Spirocycloisomerization of tethered alkylidene glycocyamidines: Synthesis of a base template common to the palau'amine family of alkaloids

Article abstract of DOI:10.1002/anie.200462069

The facile spirocyclization of tethered bisalkylidene 1 suggests a general synthetic strategy for preparing alkaloids of the palau'amine family. The development and characteristics of this core model system are discussed.

Full text of DOI:10.1002/anie.200462069

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Natural Product Synthesis
biosynthetic postulates of Al-Mourabit and Potier.[4a] The
natural product is thought to be accessible from intermediate
5, in which reductions at C20 and C6 and a net dehydration to
install the C6 diaminal would be required to produce 1.[8] The
guanidine units in 5 are now both part of glycocyamidine[9]
rings. The right-hand spirocycle is reminiscent of an inter-
mediate proposed in the oxidative synthesis of dibromo-
phakellin from dihydrooroidin by Bꢁchi and Foley.[5] Anal-
ogously, albeit at a higher oxidation state, this portion of the
palauꢀamine structure could arise from internal trapping of C-
acyl iminium ion 6 (as indicated). Notably, ion 6 may be in
equilibrium with fragmented species 7, itself another C-acyl
iminium ion. The latter could be formed by the action of
chloronium ion on pseudosymmetric bisalkylidene 8. If this
were possible, hypohalite oxidations of 8 could initiate the
formation of two rings and four new stereocenters in a single
operation (8!5).
Spirocycloisomerization of Tethered Alkylidene
Glycocyamidines: Synthesis of a Base Template
Common to the Palauꢀamine Family of
Alkaloids**
Hugo Garrido-Hernandez, Masakazu Nakadai,
Marc Vimolratana, Qingyi Li, Thomas Doundoulakis,
and Patrick G. Harran*
In 1993 Scheuer, Kinnel, and co-workers described the
structural elucidation of palauꢀamine (1), an antimicrobial
principle isolated from extracts of the marine sponge
Stylotella aurantium.[1] This substance inhibits both bacterial
and fungal growth. It has the additional ability to block
stimulated T-cell proliferation in vitro, yet remains relatively
innocuous toward resting lymphocytes. The mechanism(s)
underlying this immunosuppressive property is not known. As
a step toward the preparation of molecules useful for its
exploration, we describe herein a new spirocyclization
process—one with potential to support a synthesis of palauꢀ-
amine as well as its constitutional relatives axinellamine (2)[2]
The stereochemical outcome of such a process depends on
several factors. However, our initial goal was to validate the
construction itself. Spirocyclization within intermediate 7
requires its tethered heterocycles to stack in parallel, which
forces both the external electrophile and internal nucleophile
to approach from trajectories peripheral to this self-assem-
bled unit (A/B). Carbon–carbon bond formation would
necessarily intervene. The reactivity sought is analogous to
that observed in the oxidation of 1,5-cyclooctadiene with
halogen to give bicyclooctane products (9!10, Scheme 2).[10]
In that case, the olefins are spatially constrained and
communicate transannularly during the reaction. In our
case, substrate conformation would be relied upon to dictate
comparable results.
We first needed to assess the behavior of an isolated
alkylidene glycocyamidine toward electrophilic halogen.
Heterocycle 11[11] was condensed with isobutyraldehyde in
the presence of N,N-dimethylethylene diamine monotosylate
as catalyst[12] to afford alkylidene 13 (Scheme 3). When this
material was treated with tBuOCl in glacial AcOH, epimeric
vicinal chloroacetoxylation products 14 were produced effi-
ciently. Angular acetates 14 are themselves unstable, although
methanolysis affords isolable congeners 15—materials that
have been fully characterized. These results confirm a desired
“enamine” type reactivity of the alkylidene in 13 towards
hypohalite.[13]
and massadine (3)[3]
.
Polycyclic bisguanidines 1–3 are members of a larger
alkaloid family whose precise biosynthetic origin is a subject
of speculation.[1b,4,5] Until recently, imidazole 4 was consid-
ered likely feedstock for the group.[4a,c] New observations
challenge that idea.[6] However, from the synthetic perspec-
tive,[7] casting structures 1–3 in terms of 4 remains a useful
exercise. Substructure 4 can be traced twice within polycycles
1–3 (Scheme 1). In each instance, the monomers are oriented
head-to-head with bonds a and b forming a common embed-
ded cyclopentane. The relative stereochemistries of substitu-
ents that emanate from this core differ in 1–3. Such spatial
variations offer a rationale for how conserved events, initiated
oxidatively after or during formation of the cyclopentane ring,
could diverge to the observed ring systems—those similarly
constituted but alternately linked.[4a] We report herein a core
substrate type prone to form bond a in a spirocyclization
applicable to all three targets.
Scheme 2 details how the reaction could operate in the
palauꢀamine case. This specific example parallels the general
We next examined if similar chemistry executed on a
dimeric substrate would result in spirocyclization. The
original plan was to retain the substitution pattern of 13 in
this dimer. The condensation of 11 with dialdehydes was
unproductive. However, we did observe that glycocyamidine
[*] H. Garrido-Hernandez, M. Nakadai, M. Vimolratana, Q. Li,
T. Doundoulakis, Prof. P. G. Harran
Department of Biochemistry
University of Texas Southwestern Medical Center at Dallas
Dallas, TX 75390-9038 (USA)
Fax: (+1)214-648-6455
E-mail: pharra@biochem.swmed.edu
[14]
12 could be dehydrogenated to alkylidene 13 with SeO2
(Scheme 3). Performing this reaction twice on tethered
bisglycocyamidine 16 appeared a means to access target 17
(Scheme 4A). A synthesis of 16 was developed that begins
with 1,4-dibromo-2-butyne and elaborates symmetrically in
two directions.[15] Interestingly, with 16 in hand, it was
apparent that the properties of this molecule were not those
intended. The substance readily formed insoluble aggregates.
Under conditions in which dehydrogenation to 17 was
possible, both materials were almost completely insoluble.
Conversion was low and the isolation of even small quantities
[**] Funding provided by the NIH (RO1-GM60591), the Robert A. Welch
Foundation, and unrestricted research awards from AstraZeneca,
Eli Lilly, and Pfizer. M.N. acknowledges the JSPS for a postdoctoral
fellowship. P.H. is a fellow of the Alfred P. Sloan Foundation. We are
grateful to Dr. Radha Akella (Department of Biochemistry, UTSW)
for her expert crystallographic analyses and insight.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200462069
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765

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Scheme 1. Palau’amine (1) and its constitutional relatives axinellamine A (2) and massadine (3) can be viewed as dimeric composites of
conserved subunit 4.[4a]
Scheme 2. Retrosynthesis of palau’amine identifies a core spirocyclization relevant to structural types 1–3.
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of 17 in pure form was a painstaking exercise. Compound 17
has the functionality to test our key idea, but is otherwise a
poor model. The goal was a substrate whose heterocyclic
termini could associate internally to facilitate the cyclization
illustrated in Scheme 2. Treatment of 17 with hypochlorite
initiated no such event,[16] probably reflecting, as does its
solubility profile, an unwanted preference for bimolecular
association. The arrangement of 17 in the crystalline state
supports this idea. The unit cell contains four molecules of 17
(8 asymmetric units, space group C21/c), each in extended
conformation with their glycocyamidine rings interacting
bimolecularly through multiple H bonds (Scheme 4B).[17]
Another design was needed—one in which the guanidine
units are more properly managed. As a means to disrupt
bimolecular hydrogen bonding, we considered repositioning
the N1 benzyl unit on each heterocycle to N2. Alone, the
change was synthetically awkward, but the incorporation of
both N2 and N3 into a 2,4-benzodiazepine appeared work-
able. Compound 18 (Scheme 5) became the target. Computer
simulations suggested this molecule would adopt compact
globular forms in polar solvents, positioning the alkylidene
units appropriately for subsequent oxidative spirocyclization.
With N1 unsubstituted, Z-alkylidene geometry in 18 was
expected to be thermodynamically favored. How to install
this unsaturation was the pivotal question. We eventually
adopted an approach based on basic degradation of sulfona-
mides. Overman and Trenkle had shown that a potassium
enolate of 19 fragments with loss of the 2-trimethylsilylethyl-
sulfinate ion, affording an imine product that tautomerizes to
enamine 20 in situ.[18] A related transformation, executed
twice on bissulfonamide 23, was con-
Scheme 3. Regioselective vicinal chloroacetoxylation of an alkylidine
glycocyamidine. Reaction conditions: a) isobutyraldehyde (1.2 equiv),
N,N-dimethylethylene diamine (30 mol%), pTsOH (30 mol%), DMF,
microwave heating (1508C), 50 min (41%, E/Z=5:1); b) SeO2,
tBuOH, 758C, 2 h (75%); c) tBuOCl (1.1 equiv, neat), glacial AcOH,
room temperature, 1 h (>80%-1H NMR); d) silica gel, MeOH, room
temperature, 6 h (95%). Ts=toluene-p-sulfonyl; DMF=N,N-dimethyl-
fomamide.
sidered a method to produce 18.
Whereas the ring system in 23 was
unknown, it could be prepared con-
cisely. The route used to synthesize 16
was adapted to access the 2,7-diami-
nosuberic acid derivative 21.[15] Con-
densing this material with a twofold
excess of o-xylyldiamine-derived
methylisothiourea 22[19] provided 23
directly. The seco amides presumably
formed transiently in the reaction
cyclized spontaneously, with ejection
of methanethiol at each end of the
molecule.
With 23 available, we examined
its response to base. Exposure to
KHMDS caused degradation. How-
ever, when the compound was treated
with DBU in DMF, monoalkylidene
26 formed rapidly (Scheme 6). This
material was isolated without inci-
dent. When 26 was reexposed to
DBU, two new products emerged in
high yield. Surprisingly, neither was
Scheme 4. Bisalkylidene 17 forms highly insoluble aggregates and proves to be an intractable
model system. A) Reaction conditions: a) SeO2 (1.4 equiv), tBuOH, 708C, 4 h (5–7%). B) X-ray
crystallography indicates the glycocyamidine rings in 17 associate bimolecularly through extensive
hydrogen bonds. Partial unit-cell occupancy is shown (space group C21/c) in ORTEP format (50%
probability thermal ellipsoids, hydrogen atoms omitted for clarity).
found to be bisalkylidene 18. Rather,
they proved to be geometric isomers
of spirocycloisomerization product
24.[17] When the reaction was per-
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with signals corresponding to rearrangement products 24. The
system siphoned completely to the latter within 10 h.
We noted that the ratio of isomers 24 changed during the
experiment. It was likewise possible that the second alkene
signal reflected equilibration of 26 with an isomeric mono-
alkylidene and that target bisalkylidene 18 was not observed,
or perhaps even formed, under these conditions. To address
this question, we synthesized nonsymmetric monosulfona-
mide 25[15] and subjected it to identical elimination conditions.
Like 23, 25 rapidly converted into a monoalkylidene (namely,
27). Notably, carbamate 27 was inert to further degradation
and did not equilibrate with a second product over time,
which implies the second olefinic signal in the original
experiment is itself reflective of symmetric bisalkylidene 18.
This is precisely the behavior we had hoped to see.
Whereas 18 was designed to participate in a spirocyclization
initiated by hypohalite, the molecule is apparently so well-
poised for the reaction that a proton is sufficient provocation.
Compound 18 cannot be isolated, and available data does not
distinguish whether carbon–carbon bond formation occurs
through: 1) C directly, perhaps catalyzed by salt 28; 2) imino
tautomer D; or 3) inner salt E, the product of net proton
transfer from C12 to C12’ within D. Nevertheless, the
outcome validates the central tenet of our approach to 1–3
(Scheme 1 and Scheme 2). Moreover, the markedly different
reactivities of bisalkylidenes 17 and 18, with the former
showing no inclination to cycloisomerize (Scheme 4), suggest
the propensity for cyclization is tunable. In substrates
substituted appropriately to complete the natural products,
there should be ample opportunity to initiate analogous
spirocyclization through chlorination (or oxygenation in the
case of 3). Attempts to synthesize such substrates while
attending to stereochemical parameters associated with the
larger problem are ongoing.
Received: September 22, 2004
Published online: December 28, 2004
Please note: Minor changes have been made to this manuscript since
its publication in Angewandte Chemie Early View. The Editor.
Keywords: cyclization · guanidines · heterocycles · natural
.
products · total synthesis
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1
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768
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