Regiocontrol in MnIII-mediated oxidative heterobicyclizations: access to the core skeletons of oroidin dimers

Article abstract of DOI:10.1002/anie.200601208

(Chemical Equation Presented) In control: A cyclization reaction of an allylic β-imidazolinonyl-β-ketoester 1 is carried out in a Mn ^III-promoted radical cascade. The reaction pathway (5-exo/6-endo or 5-exo/5-exo) can be directed to construct the central cyclopentyl and cyclohexenyl core skeletons (2 and 3) of two classes of oroidin dimers with regiocontrol. An oxidative rearrangement can also be used to convert 2 into 3. PG = protecting group.

Full text of DOI:10.1002/anie.200601208

Angewandte
Chemie
Cascade Reactions
DOI: 10.1002/anie.200601208
Regiocontrol in MnIII-Mediated Oxidative
Heterobicyclizations: Access to the Core
Skeletons of Oroidin Dimers**
Xianghui Tan and Chuo Chen*
Dedicated to Professor Tien-Yau Luh
on the occasion of his 60th birthday
Pyrrole–imidazole alkaloids, also known as oroidin-family
natural products, possess diverse molecular skeletons
(Scheme 1) and many important biological properties, such
as antibiotic, antiproliferative, and immunosuppressive activ-
ities.[1] The synthesis of polycyclic oroidin dimers remains a
significant challenge.[1,2] To address this issue, we devised a
radical cascade cyclization[3] strategy to construct the central
cyclopentyl and cyclohexenyl core skeletons (4 and 5) of two
classes of oroidin dimers, ageliferin/nagelamide and massa-
dine/palauꢀamine/axinellamine.Herein, we describe two
types of MnIII-promoted cyclization cascades of allylic b-
Scheme 1. Structures of oroidin dimers.
report of Corey and Kang on the intramolecular MnIII-
promoted oxidative cyclizations of b-dicarboxylates,[5] the
reaction has grown in popularity and scope.[6,7] However,
despite extensive investigations, applications of this method
to unsaturated N-heterocyclic systems[8] and allylic b-keto-
esters[9] are rare.
As outlined in Scheme 2, our strategy involves the
oxidation of b-ketoester 1 with MnIII, thus initiating a radical
cascade cyclization reaction to give the ageliferin/nagelamide
core skeleton 4 or the massadine/palauꢀamine/axinellamine
core skeleton 5 after decarboxylation.With the controlling
element X = H, the reaction proceeds through a 5-exo/6-endo
cyclization pathway.On the other hand, the reaction can be
directed to the 5-exo/5-exo cyclization pathway when X = Cl
or CN.We have also developed an oxidative rearrangement
reaction of 2 and 4 as an alternative approach to 3 and 5.[10]
Our initial efforts focused on the 5-exo/6-endo cyclization
of 1 (1!2). b-Ketoester 6, which bears an a-methyl group,
was oxidized cleanly by [Mn(OAc)3] in HOAc at 608C to give
the ageliferin skeleton 7 in 63% yield of the isolated product
(Table 1, entry 1).No monocyclization products and other
ꢀ
imidazolinonyl-b-ketoesters 1, wherein two C C bonds and
three or four contiguous stereogenic centers are established in
a single operation (Scheme 2).
Manganese(III) acetate is well known as an effective
oxidant for enolizable carbonyl compounds.[4] Since the initial
[*] Dr. X. Tan, Prof. Dr. C. Chen
Department of Biochemistry
University of Texas
Southwestern Medical Center at Dallas
5323 Harry Hines Boulevard, Dallas, TX 75390 (USA)
Fax: (+1)214-648-0320
E-mail: Chuo.Chen@UTSouthwestern.edu
[**] We gratefully acknowledge the Southwestern Medical Foundation
and the Welch Foundation for financial support. We also thank Dr.
Radha Akella for the X-ray analyses.
1
diastereomers were observed in the H NMR spectra of the
crude product.[11] We found that [Mn(OAc)3] was the best
oxidant and HOAc the optimal solvent.Addition of Cu-
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Scheme 2. Synthetic approach to the oroidin dimers.
Table 1: MnIII-promoted 5-exo/6-endo radical cyclization reactions.[a]
inated the issue of stereospecificity.Lactone
13 was obtained as the only product from
the oxidation of 12 with [Mn(OAc)3]
(entry 4).
Entry
Substrate
Product(s)
Yield [%]
63
We then turned our attention to the
synthesis of the massadine/palauꢀamine
core skeleton and introduced a Cl atom as
X to direct the second cyclization toward
the 5-exo pathway (1!3).Oxidation of 14
proceeded through a 5-exo/5-exo cyclization
to give 15[15] along with the monocyclization
products 16 in an approximate 1:1 ratio
(Table 2, entry 1).No 6- endo products and
other diastereomers were observed in the
1H NMR spectra of the crude product.
Introduction of an electron-withdrawing
nitro group to the cinnamyl ester did not
change the product ratio significantly
(entry 2).Oxidation of the crotyl ester 20
also gave a mixture of 21 and 22 (entry 3).
Remarkably, formation of the monocycliza-
tion product can be suppressed with X =
CN.Hydantoin 24 was obtained as the
only product from the oxidation of 23 in
60% yield (entry 4).
1
2
3
4
59 (2.1:1)
50[b] (1:1.5)
60
9+10
[a] Reaction conditions: [Mn(OAc)3] (3.0 equiv), HOAc, 608C. [b] 808C.
The temporary ester linkage used in the
(OAc)2 or [Yb(OTf)3] (Tf = trifluoromethanesulfonyl) has no
significant effects.
intramolecular reaction with [Mn(OAc)3] can be removed
under decarboxylative conditions (Scheme 3).For example,
briefly treating 9 with LiOH revealed the ageliferin core 25.
We also demonstrated that the ageliferin core skeleton 25
could be oxidatively rearranged to the massadine core 26 by
mCPBA (namely, 4!5) despite epimerization at C15.Nota-
bly, in contrast to the system developed by Dilley and
Romo,[10a] ring contraction was achieved directly without
competing migration of the olefin.
Lactone 9 can also be oxidized by mCPBA after
deprotection of the triisopropylsilyl (TIPS) group (namely,
2!3).In contrast to the oxidation of 9, the reaction
proceeded with opposite facial selectivity to afford hydantoin
27, which bears the palauꢀamine spiro configuration.The ester
linker of 27 can be removed with concomitant epoxide
formation to provide 28.The modifiable C2 and C15
stereogenic centers of 28 are opposite to those of palauꢀamine.
We next explored the scope of this radical cascade
reaction.( E)-Allylic b-ketoester 8, bearing all the necessary
functional groups for the ageliferin synthesis, was oxidized
cleanly to 9 and 10 in 59% yield (d.r. = 2.1:1; entry 2). Only
two out of eight possible diastereomers were obtained.The
C11 stereogenic center controlled the diastereoselectivity of
this transformation through a moderate A1,3 strain.[12] The a-
radical of b-ketoester 8 added to the olefin preferentially from
the less-hindered b face to give 9 as the major diastereomer.
Lactones 9 and 10 contain the proper configurations at C9,
C9’, and C10 for the ageliferin and ent-ageliferin synthesis.It
is interesting to note that the oxidation of (Z)-11 proceeded
with “epimerization” at C10 and also afforded 9 and 10 (d.r. =
1:1.5; entry 3).[13,14] Constraining the (Z)-olefin in a cyclic
system not only improved the diastereoselectivity but elim-
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Angewandte
Chemie
Table 2: MnIII-promoted 5-exo/5-exo radical cyclization reactions.[a]
Keywords: cascade reactions ·
natural products · radical reactions ·
rearrangement · synthetic methods
.
Entry
Substrate
Product(s)
Yield [%]
1
87 (1:1.2)
[1] For a review of the pyrrole–imida-
zole family of natural products, see:
H.Hoffmann, T.Lindel, Synthesis
2003, 1753 – 1783.
[2] For concise syntheses of sceptrin,
see: a) P.S.Baran, A.L.Zografos,
D.P.OꢀMalley, J. Am. Chem. Soc.
2004, 126, 3726 – 3727; b) V.B.
Birman, X.-T. Jiang, Org. Lett.
2004, 6, 2369 – 2371; for an elegant
synthesis of ageliferin, see: c) P.S.
Baran, K.Li, DP..OꢀMalley, C.
Mitsos, Angew. Chem. 2006, 118,
255 – 258; Angew. Chem. Int. Ed.
2006, 45, 249 – 252; d) P.S. Baran,
D.P. OꢀMalley, A.L. Zografos,
Angew. Chem. 2004, 116, 2728 –
2731; Angew. Chem. Int. Ed. 2004,
43, 2674 – 2677; for synthetic
approaches to palauꢀamine, see:
e) H.Garrido-Hernandez, M.
Nakadai, M.Vimolratana, Q.Li,
T.Doundoulakis, PG..Harran,
2
69 (1:1)
3
4
61 (1:1.4)
60
[a] Reaction conditions: [Mn(OAc)3] (2.5 equiv), HOAc, 608C.
Angew. Chem. 2005, 117, 775 –
779; Angew. Chem. Int. Ed. 2005,
44, 765 – 769, and references
therein; for a synthetic approach
to axinellamine, see: f) J.T. Starr,
G.Koch, EM. .Carreira,
J. Am.
Chem. Soc. 2000, 122, 8793 – 8794.
[3] For a review of radical cascade
cyclizations, see: a) A.J. McCar-
roll, J.C. Walton, Angew. Chem.
2001, 113, 2282 – 2307; Angew.
Chem. Int. Ed. 2001, 40, 2224 –
2248; for reviews of cascade reac-
tions, see: b) K.C. Nicolaou, T.
Montagnon, S.A. Snyder, Chem.
Commun. 2003, 551 – 564; c) P.J.
Parsons, C.S.Penkett, A.J.Shell,
Chem. Rev. 1996, 96, 195 – 206.
[4] a) E.I.Heiba, R.M.Dessau, W.J.
Koehl, Jr., J. Am. Chem. Soc. 1968,
90, 5905 – 5906; b) J.B. Bush, Jr.,
H.Finkbeiner, J. Am. Chem. Soc.
1968, 90, 5903 – 5905.
Scheme 3. Reaction conditions: a) LiOH, THF/H2O, 508C, 15 min, 59% yield; b) mCPBA, CHCl3,
238C, 16 h, 78% yield; c) NH4F, HOAc, MeOH, 608C, 12 h, 83% yield; d) mCPBA, CHCl3, 608C,
16 h, 78% yield; e) LiOH, THF/H2O, 608C, 12 h, 72% yield. Boc=tert-butyloxycarbonyl, mCPBA=
meta-chloroperoxybenzoic acid.
[5] E.J. Corey, M.-c. Kang,
Chem. Soc. 1984, 106, 5384 – 5385.
J. Am.
In summary, we have devised a MnIII-promoted radical
cascade cyclization reaction to deliver the core skeletons of
two types of oroidin dimers, ageliferin/nagelamide and
[6] For an example of an application of this reaction to complex
natural-product synthesis, see: D.Yang, X-.Y.Ye, M.Xu, K-.W.
Pang, K.-K. Cheung, J. Am. Chem. Soc. 2000, 122, 1658 – 1663.
[7] For reviews, see: a) G.G.Melikyan, Org. React. 1997, 49, 427 –
675; b) B.B.Snider, Chem. Rev. 1996, 96, 339 – 363.
[8] a) C.-P. Chuang, S.-F. Wang, Tetrahedron Lett. 1994, 35, 1283 –
1284; b) C.-P. Chuang, S.-F. Wang, Synth. Commun. 1994, 24,
1493 – 1505; c) A.Citterio, R.Sebastiano, M.C.Carvayal, J. Org.
Chem. 1991, 56, 5335 – 5341.
ꢀ
massadine/palauꢀamine/axinellamine.Two C C bonds and
three or four contiguous stereogenic centers are established in
one step.We have also demonstrated that the ageliferin cores
thus obtained can be oxidatively rearranged to the massadine/
palauꢀamine core skeletons with controlled installation of
either spiro configurations.Efforts to synthesize selected
oroidin dimers are currently in progress.
[9] There is only one report of the oxidation of allylic b-ketoesters
with MnIII, wherein the thermodynamic trans-5-exo cyclization
products were obtained: K.Sung, Y.Y.Wang,
J. Org. Chem.
Received: March 27, 2006
2003, 68, 2771 – 2778; the MnIII-promoted cyclization of allylic b-
Published online: May 31, 2006
dicarboxylate is more common, for example, Ref.[5].
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Communications
[10] For a two-step oxidation of imidazolinone with dimethyldioxir-
[12] For a review of A1,3 strain, see: R.W. Hoffmann, Chem. Rev.
1989, 89, 1841 – 1860.
[13] For a similar stereospecificity issue, see: Ref.[6].
[14] A slightly higher temperature (808C) was required to suppress
the formation of the monocyclization product; however, the
diastereoselectivity was compromised.
[15] CCDC-290044 (15) contains the supplementary crystallographic
data for this paper.These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
ane (DMDO)/N-chlorosuccinimide (NCS) to construct the
palauꢀamine core skeleton, see: a) A.S.Dilley, D.Romo, Org.
Lett. 2001, 3, 1535 – 1538; for a similar DMDO-promoted
oxidation of imidazole, see: b) CJ..Lovely, H.Du, Y.He,
H.V.R.Dias, Org. Lett. 2004, 6, 735 – 738.
[11] The 1H NMR spectra of the crude product indicated a clean
transformation; however, the cyclization products were not
stable to column chromatography on silica gel (among all the
isolation conditions examined, Davisil 633 silica gel provided the
best yields of the isolated products).
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