Further insight from model experiments into a possible scenario concerning the origin of manzamine alkaloids

Article abstract of DOI:10.1002/anie.200800488

(Chemical Equation Presented) Without the isolation of intermediates, 5,6-dihydropyridinium salts and their nitrile derivatives were prepared as shown in the scheme. Cyclization in the presence of an aminopentadienal led to the formation of the AB ring system of the halicyclamines, or, under alternative conditions, the fused AB ring system of the manzamines. Tf= trifluoromethanesulfonyl.

Full text of DOI:10.1002/anie.200800488

Communications
DOI: 10.1002/anie.200800488
Natural Products
Further Insight from Model Experiments into a Possible Scenario
Concerning the Origin of Manzamine Alkaloids
Jean-Charles Wypych, Tuan Minh Nguyen, Philippe Nuhant, Michel BØnØchie, and
Christian Marazano*
The story of the manzamine alkaloids, natural products
extracted from sponges of the order Haplosclerida, began in
1986 with the discovery of manzamine A, followed by the
identification of a number of related derivatives.[1,2] Halicy-
clamine A, keramaphidin B, and the cyclostellettamines are
representative molecules of this series (Scheme 1).
imentally and, remarkably, provided access to keramaphi-
din B.[4] However, the efficiency of this kind of strategy was
limited as a result of a competing dihydropyridine redox
process. Furthermore, the model proposed by Baldwin and
Whitehead did not permit access to the manzamine skeleton.
Studies towards the synthesis of cyclostellettamines led us to
propose an alternate scenario based upon the chemistry of
aminopentadienal species 2, which could be formed from
malonaldehyde instead of acrolein.[5] This modification of the
initial proposal is summarized in Scheme 2. Experimental
studies revealed that only regioisomers of aminopentadienals
2 were available (isomers 14, see Scheme 5), but that these
regioisomers do add to salts 1 to deliver the AB ring system of
halicyclamine A(via analogues of 3), whereas the condensa-
tion of aminopentadienoic esters enabled the synthesis of
analogues of the AB ring system of manzamine A (via
analogues of 4).
Herein, we report further results related to the chemistry
depicted in Scheme 2. We describe a new method based on
the original proposal of Baldwin and Whitehead[3] for the
preparation of salts 1,[6] as well as a Chichibabin-like process
for the formation of a common intermediate, which, depend-
ing on the reaction conditions, rearranges to give a ring
system equivalent to the AB ring system of 3 or the AB ring
system of 4.
To our knowledge, the condensation of an unsaturated
aldehyde, such as acrolein, with aldehydes and amines to give
dihydropyridinium salts 1 has no equivalent in the literature.
However, as acrolein can be viewed as the aldol-condensation
product of acetaldehyde and formaldehyde, the reaction
could be considered to be related to the Chichibabin synthesis
of pyridines.[7] Indeed, this multicomponent procedure,
known as early as the beginning of the 20th century, involves
the condensation of three equivalents of an aldehyde with
ammonia at high temperature to give 2,3,5-trisubstituted
pyridines (Scheme 3, R1 = H). If R1 is an alkyl group, the
reaction produces the corresponding pyridinium salts. In fact,
the intermediates of this reaction are believed to be dihy-
dropyridinium salts 7, or even dihydropyridines 8, which are
oxidized spontaneously under the rather harsh and acidic
reaction conditions. The initial main drawbacks of this process
were the necessary use of the same aldehyde, which does not
allow variation of the R groups,[8] and the difficulty in
stopping the reaction at the dihydropyridinium stage.[9] The
selective synthesis of salts 1 by the route proposed in
Scheme 2 could thus be viewed as a useful modified
Chichibabin synthesis of six-membered nitrogen heterocycles.
After some unsuccessful attempts, we have now found a
sequence that mimics this process (Scheme 4). The Strecker
Scheme 1. Some natural products representative of the manzamine
family of alkaloids.
The common source of this new class of alkaloids and the
structural similarities of these compounds suggest the exis-
tence of a unique biosynthetic pathway, in a particularly
striking example of “nature diversity-oriented synthesis”. The
biosynthetic pathway is unknown to date; however, in 1992,
Baldwin and Whitehead put forward the hypothesis that
dihydropyridinium salts 1 formed through the condensation
of aminoaldehydes with acrolein may serve as key
intermediates(Scheme 2).[3] This hypothesis was tested exper-
[*] Dr. J.-C. Wypych, Dr. T. M. Nguyen, P. Nuhant, Dr. M. BØnØchie,
Dr. C. Marazano
Institut de Chimie des Substances Naturelles, CNRS
Avenue de la Terrasse, 91198 Gif-sur-Yvette Cedex (France)
Fax : (+33)1-6907-7247
E-mail: marazano@icsn.cnrs-gif.fr
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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a one-pot procedure. Moreover, the use of Zn-
(OTf)2 as a Lewis acid delivered the pyridinium
salt 16a or 16b directly in 25% overall yield (in
three steps from propionaldehyde). Finally, we
confirmed that the reduction of salt 16b afforded
the previously reported halicyclamine model com-
pounds 17b and 18b. Thus, the new procedure is
shorter than our previous approach and enables
rapid access to pyridinium derivatives 16,[14] which
are of interest for the total synthesis of the
halicyclamine class of natural products, as well as
sarains 1–3 (if one considers the aminonitrile
functionality as a masked iminium cation).[15]
The treatment of the pure adduct 15a with
acetic anhydride, followed by reduction with excess
Scheme 2. Proposed biosynthetic route (modified from the original hypothesis of
Baldwin and Whitehead[3]).
NaBH(OAc)3, afforded
a new rearrangement
Scheme 3. The Chichibabin synthesis of pyridines and pyridinium
salts. LA=Lewis acid.
adducts 9a–c were obtained in quantitative yield from the
corresponding aldehydes and benzylamine hydrochloride,
and condensed with excess acrolein to give the crude
aldehydes 10a–c, again in quantitative yield. These aldehydes
are very sensitive to acids and bases (retro-Michael process),
but were found to undergo smooth cyclization at ambient
temperature in the presence of AgBF4 (1 equiv) to give salts
11a–c.[10] Compounds 11a–c were not isolated, but were
observed in the 1H NMR spectra of the crude reaction
mixture. Their reduction with NaBH4 afforded tetrahydro-
pyridines 12a–c in 45–65% overall yield, whereas the
trapping of 11a–c with KCN afforded aminonitriles 13a–c
in 37–52% overall yield (in four steps from the starting
aldehydes). Interestingly, the treatment of 10a with a catalytic
amount of Zn(OTf)2 resulted in the direct formation of
aminonitrile 13a in 33% overall yield (in three steps from the
starting aldehyde). This new synthesis of aminonitrile deriv-
atives 13, which complements the traditional synthetic route
(the Polonovski–Potier oxidation of tetrahydropyridines),[6,11]
was thus carried out without the isolation of any intermedi-
ates.
The treatment of aldehyde 10a with ZnBr2 in the presence
of aminopentadienal 14a[12] resulted in the formation of 15a
(Scheme 5),[13] which was recovered after flash chromatog-
raphy on alumina in 22% overall yield (in three steps from
propionaldehyde). This result demonstrated that it was
possible to couple cyclization to the dihydropyridinium salt
11a with the nucleophilic addition of an aminopentadienal in
Scheme 4. A new “Chichibabin-like” three-component route to tetrahy-
dropyridines 12 and dihydropyridinium salts 11 or their cyano-substi-
tuted analogues 13: a) KCN, H2O/MeOH, room temperature, quantita-
tive; b) acrolein, neat or in CH2Cl2, room temperature, quantitative;
c) AgBF4, CH2Cl2/THF, room temperature; d) NaBH4, MeOH (see
table for yields of 12); e) KCN, H+ (see table for yields of 13);
f) Zn(OTf)2, THF, 758C, 24 h, 33% for 13a. TBDMS=tert-butyldime-
thylsilyl, Tf=trifluoromethanesulfonyl.
product, the bicyclic derivative 19, as a mixture of diastereo-
isomers in a 4:1 ratio (the orientation of the OAc group in the
two diastereoisomers is not known) and in 44% yield.[16] This
derivative possesses an AB ring motif related to that of the
manzamines. The treatment of 19 under acidic conditions
afforded the known dienal 20.[5a]
Aplausible mechanism for the rearrangement to give the
intermediate iminium salt 23 is depicted in Scheme 6.
Regioselective acylation at the oxygen atom of the amino-
pentadienal functionality of 15a would result in ring opening
to give iminium salt 21. Further acylation at the nitrogen atom
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Communications
The use of these activated species in Pictet–Spengler
reactions will be reported in the near future.
Received: January 30, 2008
Published online: June 11, 2008
Keywords: aldehydes · biomimetic synthesis · cycloaddition ·
.
natural products · nitrogen heterocycles
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Scheme 5. One-pot access to adduct 15a and halicyclamine model com-
pounds 16a,b: a) ZnBr2 (0.5 equiv), DCE, 758C, 15 h, 22%; b) Zn(OTf)2,
25% for 16a, 25% for 16b; c) NaBH4, MeOH, 40% (17b+18b).
DCE=1,2-dichloroethane.
[2] The identification of a sponge-associated bacterium respon-
sible for the biosynthesis of manzamine Aas an undescribed
species of Micromonospora (actinomycetes) suggests
a
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Scheme 6. Rearrangement of adduct 15a in the presence of acetic
anhydride: a) (CH3CO)2O (3 equiv), DCE, 608C, overnight; b) NaBH-
(OAc)3, THF, 44% from 15a; c) aqueous HCl (pH 1), EtOH, reflux,
overnight, 55%. Bn=benzyl.
[6] D. S. Grierson, M. Harris, H.-P. Husson, J. Am. Chem. Soc.
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[8] For the variation of one R group and an application of the
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milder conditions: L.-B. Yu, D. Chen, J. Li, J. Ramirez, P. G.
Wang, S. G. Bott, J. Org. Chem. 1997, 62, 208 – 211; for the
formation of dihydropyridines 8, see: T. M. Patrick, J. Org.
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in the side chain, followed by cyclization of the resulting
activated 1,3-diene 22, would then afford the bicyclic inter-
mediate 23.
In conclusion, these results with simplified models fully
demonstrate the feasibility of the chemistry depicted in
Scheme 2. Aparticularly interesting feature of this approach
is the selective formation of either the halicyclamine or the
manzamine skeleton through the condensation of five differ-
ent species in a consecutive manner.[17] The synthetic
sequence generates diversity, as the variation of four different
substituents is possible. The extension of this procedure to the
synthesis of macrocyclic derivatives is now under investiga-
tion. These new results further illustrate the synthetic interest
of aminopentadienal derivatives, introduced previously by us
to the arena of natural product synthesis.[5c,18] In particular,
the formation of derivative 19 revealed that these species can
be activated selectively by treatment with acid anhydrides.
[10] We propose the following mechanism for the formation of salts
11: For a related recent example, see: M. Movassaghi, B. Chen,
Angew. Chem. 2007, 119, 571 – 574; Angew. Chem. Int. Ed. 2007,
46, 565 – 568.
[11] For examples of recent applications, see references [4,5] and
also: a) J. Kim, R. J. Thomson, Angew. Chem. 2007, 119, 3164 –
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3166; Angew. Chem. Int. Ed. 2007, 46, 3104 – 3106; b) A. A.
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8883 – 8889.
[16] The geometry of the external double bond was assigned on the
basis of an observed NOE effect between the two olefinic
hydrogen atoms.
[17] Keramaphidine B could be viewed as derived from dihydropyr-
idinium salts 1[4] or from the halicyclamine skeleton.[5b] The
pyridinium-salt motif of the cyclostellettamines can be viewed as
derived from the oxidation of dihydropyridinium salts 1[4] or
aminopentadienals 2.[13]
[12] We recently reported the condensation of aldimines with
vinamidinium salts as a new route to aminopentadienal deriv-
atives, such as 14. This process mimics the suggested formation
of intermediates 2 (Scheme 2): J.-C. Wypych, T. M. Nguyen, M.
Bꢀnꢀchie, C. Marazano, J. Org. Chem. 2008, 73, 1169 – 1172.
[13] For previous results from our laboratory, reaction mechanisms,
and structure determination, see reference [5c].
[14] The pyridinium-salt analogue of halicyclamine Awas isolated
recently from a sponge: S. Matsunaga, Y. Miyata, R. W. M.
van Soest, N. Fusetani, J. Nat. Prod. 2004, 67, 1758 – 1760.
[18] For a related recent example, see: A. M. Kearney, C. D.
Vanderwal, Angew. Chem. 2006, 118, 7967 – 7970; Angew.
Chem. Int. Ed. 2006, 45, 7803 – 7806.
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