Organocatalytic enantioselective synthesis of 1-vinyl tetrahydroisoquinolines through allenamide activation with chiral Bronsted acids

Article abstract of DOI:10.1039/c4ra14942d

In this paper we present a new approach for the realization of tetrahydroisoquinoline scaffolds via stereoselective proto-activation of suitable allenamide precursors. The elusive and rather unstable iminium ion derived from acrylaldehyde is generated in situ and this electrophilic intermediate can be engaged in stereoselective intramolecular Friedel-Crafts-type allylic alkylation with electron-rich aromatic rings. The highest enantioselectivity for tetrahydroisoquinoline intermediates, obtained by organocatalytic transformation, is reported.

Full text of DOI:10.1039/c4ra14942d

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Organocatalytic enantioselective synthesis of
1-vinyl tetrahydroisoquinolines through allenamide
Cite this: RSC Adv., 2015, 5, 10546
activation with chiral Brønsted acids†
*
E. Manoni,‡ A. Gualandi,‡ L. Mengozzi, M. Bandini and P. G. Cozzi
In this paper we present a new approach for the realization of tetrahydroisoquinoline scaffolds via
stereoselective proto-activation of suitable allenamide precursors. The elusive and rather unstable
iminium ion derived from acrylaldehyde is generated in situ and this electrophilic intermediate can be
engaged in stereoselective intramolecular Friedel–Crafts-type allylic alkylation with electron-rich
aromatic rings. The highest enantioselectivity for tetrahydroisoquinoline intermediates, obtained by
organocatalytic transformation, is reported.
Received 20th November 2014
Accepted 5th January 2015
DOI: 10.1039/c4ra14942d
www.rsc.org/advances
The interest in this class of pharmaceutically active alkaloids
has prompted ourishing research towards the development of
innovative synthetic methods for their preparation.3 In partic-
ular, due to the presence of stereogenic centres in stereo-
chemically dened manner, enantioselective catalysis
represents the forefront of chemical research, with particular
emphasis on transition metal-based approaches.4
Introduction
Natural and synthetic 1-substituted tetrahydroisoquinolines
such as emetine, norcoclaurine, solifenacine, morphine, apor-
phinoid and erythrocarine alkaloids have received increased
attention in medicinal and synthetic chemistry due to their
wide portfolio of pharmacological applications (Fig. 1).1,2
In this segment, the asymmetric functionalization of iso-
quinoline cores focuses on the introduction of a substituent on
the C(1)-position (Reissert reaction),5 as this stereocenter is
present in most of the naturally occurring alkaloids.
The rst enantioselective catalytic version of the Reissert
reaction4 was introduced by Shibasaki in 2000,6 and the time
frame testimonies the difficulties related to these reactions.
Aerwards, a number of enantioselective methodologies has
been successively described,7 however moderate levels of ster-
eoinduction were usually recorded. Quite recently, a highly
stereoselective copper-catalysed alkynylation of tetrahy-
droisoquinoline derivatives was described by Ma,8 providing a
new methodology in the synthesis of alkaloids.9
Additionally, metal catalysed stereoselective hydrogenation
reactions of isoquinolines and 3,4-dihydroisoquinolines gave
an affordable and frequently used methodology for useful
intermediates in high ee.10
However, both the described approaches are not metal-free.
Avoiding the use of expensive and toxic metals in late-stage
synthesis may be highly desirable in view of pharmaceutical
applications. A direct approach towards tetrahydroisoquinoline
alkaloids would be the bio-mimetic Pictet–Spengler condensa-
tion11 starting from a suitably functionalized 2-phenylethyl-
amine, in the presence of Brønsted acids12 or chiral thiourea
catalysts.13 However, although a considerable number of enan-
tioselective organocatalysed Pictet–Spengler reactions has been
reported, the amine precursors were restricted to quite nucle-
ophilic tryptamines and their derivatives.14 Three important
Fig. 1 Examples of hydroisoquinoline alkaloids.
`
ALMA MATER STUDIORUM – Universita di Bologna, Dipartimento di Chimica “G.
Ciamician”, Via Selmi 2, 40126 Bologna, Italy. E-mail: piergiorgio.cozzi@unibo.it
† Electronic supplementary information (ESI) available: Procedure for the
reparation of the amides 8a–i and their fully characterization. Procedure and
complete characterization of the compounds 10a–i and 11a–e. Copies of all 1H
NMR and 13C spectra. Traces of HPLC analysis of racemic and enantioenriched
compounds. See DOI: 10.1039/c4ra14942d
‡ These authors contributed equally to this work.
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factors have contributed to establish the success of these
The generation of an equivalent of acrylaldehyde, and its
transformations: (a) the highly nucleophilic indole system, (b) synthetically exible route to a-vinyl-substituted tetrahy-
the use of N(1)-free indoles enabling the establishment of droisoquinoline building blocks, was described by Hiemstra
hydrogen bond recognition with the Brønsted acid catalyst, (c) and Rutjes in by means of [Sn(II)] or TFA-catalysed cyclization of
the peculiar electronic properties of the indolyl core, able to allylic N,O-acetals (Fig. 2).20
engage secondary interactions with aromatic substituents of the
chiral organocatalyst.
Allenamides appear as a suitable alternative synthetic route
to this important intermediates. In particular, allenamides are
A phenylethylamine moiety was employed recently in a few highly susceptible towards metal and Brønsted acid promoted
remarkable examples of stereoselective Pictet–Spengler electrophilic activation21 and this was clearly demonstrated by
´
´
condensations under binary metal/metal-free catalytic regime. Navarro-Vasquez and Domınguez in the TFA-catalysed synthesis
In details, Toda and Terada,15 reported a ruthenium-catalysed of a-vinyl-substituted tetrahydroisoquinolines (Fig. 2).22 More-
alkene isomerization combined with an enantioselective orga- over, allenamides were also used to access vinyl-isoquinolines by
nocatalysed Pictet–Spengler type cyclization promoted by chiral electrophilic activation with [gold(I)] complexes.23
phosphoric acid (Scheme 1). A fully organocatalytic enantiose-
Based on these precedents in literature and recent ndings
lective Pictet–Spengler cyclization was also described by Hiem- by some of us,24 we envisioned the use of chiral Brønsted acids
stra,16 however extra steps were required for the preparation of with allenamides, as a potent and effective way to prepare useful
suitable starting materials and for the preparation of the nal enantioenriched 1-vinyl tetrahydroisoquinoline precursors. We
alkaloids (Scheme 1). In both cases, the use of free OH groups is started our endeavour with the synthesis of the allenamides
necessary in order to enhance both enantioselectivity and 10a–i carrying different electron-withdrawing groups (EWGs) at
nucleophilic prole of the thematic moiety.
the nitrogen atom. The preparation of the amides 8a–i was
In this paper, we describe a completely metal-free strategy straightforward and it is described in the ESI.† In order to
for a stereoselective Friedel–Cras-type cyclization to afford improve the yield in the reaction with propargyl bromide, we
tetrahydroisoquinoline alkaloids, featuring synthetically ex- carried out the propargylation of amides 8a–i in THF–CH2Cl2
ible vinyl group at the a-position.17
due to the low solubility of the amides. During this step traces of
corresponding allenamides 10a–i were isolated from the reac-
tion mixture. The isolated propargylic amides 9a–i were then
treated with tBuOK in THF following the described procedure22
to give the desired allenamides 10a–i that, however, were only
obtained in low yields. Aer considerable efforts we found a
more suitable and reproducible last-step procedure employing
NaH as base (1 eq.) in the presence of a catalytic amount of
tBuOK (30 mol%). The corresponding allenamides were
obtained from low to excellent yields (Table 1).
Results and discussion
The access to a-vinyl-substituted tetrahydroisoquinolines using
the Pictet–Spengler or Bischler–Napieralski approaches is
problematic and unsuitable due to the difficulties to use acryl-
aldehyde as electrophile.18 Indeed, it has been demonstrated
that only substituted vinyl moieties can be introduced through
the Pictet–Spengler condensation.19
Therefore, allenamides 10a–e were efficiently cyclized to the
desired racemic tetrahydroisoquinolines employing the reac-
´
´
tion conditions described by Navarro-Vasquez and Domınguez
(TFA 20 mol%, CH2Cl2, rt). The vinyl-substituted heterocyclic
compounds 11a–e were isolated in moderate to good yields,
through the Friedel–Cras reaction of the iminium ion inter-
mediates. Aromatic rings carrying EWGs were not investigated
because it had already been reported that only electron-rich
nucleophiles attach the iminium ion. In fact, substrates
Scheme 1 Pictet–Spengler reactions with 2-phenylethylamine skel- Fig. 2 Different approaches to obtain the key synthetic intermediate
eton promoted by chiral phosphoric acids.
for the synthesis of 1-vinyl tetrahydroisoquinolines.
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Table 1 Preparation of the key allenamides through a propargylation– Table
2
Friedel–Crafts reaction of protected allenamides in the
rearrangement reaction sequence
presence of different Brønsted acids and conditions
Entrya Amide R1/R2/R3
EWG
Yieldb 9 (%) Yieldb 10 (%)
1
2
3
4
5
6
7
8
9
8a
8b
8c
8d
8e
8f
8g
8h
8i
H/OMe/OMe Ts
89
91
10
27
68
36
80
60
69
64
H/OMe/OMe PhCO 18
H/OMe/OMe ArCOc 27
H/OMe/OMe Boc
H/OMe/OMe CHO
68
36
63
57
51
48
H/OBn/OBn
H/–OCH2O–
CHO
CHO
Entrya
Allenamide
Solvent
cat
Yieldb 11 (%)
eec 11 (%)
OMe/OMe/H CHO
H/OMe/H CHO
a
b
1
2
3
4
5
6
7
8
10a
10b
10c
10d
10e
10e
10e
10e
10e
10e
10e
10e
10f
DCM
DCM
DCM
DCM
12a
12a
12a
12a
12a
12a
12a
12a
12b
12c
12d
12e
12f
12f
12f
12f
63
65
63
30
40
18
48
36
27
33
40
26
0
21
14
0
32
See ESI for detailed reaction conditions. Isolated yields aer
chromatographic purication. c Ar ¼ 3,5-(CF3)2C6H3.
DCM
47
Toluene
C6H5CF3
C6H5F
72d
75d
70d
79d
78d
81d
72d
—
lacking of methoxy groups on the aromatic ring failed to give
the desired product also aer prolonged reaction time and
harsh conditions (50% TFA, DMF, 70 ꢀC).22 The formation of the
six-membered ring was already shown to be favoured as
homologous compounds failed to cyclize under the standard
reaction conditions. It is worth to mention that adventitious
water can result in hydrolysis of the secondary amide, as the
iminium ion is quite unstable. Therefore, anhydrous conditions
proved to be essential for the reaction.
With the desired starting material in hand, we have investi-
gated in details the stereoselective reaction by varying Brønsted
acid, solvent, temperature, and using the differently substituted
allenamides 10a–e (Table 2). Some of the Brønsted phosphoric
acids screened were commercially available (12a, 12d), alterna-
tively, they were prepared using standard protocols.25
9
C6H5CF3
C6H5CF3
C6H5CF3
C6H5CF3
C6H5CF3
C6H5CF3
C6H5CF3
C6H5CF3
10
11
12
13
14
15
16
10g
10h
10i
0
0
0
—
—
—
a
All the reactions were carried out in dried conditions under nitrogen
˚
b
atmosphere by using 0.1 mmol of allenamides, 50 mg of 4 A MS and
10 mol% of the catalyst, in 1 mL of solvent for 24 hours. Isolated
yields aer chromatographic purication. Determined by chiral
c
HPLC analysis performed on the crude reaction mixture.
d
Determined by chiral HPLC analysis performed aer reduction with
LiAlH4 of the formyl protecting group to the corresponding methyl
group (see ESI for details).
A selection of results is depicted in Table 2, clearly outlining
the importance of the nitrogen-tethered EWGs (Table 2) in
stabilizing the allenamide. In particular, although tosyl or other
robust protecting groups (i.e. Boc, benzoyl, 3,5–(CF3)2–C6H3CO)
proved competent under optimal conditions (cat ¼ TFA or cat ¼
phosphoric acid), low enantiomeric excesses were generally
recorded with chiral promoters (Table 2, entries 1–4).
To improve the stereoselectivity of the transformation we
hypothesised that the presence of a group able to interact by
hydrogen-bond with the chiral phosphate could positively
impact onto the stereochemical outcome of the protocol. As a
result, we designed and realised the N-formyl allenamide via
treatment of the corresponding amine with ethylformate under
reuxing conditions for several hours (see ESI† for details).
Delightfully, our prediction was veried and the enantio-
meric excess for the product 11e improved considerably
(Table 2, entry 5). The ne-tuning of the solvent and the catalyst
(Table 2, entries 6–12) allowed reaching an unprecedented ee of
81% for the synthesis of vinyl tetrahydroisoquinolines
promoted by chiral Brønsted acid catalysis.
As we have already mentioned in the article, the presence of
activated molecular sieves is quite crucial to obtain satisfying
yields. Moreover, in the presence of molecular sieves the allena-
mides are not hydrolysed by adventitious water and can be recov-
ered by chromatography. When very hindered chiral Brønsted acid,
such as 12e, was used, 11e was isolated only in 26% yield (Table 2,
entry 12). The enantiomeric excesses were evaluated by chiral
HPLC analysis for all the derivatives. However, in the case of
product 11e it was necessary to transform the formyl protecting
group into the corresponding methyl by reduction (LiAlH4) in order
to properly separate the enantiomers by chiral HPLC.
In order to evaluate the scope of this procedure, we have
submitted allenamides 10f–i to the optimized reaction condi-
tions. Unfortunately, in all cases we were unable to isolate any
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110.6 (1CA), 111.3 (1CA), 111.5 (1CB), 117.0 (1CB), 117.3 (1CA),
125.2 (1CA + 1CB), 125.4 (1CA), 126.5 (1CB), 136.4 (1CA), 138.0
(1CB), 147.6 (1CB), 147.7 (1CA), 148.0 (1CA), 148.3 (1CB), 161.1
(1CA), 161.6 (1CB); ESI-MS: m/z ¼ 248.2 [M + H]+, 270.0 [M + Na]+,
495.2 [2M + H]+; HMRS calcd for C18H19N: 247.12084; found
247.12075.
Conclusions
In conclusion, we have reported the cyclization of allenamides
to 1-vinyl tetrahydroisoquinolines in the presence of chiral
phosphoric acids. The formyl protecting group turned out to be
crucial for the catalysis by improving substrate/catalyst recog-
nition via hydrogen bond interaction. The use of formyl group
in other Brønsted acid catalysed reactions is suggested.27
Fig. 3 Proposed transition state for the reaction.
traces for the desired products using less hindered Brønsted
acid 12f. In the case of the allenamides 10f, 10h and 10i the
major hindrance of the substrates is probably retarding
considerably the cyclization. In such conditions the hydrolysis
of the iminium by adventitious water is competitive and we
observed the presence of the corresponding amides. In the case
of allenamides 10g we can suggest that the reduced nucleo-
philicity of the aromatic ring can be responsible for the failure
of the reaction.
Acknowledgements
Bologna University, Fondazione Del Monte, and the European
Commission through the project MOLARNET, FP7-ICT, 318516,
are acknowledged for nancial support.
Notes and references
Finally, the absolute conguration of 11e, obtained with (R)-
12d, was established to be R, by comparison of optical rotation
with literature data.15 On the basis of this nding the model
illustrated in Fig. 3 is suggested for the transition state of the
reaction. In particular, given the importance of the formyl group,
we assume that the recognition and the high enantiomeric excess
obtained for the reaction is determined by the hydrogen bonding
of the catalyst with the hydrogen atom of the formyl group.26
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Experimental
General procedure for synthesis of compound 11e
To a solution of allenamide (0.1 mmol, 1 equiv.) in tri-
uorotoluene (1 mL) were added the Brønsted acid (0.01 mmol,
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was stirred under nitrogen for 24 h. The solvent was removed
under reduced pressure and the crude was directly puried by
ash chromatography on silica gel (cyclohexane/ethyl acetate,
3/7) to give 11e: sticky white solid, 9.9 mg, 40% yield, 81% ee;
the ee was determined via reduction of the product with LiAlH4
by HPLC analysis, Daicel Chiralpak® OD-H column: n-hexane/i-
PrOH from 90 : 10, ow rate 0.70 mL minꢁ1, 40 ꢀC, l ¼ 285 nm:
smajor ¼ 7.87 min, sminor ¼ 14.93 min; 1H NMR (400 MHz,
CDCl3) (two rotamers A : B, ratio 1 : 1): d 2.62–2.72 (2HB, m),
2.78–2.91 (2HA, m), 3.13 (1HB, ddd, J ¼ 4.7 Hz, J ¼ 11.0 Hz, J ¼
15.6 Hz), 3.45 (1HA, ddd, J ¼ 4.2 Hz, J ¼ 13.0 Hz, J ¼ 13.0 Hz),
3.65 (1HA, dd, J ¼ 5.8 Hz, J ¼ 13.2 Hz), 3.80 (3HA, s), 3.81 (3HA +
3HB, s), 3.82 (3HB, s), 4.29 (1HB, m), 5.02 (1HB, d, J ¼ 5.7 Hz),
5.09 (1HA, d, J ¼ 17 Hz), 5.11 (1HB, d, J ¼ 17.0 Hz), 5.19 (1HA, d,
J ¼ 10.1 Hz), 5.20 (1HB, d, J ¼ 10.1 Hz), 5.78 (1HA, d, J ¼ 5.6 Hz),
5.86–5.99 (1HA + 1HB, m), 6.55 (1HA, s), 6.56 (1HA + 1HB, s), 6.59
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¨
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