Structure assignment of lagunapyrone B by fluorous mixture synthesis of four candidate stereoisomers

Article abstract of DOI:10.1021/ja064812s

Techniques of fluorous mixture synthesis have been used to make four candidate stereoisomers for the natural product lagunapyrone B. A quasiracemic mixture of vinyl iodides whose component configurations at C19-21 were encoded by fluorous silyl groups was

Full text of DOI:10.1021/ja064812s

Published on Web 09/26/2006
Structure Assignment of Lagunapyrone B by Fluorous
Mixture Synthesis of Four Candidate Stereoisomers
Fanglong Yang, Jeffery J. Newsome, and Dennis P. Curran*
Contribution from the Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh,
PennsylVania 15260
Received July 6, 2006; E-mail: curran@pitt.edu
Abstract: Techniques of fluorous mixture synthesis have been used to make four candidate stereoisomers
for the natural product lagunapyrone B. A quasiracemic mixture of vinyl iodides whose component
configurations at C19-21 were encoded by fluorous silyl groups was fused to a central fragment by a
Negishi coupling. A separate quasiracemic mixture of pyrone fragments whose component configurations
at C6,7 were also encoded by fluorous silyl groups was synthesized and demixed. Stille coupling of the
resulting pure quasienantiomers with the quasiracemic mixture provided two quasi-diastereomeric samples,
which were demixed and detagged to provide all four lagunapyrone B stereoisomers. Lagunapyrone was
assigned the 6R,7S,19S,20S,21R configuration by comparison of optical rotations.
Introduction
During an investigation of the secondary metabolites of
estuarine actinomycetes, Fenical and co-workers reported the
isolation and structure assignment of lagunapyrones A, B, and
C (1-3, Figure 1).1 These compounds constitute a novel skeletal
class of natural products and feature a 24-carbon chain consisting
of an R-pyrone ring with two adjacent stereocenters (C6,7)
separated by 11 carbon atoms (four alkenes and three methylene
groups) from a second group of three stereocenters (C19-21)
terminating in another alkene. All seven of the double bonds in
the backbone of the lagunapyrones are trisubstituted, and the
three compounds differ in the nature of the group attached to
C2: 1, R ) CH3; 2, R ) C3H7; 3, R ) C4H9. Lagunapyrone B
2 exhibits moderate activity (ED50 ) 3.5 µg/mL) against a
human colon cancer cell line.
The two-dimensional structure (constitution) of the lagunapy-
rones was assigned primarily by analysis of 1D and 2D NMR
spectra. Assignment of the relative configuration as anti at C-6
and C-7 was accomplished by comparison of vicinal proton
coupling constants to calculated values and synthetic models.
The relative configuration of C-19 through C-21 was assigned
as anti,syn by converting lagunapyrone B 2 to an acetonide,
which exhibited diagnostic chemical shifts in its 13C NMR
spectrum2 and NOE effects in its 1H NMR spectrum. However,
the absolute configurations of the lagunapyrones could not be
assigned, and neither could the configurations of the two remote
groups of stereocenters be assigned relative to each other.
Accordingly, there are still four possible structures for each of
the natural products.
Figure 1. One of four possible stereostructures for lagunapyrones A-C
(1-3).
lagunapyrones.3 Recently developed techniques of fluorous
mixture synthesis4 have shown power in preparing small
stereoisomer libraries (2-32 members) of several natural
products.5-7 We set out to simultaneously prepare all four
candidate isomers for lagunapyrone B 2 by a fluorous mixture
synthesis approach, and we report herein the successful attain-
ment of this goal. By optical rotation comparison of the synthetic
(3) Piericidins coexist with the lagunapyrones and have some structural
resemblance. See: (a) Schnermann, M. J.; Boger, D. L. J. Am. Chem. Soc.
2005, 127, 15704-15705. (b) Keaton, K. A.; Phillips, A. J. Am. Chem.
Soc. 2006, 128, 408-409.
(4) Luo, Z. Y.; Zhang, Q. S.; Oderaotoshi, Y.; Curran, D. P. Science 2001,
291, 1766-1769.
(5) Short review: Zhang, W. ArkiVoc 2004, 101-109.
(6) (a) Zhang, Q. S.; Lu, H. J.; Richard, C.; Curran, D. P. J. Am. Chem. Soc.
2004, 126, 36-37. (b) Dandapani, S.; Jeske, M.; Curran, D. P. Proc. Natl.
Acad. Sci. U.S.A. 2004, 101, 12008-12012. (c) Dandapani, S.; Jeske, M.;
Curran, D. P. J. Org. Chem. 2005, 70, 9447-9462. (d) Wilcox, C. S.;
Gudipati, V.; Lu, H. J.; Turkyilmaz, S.; Curran, D. P. Angew. Chem., Int.
Ed. 2005, 44, 6938-6940. (e) Fukui, Y.; Bru¨ckner, A. M.; Shin, Y.;
Balachandran, R.; Day, B. W.; Curran, D. P. Org. Lett. 2006, 8, 301-304.
(f) Curran, D. P.; Moura-Letts, G.; Pohlman, M. Angew. Chem., Int. Ed.
2006, 45, 2423-2426. (g) Curran, D. P.; Zhang, Q.; Richard, C.; Lu, H.;
Gudipati, V.; Wilcox, C. W. J. Am. Chem. Soc. 2006, 128, 9567-9573.
(h) Curran, D. P.; Zhang, Q.; Lu, H.; Gudipati, V. J. Am. Chem. Soc. 2006,
128, 9943-9956.
Despite the novel skeleton and interesting biological activity,
there have not been any reports of synthetic efforts toward the
(1) Lindell, T.; Jenson, P. R.; Fenical, W. Tetrahedron Lett. 1996, 37, 1327-
1330.
(7) (a) Zhang, Q. S.; Rivkin, A.; Curran, D. P. J. Am. Chem. Soc. 2002, 124,
5774-5781. (b) Zhang, Q. S.; Curran, D. P. Chem.-Eur. J. 2005, 11, 4866-
4880.
(2) Rychnovsky, S. D.; Rogers, B. N.; Richardson, T. I. Acc. Chem. Res. 1998,
31, 9-17.
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Synthesis of Lagunapyrone B
A R T I C L E S
Figure 2. Retrosynthetic analysis of lagunapyrone B identifies three fragments.
compounds to the natural sample, we assign the 6R,7S,19S,-
20S,21R configuration to lagunapyrone B.
Results and Discussion
Figure 2 shows a partial retrosynthesis of lagunapyrone B 2.
We divided the molecule into left and right fragments, 4 and 6,
which we planned to make as quasiracemic mixtures7 with
configurations encoded by fluorous tags in the protecting groups
(PG). These fragments would then be bridged together with
symmetric middle fragment 5 by a Negishi coupling8 to make
the conjugated diene (C15,16) and a Stille coupling9 to make
the skipped diene (C10,11). This provides a mixture of four
fluorous-tagged quasi-isomers ready for demixing and detagging
to provide lagunapyrone and three isomers. This kind of double
tagging strategy was recently validated in the synthesis of
passifloricin.6f In practice, we deviated from the plan by using
a middle fragment of lower symmetry than 5 to facilitate the
couplings, and by demixing and detagging of one of the
fragments (4) prior to the final coupling because of steric
problems with that fragment.
The premix stage of the synthesis of R-pyrone fragment is
summarized in eq 1. (Z)-2-Buten-1,4-diol 7 was converted to
the bis-PMB ether 8 under standard conditions. Ozonolysis and
Wittig reaction then provided aldehyde 9 as a single E-isomer.
This aldehyde was then subjected to a Paterson anti-aldol
reaction10 with (S)-10 to provide (R,S)-11 as a single isomer in
71% isolated yield after flash chromatography. The hydroxy
group of 11 was protected by silylation with in situ generated
fluorous TIPS triflate11 bearing the C6F13 group to encode the
6R,7S configuration in (R,S)-12a. Likewise, reaction of 9 with
(R)-10 (not shown) and silylation with the fluorous TIPS triflate
TfOSi(iPr)2(CH2)2C4F9 provided the quasienantiomer (S,R)-12b
with the 6S,7R configuration encoded by the C4F9 group.
Quasienantiomers 12a and 12b were then mixed in a 1/1 ratio,
and the synthesis was continued with quasiracemate M-12a,b
as summarized in Scheme 1. (From here on, structures are drawn
with configurations of the natural product lagunapyrone for
simplicity; however, all compounds bearing the “M” prefix are
fluorous-tagged mixtures of two quasienantiomers.) Standard
cleavage of the R-benzoyloxyketone M-12a,b provided aldehyde
M-13a,b in 90% overall yield.12 This aldehyde was used without
success as a precursor for a number of standard R-pyrone
syntheses;13,14 accordingly, we deployed a new R-pyrone
synthesis to build this key group.
Addition of allylmagnesium bromide to M-13a,b provided a
stereoisomeric mixture of alcohols M-14a,b that was not
separated but was instead directly silylated with TESCl to
provide M-15a,b. Next, oxidative cleavage and Still-Gennari
reaction15 of the resulting aldehyde provided M-17a,b as the
Z-isomer in 65% yield. Conversion of ester 17 to acid M-18a,b
was effected by DIBAL reduction (60%) and oxidation (Dess-
Martin, 92%, followed by NaClO2, 90%) rather than standard
saponification because of the base sensitivity of this series of
molecules. Removal of the TES group with dichloroacetic acid
(71%) followed by Dess-Martin oxidation (96%) provided keto
acid M-19a,b, which was smoothly dehydrated to R-pyrone
M-20a,b (87%) upon exposure to acetic anhydride and dichlo-
roacetic acid. DDQ oxidation16 removed the PMB group (95%),
(8) (a) Negishi, E.; Hu, Q.; Huang, Z. H.; Qian, M. X.; Wang, G. W.
Aldrichimica Acta 2005, 38, 71-88. (b) Huo, S.; Negishi, E.-i. In Handbook
of Organopalladium Chemistry for Organic Synthesis; Negishi, E.-I., de
Meijere, A., Eds.; Wiley-Interscience: New York, 2002; Vol. 1, pp 335-
408.
(9) Farina, V.; Krishnamurthy, V.; Scott, W. L. Org. React. 1997, 50, 1-652.
(10) (a) Paterson, I.; Wallace, D. J.; Cowden, C. J. Synthesis 1998, 639-652.
(b) Cowden, C. J.; Paterson, I. Org. React. 1997, 51, 1-200.
(11) (a) Zhang, W.; Luo, Z.; Chen, C. H. T.; Curran, D. P. J. Am. Chem. Soc.
2002, 124, 10443-10450. (b) Fluorous silanes and silica products were
purchased from Fluorous Technologies, Inc. DPC owns an equity interest
in this company.
(12) (a) Makabe, H.; Higuchi, M.; Konno, H.; Murai, M.; Miyoshi, H.
Tetrahedron Lett. 2005, 46, 4671-4674. (b) Sinha, S.; Sinha-Bagchi, A.;
Keinan, E. J. Org. Chem. 1993, 58, 7789-7796.
(13) Afarinka, K.; Vindev, V. Sci. Synth. 2003, 14, 275-346.
(14) Yang, F. Ph.D. Thesis, University of Pittsburgh, 2006.
(15) Still, W. C.; Gennari, C. Tetrahedron Lett. 1983, 24, 4405-4408.
(16) Nakajima, N.; Abe, R.; Yonemistu, O. Chem. Pharm. Bull. 1988, 36, 4244.
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A R T I C L E S
Yang and Curran
Scheme 1. Quasiracemic Synthesis of the Left Fragment
Scheme 2. Premix Stage of the Right Fragment Synthesis
and the resulting alcohol was acetylated as usual to provide allyl
acetate M-21a,b in 100% yield.
The configuration of (R,R)-24 was proved by X-ray crystal-
lography (see Supporting Information). Further, we exchanged
spectra with Prof. Hamada and learned that indeed his TiCl4
product was different from ours, not the same. Thus, his structure
assignment is also correct, and we currently do not understand
why our results and his differ significantly. Nonetheless, because
we needed both enantiomers of the aldol adduct and because
the conditions proved reliable, we scaled up the synthesis of
(R,R)-24 and tagged this adduct with a fluorous TIPS triflate
bearing the C3F7 group to provide (R,R)-25a. Starting from the
enantiomer of (S)-22 (not shown), we prepared the enantiomer
of (R,R)-24 by aldol reaction and then tagged this with the C4F9
variant of the fluorous TIPS group to give (S,S)-25b. Quasir-
acemate M-25a,b was then made by mixing equal amounts of
the corresponding quasienantiomers.
The fluorous mixture synthesis steps to make fragment
M-32a,b are summarized in Scheme 3. Reductive removal of
the auxiliary (LiBH4, 70%) from quasiracemate M-25a,b
followed by PCC oxidation (91%) provided M-26a,b. Allylation
with allyl trimethylsilane and dimethylaluminum chloride20 then
provided anti,syn-M-27a,b as a single isomer in 78% yield. The
1,3-anti configuration of the diol was confirmed by deprotection
of the F-TIPS group to provide a true racemate, followed by
Throughout this work, the quasiracemic mixtures were treated
like standard racemic mixtures for separation and identification.7
1
Relevant chemical shifts and coupling constants in the H and
13C NMR spectra of M-21a,b matched very well with those
reported for lagunapyrone B 2, thereby supporting the assign-
ment of this part of the structure.
The premix stage of the synthesis of right fragment involved
asymmetric aldol reaction and tagging, as summarized in
Scheme 2. The Evans aldol reaction17 of (S)-22 with tiglic
aldehyde 23 under standard conditions to provide Evans syn-
aldol adduct (S,S)-24 (Bu2BOTf) was reported to be low yielding
by Hamada,18 and indeed we also experienced problems with
this reaction. Hamada reported that the yield of the standard
Evans aldol product (S,S)-24 could be increased significantly
by using TiCl4/iPr2NEt conditions;17b however, in our hands
these conditions produced a separable mixture of the non-Evans
syn-product (R,R)-24 (isolated in 66% yield)19 along with the
anti-aldol product (S,R)-24 (isolated in 9% yield). Only a trace
of the expected Evans syn-product (S,S)-24 (<2%) was formed.
(17) (a) Evans, D. A.; Nelson, J. V.; Vogel, E.; Taber, T. R. J. Am. Chem. Soc.
1981, 103, 3099-3111. (b) Evans, D. A.; Reiger, D. L.; Bilodeau, M. T.;
Urpi, F. J. Am. Chem. Soc. 1991, 113, 1047-1049.
(18) Makino, K.; Henmi, Y.; Hamada, Y. Synlett 2002, 613-615.
(19) Crimmins, M. T.; King, B. W.; Tabet, E. A.; Chaudhary, K. J. Org. Chem.
2001, 66, 894-902.
(20) Evans, D. A.; Allison, B. D.; Yang, M. G.; Masse, C. E. J. Am. Chem.
Soc. 2001, 123, 10840-10852.
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Synthesis of Lagunapyrone B
A R T I C L E S
Scheme 3. Quasiracemic Synthesis of the Right Fragment
Scheme 4. Fragment Coupling To Make Four Protected Lagunapyrones
conversion to the acetonide and 13C NMR analysis2 (not shown,
see Supporting Information). The data for this analysis were
very similar to those reported by Fenical,1 thereby confirming
his assignment of the relative configuration of this part of
lagunapyrone.
served as the lynch pin for fragment coupling (reagent equivalent
of 5 in Figure 2) as shown in Scheme 4. Negishi coupling8 of
M-32a,b and 34 provided a conjugated diene (93%), whose
alkynyl silane was then removed with KOH to provide M-35a,b
in 73% yield. Treatment of 35 under standard conditions with
Cp2ZrCl2 and Me3Al,22 followed by iodination of the so-formed
intermediate, provided E-vinyl iodide M-36a,b in 70% yield.
Halogen-lithium exchange followed by quenching with Me3-
SnCl then provided the corresponding E-vinyl stannane M-37a,b.
Unfortunately, attempted coupling of M-37a,b with allyl
acetate M-21a,b under a variety of conditions did not result in
detectable amounts of the expected four-compound mixture of
protected lagunapyrones. Because stannane M-37a,b coupled
with simple model compounds,14 we began to suspect that the
problem was with the fluorous TIPS ether in the allylic position
(O7) of allyl acetate M-21a,b. To probe the effect of this
substituent further, we conducted a simple but insightful series
of model couplings shown in eq 2. The free alcohol 38a and its
derived TMS 38b and TIPS 38c ethers were coupled with vinyl
stannane 39 under identical conditions (Pd2(dba)3, LiCl, DMF).
The reaction with free alcohol 38a was complete in only 3 h at
25 °C and provided 40a in 82% yield after chromatographic
Protection of alcohol M-27a,b with TBDPSCl was slow, but
after 3 days provided a 77% yield of M-28a,b alongside 17%
of recovered 27.21 Regioselective dihydroxylation of M-28a,b
was accomplished with AD-mix-R, and the resulting diol was
cleaved to aldehyde M-29a,b with NaIO4 (95%). Conversion
to the dibromide by the Corey-Fuchs method (CBr4, PPh3,
81%), followed by treatment with butyllithium and in situ
methylation of the resulting anion, provided alkyne M-31a,b
in 92% yield. Hydrozirconation of 31 followed by iodinolysis
provided M-32a,b and M-33a,b in good yield (77%), but with
relative low regioselectivity in favor of 32 (2/1) despite the
nearby bulky TBDPS ether. The major regioisomer M-32a,b
was carefully separated by chromatography in preparation for
union of the fragments.
Vinyl zinc reagent 34 was readily available from the
corresponding vinyl iodide (see Supporting Information) and
(21) This sequence of steps was also conducted with a TBS protecting group,
and this is described in ref 14 and in the Supporting Information.
(22) Wipf, P.; Lim, S. Angew. Chem., Int. Ed. Engl. 1993, 32, 1095-1097.
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A R T I C L E S
Yang and Curran
purification. Reaction of the TMS ether 38b required 20 h at
40 °C and provided 40b in only 50% yield, while the experiment
with the standard TIPS ether 38c did not provide any detectable
amount of 40c over 20 h at 40 °C.
These results pinpoint the problem in the failed couplings of
37 and 21 and, more importantly, suggest an expedient solution.
The fluorous TIPS ether group in 21 is the culprit that blocks
coupling, but it is apparently the size of the silyl group and not
its fluorous substituent that causes the problem. The solution is
simply to remove this group before coupling. This adds three
reactions to the synthesis because M-21a,b must be demixed
before it is deprotected (two extra reactions because in the
original plan all protecting groups were to be removed simul-
taneously) and coupled (one extra reaction). We deemed that a
small price to pay for success.
Demixing of M-21a,b by preparative fluorous HPLC23
occurred smoothly to provide quasienantiomers (R,S)-21a and
(S,R)-21a, which were then deprotected to provide true enan-
tiomers (R,S)-41 and (S,R)-41 (Scheme 4). Gratifyingly, indi-
vidual coupling of (R,S)- and (S,R)-41 with quasiracemate
M-37a,b provided quasi-diastereomeric two-compound mixtures
M-42a,b and M-43a,b in 60% yield. These were preparatively
demixed, and the products were detagged to provide all four
target isomers of 2 in individual pure form in amounts ranging
from 4.2 to 6.0 mg. The complete structures of these final
products are shown in Figure 3 (labeled as 2.1-2.4) along with
their optical rotations and the fluorous tagging scheme (for
reference).
Carbon-13 and 1H NMR spectra (151 and 600 MHz,
respectively) of all four of these stereoisomers were, to the best
of our ability to assess, identical. This is expected for two pairs
of compounds (2.1/2.4 and 2.2/2.3) because they are enanti-
omers. However, the spectra of the diastereomeric compounds
were also identical, indicating that the long spacer between the
remote pairs of stereocenter groups prohibits communication
of these groups, at least under these standard NMR recording
conditions. Importantly, all of the spectra also matched very
well with both the tabulated spectra for lagunapyrone B1 and
the copies of the spectra kindly provided by Dr. Fenical.
Thus, the constitutional and partial stereochemical assignment
of 2 by Fenical and co-workers is correct, and there remains
only the question of absolute configuration of the fragments.
Preferred ways to answer this question are by chiral HPLC
analysis or by synthesis of a chiral derivative, but unfortunately,
after almost a decade of storage, the remaining natural sample
of lagunapyrone B had decomposed. Accordingly, we turned
Figure 3. Four candidate structures 2.1-2.4 for lagunapyrone with optical
rotations (c ) 0.2); 2.3 is lagunapyrone.
to polarimetry and measured the optical rotations indicated in
Figure 3 under conditions similar to those used for the natural
product (CH2Cl2, c ) 0.2).24 Enantiomeric pairs gave rotations
that were opposite in sign and approximately equal in magnitude,
as expected. The rotation of the natural product (+10.9) under
these conditions matched very well to that of the 6R,7S,19S,-
20S,21R isomer 2.3 (+11.5), and accordingly we assign this
configuration to lagunapyrone B, and, by analogy, to lagunapy-
rones A and C.
Conclusions
The convergent synthesis of lagunapyrone B described herein
requires 18 linear steps starting from the Paterson anti-aldol
reaction of readily available aldehyde 9. By using the technique
of fluorous quasiracemic synthesis and the “mix early/demix
late” principle, most of these steps only had to be conducted
once, even though two and ultimately four compounds were
being made. The synthesis features key Negishi and Stille
couplings of the quasiracemic fragments to a simple lynch pin
fragment to build the lagunapyrone backbone in short order.
One of the demixings had to be conducted one step earlier than
planned because a steric effect of the protecting group shut down
the Stille coupling, but the work around by early deprotection
was straightforward thanks to the convergent strategy. A new
R-pyrone synthesis involving Still-Gennari reaction of a
â-silyloxyaldehyde to give a δ-silyloxy-R,â-unsaturated ester,
followed by conversion to the δ-keto-R,â-unsaturated acid and
dehydration, was deployed to make the pyrone fragment.
The techniques of quasiracemic synthesis used herein are
especially straightforward because the quasiracemic mixtures
(24) Fenical also measured the rotation of lagunapyrone B in CH2Cl2 but at c
) 3.7. In addition to the rotation measured at c ) 0.2 in Figure 3, we also
measured two rotations at higher concentrations to probe for possible
concentration dependence: 2.3, [R]D ) +11.5 at c ) 0.5; 2.2, [R]D ) -11.4
at c ) 0.4. We thank Mr. X. Wang for conducting these experiments.
(23) Curran, D. P. In The Handbook of Fluorous Chemistry; Gladysz, J. A.,
Curran, D. P., Horvath, I. T., Eds.; Wiley-VCH: Weinheim, 2004; pp 101-
127.
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Synthesis of Lagunapyrone B
A R T I C L E S
typically behave like standard racemic mixtures in standard
separation methods and spectroscopic analyses.7 However,
unlike true enantiomers, the quasienantiomers can be sepa-
rated and identified at any time by using fluorous HPLC
techniques. The double tagging method used here (each frag-
ment gets its own fluorous tag) has only been introduced re-
cently in a proof-of-principle publication,6f and this is the first
example of use of the technique to solve a structural problem.
That the tagging and demixing were successful could not be
proved by standard spectroscopic analysis because the com-
pounds exhibit substantially identical spectra, but it was clearly
shown by optical rotation experiments. The results encourage
the continued application and expansion of fluorous mixture
synthesis methods for the preparation of natural product
stereoisomer libraries.
structure. We also thank Dr. W. Fenical for copies of spectra
of lagunapyrone B. We dedicate this paper to Professor
Theodore Cohen in tribute to his 50th anniversary at the
University of Pittsburgh.
Note Added after ASAP Publication. After this paper was
published ASAP on September 26, 2006, a third author was
added and the Acknowledgment modified accordingly; changes
were made to the first sentences of paragraphs 6, 13, and 15
and the first, second, and fourth sentences of paragraph 12 in
the Results and Discussion section; and a correction was made
to the explanatory text under compound M-43a,b in Scheme 4.
The corrected version was published ASAP on October 5, 2006.
Supporting Information Available: Full experimental details
and key characterization data of new compounds; a CIF file of
the X-ray crystal structure. This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the National Institutes of Health
(National Institute for General Medical Sciences) for funding
of this work. We thank Dr. Steven Geib for solving the crystal
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