerically Enriched Enones
product (+)-3 as having the stereochemistry shown in
Scheme 5. This means that (-)-21 is the minor exo-product
and must have the opposite stereochemistry at the spiro
stereocenter (Scheme 5). Therefore, the remaining two
compounds were obviously 20 and 22, but at this stage we
did not know which was which. Further confirmation of the
stereochemical assignment of 3 and 21 comes from analysis
of the 13C NMR data for the carbonyl present in the
spirochroman ring system, with compound (+)-3 having a
δCdO ) 208.3 ppm and (-)-21 having a δCdO ) 213.1 ppm.
These values are in accord with those observed in similar
spirochromans present in the dimer of 5-oxotaxinine (δCdO
) 206.4 ppm)13 and tagalsin I (δCdO ) 213.4 ppm),14
respectively, whose structures have been solved by X-ray
crystallography. Inspection of the published X-ray structures
reveals that the ethereal C-O bond of the dihydropyran
fragment of the dimer of 5-oxotaxinine is in an equatorial
position on the cyclohexanone fragment, and the equivalent
ethereal C-O bond of the dihydropyran fragment of tagalsin
I is in an axial position on the cyclohexanone fragment of
the corresponding spirochroman ring system. Applying this
knowledge to our data, we can see that spirochromans 3,
Figure 2.
Correlation of spirochroman configuration with 13C data.
and spirochromans 21, 22, and 23 possess the stereochemistry
represented by structure B as they all have δCdO ) 213 ( 1
ppm. The ability to assign configuration at the spirochroman
spirocenter (relative to the protected hydroxymethyl sub-
stituent on the cyclohexanone ring) using 13C NMR allows
us to propose the stereochemistry shown in Figure 1 for
dihypoestoxide, as the natural product has δCdO ) 207.5
ppm, which fits extremely well with our data for the core
structure 3.
In conclusion, we have shown that hetero-Diels-Alder
cycloaddition/dimerization could account for the formation
of dihypoestoxide from hypoestoxide, although the conditions
required to accomplish this transformation in vivo are yet
to be established. Furthermore, we have been able to propose
a complete stereochemical assignment of dihypoestoxide
based upon analysis of its 13C NMR data.
Acknowledgment. We thank the HEC (Pakistan) for
providing a Scholarship (MU) and also AstraZeneca and
Pfizer for providing additional financial support of this work.
Supporting Information Available: Copies of the 1H and
13C NMR spectra and detailed experimental procedures for
the preparation of all new compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
(12) (a) Desimoni, G.; Tacconi, G. Chem. ReV. 1975, 75, 651. (b)
Colonge, J.; Descotes, G. R,ꢀ-Unsaturated Carbonyl Compounds as Dienes.
In Organic Chemistry; Hamer, J., Ed.; Academic Press: New York, 1967;
Vol. 8, pp 217-253.
(13) Hosoyama, H.; Shigemori, H.; In, Y.; Ishida, T.; Kobayashi, J.
Tetrahedron Lett. 1998, 39, 2159.
(14) Zhang, Y.; Lu, Y.; Mao, L.; Proksch, P.; Lin, W. Org. Lett. 2005,
7, 3037.
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