Catalytic enantioselective synthesis of oxindoles and benzofuranones that bear a quaternary stereocenter

Article abstract of DOI:10.1002/anie.200351666

A common feature in indole- and benzofuran-derived natural products is a quaternary stereocenter in the 3-position of the heterocycle. The catalyst (-) -1 promotes the asymmetric synthesis of such compounds 3 (with up to 99% ee) through the rearrangement of O-acylated benzofuranones (X = O) and oxindoles (X = NR²) 2.

Full text of DOI:10.1002/anie.200351666

Angewandte
Chemie
Stereogenic Quaternary Centers
Catalytic Enantioselective Synthesis of Oxindoles
and Benzofuranones That Bear a Quaternary
Stereocenter**
Ivory D. Hills and Gregory C. Fu*
A diverse array of indole alkaloids and benzofuran-derived
natural products bear quaternary stereocenters in the 3-
position of the heterocycle.[1,2] Although noteworthy progress
has been described in the development of strategies for the
enantioselective synthesis of such compounds,[3] there
remains a need for additional approaches.
In 1986, Black et al. reported that 4-dimethylaminopyr-
idine (DMAP) catalyzes the rearrangement of O-acylated
benzofuranones to give their C-acylated isomers [Eq. (1)].[4]
During the course of studies directed toward the synthesis of
[*] Prof. Dr. G. C. Fu, I. D. Hills
Department of Chemistry, Massachusetts Institute of Technology
Cambridge, MA 02139 (USA)
Fax: (+1)617-324-3611
E-mail: gcf@mit.edu
[**] We thank Dr. J. Craig Ruble for preliminary experiments. Support
has been provided by the American Chemical Society (Organic
Division Fellowship to I.D.H., sponsored by Abbott Laboratories),
the National Institutes of Health (National Institute of General
Medical Sciences, R01-GM57034), Merck, and Novartis. Funding
for the MIT Department of Chemistry Instrumentation Facility has
been furnished in part by NSF CHE-9808061 and NSF DBI-9729592.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2003, 42, 3921 –3924
DOI: 10.1002/anie.200351666
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Communications
the originally assigned incorrect structure of the potent
anticancer agent diazonamide A,[5] Moody et al. employed a
non-asymmetric Black-type C-acylation to generate the
Our initial studies focused on O-acylated oxindoles, a
family of compounds that had not previously been explored in
the context of O-to-C rearrangements. We generated a
representative substrate by treating an oxindole with methyl
chloroformate, and we were pleased to discover that PPY
derivative 1 did, indeed, catalyze the rearrangement of the
resulting carbonate, thus providing a newquaternary stereo-
center with promising enantioselectivity (Table 1, entry 1).
Table 1: Effect of the acyl group on the enantioselectivity of O-to-C
rearrangements.
quaternary stereocenter of the benzofuran-derived core.[6]
A
Entry
R
ee [%][a]
fewyears later, also in the context of an approach to the
synthesis of the incorrect structure of diazonamide A, Vedejs
and Wang described a diastereoselective variant of the Black
rearrangement reaction [Eq. (2)] (Troc = trichloroethoxycar-
bonyl).[7] Notably, for the correct structure of diazonamide A
(above), a corresponding C-acylation strategy would employ
an oxindole rather than a benzofuranone as the substrate.
1
2
3
Me
Et
tBu
58
63
–
[b]
4
98
[a] The data are an average of two runs. [b] No rearrangement was
observed.
We subsequently determined that an increase in the bulk of
the carbonate group (Me!Et) led to an increase in the
enantioselectivity (Table 1, entries 1 and 2, 58!63% ee).
Unfortunately, in the case of a tert-butyl substituent, the
rearrangement did not proceed, presumably due to a steric
effect (Table 1, entry 3). However, we were able to overcome
this lack of reactivity through electronic activation, specifi-
cally, the use of a trichloro-tert-butyl group: rearrangement of
this carbonate furnished the desired product with very good
enantioselectivity (Table 1, entry 4, 98% ee).[9]
With the trichloro-tert-butoxycarbonyl substituent as the
migrating group, catalyst 1 promotes the rearrangement of a
variety of oxindole derivatives with high enantioselectivity
(Table 2).[10,11] The reaction proceeds cleanly with either
aromatic or heteroaromatic groups in the 3-position
(Table 2, entries 1 and 2).[12] 3-Alkyl-substituted O-acylated
oxindoles can also be employed as substrates, although these
rearrangements are slower and require a 10% catalyst
loading to obtain a good yield (Table 2, entries 3 and 4).
Substitution on the six-membered ring is tolerated, furnishing
a product suitable for further functionalization (Table 2,
entry 5). Finally, the reaction is not limited to N-methyl-
substituted oxindoles—catalyst 1 also promoted the rear-
rangement of an N-benzyl-protected heterocycle with high
enantioselectivity (Table 2, entry 6).[13]
In recent years, we have been pursuing the development
of applications of chiral derivatives of DMAP and PPY
(PPY= 4-(pyrrolidino)pyridine; e.g., 1 and 2) to a range of
enantioselective nucleophile-catalyzed transformations.[8] In
viewof the potential significance of the reaction products, we
decided to explore the use of these catalysts in asymmetric
rearrangements of O-acylated benzofuranones and oxindoles.
Herein we provide the first examples of enantioselective
variants of these processes, demonstrating that catalyst 1
generates the newquaternary stereocenter with very good
enantiomeric excess [Eq. (3)].
The conditions that we employed for O-to-C rearrange-
ments of oxindole derivatives (Table 2) are directly applicable
to the corresponding reactions of O-acylated benzofuranones
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Chemie
Table 2: Catalytic enantioselective rearrangement of oxindole deriva-
tives.
Entry
R1
R2
R3
ee [%][a]
Yield [%][a]
1
2
Ph
Me
Me
Me
Me
Me
Bn
H
H
H
H
I
99
95
94
93
98
98
91
81
82
72
94
88
2-thienyl
benzyl
Me
Ph
Ph
3[b]
4[b]
5
Figure 1. X-ray crystal structure of an ion pair derived from catalyst 1
and an O-acylated benzofuranone.
6
H
[a] Yield of isolated products. The data are an average of two runs.
[b] Catalyst loading: 10%.
In summary, we have developed the first method for the
catalytic enantioselective rearrangement of O-acylated ben-
zofuranones and oxindoles, an efficient carbon–carbon bond-
forming reaction that generates a quaternary stereocenter.
On the mechanistic side, we have crystallographically char-
acterized the presumed intermediate in this process. In view
of the abundance of important indole- and benzofuran-
derived natural products that bear a quaternary stereocenter
in the 3-position of the heterocycle, we believe that this
method may prove useful in asymmetric synthesis.
(Table 3).[14] Thus, for both 3-aryl- and 3-alkyl-substituted
compounds, catalyst 1 promotes the generation of the new
quaternary stereocenter with very good enantioselectiv-
ity.[15,16]
Table 3: Catalytic enantioselective rearrangement of benzofuranone
derivatives.
Received: April 15, 2003 [Z51666]
Published online: July 3, 2003
Entry
R1
R2
ee [%]
Yield [%][a]
Keywords: asymmetric synthesis · heterocycles ·
homogeneous catalysis · ion pairs · rearrangements
.
1
Ph
Bn
Me
H
H
Me
97
88
90
81
95
93
2
3[b]
[1] For a sampling of recent total syntheses, see: a) gelsemine: H.
Lin, S. J. Danishefsky, Angew. Chem. 2003, 115, 38 – 53; Angew.
Chem. Int. Ed. 2003, 42, 36 – 51; b) strychnofoline: A. Lerchner,
E. M. Carreira, J. Am. Chem. Soc. 2002, 124, 14826 – 14827;
c) alantrypinone: D. J. Hart, N. A. Magomedov, J. Am. Chem.
Soc. 2001, 123, 5892 – 5899; d) spirotryprostatin B: L. E. Over-
man, M. D. Rosen, Angew. Chem. 2000, 112, 4768 – 4771; Angew.
Chem. Int. Ed. 2000, 39, 4596 – 4599.
[2] For a reviewof catalytic asymmetric methods that generate
quaternary stereocenters, see: a) E. J. Corey, A. Guzman-Perez,
Angew. Chem. 1998, 110, 402 – 415; Angew. Chem. Int. Ed. 1998,
37, 388 – 401; see also: b) J. Christoffers, A. Mann, Angew. Chem.
2001, 113, 4725 – 4732; Angew. Chem. Int. Ed. 2001, 40, 4591 –
4597.
[a] Yield of isolated product. [b] This reaction was run at ꢀ128C with
10% catalyst.
Black et al. suggested that DMAP-catalyzed rearrange-
ments of O-acylated benzofuranones proceed through the
mechanism illustrated in Scheme 1.[4] We believe that asym-
metric reactions of O-acylated benzofuranones and oxindoles
catalyzed by PPY derivative 1 followan analogous pathway.
Indeed, we have been able to obtain a low-resolution X-ray
crystal structure of the ion pair corresponding to
3
(Figure 1).[17–19]
[3] For example, the intramolecular asymmetric Heck reaction has
proved to be useful; for leading references, see: a) A. D.
Lebsack, J. T. Link, L. E. Overman, B. A. Stearns, J. Am.
Chem. Soc. 2002, 124, 9008 – 9009; b) Y. Donde, L. E. Overman
in Comprehensive Asymmetric Catalysis; (Eds.: E. N. Jacobsen,
A. Pfaltz, H. Yamamoto), Springer, NewYork, 1999, pp. 675 –
697.
[4] a) T. H. Black, S. M. Arrivo, J. S. Schumm, J. M. Knobeloch, J.
Chem. Soc. Chem. Commun. 1986, 1524 – 1525; b) T. H. Black,
S. M. Arrivo, J. S. Schumm, J. M. Knobeloch, J. Org. Chem. 1987,
52, 5425 – 5430.
[5] For the correct structural assignment of diazonamide A, see:
a) J. Li, A. W. G. Burgett, L. Esser, C. Amezcua, P. G. Harran,
Angew. Chem. 2001, 113, 4906 – 4909; Angew. Chem. Int. Ed.
2001, 40, 4770 – 4773; for an overviewof diazonamide chemistry,
see: b) T. Ritter, E. M. Carreira, Angew. Chem. 2002, 114, 2601 –
Scheme 1. Proposed mechanism for DMAP-catalyzed rearrangements
of O-acylated benzofuranones.
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Communications
2606; Angew. Chem. Int. Ed. 2002, 41, 2489 – 2495; for the total
supplementary crystallographic data for 3. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrie-
ving.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB21EZ, UK; fax: (+ 44)1223-
336-033; or deposit@ccdc.cam.ac.uk).
synthesis of the corrected structure of diazonamide A, see:
c) K. C. Nicolaou, M. Bella, D. Y.-K. Chen, X. Huang, T. Ling,
S. A. Snyder, Angew. Chem. 2002, 114, 3645 – 3649; Angew.
Chem. Int. Ed. 2002, 41, 3495 – 3499; K. C. Nicolaou, P.
Bheema Rao, J. Hao, M. V. Reddy, G. Rassias, X. Huang,
D. Y.-K. Chen, S. A. Snyder, Angew. Chem. 2003, 115, 1795 –
1800; Angew. Chem. Int. Ed. 2003, 42, 1753 – 1758.
[19] The ee of the product does not erode with time, indicating that C-
acylation of the enolate is irreversible; 1H NMR studies show
that for the benzofuranone chemistry, the resting state of the
catalyst is the N-acylated derivative, whereas for the oxindole
chemistry, the resting state is the catalyst itself (not acylated).
[6] C. J. Moody, K. J. Doyle, M. C. Elliott, T. J. Mowlem, J. Chem.
Soc. Perkin Trans. 1 1997, 2413 – 2419.
[7] E. Vedejs, J. Wang, Org. Lett. 2000, 2, 1031 – 1032.
[8] For an early overview, see: a) G. C. Fu,Acc. Chem. Res. 2000, 33,
412 – 420; for intra- and intermolecular C-acylation reactions,
see: b) J. C. Ruble, G. C. Fu, J. Am. Chem. Soc. 1998, 120,
11532 – 11533; A. H. Mermerian, G. C. Fu, J. Am. Chem. Soc.
2003, 125, 4050 – 4051; for additional applications, see: c) S. Arai,
S. Bellemin-Laponnaz, G. C. Fu, Angew. Chem. 2001, 113, 240 –
242; Angew. Chem. Int. Ed. 2001, 40, 234 – 236; B. L. Hodous,
G. C. Fu, J. Am. Chem. Soc. 2002, 124, 1578 – 1579.
[9] 2,2,2-Trichloro-1,1-dimethylethyl chloroformate is commercially
available. The trichloro-tert-butoxycarbonyl group has been
employed in kinetic resolutions of secondary alcohols by an N-
acylated chiral derivative of DMAP: a) E. Vedejs, X. Chen, J.
Am. Chem. Soc. 1996, 118, 1809 – 1810. Catalyst (ꢀ)-2 affords
essentially identical enantioselectivity as (ꢀ)-1 (97% ee; same
enantiomer of the product as (ꢀ)-1); we chose to focus our
studies on catalyst 1, since it is particularly stable.
[10] The absolute stereochemistry of the product of Table 2, entry 3
was determined by X-ray crystallography (see the Supporting
Information); the other configurations were assigned by analogy.
[11] General procedure: The substrate (1.00 equiv) and catalyst (ꢀ)-
1 (0.050 equiv) were added, exposed to the air, to a vial that
contained a stirrer bar. The vial was sealed with a septum and
purged with argon. CH2Cl2 ([substrate] = 1.0m) was then added
to the vial through a syringe, and the reaction mixture was
heated at 358C for 48 h. The reaction mixture was then applied
directly to a silica-gel column for purification by flash chroma-
tography (typically, ~ 85% of the catalyst was recovered).
[12] The slight difference in enantiomeric excess between Table 1,
entry 4 and Table 2, entry 1 is due to the difference in the scale of
the reactions. See the Supporting Information for additional
details.
[13] In preliminary studies, we selectively hydrolyzed (aqueous
NaOH) and transesterified (NaOMe) the trichloro-tert-butyl
ester group.
[14] The absolute stereochemistry of the product of Table 3, entry 1
was determined by X-ray crystallography (see the Supporting
Information); the other configurations were assigned by analogy.
[15] We employed the product of Table 3, entry 3 in a formal total
synthesis of debromoaplysin (I. D. Hills, unpublished results).
[16] Under our standard reaction conditions, the benzofuranone-
derived substrates react more rapidly than do the oxindole-
derived compounds.
[17] In the original studies of Black et al., a solid was generated
under certain conditions and speculated to be the ion-pair
intermediate. Unfortunately, the solid could not be character-
ized.[4]
[18] The quality of the crystal was sufficiently high to unambiguously
assign the structure of the ion pair, but not sufficiently high to
accurately determine bond lengths. CCDC-208287 contains the
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