On the structure of the Phytophthora α1 mating hormone: synthesis and comparison of four candidate stereoisomers

Article abstract of DOI:10.1016/j.tetlet.2007.09.054

Two two-compound mixtures of candidate structures for the Phytophthora α1 mating hormone have been synthesized. The mixtures were designed to have differing configurations at C3, but proved to be identical. This suggests that epimerization occurred at C3,

Full text of DOI:10.1016/j.tetlet.2007.09.054

Tetrahedron Letters 48 (2007) 7965–7968
On the structure of the Phytophthora a1 mating hormone:
synthesis and comparison of four candidate stereoisomers
Reena Bajpai, Fanglong Yang and Dennis P. Curran*
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA
Received 23 August 2007; revised 6 September 2007; accepted 7 September 2007
Available online 12 September 2007
Dedicated to the memory of Professor Yoshihiko Ito, 1937–2006
Abstract—Two two-compound mixtures of candidate structures for the Phytophthora a1 mating hormone have been synthesized.
The mixtures were designed to have differing configurations at C3, but proved to be identical. This suggests that epimerization
occurred at C3, and comparison with the data for the natural samples suggest that this is also a mixture of isomers.
Ó 2007 Elsevier Ltd. All rights reserved.
Phytophthora, derived from Greek ‘the plant destroyer’,
is a parasitic genus of organism whose members live in
the soil and attack the roots and basal stems of plants.
These fungi-like parasites do considerable damage to
assorted hosts.1 For example, Phytophthora infestans was
responsible for the blight that precipitated the Irish po-
tato famine in the mid-19th century. Other Phytophthora
induce rot in plants as diverse as strawberries, tomatoes,
tobacco, azaleas, coconut trees, and oak trees, among
others. The control of these organisms remains difficult.
O
3
7
11
15
17
RO
OR
HO
20
19
18
hormone α1 1, R = H
bis-4-bromobenzoate 2, R = C(O)-4-BrC H
6
4
Figure 1. Two-dimensional structure of the Phythophera a1 mating
hormone and its bis-4-bromobenzoate.
sal hormone for all heterothallic species of
Phytophthora. Yajima and co-workers recently made a
mixture that presumably contains all 16 stereoisomers
of 1, and found that this mixture exhibited about 20%
of the oospore-inducing activity of the natural sample.4
They also commented that it was impossible to discrim-
inate between the diastereomers of 1 and 2 by NMR
analysis.
Phytophthora are classified as homothallic and hetero-
thallic depending on whether they reproduce asexually
or sexually. The sexual reproduction of heterothallic
phytophthora is regulated by hormone secretion,2 and
in 2005 Ojika and co-workers isolated about 1 mg of
hormone a1 from the A1 mating type of Phytophthora
nicotianae.3 Based on spectroscopic analysis of this
sample and its bis-4-bromobenzoate derivative 2, Ojika
proposed that the two-dimensional structure (constitu-
tion) of hormone a1 was 1,11,16-trihydroxy-3,7,11,15-
tetramethylhexadecan-4-one 1 (Fig. 1). With four
stereocenters 1 has 16 stereoisomers, but no information
on the three-dimensional structure (configuration) of 1 is
yet available.
We have recently used techniques of fluorous mixture
synthesis to make stereoisomer libraries of a number
of natural products.5,6 As a prelude to employing this
technique to make a library of a1 hormones, we decided
to execute a traditional synthesis of the hormone. The
goals of this work were: (1) to put in place a route that
could subsequently be used for fluorous mixture synthe-
sis, (2) to prove the constitution of 1, and (3) to provide
the preliminary information about configuration of 1.
Assigning the configuration of 1 is especially an impor-
tant goal because Ojika has proposed that 1 is a univer-
*
Herein we communicate the synthesis of four isomers of
Corresponding author. Tel.: +1 412 624 8240; fax: +1 412 624
9861; e-mail: curran@pitt.edu
1. Like Yajima,4 we confirm the constitution of 1. We
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.09.054

7966
R. Bajpai et al. / Tetrahedron Letters 48 (2007) 7965–7968
also find the stereoisomers difficult to differentiate spec-
troscopically from each other and the natural product;
however, we do find clear indications that there are
small but meaningful differences in the spectra of some
diastereomers. Based on our analysis, we suggest that
the natural sample may not be a single isomer. We com-
ment on the problems that need to be solved to provide
a rigorous assignment of the configuration of 1.
O
O
O
O
S
TBSO
TBSO
NaHDMS
N
N
O
O
CH I
3
S
S
Ph
Ph
(S
)-6
(
S
,
S
)-7, 87%
O
1) LiOOH, 95%
The latent symmetry of the C6–C16 fragment of 1
makes the retrosynthetic cleavage of the C5–C6 bond
especially attractive. As summarized in Figure 2, we
envisioned that ene diyne 3 could be readily converted
to 1 by hydrogenation and deprotection. Now cleavage
of the C5–C6 bond of 3 provides a simple ketoalcohol 4
(C1–C5) and a symmetrical protected triol 5 (C6–C16).
Triol 5 has four stereoisomers whose syntheses are facil-
itated by symmetry (two have a mirror plane while two
have ‘pseudo-C2 symmetry’7). But using the symmetry
in this way exacts a price because the symmetry of frag-
ment 5 must be broken prior to the union of the two
fragments.7 Purposefully refusing to pay that price, we
decided to generate a two-compound mixture during
the symmetry-breaking step of the synthesis to see if it
were possible at any stage to differentiate or perhaps
even separate the stereoisomers.
TBSO
2) CDI, MeNHOMe, 94%
3) MeMgBr, 91%
(S)-4
Scheme 1. Synthesis of the C1–C5 fragment 4.
synthesis of the (S)-4 enantiomer is summarized in
Scheme 1. Alkylation of readily available (S)-6 (see Sup-
plementary data) with NaHMDS and CH3I gave (S,S)-7
in 87% yield. Hydrolysis with lithium hydroperoxide,
conversion to the Weinreb amide and addition of
CH3MgBr provided (S)-4 in good overall yield.9 Like-
wise, (R)-4 was made by the same route starting from
(R)-6 (not shown). Chiral GC analysis showed that the
samples of 4 were enantiopure.
This unconventional decision was driven in part by our
expectation that it might be difficult to decide whether
an individual synthetic stereoisomer of 1 matched the
natural sample or not. If we made two stereoisomers
of 1 as a mixture, would we be able to differentiate them
from each other? Assuming (without basis) that the nat-
ural product is a single isomer, then at least one of the
two samples of the mixture (possibly both) would not
be the natural product. Would it be possible to differen-
tiate that isomer from the natural product?
Starting from readily available (R)-8 (see Supplementary
data), we prepared a two-compound mixture of C6–C16
fragment 10 as summarized in Scheme 2. Deprotonation
of 2 equiv of (R)-8 and addition of 1 equiv acetyl chlo-
ride provided pseudo-C2 symmetric alcohol (R,R)-5.
Removal of both TBS groups with TBAF gave a triol
whose symmetry was broken in a non-selective fashion
by statistical monosilylation. This provided an insepara-
ble mixture of two compounds 9 whose configurations
differed at C11 in 45% yield.10 Oxidation of the free pri-
mary alcohol of 9 with the Dess–Martin periodinane11
followed by silylation of the remaining tertiary alcohol
with TMSCl provided 10, again as an inseparable mix-
ture of epimers at C11.
Both enantiomers of protected ketoalcohol 4 were
synthesized by standard Evans aldol methods,8 and the
O
1
5
11
HO
15
17
HO
6
EtMgBr
AcCl
6
16
OH
R
R
TBSO
OTBS
TBSO
20
19
18
OH
)-5
1
(R
)-8, (2 equiv)
(
R,R
O
16
5
TBSO
OTBS
6
R
R
1) TBAF, 96%
2) TBSCl, 45%
11
HO
OTBS
OTBS
R
/
S
TMSO
HO
3
(
R
,
R
/S
,
R
)-9
6
16
O
TBSO
OTBS
+
TBSO
R
R
1) DMP, 96%
11
R/S
O
2) TMSCl, 79%
HO
TMSO
(S)-4 and (R)-4
5
(R,R/S,R
)-10
Figure 2. Retrosynthetic analysis of 1 based on the latent symmetry of
the C6–C16 fragment.
Scheme 2. Synthesis of the C6–16 fragment 10.

R. Bajpai et al. / Tetrahedron Letters 48 (2007) 7965–7968
7967
O
O
R
R
1) LDA, 80%
OTBS
O
TBSO
+
R/S
HO
OH
2) MsCl, Et N
3
85%
TMSO
HO
(S
)-4
(R
,R
/S
,R
)-10
1
O
OH
1) H , Pd-C
2
91%
O
OH
S
R
R
TBSO
OTBS
R/S
HO
11
Figure 3. Proposed ketoalcohol–hemiacetal equilibrium.
2) TBAF, 65%
TMSO
(
S,R
,R/
S,
R
)-3
O
S
3
R
R
/S
R
RO
made from (
S
)-4 and
OR
spectra of the bis-4-bromobenzoate samples 2 were more
informative.
(R,R/S,R)-10
HO
(
S,R
,R
/S
,R
)-1, R = H
(
S
,
R,R
/
S
,
R
)-2, R = C(O)-4-BrC H
Though the purified bis-4-bromobenzoate samples 2
exhibited a single spot on standard silica TLC and a
single peak on reverse phase HPLC analysis, there were
clear suggestions that they were not single components.
6
4
O
R
R
R
/S
R
RO
made from (
R
)-4 and
OR
3
(R,R/S,R)-10
1
HO
The H NMR spectra of the two synthetic samples and
the natural bis-4-bromobenzoate sample were substan-
tially identical. All three spectra showed single reso-
nances for all non-overlapping protons, with an
important exception. The methyl group at C19 (bonded
to C7) appeared as an apparent triplet, which we inter-
pret as two overlapping doublets (d 0.83 and 0.82), each
arising from a different stereoisomer. The natural
sample also showed this feature, and this suggests that
it may not be isomerically pure.
(
(R
R
,
,
R
,
R
/
/
S
S
,
,
R
R
)-1, R = H
)-2, R = C(O)-4-BrC H
R,R
6
4
Scheme 3. Fragment coupling and completion of the synthesis of 1.
The coupling of the two fragments (Scheme 3) was
accomplished by aldol reaction of (S)-4 and (R,R/
S,R)-10 (LDA, THF, 80%), followed by mesylation
and in situ dehydration (MsCl, Et3N, 85%). The result-
ing ene diyne (S,R,R/S,R)-3 must again be a mixture of
two isomers. The alkene and alkynes were saturated by
catalytic hydrogenation to provide a protected ketotriol,
which was then deprotected with TBAF in THF at room
temperature to give the first two-compound mixture
(S,R,R/S,R)-1. The second two-compound mixture
(R,R,R/S,R)-1 was prepared in the same way by cou-
pling (R)-4 with (R,R/S,R)-10, then taking the coupled
product through the same sequence of steps. Both sam-
ples of 1 were converted to the bis-4-bromobenzoate
derivatives 2 by standard acylation (77%).
The 13C spectra of the two synthetic samples 2 showed
single resonances for many of the carbons. However,
some of the resonances were clearly doubled, and one
(C18) was even quadrupled. Figure 4 summarizes the
doubled and quadrupled resonances (see Supplementary
data for a tabulation and overlaid, expanded spectra). In
addition, HPLC analysis on a Chiralcel OD column
showed four overlapping peaks for both samples of 2.
Taken together, these results suggests that epimerization
occurred at some point during the synthesis and that the
final products 1 and 2 are four-isomer mixtures rather
than the expected two-isomer mixtures. We suggest that
epimerization at C3 occurred during the aldol/elimina-
tion sequence (or thereafter), and this would mean that
the two samples of 1 are identical. So far, all data that
we have collected on the samples show them to be
identical.
After detailed characterization, we then compared the
spectroscopic data of the two two-compound mixtures
of 1 with each other and with those reported for the nat-
ural product. Ojika and co-workers commented that the
spectra of the natural product ‘suggested the presence
of unknown trace components’,3 and we saw similar
small resonances (<10%) in our spectra. Accordingly,
we conclude that these do not come from an impurity
per see, but instead arise from hemiacetal 11 (addition
of OH at C1 to ketone at C4) present at equilibrium
(Fig. 3).12
This work helps to advance the stereostructure assign-
1
ment of the hormone a1. The H NMR spectra of the
O
1
*
*
*
*
*
11
4-BrC H CO
6
4
The 500 MHz H NMR spectra of the two synthetic
*
*
15
OC(O)-4-BrC H
*
6
4
3
7
*
*
samples of 1 were substantially identical, and there
was no indication that these samples were isomer mix-
tures. Further, both the spectra were substantially iden-
tical to that of the natural sample. The 150 MHz 13C
NMR spectra of the two samples did show doubling
of a few resonances (see Supplementary data), but the
HO
#
*
* = resonace doubled
# = resonance quadrupled
Figure 4. Doubled and quadrupled 13C NMR resonances of the two
samples of 2.

7968
R. Bajpai et al. / Tetrahedron Letters 48 (2007) 7965–7968
isomers of 1 that we have made are identical, while the
13C spectra are very similar but not identical. The spec-
tra of the bis-4-bromobenzoate derivatives are better
resolved and both 1H and 13C NMR spectra show small
differences for stereoisomers. Because some of these dif-
ferences also appear in the bis-4-bromobenzoate derived
from the natural sample, it seems likely that sample is
also a mixture of stereoisomers, probably at C3. Finally,
the purposeful synthesis of mixtures of isomers at C11
did prove helpful in looking for differences in spectra.
But it is probably not a good idea for an expanded
synthesis of a stereoisomer library by fluorous mixture
synthesis or other methods because it did not prove
practical to separate the isomers at any stage and
because of the added complication posed by the
observed epimerization at C3. To make a stereoisomer
library would require a stereoselective synthesis of each
isomer.
References and notes
1. (a) Erwin, D. C.; Bartnicki-Garcia, S.; Tsao, P. H.
Phytophthora: Its Biology, Taxonomy, Ecology, and
Pathology; American Phytopathological Society: St. Paul,
Minn, 1983; (b) Fry, W. E.; Goodwin, S. B. Bioscience
1997, 47, 363–371.
2. Chern, L. L.; Tang, C. S.; Ko, W. H. Bot. Bull. Acad. Sin.
1999, 40, 79–85.
3. Qi, J.; Asano, T.; Jinno, M.; Matsui, K.; Atsumi, K.;
Sakagami, Y.; Ojika, M. Science 2005, 309, 1828.
4. Yajima, A.; Kawanishi, N.; Qi, J.; Asano, T.; Sakagami,
Y.; Nukada, T.; Yabuta, G. Tetrahedron Lett. 2007, 48,
4601–4603.
5. Short overviews: (a) Zhang, W. Arkivoc 2004, 101–109; (b)
Curran, D. P. In The Handbook of Fluorous Chemistry;
Gladysz, J. A., Curran, D. P., Horvath, I. T., Eds.; Wiley-
VCH: Weinheim, 2004; pp 128–156.
6. (a) Luo, Z. Y.; Zhang, Q. S.; Oderaotoshi, Y.; Curran, D.
P. Science 2001, 291, 1766–1769; (b) Dandapani, S.; Jeske,
M.; Curran, D. P. Proc. Nat. Acad. Sci. 2004, 101, 12008–
12012; (c) Curran, D. P.; Zhang, Q. S.; Richard, C.; Lu, H.
J.; Gudipati, V.; Wilcox, C. S. J. Am. Chem. Soc. 2006,
128, 9561–9573; (d) Curran, D. P.; Zhang, Q. S.; Lu, H. J.;
Gudipati, V. J. Am. Chem. Soc. 2006, 128, 9943–9956; (e)
Curran, D. P.; Moura-Letts, G.; Pohlman, M. Angew.
Chem., Int. Ed. 2006, 45, 2423–2426; (f) Yang, F.;
Newsome, J. J.; Curran, D. P. J. Am. Chem. Soc. 2006,
128, 14200–14205.
Note added in proof
Ojika et al.13 have just provided additional evidence that
a natural hormone is a mixture of stereoisomers.
Acknowledgments
7. Poss, C. S.; Schreiber, S. L. Acc. Chem. Res. 1994, 27, 9–17.
8. Evans, D. A.; Takacs, J. M.; McGee, L. R.; Ennis, M. D.;
Mathre, D. J.; Bartroli, J. Pure Appl. Chem. 1981, 53,
1109–1127.
9. (a) Nahm, S.; Weinreb, S. M. Tetrahedron Lett. 1981, 22,
3815–3818; (b) Garg, N. K.; Caspi, D. D.; Stoltz, B. M. J.
Am. Chem. Soc. 2004, 126, 9552–9553.
We thank the National Institute of General Medical Sci-
ence of the National Institutes of Health for funding of
this work. We also thank Professor M. Ojika and Dr. A.
Yajima for sending copies of the NMR spectra of their
samples.
10. The spectra of the two mixed isomers in these compounds
were substantially identical, and no doubling of reso-
nances was observed. However, in related compounds
with saturated alkynes, two resonances for C11 and C18
could sometimes be observed in the 13C NMR spectra.
11. (a) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48,
4155–4156; (b) Denmark, S. E.; Fujimori, S. Synlett 2001,
1024–1029.
12. Cavicchioli, S.; Savoia, D.; Trombini, C.; Umani-Ronchi,
A. J. Org. Chem. 1984, 49, 1246–1251.
13. Ojika, M.; Qi, J.; Kito, Y.; Sakagami, Y. Tetrahedron:
Asymmetry 2007, 18, 1763–1765.
Supplementary data
Contains full experimental and compound characteriza-
tion details, along with copies of the NMR spectra of 1
and 2 and a tabular listing of resonances (78 pages).
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.2007.
09.054.