Studies toward a marine toxin immunogen: Enantioselective synthesis of the spirocyclic imine of (-)-gymnodimine

Article abstract of DOI:10.1021/ol051840r

(Chemical Equation Presented) An enantioselective Diels-Alder reaction catalyzed by an Evans' copper-bis(oxazoline) complex was utilized to construct a highly functionalized spirolactam, a key intermediate in our projected total synthesis of the marine to

Full text of DOI:10.1021/ol051840r

ORGANIC
LETTERS
2005
Vol. 7, No. 23
5127-5130
Studies toward a Marine Toxin
Immunogen: Enantioselective Synthesis
of the Spirocyclic Imine of
(−)-Gymnodimine
Ke Kong, Ziad Moussa, and Daniel Romo*
Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012,
College Station, Texas 77842-3012
romo@mail.chem.tamu.edu
Received August 1, 2005 (Revised Manuscript Received September 29, 2005)
ABSTRACT
An enantioselective Diels−Alder reaction catalyzed by an Evans’ copper−bis(oxazoline) complex was utilized to construct a highly functionalized
spirolactam, a key intermediate in our projected total synthesis of the marine toxin, gymnodimine. Additional transformations, including a mild
N-tosyl group deprotection, afforded a keto spirocyclic imine moiety, the proposed pharmacophore of gymnodimine. Thus, the prepared
ketone is a potentially useful intermediate for conjugation to provide an immunogen for eventual monitoring of gymnodimine and congeners.
In 1994, oysters from Foveaux Strait, South Island, New
Zealand, were found to be toxic and led to the isolation and
structural elucidation of a novel marine toxin, (-)-gym-
nodimine (1) (Figure 1).1 Subsequently, Blunt, Munro, and
co-workers established the absolute stereochemistry of gym-
nodimine by X-ray crystallographic analysis of its p-
bromobenzamide derivative.2 More recently, two new ana-
logues, gymnodimine B and C, have been reported.3
Gymnodimine and analogues exhibit unusual structural
features, including a spirocyclic imine ring system and a
trisubstituted tetrahydrofuran embedded within a 16-mem-
bered macrocycle. The presence of the spirocyclic imine
places this marine toxin in the same family with the
spirolides,4 the pinnatoxins,5 and prorocentrolide.6 The toxic-
ity of the spirolides has been attributed to the imine
Figure 1. Retrosynthetic analysis of gymnodimine.
(1) Seki, T.; Masayuki, S.; Mackenzie, L.; Kaspar, H. F.; Yasumoto, T.
Tetrahedron Lett. 1995, 36, 7093.
(2) Stewart, M.; Blunt, J. W.; Munro, M. H. G.; Robinson, W. T.;
Hannah, D. J. Tetrahedron Lett. 1997, 38, 4889.
(3) (a) Miles, C. O.; Wilkins, A. L.; Stirling, D. J.; MacKenzie, A. L. J.
functionality as imine reduction leads to nontoxic deriva-
tives.7 However, the exact mode of action of these toxins
Agric. Food Chem. 2000, 48, 1373. (b) Miles, C. O.; Wilkins, A. L.; Stirling,
D. J.; MacKenzie, A. L. J. Agric. Food Chem. 2003, 51, 4838.
(4) Hu, T.; Curtis, J. M.; Oshima, Y.; Quilliam, M. A.; Walter, J. A.;
Wright, W. M.; Wright, J. L. C. J. Chem. Soc., Chem. Commun. 1995,
2159.
(5) Uemura, D.; Chou, T.; Haino, T.; Nagatsu, A.; Fukuzawa, S.; Zheng,
S.; Chem, H. J. Am. Chem. Soc. 1995, 117, 1155.
(6) Torigoe, K.; Murata, M.; Yasumoto, T.; Iwashita, T. J. Am. Chem.
Soc. 1988, 110, 7876.
10.1021/ol051840r CCC: $30.25
© 2005 American Chemical Society
Published on Web 10/14/2005

has not been elucidated as the only activity reported for the
spirolides is weak activation of type L calcium channels (at
1.7 mM), and this activity does not correlate with the potent
toxicity of these compounds. The potential for human
consumption of shellfish contaminated with toxic concentra-
tions of gymnodimine is prompting efforts to develop assays
to monitor gymnodimine and congeners.8 This is even more
pressing given the recent finding that gymnodimine persists
in shellfish for extended periods.9
the spirocyclic imine precursor 3, and finally macro-
cyclization to provide the core framework of gymno-
dimine (Figure 1). Building on our success in the racemic
series, we envisioned an asymmetric version of the Diels-
Alder reaction between R-methylene lactam 4 and diene
5 to provide optically active spirocyclic lactam 3.12a There
are relatively few examples of catalytic, asymmetric Diels-
Alder processes employed in natural product total synthe-
ses.17
In addition to bioactivity, the intriguing structures of
gymnodimine (1, Figure 1) and related natural products have
led to considerable attention from synthetic groups. An
elegant total synthesis of pinnatoxin reported by Kishi
employed an intramolecular Diels-Alder reaction to install
Initial attempts to promote an enantioselective Diels-
Alder reaction with the previously employed diene (Z)-5
revealed a complete lack of reactivity with several chiral
catalysts. We reasoned that the lack of reactivity may be
due to the olefin geometry,18 and thus pursued the synthesis
of diene (E)-5. Synthesis of this diene commenced with
stannylcupration19 of 2,4-hexadiyne 6, affording (E)-vinyl
stannane 7 as the major isomer (89%, 5:2 mixture of
regioisomers, Scheme 1). While radical and metal-catalyzed
the quaternary carbon of the spirocyclic core structure.10
A
recent extension of this strategy by the same group employ-
ing iminium dienophiles toward gymnodimine was
recently disclosed.11 Our initial studies demonstrated the
potential of an intermolecular Diels-Alder process be-
tween an R-methylene lactam 4b and dienes 5 to rapidly
assemble the spirocyclic moiety common to the gymnodi-
mines (Figure 1). This [4 + 2] cycloaddition was promoted
by Et2AlCl and proceeded in good yield and excellent
diastereoselectivity.12a Subsequently, Murai13 and White14
disclosed their routes to the spirocyclic center involving
Diels-Alder variants. Murai’s intermolecular, double-dias-
teroselective Diels-Alder process employed Ellman’s cop-
per bis(sulfinyl)-imidoamidine complex15 on an ad-
vanced chiral intermediate and proceeded with high yield
and diastereoselectivity.13b In White’s approach, an inter-
molecular Diels-Alder reaction employing an optically
active diene was utilized to form the spirocyclic sys-
tem, albeit with low diastereoselectivity.14b Brimble has also
described approaches to the spirocyclic imine of gymnodi-
mine.16 Herein, we report the asymmetric synthesis of the
spirocyclic moiety via an enantioselective Diels-Alder
process and subsequent mild conversion to a model spiro-
cyclic imine moiety of gymnodimine. This compound will
be studied as an immunogen for the development of an assay
to monitor the concentration of gymnodimine and congeners
in shellfish and coastal regions.
Scheme 1. Synthesis of Diene (E)-5
hydrostannylations gave higher yields, the regioselectivity
was less satisfactory.20 Pd-catalyzed hydrostannylation gave
the undesired regioisomer as the sole product. Tin-lithium
exchange of stannane 7 followed by reaction with amide 8
gave the corresponding ketone. Subsequent enolization and
silylation gave diene (E)-5.
After surveying several chiral catalysts, we found that
Evans’ copper-bis(oxazoline) hexafluoroantimonate com-
plex (9)21 promoted the Diels-Alder reaction of lactam 4a
Our retrosynthesis relies on butenolide annulation onto the
spirocyclic intermediate 3, coupling of tetrahydrofuran 2 with
(17) (a) For a review of catalytic, asymmetric Diels-Alder reactions,
see: Dias, L. C. J. Braz. Chem. Soc. 1997, 8, 289. (b) For more recent
examples, see: Usuda, H.; Kuramochi, A.; Kanai, M.; Shibasaki, M. Org.
Lett. 2004, 6, 4387 and references cited therein.
(18) In general, (Z)-dienes are known to be poor reactants in Diels-
Alder reactions. However, some notable exceptions have been reported.
For example, see: Roush, W. J.; Barda, D. A. J. Am. Chem. Soc. 1997,
119, 7402.
(19) Betzer, J.-F.; Ardisson, J., Lallemand, J.-Y.; Pancrazi, A. Tetrahe-
dron Lett. 1997, 38, 13, 2279.
(20) For a review: Smith, N. D.; Mancuso, J.; Lautens, M. Chem. ReV.
2000, 100, 3257.
(21) (a) Evans, D. A.; Miller, S. J.; Lectka, T.; Matt, P. V. J. Am. Chem.
Soc. 1999, 121, 7559. (b) Evans, D. A.; Barnes, D. M.; Johnson, J. S.;
Lectka, T.; Matt, P. V.; Miller, S. J.; Murry, J. A.; Norcross, R. D.;
Shaughnessy, E. A.; Campos, K. R. J. Am. Chem. Soc. 1999, 121, 7582.
(c) For use of the triflate salt in related Diels-Alder reactions towards
pinnatoxins, see: Ishihara, J.; Horie, M.; Shimada, Y.; Tojo, S.; Murai, A.
Synlett 2002, 404. (d) For related studies which appeared when our work
was in progress, see: Brimble, M. A.; Crimmins, D.; Trzoss, M. ARKIVOC
2005, 1, 39.
(7) Hu, T.; Curtis, J. M.; Walter, J. A.; Wright, J. L. C. Tetrahedron
Lett. 1996, 37, 7671-7674.
(8) (a) McNabb, P.; Selwood A. I.; Holland P. T. J. AOAC Int. 2005,
88, 761. (b) MacKenzie, L.; Holland, P.; McNabb, P. Toxicon 2002, 40,
1321.
(9) Stirling, D. NZ J. Mar. Freshwat. Res. 2001, 35, 851.
(10) McCauley, J. A.; Nagasawa, K.; Lander, P. A.; Mischke, S. G.;
Semones, M. A.; Kishi, Y. J. Am. Chem. Soc. 1998, 120, 7647.
(11) Johannes, J. W.; Wenglowsky, S.; Kishi, Y. Org. Lett. 2005, 7, 3997.
(12) (a) Yang, J.; Cohn, S. T. Romo, D. Org. Lett. 2000, 2, 763. (b)
Ahn, Y.; Cardenas, G. I.; Yang, J.; Romo, D. Org. Lett. 2001, 3, 751.
(13) (a) Ishihara, J.; Miyakawa, J.; Tsujimoto, T.; Murai, A. Synlett 1997,
1417. (b) Tsujimoto, T.; Ishihara, J.; Horie, M.; Murai, A. Synlett 2002,
399.
(14) (a)White, J. D.; Wang, G.; Quaranta, L. Org. Lett. 2003, 5, 4109.
(b) White, J. D.; Wang, G.; Quaranta, L. Org. Lett. 2003, 5, 4983.
(15) Owens, T. D.; Hollander, F. J.; Oliver, A. G.; Ellman, J. A. J. Am.
Chem. Soc. 2001, 123, 1539.
(16) Brimble, M. A.; Trzoss, M. Tetrahedron 2004, 60, 5613.
5128
Org. Lett., Vol. 7, No. 23, 2005

and diene (E)-5 to afford spirolactam 3a as a single dia-
stereomer in 85% yield (Scheme 2). The enantioselectivity
of the alkyne.24 In fact, the s-cis conformation of diene (Z)-5
is lower in energy than s-trans (Figure 2). As expected, the
Scheme 2. Enantioselective Diels-Alder Reaction Leading to
Spirolactam 3A
Figure 2. Calculated relative energies for dienes (Z)- and (E)-5.
s-trans conformer of diene (E)-5 is more stable than the s-cis
conformer but the latter conformer is accessible at room
temperature. Taken together, these data suggest that the
unreactivity of diene (Z)-5 in the asymmetric reaction is due
to steric interactions with the catalyst-ligand complex
proceeding through a more concerted process (vide infra)
and not inaccessibility of the reactive s-cis conformer.
1
of the process was determined initially by H NMR using
chiral shift reagents22 and then independently verified by
Mosher ester analysis of a derived alcohol. Attempted
reduction of the Cbz-lactam led to partial deprotection of
the Cbz group demonstrating the congested nature of the
carbonyl carbon due to the R-quaternary center. In related
studies, we found that addition of n-BuLi led to exclusive
Cbz deprotection (vide infra, Scheme 3). This allowed a
chemoselective deprotection and conversion to the N-tosyl
lactam. Reduction to alcohol 11 with LiBH4 proceed
smoothly and this was converted to Mosher ester 12. Analysis
of the crude mixture by 19F NMR indicated that the Diels-
Alder reaction leading to spirolactam 3a had proceeded
with high enantioselectivity (95% ee). The absolute stereo-
chemistry was established by anamolous dispersion via
X-ray crystallographic analaysis of lactam 3b (Scheme 2
inset).
Figure 3. Favored (A) and unfavored (B) transition state arrange-
ments that rationalize the differential reactivity of dienes (E) and
(Z)-5 in the Diels-Alder process.
The Diels-Alder reaction leading to spirolactam 3a with
both high diastereoselectivity and enantioselectivity has a
few features worthy of comment. An interesting observation
is the complete unreactivity of diene (Z)-5. In contrast, we
previously determined that both geometric isomers of diene
5 could be used to provide the same diastereomer in high
yield using Et2AlCl as promoter.12a These results suggest
either different mechanisms for these Diels-Alder reactions
(∼concerted vs completely stepwise) or the potential for
olefin isomerization with the stronger Lewis acid Et2AlCl23
but not with the weakly Lewis acidic chiral catalyst.
Calculations suggest that the energy difference between s-cis
and s-trans conformers is not sufficient to rationalize the
unreactivity of diene (Z)-5 due to the low steric requirement
Another noteworthy feature is the high diastereoselectivity
resulting from an exo-selective reaction. This selectivity is
not unusual for this type of conformationally restricted (s-
cis), exocyclic alkene and has been attributed to favorable
dipole cancellation in the exo-transition state.25 In the present
enantioselective process, the preference for exo selectivity
is more pronounced due to interactions of the 2-siloxysub-
stituent of diene 5 with the chiral ligand in the endo transition
state (not shown, cf. Figure 3) as previously observed by
Evans in reactions of 3-methyl-1-acetoxybuadienes.21b The
(24) DFT calculations were performed using the Gaussian 03 program
package (B3LYP/6-31G* basis set + zpe). See the Supporting Information
for further details.
(22) See the Supporting Information for details.
(25) For Diels-Alder reactions with exocyclic alkenes leading to exo
selectivity, see: (a) Roush, W. R.; Brown, B, B. J. Org. Chem. 1992, 57,
3380. (b) Fotiadu, F.; Michel, F.; Buono, G. Tetrahedron Lett. 1990, 31,
4863.
(23) We have determined that pure diene (Z)-5 undergoes isomerization
to a ∼1:1 mixture of geometrical isomers within ∼25 min under the Diels-
Alder reaction conditions with Et2AlCl as determined by aliquot NMR.
Org. Lett., Vol. 7, No. 23, 2005
5129

stereochemistry of adduct 3a thus results from an exo
trajectory of the diene (E)-5 from the most accessible face
opposite to the tert-butyl substituent (sphere) of the catalyst-
dienophile complex (Figure 3A). The severe steric interaction
of diene (Z)-5 with the catalyst-ligand complex readily
explains its low reactivity in the enantioselective Diels-Alder
reaction (Figure 3B).
sequence to convert a N-tosyl group into a more labile
N-trifluoroacetyl group. The sequence builds on related
processes employed to cleave N-O bonds29 and applied by
us and others30 to cleave N-benzyloxy and N-tosyl-â-lactams.
Treatment of tosylamine 14a with trifluoroacetic anhydride
in the presence of base led to the formation of a putative
intermediate N-tosyl N-trifluoroacetyl amide, which was
subsequently reduced with samarium iodide to provide
trifluoroacetamide 14b. Cleavage of the trifluoroacetamide
with warm ammonium hydroxide led to cleavage of the
trifluoroacetyl group and concomitant cyclization to imine
15. The mildness of this deprotection method makes this an
attractive procedure for cleavage of primary N-tosyl amides.
Desilylation of the silylenol ether under basic conditions
provided spirocyclic ketoimine 16. The keto group present
in this spirocycle will enable linker attachment (e.g., oxime
or enamine formation) and allow coupling to a carrier protein
for antibody production.
In conclusion, we have completed an efficient synthesis
of the spirocyclic imine moiety of gymnodimine via a highly
diastereo- and enantioselective Diels-Alder reaction. Sub-
sequent selective monoalkylation of a derived N-tosyl lactam
with an alkyllithium furnished a ketosulfonamide precursor
to the cyclic imine. A mild deprotection, cyclization sequence
was developed to convert the sulfonamide to the cyclic imine.
Desilylation provided a ketone readied for conjugation and
evaluation as a possible immunogen for development of an
immunoassay for gymnodimine and congeners. These studies
and efforts toward the total synthesis will be the subject of
future reports.
We previously described a one-pot Hua reaction to prepare
the cyclic imine found in gymnodimine.12b Unfortunately,
this reaction could not be translated to the real system. We
ultimately found that direct addition of alkyllithiums to the
N-tosyl lactam 3b proceeded smoothly to provide monoalkyl-
ated products if careful attention was given to reaction
temperature.26 Thus, treatment of N-tosyl lactam 3b with (4-
methyl-3-pentenyl)lithium (13b)27 in the presence of TME-
DA at -20 °C afforded the desired amino ketone 14a in
good yield (Scheme 3). It is worth noting that the combina-
tion of an activated lactam with a reactive lithium species
enabled this addition despite the proximity to a quaternary
carbon center.28
Scheme 3. Formation of the Spirocyclic Ketoimine 16a
Acknowledgment. We thank the NIH (GM52964), the
Welch Foundation (A-1280), and Pfizer for support of these
investigations. We thank Dr. Nattamai Bhuvanesh (TAMU)
for obtaining the X-ray structure of lactam 3b. We thank
Dr. Huda Henry-Riyad for performing the DFT calculations
and Dr. Lisa Perez in the Laboratory for Molecular Simula-
tion (TAMU) for assistance.
Supporting Information Available: Experimental pro-
cedures and characterization data for compounds 3a, 3b, (E)-
5, 7, 10, 11, 12, 14a, 14b, 15, 16. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL051840R
Efforts to keep the silyl enol ether intact for the purposes
of the total synthesis, led us to develop a mild, single-pot
(28) For related examples of ring cleavage reactions of N-protected
lactams with nucleophiles, see: (a) Flynn, D. L.; Zelle, R. E.; Grieco, P.
A. J. Org. Chem. 1983, 48, 2424. (b) Giovannini, A.; Savoia, D.; Umani-
Ronchi, A. J. Org. Chem. 1989, 54, 228.
(29) Keck, G. E.; McHardy, S. F.; Wager, T. T. Tetrahedron Lett. 1995,
36, 7418.
(30) (a) Rzasa, R. M.; Shea, H. A.; Romo, D. J. Am. Chem. Soc. 1998,
120, 591. (b) Taggi, A. E.; Hafez, A. M.; Wack, H.; Young, B.; Drury, W.
J.; Lectka, T. J. Am. Chem. Soc. 2000, 122, 7831.
(26) The absence of dialkylated adducts is unexpected as addition of
alkyllithiums to simple R,R′-dimethyl-N-tosyllactams gave predominantly
alcohols resulting from double addition even when 1 equiv of alkyllithium
was employed (-78 f +23 °C).
(27) Hudlicky, T.; Govindan, S. V.; Frazier, J. J. Org. Chem. 1985, 50,
4166.
5130
Org. Lett., Vol. 7, No. 23, 2005