Enantioselective thiourea-catalyzed acyl-Mannich reactions of isoquinolines

Article abstract of DOI:10.1002/anie.200502277

(Chemical Equation Presented) Inexpensive aromatic feedstocks are substrates for highly enantioselective acylative Mannich reactions catalyzed by a thiourea chiral hydrogen-bond donor 1. This methodology provides access to useful 1-substituted dihydroisoq

Full text of DOI:10.1002/anie.200502277

Communications
Asymmetric Catalysis
DOI: 10.1002/anie.200502277
Enantioselective Thiourea-Catalyzed Acyl-
Mannich Reactions of Isoquinolines**
Mark S. Taylor, Norihito Tokunaga, and
Eric N. Jacobsen*
Aromatic molecules represent an attractive class of feedstock
compounds for organic synthesis because of their ready
availability, their stability, and the wealth of classical and
modern chemistry available for their preparation and manip-
ulation. The development of enantioselective, catalytic meth-
odologies that engage aromatic p systems as substrates offers
particular promise for synthetic applications. Despite this
potential, it is only recently that aromatic frameworks have
been employed successfully as electrophiles[1] or nucleo-
philes[2] in asymmetric, catalytic reactions, and many impor-
tant challenges in this area remain unmet. The addition of
carbon-centered nucleophiles to nitrogen-containing hetero-
aromatic compounds is a particularly interesting problem, in
light of the potential impact of such a methodology on
alkaloid synthesis. Diastereoselective reactions controlled by
chiral auxiliaries currently constitute the state-of-the-art
methods for the majority of stereocontrolled transformations
of this type.[3] The elegant alkaloid syntheses of Comins et al.
based on diastereoselective nucleophilic additions to chiral
(4-methoxy)acylpyridinium derivatives illustrate the utility of
such approaches.[4] Only one enantioselective, catalytic
method for the addition of carbon-centered nucleophiles to
aromatic nitrogen heterocycles has been developed to date:
the aluminum-catalyzed acylcyanation of quinolines, isoqui-
nolines, and pyridines (the Reissert reaction) developed by
Shibasaki and co-workers.[1a–d] Herein, we report the first
example of an asymmetric, catalytic addition of enolate
equivalents to heteroaromatic electrophiles. This acyl-Man-
nich reaction,[5] catalyzed by a chiral thiourea derivative,
provides access to useful enantioenriched dihydroisoquino-
line building blocks.[6]
N-Alkylations or N-acylations of nitrogen-containing
heteroaromatic compounds give rise to highly electrophilic
[*] M. S. Taylor, N. Tokunaga, Prof. Dr. E. N. Jacobsen
Department ofChemistry and Chemical Biology
Harvard University
Cambridge, MA 02138 (USA)
Fax: (+1)617-496-1880
E-mail: jacobsen@chemistry.harvard.edu
[**] This work was supported by the NIH (GM-43214 and P50
GM069721). M.S.T. gratefully acknowledges Bristol-Myers-Squibb
for a Graduate Fellowship and Harvard University for a John Parker
Graduate Scholarship. N.T. thanks the Japan Society for the
Promotion of Science for the award of a fellowship for graduate
students. Dr. Richard Staples is acknowledged for the determination
ofthe X-ray crystal structure of 1.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Chemie
iminium or acyliminium ions that are susceptible to a variety
of addition reactions. There are very few examples of the
asymmetric catalysis of such additions,[1a–e] a fact that may
reflect, to some extent, the difficulty in the development of
catalysts capable of the activation of such intermediates. In
that context, we were highly encouraged by the discovery that
chiral thiourea 1 catalyzes an enantioselective acylative
variant of the Pictet–Spengler reaction (Scheme 1). The
lective Pictet–Spengler reactions, furnished the acyl-Mannich
product in poor enantiomeric excess (28% ee; entry 1). More
encouraging results were obtained using chloroformates
(entries 2–5), with which tuning of the alkoxy substituent
had a pronounced effect upon enantioselectivity. The dihy-
droisoquinoline product was obtained in a promising 82% ee
by using 2,2,2-trichloroethyl chloroformate (TrocCl). Further
improvement upon this result was realized by variation of the
structure of the nucleophile
(entries 5–7), with the best
result provided by the silyl
ketene acetal derived from iso-
propyl acetate. The acyl-Mannich
reaction of isoquinoline pro-
ceeded in 80% yield with
86% ee under the optimized con-
ditions.
Despite markedly different
dependences of enantiomeric
excess upon the structure of the
acyl group, the acyl-Mannich and
acyl-Pictet–Spengler
reactions
Scheme 1. Enantioselective acyl-Pictet–Spengler reaction catalyzed by 1.
share several common features.
Pronounced solvent effects were
possibility that 1 can activate a putative intermediate
acyliminium ion towards enantioselective cyclization by
hydrogen-bond donation[7,8] prompted us to study its appli-
cation to the reactions of acyliminium ions derived from
nitrogen heterocycles.
In light of the prevalence of the 1-substituted tetrahy-
droisoquinoline motif in alkaloid architecture,[9] we selected
isoquinoline as a model substrate (Table 1). The enantiose-
lectivity of acyl-Mannich-type reactions catalyzed by 1 was
found to depend strongly on the nature and structure of the
acylating agent and nucleophile. A screen of acylating agents
was performed using the O-tert-butyldimethylsilyl (TBS)
ketene acetal derived from methyl acetate as the nucleo-
phile.[10] Acetyl chloride, the optimal reagent for enantiose-
observed in both cases, with diethyl ether affording the
highest enantioselectivity. In addition, 1 is the optimal catalyst
identified to date for both reactions.[11] In particular, the
ee values of both the acyl-Mannich and acyl-Pictet–Spengler
reaction exhibit a dramatic dependence upon the substitution
pattern of the pyrrole moiety (Table 2). The crystal structure
of 1 may help to explain this behavior (Figure 1).[12] The
orientation of the 2-methyl-5-phenylpyrrole structural motif
places the phenyl group in position to interact closely with any
species that undergoes hydrogen-bonding interactions with
the acidic thiourea protons (Figure 1).[13] In the solid state, 1
exists as a dimeric structure through bifurcated hydrogen-
Table 2: Dependence ofreaction enantioselectivity on catalyst structure.
Table 1: Optimization ofthe acylating agent and nucleophile.
Entry
R
R’
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
7
Me
OBn
Me
Me
Me
Me
Me
Bn
80
60
70
75
65
80
80
28
41
47
64
82
73
86[c]
OC(CH3)2CCl3
OCH2(fluorenyl)
OCH2CCl3
OCH2CCl3
OCH2CCl3
Entry
R
R’
Yield [%][a]
ee [%][b]
1
2
3
Me
Ph
Me
Me
Ph
Ph
60
55
55
30
78
85
iPr
[a] Yield ofisolated product atfer column chromatography. [b] Enantio-
meric excess determined by supercritical fluid chromatography (SFC)
using commercially available chiral stationary-phase columns. [c] Reac-
tion temperature was ꢀ708C.
[a] Yield ofisolated product atfer column chromatography. [b] Enantio-
meric excess determined by SFC using commercially available chiral
stationary-phase columns.
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utility for alkaloid synthesis from stable, readily accessible
aromatic starting materials. For certain applications, this
methodology may prove complementary to the Pictet–
Spengler reaction, both in terms of electronic requirements
(both electron-rich and electron-poor products may be
accessed) and regioselectivity (the substitution pattern of
the product is determined only by the choice of the isoquino-
line starting material). In addition, this study demonstrates
that thiourea catalyst 1 mediates two distinct enantioselective
transformations of N-acyliminium ions: intramolecular Frie-
del–Crafts reactions of acyliminium ions derived from acyclic,
aliphatic imines (the acyl-Pictet–Spengler reaction) and
intermolecular Mannich reactions of acylisoquinolinium
ions. Further extension of this mode of catalysis to encompass
other reactions of N-acyliminium intermediates and inves-
tigation of the mechanism by which these electron-poor
species are activated by a hydrogen-bond donor are the focus
of future study.
Figure 1. Solid-state structure ofcatalyst 1.
ꢀ
bonding interactions between the thiourea N H protons and
the amide carbonyl group.[14]
A number of substituted dihydroisoquinolines are acces-
sible through this new methodology (Table 3) and serve as Experimental Section
General procedure for acyl-Mannich reactions catalyzed by 1: 1-
Isopropoxycarbonylmethyl-1H-isoquinoline-2-carboxylic acid 2,2,2-
trichloroethyl ester (Table 3, entry 1): Isoquinoline (61 mL,
0.50 mmol; 97% purity) was dissolved in diethyl ether (5.0 mL) in a
flame-dried round-bottomed flask and cooled to 08C. 2,2,2-Trichloro-
ethyl chloroformate (76 mL, 0.55 mmol, 1.1 equiv; 98% purity) was
added dropwise by syringe, and the resulting white suspension was
warmed to 238C, stirred for 30 min, and then cooled to ꢀ788C (dry
ice/isopropanol bath). Catalyst 1 (26.9 mg, 0.050 mmol, 10 mol%) in
Table 3: Acyl-Mannich reaction ofsubstituted isoquinolines.
diethyl ether (4.0 mL
butyldimethylsilyloxy)-1-isopropoxyethene
+
1.0 mL rinse volume) and then 1-(tert-
(216 mg, 1.0 mmol,
Entry
R
Yield [%][a]
ee [%][b]
2.0 equiv) were added. The reaction mixture was warmed to ꢀ708C
(isopropanol bath equipped with immersion cooler) and stirred for
14 h. Cooling was stopped and the bath allowed to warm to 238C over
3 h. After the solvent was removed in vacuo, the residue was purified
by chromatography on silica gel (0!5% ethyl acetate/hexanes), thus
1
2
3
4
5
6
7
8
H
3-Me
4-Br
5-Br
5-OTBS
5-NO2
6-OSO2CF3
7-OTBS
80
75
78
77
77
71
67
86
86
92
91
87
83
71
83
60
yielding
a colorless oil (161 mg, 0.40 mmol, 80% yield). The
enantiomeric excess was determined to be 86% by SFC using a
commercial chiral stationary phase (Chiralpak OD-H, 5% methanol/
CO2, 5 mLminꢀ1
, 508C, 285 nm; tr(minor): 2.23 min, tr(major):
2.66 min); aD ꢀ2408 (c = 1.1 g 100 mLꢀ1
,
CH2Cl2); 1H NMR
[a] Yield ofisolated product atfer column chromatography. [b] Enantio-
meric excess determined by SFC or HPLC using commercially available
chiral stationary-phase columns.
(500 MHz, CDCl3): the compound exists as a 1.7:1 mixture of
carbamate rotamers; signals corresponding to the major rotamer: d =
7.26–7.17 (3H, m), 7.10 (1H, d, J = 7.5 Hz), 6.90 (1H, d, J = 7.5 Hz),
5.99 (1H, d, J = 8.0 Hz), 5.84–5.82(1H, m), 4.97–4.88 (2H, m), 4.78
(1H, d, J = 12.0 Hz), 2.67–2.58 (2H, m), 1.20 (3H, d, J = 6.0 Hz),
1.16 ppm (3H, d, J = 6.5 Hz); representative signals corresponding to
the minor rotamer: d = 6.90 (1H, d, J = 7.5 Hz), 6.04 (1H, d, J =
8.0 Hz), 4.82(1H, d, J = 11.5 Hz), 2.77 (1H, dd, J = 14.0, 9.0 Hz),
1.11 ppm (3H, d, J = 5.5 Hz); 13C NMR (100 MHz, CDCl3), signals
corresponding to both rotamers: d = 169.6, 169.5, 151.6, 151.2, 131.4,
131.2, 129.9, 129.8, 128.6, 128.5, 127.7, 127.5, 126.7, 126.6, 125.5, 125.3,
124.5, 123.6, 110.7, 110.6, 95.2, 95.2, 75.7, 75.6, 68.4, 68.4, 53.3, 53.1,
40.8, 40.1, 22.0, 22.0, 21.9, 21.9 ppm; IR (neat): n˜ = 3063
precursors to enantioenriched 1-substituted tetrahydroisoqui-
nolines: hydrogenation of the enamide moiety and reductive
cleavage of the trichloroethyl carbamate group occur in good
yield without detectable racemization (Scheme 2).[15] Thus,
this method represents a straightforward approach to the
preparation of enantioenriched heterocycles with potential
(w), 2980 (m), 2936 (w), 1728 (s), 1651 (s), 1452 (m), 1262
(m), 1109 (m), 968 cmꢀ1 (w); HRMS (ES): m/z calcd for
[C17H18Cl3NO4+H]+: 406.0380; found: 406.0380.
Received: June 29, 2005
Published online: September 21, 2005
Keywords: asymmetric catalysis · heterocycles ·
.
Scheme 2. Synthesis ofan enantioenriched tetrahydroisoquinoline. TFA =trifluoroacetic
acid.
hydrogen bonds · Mannich reaction · urea derivatives
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[10] A preliminary survey of ketone enolates did not yield satisfac-
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went addition to isoquinoline in the presence of TrocCl and 1 to
yield the corresponding dihydroisoquinoline in 14% ee (unopti-
mized).
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[11] A list of solvents and other catalysts tested may be found in the
Supporting Information.
[12] Slow evaporation of a solution of 1 in hexanes/diethyl ether
yielded crystals suitable for X-ray analysis; crystal data for 1:
C32H50N4OS, Mr = 538.82, colorless prism, 0.20 ” 0.18 ” 0.14 mm,
orthorhombic, a = 16.590(3), b = 17.202(3), c = 22.411(4) Š, V=
6396(2) Š3, T= 193(2) K, space group C2221, Z = 8, 1calcd
=
1.119 gcmꢀ3, m = 0.130 mmꢀ1; a total of 22451 reflections were
measured, 7659 independent, final residuals were R1 = 0.0394
and wR2 = 0.0877 for 7659 observed reflections with I > 2s(I),
543 parameters, GOF = 0.960, maximum residual electron
density 0.438 eŠꢀ3; data were collected on a Bruker SMART
CCD (charge-coupled device) based diffractometer equipped
with an Oxford Cryostream low-temperature apparatus; data
were measured by using scans of 0.38 per frame for 45 s, such that
a hemisphere was collected; a total of 1271 frames were
collected with a maximum resolution of 0.76 Š; the first
50 frames were recollected at the end of data collection to
monitor for decay; the structure was solved by the direct method
(G. M. Sheldrick, SHELXL-97, Program for the Solution of
Crystal Structures, University of Göttingen, Germany, 1997)
followed by refinement by the least-squares method on F2
(SHELXL-97); all non-hydrogen atoms were refined anisotropi-
cally; the positions of hydrogen atoms were found by difference
Fourier methods and refined isotropically. CCD-275454 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.
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diastereoselective variant that employs a chiral acyl chloride,
see reference [3f].
[6] a) Examples of enantioselective additions of silyl ketene acetals
to 6,7-dialkoxy-3,4-dihydroisoquinoline N-oxides have been
reported: S.-I. Murahashi, Y. Imada, T. Kawakami, K. Harada,
Y. Yonemushi, N. Tomita, J. Am. Chem. Soc. 2002, 124, 2888 –
2889; b) proline-catalyzed Mannich reactions of 3,4-dihydro-b-
carbolines have been reported: T. Itoh, M. Yokoya, K. Miyauchi,
K. Nagata, A. Ohsawa, Org. Lett. 2003, 5, 4301 – 4304.
[7] For enantioselective reactions catalyzed by chiral urea or
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[13] A neutral chloroamide structure, rather than an ion pair, may
represent a more accurate depiction of the bonding interactions
in N-acyliminium chlorides in nonpolar organic solvents (for an
NMR spectroscopic study that supports this assertion, see: A. K.
Bose, G. Spiegelman, M. S. Manhas, Tetrahedron Lett. 1971,
3167 – 3170). Calculations that use the density-functional theory
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Communications
(B3LYP/6-31G + + (d,p)) predict substantial chloroamide char-
acter for N-acylisoquinolinium chloride (M. S. Taylor, E. N.
Jacobsen, unpublished results), and the strong dependence of
enantioselectivity upon solvent polarity observed in both acyl-
Mannich and acyl-Pictet–Spengler reactions may be a reflection
of the importance of the pairing of the N-acyliminium ions in
these processes. These reactions are also subject to dramatic
leaving-group effects upon reactivity and enantioselectivity, a
phenomenon that seems difficult to reconcile with a fully ionized
N-acyliminium ion as the reactive species. The possibility that 1
activates intermediate chloroamides toward substitution reac-
tions by hydrogen-bonding interactions with the carbonyl group
represents an intriguing, albeit speculative, mechanistic hypoth-
esis that is consistent with these observations.
[14] For a discussion of the crystallization and cocrystallization of
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actions, see: a) M. C. Etter, T. W. Panunto, J. Am. Chem. Soc.
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[15] Transesterification of the resulting product permitted the
absolute configuration to be assigned (see the Supporting
Information for details).
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