Synthesis of marine alkaloids from the oroidin family

Article abstract of DOI:10.1002/anie.200801793

Oxidation with a difference: The complexity of the oroidin alkaloid family of marine natural products is generated biosynthetically from one simple alkene precursor, but has left the synthetic community with problems. The first total synthesis of the tetracyclic axinellamines (see structure) was recently completed. Key to the success was the chemoselective oxidative functionalization of imidazol(in)e rings (green). (Chemical Equation Presented).

Full text of DOI:10.1002/anie.200801793

Angewandte
Chemie
DOI: 10.1002/anie.200801793
Alkaloids
Synthesis of Marine Alkaloids from the Oroidin
Family**
Hans-Dieter Arndt* and Matthias Riedrich
alkaloids · natural products · oroidin ·
synthetic methods · total synthesis
P
olyheterocyclic, nitrogen-rich alkaloids probably rank
sceptrin (1), the axinellamines (2 and 3), palauꢀamine (4), and
ageliferin (5). These marine natural products arise from one
precursor, the rather inconspicuous pyrrolo-imidazole alkene
oroidin (6) first identified in 1971.[2] Dimerization of 6 and
consecutive functionalizations are currently believed to give
rise to this impressive array of densely functionalized, highly
oxidized, polycyclic oroidin alkaloids.[1e,3] The similarity of
these molecules and their often simultaneous occurrence is
indicative of common biosynthetic pathways, and further-
more suggests the generation of a divergent natural product
compound “library” from one simple precursor.[4]
among the most challenging synthetic targets in organic
synthesis. In this regard the oroidin class of alkaloids has
received much attention recently (Scheme 1),[1] among them
Until lately, palauꢀamine (4) did not fit well into this
unifying picture, mostly for stereochemical reasons. In the
original work the junction of the two five-membered rings had
been assigned as cis.[5] However, thorough spectroscopic
investigation[6,1e] was recently complemented by synthesis
(vide infra),[7] which likewise suggested the C-11/C-12 ring
fusion in 4 to be in the thermodynamically less stable trans
configuration (as shown in Scheme 1). This structural revision
now makes palauꢀamine a full member of the oroidin alkaloid
group and raises hope that integrative strategies for their total
synthesis might be developed in the near future.
All these pyrrole–imidazole alkaloids feature a four-, five-,
or six-membered central carbocyclic ring and an individual
connectivity of the pendant side chain heterocycles. These
unique patterns have stimulated many synthetic efforts and
led to distinct solutions for each of the scaffolds (Scheme 2).[8]
In one early hypothesis on the biosynthesis, the six-membered
ring of the ageliferins 7 was proposed to arise from a [4
2] cycloaddition.[3a] This was implemented in synthesis first by
Ohta et al. (8!7).[9] A MnIII-promoted radical cascade
annulation from the imidazolone 9 was developed by Chen
and Tan.[10] In their total synthesis of 5, Baran et al. success-
fully implemented a double ring-enlargement of the four-
membered-ring precursor sceptrin (1) to 5 under high-
temperature conditions.[11] The sceptrin scaffold 10 itself has
been elaborated by [2 + 2] photocycloadditions, for instance
from (E)-1,4-dichloro-2-butene and maleic anhydride[12] or by
fragmentation of the photochemically accessible oxaquadri-
cyclane 11.[13]
Scheme 1. Selected oroidin alkaloids of marine origin. Oroidin-derived
central bonds are highlighted in red.
[*] Dr. H.-D. Arndt, M. Riedrich
Technische Universität Dortmund
Fakultät Chemie, Otto-Hahn-Strasse 6, 44221 Dortmund
and
Max-Planck-Institut für Molekulare Physiologie
Abteilung Chemische Biologie, Otto-Hahn-Strasse 11
44227 Dortmund (Germany)
The biggest challenge is probably posed by the fully
substituted five-membered-ring scaffolds 12 present in the
axinellamines and palauꢀamines. For these, biomimetic linear
assembly, ring enlargements of four-membered sceptrin-like
precursor 10, or oxidative ring contractions of six-membered
Fax: (+49)231-133-2498
E-mail: hans-dieter.arndt@mpi-dortmund.mpg.de
[**] Research by the authors was supported by the Deutsche For-
schungsgemeinschaft (Emmy-Noether young investigator grants
AR493-1 and -2 to H.D.A.) and the Fonds der Chemischen Industrie.
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Highlights
20 in four standard transformations.[17]
Re-installation of PMB protecting
a
group on the secondary alcohol and
ozonolysis provided diketone 21, which
was a,w-dibrominated via the bis-silyle-
nol ether. Intramolecular aldol addition
under solvent-free conditions, exchange
of the most reactive bromide for a more
stable chloride, and deprotection gave
diol 22. To install the third stereogenic
substituent, the tertiary hydroxy group
had to be eliminated and the secondary
hydroxy group displaced by Clꢀ, which
was achieved in a one-pot reaction with
SO2Cl2. Regioselective displacement of
the bromide substituents with protected
guanidine was then achieved after Luche
reduction of the enone carbonyl (!23).
Upon reoxidation to the enone, spirocyc-
lization occurred, but high temperatures
were found to be essential to favor the
correct diastereomer (1.3:1). The 2-ami-
noimidazole was then introduced by dis-
placement and in situ condensation with
Boc-guanidine, and spirocycle 24 could be
purified after derivatization (Boc2O).
Only 16 steps were required to generate
Scheme 2. Some synthetic routes to the related core structures of the oroidin alkaloids.
Bn=benzyl, Boc=tert-butoxycarbonyl, TIPS=triisopropylsilyl, TBS=tert-butyldimethylsilyl.
ageliferin-like precursors 7 have been proposed.[1,3] While the
ring-enlargement 10!12 still waits to be realized experimen-
tally, ring-contractions 7!12 have been executed with con-
siderable success.[14] However, “abiotic” syntheses of scaffolds
12 have proven to be tantamount.[1] For instance, Carreira
et al. already reported the first enantioselective synthesis of
the axinellamine core 12 by desymmetrization of anhydride
13 in 2000.[15] Recently, Baran et al. completed the first total
synthesis of axinellamines[16] (2, 3) in racemic form by using a
ring contraction of the cyclohexene 14.[17]
the protected axinellamine precursor 24 from 14 with two
heterocycles already installed (overall yield 0.7%).
Starting point for the enantioselective synthesis[15] of 19
was the readily available Diels–Alder adduct 15 (Scheme 3),
which was converted to the sterically more congested
anhydride 13 in sixsteps. Desymmetrization of meso-13 to
the chiral monoester 16 was achieved in 93% ee by using the
method of Bolm et al.,[18] and epimerization of the more
acidic ester a-CH group followed by reduction, introduction
of the nitrogen substituents by Mitsunobu displacement, and
chemoselective degradation of the vinyl group, provided
aldehyde 17. A third nitrogen atom was now introduced by
oxidation of 17 and Curtius degradation, and ozonolysis of the
alkene followed by thermodynamically driven epimerization
delivered the all-trans dialdehyde 18. The aldehyde group
proximal to the carbamate nitrogen atom could now be
regioselectively converted into its monoacetal, and degrada-
tion of a Barton ester derived from the remaining aldehyde
function installed the secondary chloride 19 stereoselectively.
Overall, 21 steps gave access to the axinellamine scaffold 19
with complete stereocontrol in 6.4% yield from 15.[15]
Scheme 3. Enantioselective synthesis of the axinellamine core (Carreira
et al.).[15] a) Quinine, MeOH, CCl4, toluene; b) lithium diisopropyl-
amide, Et2O; c) LiAlH4, Et2O; d) phthalimide, DEAD, PPh3; e) 5%
OsO4·(DHQD)2Pyr, NMO, THF/H2O; f) NaIO4, K2CO3, THF/H2O;
g) NaClO2, DMSO, tBuOH/H2O; h) (COCl)2, CH2Cl2; i) NaN3, DMSO;
j) benzene, reflux; k) LiOBn, THF; l) O3, CH2Cl2, then PPh3, K2CO3;
m) 1,3-propanediol, PPTS, Et2O; n) KMnO4, tBuOH/H2O; o) thiopyr-
idine-N-oxide, EDC, DMAP, CCl4. PhtN=phthalimido, Cbz=benzylox-
ycarbonyl, DEAD=diethylazodicarboxylate, (DHQD)2Pyr=hydro-
quinidine-2,5-diphenyl-4,6-pyrimidindiyl diether, NMO=4-methylmor-
pholine-N-oxide, PPTS=pyridinium-p-toluenesulfonate, EDC=N’-(3-di-
methylaminopropyl)-N-ethylcarbodiimide, DMAP=4-dimethylamino-
pyridine.
The first completed total synthesis of the axinellamines[16]
followed a very straightforward approach (Scheme 4). First,
the racemic Diels–Alder product 14 was elaborated to diazide
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The same in situ protection probably helped with the
following transformation as well: The regioselective oxidation
with the AgII complex 26, which gave tetracycle 27 in only
four steps and 40% yield from 24. While it is not uncommon
that methylene groups a to nitrogen atoms can be oxidized,
the regioselectivity and control of overoxidation in this
polyfunctionalized scaffold remains breathtaking! The ax-
inellamides 2 and 3 were then swiftly reached by reduction of
the azide groups followed by acylation with suitable pyrrole
building blocks. The first total synthesis of 2 and 3 was thereby
completed in 22 steps from 14 (overall yield 0.2%).
What remains to be solved is the palauꢀamine problem.
Overman et al. recently provided synthetic material proving
the newly assigned stereochemistry of 4 (Scheme 6). Key to
the synthesis[7b] of the epi-palauꢀamine scaffold 28 was a
distinctive bicyclization, which was achieved by an intra-
molecular 1,3-dipolar cycloaddition, to give the tetracycle 30
from the dihydropyrrole 29 in 70% yield.[7a] This elegant
transformation fixed three contiguous stereocenters, two of
Scheme 4. Diastereoselective synthesis of the axinellamine core (Baran
et al.).[16,17] a) LiAlH4, THF; b) MsCl, pyridine; c) NaN3, DMF, 1008C;
d) TBAF; e) PMBCl, NaH, DMF; f) O3, MeOH; g) TMSOTf, EtNiPr2,
then NBS; h) SiO 2, no solvent, 478C; i) LiCl, DMF; j) 10% TFA;
k) SO2Cl2, 2,6-lutidine, CH2Cl2; l) NaBH4, CeCl3, MeOH; m) N,N’-bis-
Boc-guanidine, DBU, DMF; n) IBX, benzene, reflux; o) Boc-guanidine,
THF, reflux; p) Boc2O, NEt3, cat. DMAP, CH2Cl2. R=COOMe, R’=Boc,
PMB=para-methoxybenzyl, MsCl=methylsulfonyl chloride, TBAF=te-
trabutylammonium fluoride, TMS=trimethylsilyl, NBS=N-bromosuc-
cinimide, TFA=trifluoroacetic acid, DBU=1,8-diazabicyclo-
[5.4.0]undec-7-ene, IBX=o-iodoxybenzoic acid.
Only two oxidations now separated 24 from the axinella-
mide target connectivity (Scheme 5).[16] Baran et al. realized
that the imidazole double bond might be selectively oxidi-
zed,[1c] and indeed found that after Boc deprotection of 24 the
respective diol could be formed by DMDO, which on
treatment with TFA condensed to aminal 25. It can be
speculated that in the aqueous reaction medium unwanted
oxidations of the nitrogen atoms are suppressed by proto-
nation.
Scheme 6. Synthesis of the fully elaborated epi-palau’amine scaffold
(Overman et al.).[7] a) Thiosemicarbazide, EtOH, 1108C; b) SmI2, THF/
MeOH; c) MeI, EtNiPr2, DMAP, CH2Cl2; d) TeocCl, EtNiPr2, CH2Cl2;
e) Cbz-NCS, CH2Cl2; f) EDC, oNBn-NH2, EtNiPr2, CH2Cl2; g) 10% TFA;
h) TeocCl, EtNiPr2, CH2Cl2; i) NaBH4, MeOH/THF; j) Ac2O, pyridine,
DMAP; k) TBAF, THF; l) IBX, DMSO; m) NaBH4, MeOH, 08C;
n) mCPBA, CH2Cl2; o) NH3, CH2Cl2, ꢀ788C; p) hn, dioxane; q) H2,
Pd/C, aq dioxane. Sem=2-(trimethylsilyl)ethoxymethyl, Teoc=trimeth-
ylsilylethyloxycarbonyl, oNBn=ortho-nitrobenzyl, NCS=N-chloro-
succinimide, mCPBA=m-chloroperoxybenzoic acid.
Scheme 5. Completion of the axinellamine total synthesis (Baran
et al.).[16] a) 67% TFA; b) DMDO, H2O, 08C, c) 26, H2O, 508C; d) 1,3-
propanedithiol, NEt3, MeOH; e) 4,5-dibromopyrrole-2-yl-trichlorometh-
yl ketone, EtNiPr2, DMF, 458C. DMDO=2,2-dimethyldioxirane.
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Highlights
ꢀ
[2] S. Forenza, L. Minale, R. Riccio, E. Fattorusso, J. Chem. Soc. D
1971, 1129 – 1130.
them quaternary. Cleavage of the assisting N N bond,
thiohydantoin protection, and amine acylation provided the
thiourea 31, which was transformed into the protected
imidazolone, doubly reduced, and protected to give the bis-
aminal 32. The use of TBAF induced the removal of the Teoc-
and TBS protecting groups and closure of the ketopiperazine
ring as well, and inversion of the secondary alcohol by an
oxidation–reduction sequence provided the hemiaminal 33.
Conversion of the isothiourea into the guanidine and
deprotection yielded the cis-configured compound 28 in
17 steps and 14% total yield from 29 (ca. 31 steps and 2.4%
yield from monoprotected 2-butene-diol), which was found by
NMR spectroscopy to differ significantly from palauꢀamine
(4). Nevertheless, this synthesis is characterized by thorough
optimization and consecutive elegant buildup of the difficult
heterocycles, and should be instrumental for future total
synthesis efforts.
[3] a) R. Kinnel, H.-P. Gehrken, R. Swali, G. Skoporowski, P. J.
Scheurer, J. Org. Chem. 1998, 63, 3281 – 3286; b) A. Al-
Mourabit, P. Potier, Eur. J. Org. Chem. 2001, 237 – 243; c) H.
Garrido-Hernandez, M. Nakadai, M. Vimolratana, Q. Li, T.
Doundolakis, P. G. Harran, Angew. Chem. 2005, 117, 775 – 779;
Angew. Chem. Int. Ed. 2005, 44, 765 – 769.
[4] a) F. C. Schröder, J. J. Farmer, A. B. Attygale, S. R. Smedley, T.
Eisner, J. Meinwald, Science 1998, 281, 428 – 431; b) R. Brein-
bauer, I. Vetter, H. Waldmann, Angew. Chem. 2002, 114, 3002 –
3015; Angew. Chem. Int. Ed. 2002, 41, 2878 – 2890; c) E. Gravel,
E. Poupon, Eur. J. Org. Chem. 2008, 27 – 42.
[5] R. B. Kinnel, H.-P. Gehrken, P. J. Scheurer, J. Am. Chem. Soc.
1993, 115, 3376 – 3377; see also ref. [3a].
[6] A. Grube, M. Köck, Angew. Chem. 2007, 119, 2372 – 2376;
Angew. Chem. Int. Ed. 2007, 46, 2320 – 2324.
[7] a) J. D. Katz, L. E. Overman, Tetrahedron 2004, 60, 9559 – 9586;
b) B. A. Lanman, L. E. Overman, R. Paulini, N. S. White, J. Am.
Chem. Soc. 2007, 129, 12896 – 12900.
The oroidin alkaloids have helped to revitalize and
advance heterocyclic chemistry by providing a stimulus for
new chemical developments and elegant methodology.[1] With
the stereochemical assignment of palauꢀamine (4) now
proven, and the first total synthesis of the axinellamines (2
and 3) completed, the stage is set for new developments. Late-
stage oxidations such as innovatively used by Baran et al.
provide great opportunities for designing new synthesis
routes and for the generation of complexity from common
[8] For more complete coverage see refs. [1].
[9] I. Kawasaki, N. Sakaguchi, N. Fukushima, N. Fujioka, F. Nikaido,
M. Yamashita, S. Ohta, Tetrahedron Lett. 2002, 43, 4377 – 4380.
[10] X. Tan, C. Chen, Angew. Chem. 2006, 118, 4451 – 4454; Angew.
Chem. Int. Ed. 2006, 45, 4345 – 4348.
[11] a) P. S. Baran, D. P. OꢀMalley, A. L. Zografos, Angew. Chem.
2004, 116, 2728 – 2731; Angew. Chem. Int. Ed. 2004, 43, 2674 –
2677; b) B. H. Northrop, D. P. OꢀMalley, A. L. Zografos, P. S.
Baran, K. N. Houk, Angew. Chem. 2006, 118, 4232 – 4236;
Angew. Chem. Int. Ed. 2006, 45, 4126 – 4130.
[12] V. B. Birman, X.-T. Jiang, Org. Lett. 2004, 6, 2369 – 2371.
[13] a) P. S. Baran, A. L. Zografos, D. P. OꢀMalley, J. Am. Chem. Soc.
2004, 126, 3726 – 3727; b) D. P. OꢀMalley, K. Li, M. Maue, A. L.
Zografos, P. S. Baran, J. Am. Chem. Soc. 2007, 129, 4762 – 4775.
[14] a) A. S. Dilley, D. Romo, Org. Lett. 2001, 3, 1535 – 1538; b) C. J.
Lovely, H. Du, Y. He, H. V. Rasika-Dias, Org. Lett. 2004, 6, 735 –
738; c) see also refs. [1b] and [3c].
ꢀ
precursors. Indeed, exciting developments in C H activation
lend credit to the notion that such methods might become
even more useful and broadly applicable in the future.[19] It is
tangible that the findings highlighted here mark a beginning
for many more future discoveries in this field.
Published online: May 30, 2008
[15] J. T. Starr, G. Koch, E. M. Carreira, J. Am. Chem. Soc. 2000, 122,
8793 – 8794.
[16] D. OꢀMalley, J. Yamaguchi, I. S. Young, I. B. Seiple, P. S. Baran,
Angew. Chem. 2008, 120, 3637 – 3639; Angew. Chem. Int. Ed.
2008, 47, 3581 – 3583.
[17] J. Yamaguchi, I. B. Seiple, I. S. Young, D. P. OꢀMalley, M. Maue,
P. S. Baran, Angew. Chem. 2008, 120, 3634 – 3636; Angew. Chem.
Int. Ed. 2008, 47, 3578 – 3580.
[18] C. Bolm, A. Gerlach, C. L. Dinter, Synlett 1999, 195 – 196.
[19] A recent highlight: M. Christmann, Angew. Chem. 2008, 120,
2780 – 2783; Angew. Chem. Int. Ed. 2008, 47, 2740 – 2742.
[1] Some reviews: a) H.-D. Arndt, U. Koert in Organic Synthesis
Highlights 4 (Ed.: H.-G. Schmalz), Wiley-VCH, Weinheim, 2000,
pp. 241 – 250; b) H. Hoffmann, T. Lindel, Synthesis 2003, 1753 –
1783; c) H. Du, Y. He, R. Sivappa, C. J. Lovely, Synlett 2006,
965 – 992; d) S. M. Weinreb, Nat. Prod. Rep. 2007, 24, 931 – 948;
e) M. Köck, A. Grube, I. B. Seiple, P. S. Baran, Angew. Chem.
2007, 119, 6706 – 6714; Angew. Chem. Int. Ed. 2007, 46, 6586 –
6594.
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