Mechanistic studies on the cis to trans epimerization of trisubstituted 1,2,3,4-tetrahydro-β-carbolines

Article abstract of DOI:10.1021/jo1003778

Figure presented It is well-known that N_b-benzyltryptophan alkyl esters undergo the Pictet-Spengler reaction with aldehydes to furnish both cis- and trans-1,2,3,4-tetrahydro-β-carbolines, with the trans isomer predominating. Epimerization at C-1 took place under acidic conditions to produce, exclusively, the thermodynamically more stable trans diastereomer via internal asymmetric induction. Recent kinetic experiments provided insight into the cis to trans epimerization mechanism involved in the Pictet-Spengler reaction of 1,2,3-trisubsituted tetrahydro-β-carbolines. Since the epimerization reaction had been shown to be sensitive to electronic effects at C-1, the rate data for a series of 1-phenyl-substituted 1,2,3,4-tetrahydro- β-carbolines was investigated via a Hammett study. Analysis of the data supported the presence of a positively charged intermediate with a φ value of -1.4, although the existence of an iminium ion intermediate or a carbocationic intermediate could not be determined from this data alone. Analysis of the rate of epimerization demonstrated first-order kinetics with respect to TFA following the initial protonation of the substrate. This observation was consistent with the formation of a doubly protonated intermediate as the rate-determining step in the carbocation-mediated cis to trans epimerization process. In addition, the observed first-order rate dependence was inconsistent with the retro-Pictet-Spengler mechanism since protonation at the indole-2 position was not rate determining as demonstrated by kinetic isotope effects. Based on this kinetic data, the retro-Pictet-Spengler pathway was ruled out for the cis to trans epimerization of 1,2,3-trisubstituted 1,2,3,4-tetrahydro-β-carbolines, while the olefinic mechanism had been ruled out by experiments carried out in TFA-d.

Full text of DOI:10.1021/jo1003778

pubs.acs.org/joc
Mechanistic Studies on the Cis to Trans Epimerization of Trisubstituted
1,2,3,4-Tetrahydro-β-carbolines
Michael L. Van Linn and James M. Cook*
Department of Chemistry and Biochemistry, University of Wisconsin;Milwaukee, 3210 North Cramer
Street, Milwaukee, Wisconsin 53211
capncook@uwm.edu
Received February 28, 2010
It is well-known that Nb-benzyltryptophan alkyl esters undergo the Pictet-Spengler reaction with
aldehydes to furnish both cis- and trans-1,2,3,4-tetrahydro-β-carbolines, with the trans isomer
predominating. Epimerization at C-1 took place under acidic conditions to produce, exclusively,
the thermodynamically more stable trans diastereomer via internal asymmetric induction. Recent
kinetic experiments provided insight into the cis to trans epimerization mechanism involved in the
Pictet-Spengler reaction of 1,2,3-trisubsituted tetrahydro-β-carbolines. Since the epimerization
reaction had been shown to be sensitive to electronic effects at C-1, the rate data for a series of
1-phenyl-substituted 1,2,3,4-tetrahydro-β-carbolines was investigated via a Hammett study. Analy-
sis of the data supported the presence of a positively charged intermediate with a F value of -1.4,
although the existence of an iminium ion intermediate or a carbocationic intermediate could not be
determined from this data alone. Analysis of the rate of epimerization demonstrated first-order
kinetics with respect to TFA following the initial protonation of the substrate. This observation was
consistent with the formation of a doubly protonated intermediate as the rate-determining step in the
carbocation-mediated cis to trans epimerization process. In addition, the observed first-order rate
dependence was inconsistent with the retro-Pictet-Spengler mechanism since protonation at the
indole-2 position was not rate determining as demonstrated by kinetic isotope effects. Based on this
kinetic data, the retro-Pictet-Spengler pathway was ruled out for the cis to trans epimerization of
1,2,3-trisubstituted 1,2,3,4-tetrahydro-β-carbolines, while the olefinic mechanism had been ruled out
by experiments carried out in TFA-d.
Introduction
medicinal chemistry include potential implications for β-car-
bolines in the treatment of alcoholism.4,5 The enantiospecific
Pictet-Spengler reaction, in particular, has stimulated wide-
spread interest in both organic and medicinal chemistry6-19
The Pictet-Spengler reaction,1 which involves the cycliza-
tion of electron-rich aryl or heteroaryl groups onto imine or
iminium ion electrophiles, has long been a standard method
for the construction of both tetrahydroisoquinolines1 and
tetrahydro-β-carbolines.2 These privileged scaffolds appear
in a diverse array of biologically active compounds; both of
these ring systems are popular choices for combinatorial
libraries targeted at drug discovery.3 Recent advances in
(4) Harvey, S. C.; Foster, K. L.; McKay, P. F.; Carroll, M. R.; Seyoum,
R.; Woods, J. E., II; Grey, C.; Jones, C. M.; McCane, S.; Cummings, R.;
Mason, D.; Ma, C.; Cook, J. M.; June, H. L. J. Neurosci. 2002, 22, 3765.
(5) Foster, K. L.; McKay, P. F.; Seyoum, R.; Milbourne, D.; Yin, W.;
Sarma, P.; Cook, J. M.; June, H. L. Neuropsychopharmacology 2004, 29, 269.
(6) Ungemach, F.; DiPierro, M.; Weber, R.; Cook, J. M. J. Org. Chem.
1981, 46, 164.
(7) Czarnocki, Z.; Suh., D.; MacLean, D. B.; Hultin, P. G.; Szarek, W. A.
Can. J. Chem. 1992, 70, 1555.
(8) Reddy, M. S.; Cook, J. M. Tetrahedron. Lett. 1994, 35, 5413.
(9) Waldmann, H.; Schmidt, G.; Henke, H.; Burkard, M. Angew. Chem.,
Int. Ed. 1995, 34, 2402.
(1) Pictet, A.; Spengler, T. Ber. Dtsch. Chem. Ges. 1911, 44, 2030.
(2) Tatsui, G. Yakugaku Zasshi 1928, 48, 453.
(3) Bunin, B. A.; Dener, J. M.; Kelly, D. E.; Paras, N. A.; Tario, J. D.;
Tushup, S. P. J. Comb. Chem. 2004, 6, 487.
DOI: 10.1021/jo1003778
r
Published on Web 04/29/2010
J. Org. Chem. 2010, 75, 3587–3599 3587
2010 American Chemical Society

Van Linn and Cook
JOCArticle
FIGURE 1. Proposed intermediates in the cis to trans epimerization.
and has resulted in the total synthesis of a number of natural
products including vincamajinine, alstonisine, macralstoni-
dine, (-)-alstophylline, (-)-macralstonine, ajmaline, and
(-)-raumacline.20 Alternatively, several groups have extended
the scope to examine the ratio of cis/trans isomers of the
Pictet-Spengler reaction employing tryptophan derivatives
with aldehydes and this has been reviewed.21,22 The develop-
ment of chiral auxiliaries by Reddy8 and Nakagawa23,24
provided the basis for asymmetric Pictet-Spengler reactions.
In addition, Nakagawa10 had also explored Brønsted acid
assisted Lewis acids in stoichiometric amounts to induce stereo-
selectivity in the Pictet-Spengler reaction. Recently, important
examples of external asymmetric induction via organocatalysis
have been applied to the Pictet-Spengler cyclization. Jacobsen
et al.12,16,17 have developed chiral thiourea catalysts for the
asymmetric Pictet-Spengler reaction of tryptamines. Alterna-
tively, List13 and van Maarseveen14,15,18 have investigated chiral
phosphoric acid catalysts with much success.
Although progress has been made in regard to synthesis,
interestingly, the mechanism of the cis to trans isomerization
in the presence of Brønsted acids has not been fully defined.
Three potential mechanistic pathways have been proposed
and their intermediates are depicted in Figure 1. The olefinic
pathway had been previously ruled out with aliphatic
substituents at the C-1 position22 and was therefore not
considered in the kinetic analysis. A structural basis for the
development of the proposed carbocationic intermediate
had been described by Han.25 The conformation of the
C-ring was found to be important during the epimerization
process; ultimately, the p-orbital of the developing car-
bocation was thought to overlap well with the indole π-
electrons. In efforts aimed to investigate the mechanism of
the isomerization at C-1, a set of readily accessible electron-
donating and electron-withdrawing substituted aromatic
aldehydes have been reacted with L-tryptophan ethyl ester,
followed by Nb-benzylation, to provide optically active cis
and trans 1,2,3-trisubsituted tetrahydro-β-carbolines.26
In a general sense, electron-rich substituents on aromatic
rings at the C-1 position enhanced the rate of the cis to trans
epimerization.26 This result was in agreement with the pro-
posed carbocationic mechanism of epimerization, although
the retro-Pictet-Spengler process could not be completely
ruled out at that time.26 In this work, the kinetics of the
cis to trans epimerization of 1,2,3-trisubsituted tetrahydro-β-
carbolines was studied to elucidate the mechanism of this
epimerization.
Results and Discussion
Since the general trend of rates of epimerization was in
place, a detailed kinetic study was designed. A Hammett
linear free energy relationship is a common method in the
application of kinetic data to mechanistic problems.27,28 In
order to use this method, the parent 1-phenyltetrahydro-β-
carboline (7d) was employed as one of the molecules to be
studied. This parent substrate was obtained from benzalde-
hyde and L-tryptophan ethyl ester when stirred under the
conditions of the Pictet-Spengler cyclization previously
described.26 In addition, substrates which carried a series
of substituents with a range of electron densities at the C-1
position were easily obtained by a Pictet-Spengler cyclization,
followed by separation of the cis and trans diastereomers and
subsequent Nb-benzylation, as previously described (Scheme 1
and Table 1).26 Several of the analogues were very sensitive to
acid-induced epimerization; therefore, large quantities of base
were used during the alkylation reaction.
(10) Yamada, H.; Kawate, T.; Matsumizu, M.; Nishida, A.; Yamaguchi,
K.; Nakagawa, M. J. Org. Chem. 1998, 63, 6348.
(11) Gremmen, C.; Willemse, B.; Wanner, M. J.; Koomen, G. J. Org.
Lett. 2000, 2, 1955.
(12) Taylor, M. S.; Jacobsen, E. N. J. Am. Chem. Soc. 2004, 126, 10558.
(13) Seayad, J.; Seayad, A. M.; List, B. J. Am. Chem. Soc. 2006, 128, 1086.
(14) Wanner, M. J.; van der Haas, R. N. S.; de Cuba, K. R.; van
Maarseveen, J. H.; Hiemstra, H. Angew. Chem., Int. Ed. 2007, 46, 7485.
(15) Wanner, M. J.; Boots, R. N. A.; Eradus, B.; de Gelder, R.; van
Maarseveen, J. H.; Hiemstra, H. Org. Lett. 2009, 11, 2579.
(16) Reisman, S. E.; Doyle, A. G.; Jacobsen, E. N. J. Am. Chem. Soc.
2008, 130, 7198.
(17) Klausen, R. S.; Jacobsen, E. N. Org. Lett. 2009, 11, 887–890.
(18) Sewgobind, N. V.; Wanner, M. J.; Ingemann, S.; de Gelder, R.; van
Maarseveen, J. H.; Hiemstra, H. J. Org. Chem. 2008, 73, 6405.
(19) Shi, X.-X.; Liu., S.-L.; Xu, W.; Xu, Y.-L. Tetrahedron: Asymmetry
2008, 19, 435.
(20) Edwankar, C. R.; Edwankar, R. V.; Rallapalli, S.; Cook, J. M. Nat.
Prod. Commun. 2008, 3, 1839.
(21) Cox, E. D.; Cook, J. M. Chem. Rev. 1995, 95, 1797.
(22) Czerwinski, K. M.; Cook, J. M. In Advances in Heterocyclic Natural
Products Synthesis; Pearson, W., Ed.; JAI Press: Greenwich, CT, 1996;
Vol. 3, p 217.
Since the synthesis of a series of electronically altered
molecules had been completed (see Supporting Information
for details), the Hammett analysis was undertaken. Instead
of removing aliquots of the reaction mixture during the
epimerization process with TFA as previously executed,26
(23) Soe, T.; Kawate, T.; Fukui, N.; Hino, T.; Nakagawa, M. Hetero-
cycles 1996, 42, 347.
(24) Kawate, T.; Yamanaka, M.; Nakagawa, M. Heterocycles 1999, 50,
1033.
(26) Kumpaty, H. J.; Van Linn, M. L.; Kabir, M. S.; Forsterling, F. H.;
Deschamps, J. R.; Cook, J. M. J. Org. Chem. 2009, 74, 2771.
(27) Hammett, L. P. Chem. Rev. 1935, 17, 125.
€
(25) Han, D. M.; Forsterling, F. H.; Deschamps, J. R.; Parrish, D.; Liu,
X. X.; Yin, W. Y.; Huang, S. M.; Cook, J. M. J. Nat. Prod. 2007, 70, 75.
(28) Hammett, L. P. J. Am. Chem. Soc. 1937, 59, 96.
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SCHEME 1. Synthesis of Trisubstituted 1,2,3,4-Tetrahydro-
β-carbolines
Experiments designed to determine useful reaction condi-
tions to study the epimerization process in situ were not
trivial. Sampling rate of the detection method (1H NMR),
detection interference of either reagents or intermediates, as
well as detection of both the cis and trans epimers proved to be
of importance in the experimental design. First, the reaction
conditions were set up in order to observe the reaction on a
reasonable time scale with the ability to monitor the cis to
trans conversion. Also, reactants or intermediates must not
interfere with the detection of either isomer. In order to obtain
useful data, the acid (and its concentration), the cis isomer
concentration, and the temperature were parameters that
were altered.
Initially, the effect of the acid strength on rate was studied.
Several acids were employed in the epimerization reaction at
different concentrations. Acetic acid proved to be insuffi-
cient since epimerization of 7d (12.5 mM) was very slow
under the concentrations used (8.7 M). In addition, the
significant signal of the acetic acid hydrogen atoms in
1
the H NMR spectrum significantly reduced the signals of
the tetrahydro-β-carbolines in the spectrum, which indicated
acetic acid may not be the optimal catalyst to study the cis to
trans epimerization by this method. In an attempt to increase
the rate of epimerization, formic acid was employed in a
higher concentration (13.3 M) than that employed for acetic
acid. In this case, the ratio of cis/trans isomers was unable to
be determined since the formic acid signal was too strong to
permit observation of the signals from the tetrahydro-β-
carbolines of interest. To reduce the formic acid signal, the
concentration was reduced to 6.6 M. The clarity of the
spectrum remained poor, and the cis/trans ratio could not
be determined; therefore, the use of formic acid was aban-
doned. Since this issue complicated data analysis, an acid
which does not contain unnecessary hydrogen atoms asso-
ciated with the molecule would be beneficial. The use of
deuterated acids would be useful in this case; however, the
inherent cost of these acids was not the optimal solution.
Trifluoroacetic acid (TFA) was chosen on the basis of this
criteria as well as its increased acidity.
TABLE 1. Products from the Pictet-Spengler Reaction and Nb-Benzylation
Pictet-Spengler products
alkylation products
yield,a,b
(%)
yield,a,c
(%)
yield,a,c
(%)
X
cis
trans
cis
trans
Initial attempts with TFA suggested that this acid would
be useful with regard to the clarity of the spectrum; however,
the protons at C-4 were broadened in the 1H NMR spectrum.
The integration of these signals could not be used as pre-
viously described26 to determine the epimeric ratio; however,
the triplet from the ethyl ester function of the cis isomer 7d (δ
1.26 ppm) was noticeably shifted upfield as compared to the
trans isomer 8d (δ1.16 ppm) (see Figures S1 and S2, Support-
ing Information). This chemical shift difference permitted
the concentration of both isomers to be monitored over the
course of the epimerization process.
Et
5a
5b
5c
5d
5e
5f
5g
5h
5i
6a
6b
6c
6d
6e
6f
6g
6h
6i
90
91
87
95
91
76
88
88
82
7a
7b
7c
7d
7e
7fd
7g
7hd
7i
69
55
61
64
75
85
64
82
71
8a
8b
8c
8d
8e
8fd
8g
8hd
8i
68
59
60
68
72
82
68
88
72
iPr
tBu
H
F
Cl
Br
NO2
OMe
aYields are not optimized. bcis þ trans. Cis and trans compounds
c
were alkylated individually. d1.5 equiv of DIPEA at reflux.
Due to the increased acidity of TFA as compared to either
acetic or formic acid, TFA concentrations of 1.7 M of TFA
promoted the rate of the epimerization of the parent 7d
([7d] = 12.5 mM) in less than 1 min. Since the standard 1H
spectrum required approximately 1 min (16 scans), molar
concentrations would not permit observation of the rate of
the process. In this regard, the concentration of TFA was
reduced significantly by means of dilution in CDCl3 to
2.7 mM while the concentration of 7d was kept constant.
After 1 h, the proton spectrum was taken of the reaction
mixture. Approximately 20% of the trans isomer was pre-
sent, based on the integration of the NMR spectrum. These
more detailed observations were obtained by observing the
cis/trans ratio in situ. In the present case, 1H NMR spectro-
scopy had proven useful as a means to observe both of these
species. In the following experiments, the cis to trans ratio
was determined by integration of 1H NMR spectra while the
epimerization of the cis isomer into its trans counterpart was
followed by 1H NMR spectroscopy. Beside the less laborious
nature of observations of the reaction in the spectrometer,
the real strength of this method was the ability to obtain
more data over the course of the reaction progress for the
rate studies.
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Van Linn and Cook
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TABLE 2. Effect of TFA Concentration on the Rate of Epimerization
now be employed in the epimerization experiments. Since
rate data was obtained for p-ethyl analogue 7a, any substit-
uents with a σþ value greater than -0.30 could be used in the
Hammett analysis.29
In this regard, the series of molecules illustrated in
Scheme 1 and Table 1 (7a-h) was employed to determine if
a linear free energy relationship existed for the epimerization
reaction, which, in turn, would provide insight into the
mechanism of the epimerization. Each kinetic experiment
was set up with the following conditions: [cis] = 12.5 mM,
[TFA] = 13.5 mM, T = 303 K. Data for compounds 7a-g
were obtained over a reasonable time frame; i.e., complete
epimerization occurred in 15-30 min for this series of
substrates. Because the rates could be observed for this series
of molecules, the data was manipulated further; each spec-
trum of a kinetic experiment was integrated manually to
obtain the concentration of the cis isomer and was then
plotted for each compound over time. Essentially, a first-
order decay of the cis isomer was observed for each com-
pound; however, the initial region of each plot appeared to
be more linear in nature than an actual exponential decay
(see Figure S5, Supporting Information). It was thought this
could be due to an insufficient amount of TFA present
during the initial stages of the reaction. If this were the case,
a lower concentration of TFA would increase this effect; the
linear portion of the plot would extend further throughout
the process. To test this hypothesis, the acid concentration
was reduced to catalytic amounts (see Figure S6, Supporting
Information). Indeed, as the amount of acid catalyst was
reduced, the reaction plot became more linear over the
epimerization process. This observation can be understood
because the catalytic amount of TFA had cycled via equilib-
rium, presumably, through the epimerization process to the
point wherein equimolar concentrations were present bet-
ween the cis isomer and TFA. Just past the equimolar
point, TFA would become an excess reagent, which would
then make the reaction essentially pseudo-first-order, which
resulted in a first-order exponential decay.
[TFA] (mM)
kobsa (s-1
)
std dev
std error
13.5
18.0
22.5
27.0
67.5
0.0016
0.0025
0.0031
0.0049
0.0122
0.0001
0.0001
0.0003
0.0004
0.0007
<0.0001
0.0001
0.0002
0.0003
0.0005
aAverage of two runs; [7d] = 5.0 mM for all experiments; T = 303.0 K.
conditions could be used to monitor the reaction; however,
in an effort to minimize reaction times and collect more data
in a timely fashion, the TFA concentration was increased. It
was found that TFA concentrations up to 67 mM could be
used to obtain measurable rate constants when the concen-
tration of the substrate (7d) was equal to 5.0 mM (Table 2).
Since it was known that electron-donating substituents on
the phenyl ring at C-1 increased the rate of the epimerization,
the 4-methoxy-substituted substrate 7i was subjected to the
epimerization process under the same conditions as 7d; [7i] =
12.5 mM, [TFA] = 13.5 mM. Since the rates are compared to
one another in a free-energy relationship, all reaction con-
ditions were kept exactly the same. Upon addition of TFA to
the cis isomer of 7i, unfortunately, the cis to trans epimeriza-
tion took place in less than 3 min. Analysis of the first proton
spectrum in the kinetic experiment showed that all the cis
isomer had been converted completely into the trans diaster-
eomer. On the other hand, when catalytic amounts of TFA
were employed in order to measure the rate of this reaction
7if8i, this provided excellent results (see Figure S3, Sup-
porting Information); however, the same reaction conditions
were not ideal for phenyl rings with electron-withdrawing
substituents at the C-1 position due to the extremely long
reaction times. For example, data could not be collected in a
reasonable time frame for the p-nitrophenyl analogue 7h since
epimerization was only 70% complete after 15 h (Figure S4,
Supporting Information). Since the methoxy group was
strongly electron donating (σþ=-0.78), it was believed rate
data obtained by 1H NMR spectroscopy for this substrate (7i)
would not be possible under the same reaction conditions
required for the remaining compounds in the series and it was
therefore excluded from the kinetic analysis.
Since the p-methoxyphenyl analogue 7i reacted too ra-
pidly to obtain accurate rate data, the upper limit of reacti-
vity (electron donating) in order to observe the rate of
epimerization by NMR was determined. The σþ value of
an ethyl function located in the para position of a phenyl
group was -0.30, which was less able to stabilize a positively
charged intermediate when compared to that of a methoxy
substituent located in the same position (-0.78); therefore
the p-ethyl substituted analogue (7a) should not react as fast
as the p-methoxy-substituted analog (7i). When the p-ethyl-
phenyl substrate 7a was subjected to the standard epimeriza-
tion conditions, fortunately the rate could be followed by 1H
NMR spectroscopy. Because of this observation, a series of
1-(para-substituted)-phenyltetrahydro-β-carbolines could
Since it was now known that a slight excess of TFA
(13.5 mM) was insufficient to produce a pseudo-first-order
decay from time = 0, the concentration of TFA was in-
creased to 27.0 mM. Typically, 10 equiv of reagent were
required to satisfy pseudo-first-order conditions; however,
since this epimerization reaction was catalytic, an excess of
TFA completely protonated the cis isomer. A study of the
amount of TFA required to completely protonate the cis
isomer indicated that 1.5 equiv was the minimum amount of
TFA that would produce a first-order decay as the cis isomer
was converted into the trans diastereomer over time (see
Figure S7, Supporting Information).
Once the final reaction conditions were set, the series of
para-substituted substrates (7a-h) were subjected to the
epimerization process and followed by H NMR spectros-
1
copy. The plots fit a first-order model very well (Figure S8,
Supporting Information), which permitted extraction of the
pseudo-first-order rate constants (see Table 3). Since all of
the kinetic experiments were carried out under the same
reaction conditions, a Hammett plot could be constructed.
Here, the observed rate constants were compared directly
to that of the unsubstituted parent compound, 7d. As
(29) Hansch, C.; Leo, A.; Taft, R. W. Chem. Rev. 1991, 91, 165.
3590 J. Org. Chem. Vol. 75, No. 11, 2010

Van Linn and Cook
JOCArticle
TABLE 3. Observed Rate Constants of the Cis to Trans Isomerization
values should be employed in the Hammett plots. Indeed, this
was the exact observation. Hence, analysis of this observation
indicated that a positively charged intermediate was in direct
resonance with the substituent at the para (40) position under
study, and the Hammett F value was found to be -1.4. The
negative slope was in agreement with the expected value since
positive charge builds up throughout the course of the reaction
process. However, the observed F value of -1.4 was consider-
ably less than typical F values for benzylic cations. The
decreased sensitivity to changes in the substitution pattern
could be attributed to electron delocalization from the neigh-
boring indole ring, which also stabilized the proposed carbo-
cationic intermediate.
The Hammett plot produced here could not be used to
unequivocally distinguish between the carbocationic me-
chanism or the retro-Pictet-Spengler pathway for the epi-
merization process. Both the carbocationic intermediate as
well as the iminium ion intermediate are in direct resonance
with a substituent at the para position of the phenyl ring at
C-1. However, by careful examination of both proposed
mechanisms, differences could be noticed when the concen-
tration of TFA was altered.
compd
substituent
kobsa (s-1
)
std dev
std error
7a
7b
7c
7d
7e
7f
Et
0.0073
0.0062
0.0062
0.0018
0.0026
0.0017
0.0015
0.0001
0.0006
0.0003
0.0004
<0.0001
<0.0001
<0.0001
0.0001
0.0003
0.0002
0.0002
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
iPr
tBu
H
F
Cl
Br
NO2
7g
7h
<0.0001
aAverage of three experiments.
mentioned previously, electron-donating substituents (either
by resonance or by induction) increased the rate of epimeri-
zation and, therefore, are expected to produce a negative
slope in the Hammett plot. In fact, this was exactly what was
observed (see Figure 2). Since the data was scattered around
the linear regression line, it was clear the data did not fit the
Hammett linear free energy relationship well when the
standard σpara values were employed. When σþ
values
were used, however, the data correlated much better (Figure 3).
para
The key substrate (7e) which was used to confirm that σþ
para
values should be used in this mechanistic study had been
prepared (see the Supporting Information). The σpara value
for a fluorine atom in the para position of a benzene ring was
value was -0.07. If the parent 7d
para
epimerized at C-1 slower than the p-fluoro analogue 7e, σþ
para
Observation of the data shown in Figure 4 clearly indi-
cated the rate of epimerization of 7d increased as the
concentration of TFA increased and indicated first-order
kinetics after protonation of the starting material. Since this
reaction was carried out with excess quantities of acid
0.06 while the σþ
FIGURE 2. Hammett plot with σpara values.
FIGURE 3. Hammett plot with σþpara values.
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Van Linn and Cook
JOCArticle
FIGURE 4. Effect of the TFA concentration on the rate.
catalyst, the rate should not change to a significant degree as
the catalyst (TFA) concentration was altered if one catalyst
molecule was involved in the epimerization process. How-
ever, this was not the case, which suggested that the acid
catalyst was more involved in the epimerization mechanism
than a single protonation. In this case, it was believed that
TFA was involved in a double protonation. In the proposed
carbocationic mechanism, the initial protonation took place
at the Nb-nitrogen atom. As this mechanistic route pro-
gressed and the carbocationic intermediate was formed, a
second protonation took place on the Nb-nitrogen atom (see
Scheme 2). First-order kinetics was observed with respect
to the concentration of TFA as would be expected when
k-1 . k2[TFA].
SCHEME 2. Carbocation-Mediated Mechanism Including a
Second Protonation
In the carbocationic mechanism, a doubly protonated
intermediate such as 9 would form following C-N bond
cleavage (Scheme 2). The rate at which 9 would form would
increase as the concentration of TFA was increased. In order
for cyclization to take place, 9 would have to be deproto-
nated, which would be slowed in the presence of a higher
concentration of TFA; however, the rate of the reaction
would not be affected to a large extent as the concentration of
such an intermediate would remain small over time due to its
instability. Therefore, this step was not considered as the rate
determining step. In this case, a rate enhancement would be
observed for each mechanistic step when the TFA concen-
tration was increased. This was exactly what was observed
(Figure 4). In fact, after initial protonation of the starting
material (as detected by 1H NMR spectroscopy), first-order
kinetics with respect to the concentration of TFA was
observed, therefore the rate was dependent upon k2 and
the concentration of TFA.
The acid dependence of these reactions suggested that a
dicationic electrophile (diprotonated imine) was the reactive
intermediate.30 The same concept could be applied to the cis to
trans epimerization mechanism under study here, as shown in
Scheme 2. The second protonation would take place after the
carbocation had formed. During the time of the second pro-
tonation, the C-3/C-4 bond would have time to rotate to the
more stable trans configuration, followed by Nb-deprotonation
As reported by Shudo et al.,30 doubly charged intermediates
have been proposed in the tetrahydroisoquinoline series. In these
cases, strong acids, such as TFA and trifluoromethanesulfonic
acid (TfOH), were used to catalyze the Pictet-Spengler reaction.
(30) Yokoyama, A.; Ohwada, T.; Shudo, K. J. Org. Chem. 1999, 64, 611.
3592 J. Org. Chem. Vol. 75, No. 11, 2010

Van Linn and Cook
JOCArticle
SCHEME 3. Retro-Pictet-Spengler Mechanism
SCHEME 4. Cis to Trans Epimerization Process in TFA-d on
1-Substituted Phenyltetrahydro-β-carbolines 7a, 7d, and 7g
TABLE 4. KIE Experiments on 1-Substituted Phenyltetrahydro-
β-carbolines
and recyclization to form the trans product. This type of mecha-
nism would be consistent with the rate data shown in Figure 4
since the reaction rate was first order in the concentration of TFA
even after initial protonation of the starting material.
compd
substituent X
kobs avg (s-1
)
k-dobs avg (s-1)
KIE
7a
7d
7g
Et
H
Br
0.0073
0.0018
0.0016
0.0060
0.0015
0.0013
1.21
1.20
1.30
On the other hand, if the retro-Pictet-Spengler pathway
was involved, the initial protonation step (step A, Scheme 3)
to form 10 would increase in rate as the concentration of
TFA increases. However, the rate of the subsequent depro-
tonation to form 11 (step B, Scheme 3) would decrease in rate
as the concentration of TFA was increased. When a high
concentration of TFA was present, the concentration of 10
would increase over the course of the reaction. In all of these
steps, the Nb-nitrogen would remain protonated since its pKa
was much greater than the carbon at the indole-2 position. In
fact, the retro-Pictet-Spengler mechanism cannot proceed
with this Nb-nitrogen atom protonated. Although the pro-
tonated (10) and unprotonated (11) forms are in equilibrium,
the likelihood of the free amine being present becomes
increasingly smaller as the concentration of TFA increases.
Therefore, deprotonation (step B, Scheme 3) was presum-
ably the rate-determining step. This was contrary to the data
depicted in Figure 4 and was inconsistent with first-order
kinetics with respect to the concentration of TFA.
To determine if protonation at the indole-2 position was
or was not the rate-determining step in the retro-Pictet-
Spengler process, kinetic isotope experiments were de-
signed and executed. Since primary KIEs are observable
when an acid is involved in the rate-determining step, such
experiments, in support of the other kinetic experiments,
would provide useful mechanistic information about the
epimerization process. If, in fact, this protonation was
the rate-determining step, strong primary kinetic isotope
effects (KIE) would be observed. The rate would be notice-
ably decreased when the epimerization was run in TFA-d.
Since protonation/deprotonation of the nitrogen atom is fast
(due to basicity), the isotope effect would not be observed
for these mechanistic steps. In fact, similar experiments have
(31) Maresh, J. J.; Giddings, L.; Friedrich, A.; Loris, E. A.; Panjikar, S.;
Trout, B. L.; Stockigt, J.; Peters, B.; O’Connor, S. E. J. Am. Chem. Soc. 2008,
130, 710.
J. Org. Chem. Vol. 75, No. 11, 2010 3593

Van Linn and Cook
JOCArticle
been carried out by O’Connor et al. in regard to the
biosynthesis of indole alkaloids.31 A deuterium atom was
synthetically installed at the indole-2 position of tryptamine,
followed by a Pictet-Spengler reaction with propanal under
acidic conditions. KIEs between 2.0 and 2.6 were observed
by O’Connor et al., which suggested that deprotonation at
the indole-2 position was the rate-determining step in the
Pictet-Spengler reaction.31 Similar results were expected in
these experiments if, in fact, protonation (or deprotonation)
at the indole-2 position was the rate-determining step in the
retro-Pictet-Spengler process. Each experiment was set up
to keep the same reaction conditions as the reactions run
earlier in TFA: that is to say, the concentration of starting
cis isomer, concentration of TFA-d, and reaction tempera-
ture (303 K) in the NMR spectrometer (Scheme 4) were
maintained as before. A small primary kinetic isotope effect
(1.2-1.3) was observed (Table 4). The KIEs observed here
were much smaller than those observed by O’Connor et al.31
This data was inconsistent with the retro-Pictet-Spengler
mechanism for the cis to trans epimerization process since a
strong KIE was not observed. Therefore, it was concluded
SCHEME 5. Cis to Trans Epimerization in the Presence of
Water
SCHEME 6. Proposed Mechanism of the Retro-Pictet-Spengler Process Observed by Hamaker34
3594 J. Org. Chem. Vol. 75, No. 11, 2010

Van Linn and Cook
JOCArticle
that protonation at the indole-2 position was not the
rate-determining step in the cis to trans epimerization of
1-(40-substituted)-phenyltetrahydro-β-carbolines.
Na-sulfonamide derivative 16 was reacted in ethanolic HCl.
The Nb-benzyl tryptophan derivative 21 was the exclusive
product of this process in the Na-sulfonamido case of
Hamaker;34 the trans isomer 18 was not observed (see
Scheme 6). It was believed this result was observed because
the electron withdrawal of the sulfonamide retarded stabili-
zation of the carbocationic intermediate 17 which prevented
cleavage of the C-N bond. However, continued heating in
ethanolic HCl resulted in the production of the tryptophan
alkyl ester 21, presumably via a retro-Pictet-Spengler reac-
tion through iminium ion intermediate 20. Since the retro-
Pictet-Spengler mechanism had been postulated to take
place in the Na-sulfonamide case, the cis to trans epimeriza-
tion of the Na-H case, 7h, was carried out under similar
reaction conditions (Table 4). In this case, hydrolysis of
the tetrahydro-β-carboline ring system to give Nb-benzyl-
tryptophan ethyl ester 15 or the corresponding aryl alde-
hyde (or acetal) did not occur (see Table 5). After 3 h at
reflux in concentrated ethanolic HCl, a mixture of cis and
trans isomers was detected by TLC. This was confirmed by
1H NMR spectroscopy following aqueous workup. The
products of this reaction were then resubjected to the same
reaction conditions overnight. The following day, decom-
position was observed by TLC, but no Nb-benzyltrypto-
phan ethyl ester 15 or p-nitrobenzaldehyde was observed by
TLC or by NMR spectroscopy at any time during these
reactions.
To minimize the possibility of decomposition, a 1:2 dilu-
tion of concentrated ethanolic HCl with EtOH was used as
the solvent/reagent system. After 2 days at reflux, a mixture
of cis and trans isomers was again observed by TLC in the
1-(40-nitro)-phenyltetrahydro-β-carboline case, 7h. Addi-
tional ethanolic HCl was added to the reaction mixture in
order to increase the rate of epimerization. The appearance
of more of the trans 1-(40-nitro)-phenyltetrahydro-β-carbo-
line diastereomer 8h was observed after 4 h. The reaction
was continued for an additional 2 days at which point a
mixture of cis and trans isomers remained, accompanied by
In addition to a study of the rates of epimerization for 1-(40-
substituted)-phenyltetrahydro-β-carbolines, chemical experi-
ments were also conducted in order to determine the mechan-
ism of the cis to trans epimerization. Czerwinski32 had shown
that deuterium was not incorporated into the trans product in
the presence of deuterated TFA which confirmed that the
olefinic mechanism had not occurred in the cis to trans
isomerization. This same experiment was carried out in the
1-(40-substituted)-phenyltetrahydro-β-carboline series (7a, 7d,
7g) to determine if different substituent patterns altered the
outcome of that process. The cis substrate (7a, 7d, 7g) was
dissolved in CDCl3, and TFA-d was added (Scheme 4). The cis
to trans epimerization process was allowed to take place, and
the 1H NMR spectrum was taken following aqueous workup.
Full integration of the C-1 proton was observed in the spec-
trum. This was identical to the result observed by Czerwinski32
which unequivocally indicated that the olefinic mechanism did
not occur in the 1-(40-substituted)-phenyltetrahydro-β-carbo-
line series which was in agreement with the earlier work of Joule
et al.33 Therefore, this mechanism did not occur with 1-alkyl or
1-aryl substituents.
If the cis to trans epimerization proceeded through
the retro-Pictet-Spengler mechanism, Nb-benzyltryptophan
ethyl ester 15 would be an expected product if the reaction
was carried out in the presence of water. Here, the iminium
ion intermediate would be hydrolyzed to the tryptophan
ethyl ester derivative 15 and the corresponding aldehyde. In
this vein, the parent 7d (12.5 mM) was dissolved in CDCl3.
Prior to the addition of TFA, 1.0 volume equiv of water (13.9
M) was added to the reaction medium. The TFA (27.0 mM)
was then added, and the epimerization reaction was allowed
to take place (Scheme 5). The reaction was monitored by
TLC, and the 1H NMR spectrum was acquired when the cis
to trans epimerization was complete. At no time was Nb-
benzyltrytophan ethyl ester 15 or the corresponding aryl
1
aldehyde observed by TLC or by H NMR spectroscopy.
Hydrolysis of the ethyl ester was also not observed. With a
large excess of water present in the reaction medium, Nb-
benzyltryptophan ethyl ester would be expected if the imi-
nium ion was formed in a retro-Pictet-Spengler process
since this would be favored entropically. Since the epimeri-
zation was complete for the parent 7d, the same reaction was
carried out for both the p-ethylphenyl, 7a, and p-bromophe-
nyl, 7g, substrates. The same result was realized in both of
these cases, which suggested that the mechanism of epimeri-
zation did not change as the electronic nature at the C-1
position was varied. This data was consistent with the
carbocationic mechanism since hydrolysis of a proposed
iminium ion which would arise in the retro-Pictet-Spengler
process was not observed.
TABLE 5. Cis to Trans Epimerization of 1-(40-Nitro)tetrahydro-
β-carboline 7h under the Conditions of Hamaker34
In a separate earlier experiment, Hamaker34 had shown
that a retro-Pictet-Spengler mechanism was forced to occur
if a strong electron-withdrawing group was installed on the
indole Na-nitrogen atom. This result was observed when a
ethanolic HCl
time
7h (%)
8h (%)
15 (%)
saturated solution
3 h
90
ND
60
10
ND
40
<1a
ND
10 hc
2 d
(32) Czerwinski, K. M. Ph.D. Thesis, University of Wisconsin-
Milwaukee, 1995.
(33) Gaskell, A. J.; Joule, J. A. Tetrahedron 1967, 23, 4053.
(34) Hamaker, L. K. Ph.D. Thesis, University of Wisconsin-Milwaukee,
1995.
1:2 dilution with EtOH
<1b
4 d
25
75
<1b
aAnalyzed by 1H NMR. bBy TLC. cDecomposition occurred.
J. Org. Chem. Vol. 75, No. 11, 2010 3595

Van Linn and Cook
JOCArticle
TABLE 6. Temperature Effect on Ratea
TABLE 7. Energies of Activationa
temp (K)
kobsb (s-1
)
compd
substituent X
σF
Ea (kJ/mol)
313.0
308.0
303.0
300.0
298.0
0.0132
0.0059
0.0018
0.0012
0.0007
7a
7b
7c
7d
7e
7f
Et
0.4187
0.3908
0.3629
0.0000
0.0977
-0.1535
-0.2093
-1.1025
152.3a
152.3a
152.3a
152.7
iPr
tBu
H
F
Cl
Br
NO2
152.6a
152.9a
152.9a
153.8a
a[cis] = 12.5 mM; [TFA] = 27.0 mM. bAverage of three experiments.
7g
7h
slight decomposition as observed by TLC. The reaction
was stopped at this point since it was clear that hydrolysis
of an iminium ion intermediate from a retro-Pictet-
Spengler mechanism did not occur vs an authentic sample
of 15.
a
[cis] = 12.5 mM; [TFA] = 27.0 mM. Data were calculated
based on the Hammett plot and assumed negligible change in ΔSq.
The rate data collected as shown in Table 6 was then
plotted according to the Arrhenius equation (Figure 5)
to obtain energy of activation (Ea) data for the parent
substrate, 7d, and was determined to be 153.7 kJ/mol.
Since the Arrhenius equation correlated rate data directly
to the energy of activation, the differences in rates, namely
the F value from Figure 3, was employed to determine the
Ea for the series of compounds under study. The activation
energies ranged from 152.3 to 153.8 kJ/mol for the series of
1-(40-substituted)-phenyl-1,2,3,4-tetrahydro-β-carbolines
(see Table 7).
In order to obtain more specific thermodynamic data, the
rate data from Table 6 for 1-phenyltetrahydro-β-carboline
7d was employed to construct an Eyring plot (Figure 6).
The enthalpy of activation (ΔHq) and the entropy of activa-
tion (ΔSq) were extracted from the linear plot and were
Electron withdrawal at the C-1 position via the p-nitro
group did not promote a retro-Pictet-Spengler process as
was the case with the Na-sulfonamide species of Hamaker.34
In the cases with an indole, or an electron-rich indole,
electron density flowed away from the indole ring during
the formation of the proposed carbocationic intermediate.
This was contrary to a retro-Pictet-Spengler-type mechan-
ism wherein electrons flow into the indole ring when the
iminium ion was formed.34 Therefore, it was reasonable to
believe that the retro-Pictet-Spengler mechanism did not
occur in the cis to trans epimerization process of 1,2,3,4-
tetrahydro-β-carbolines when indole, or an electron-rich
indole, was employed.
Enthalpies and entropies of activation were determined
from kinetic data. Such data was acquired for the parent 7d
at temperatures ranging from 298 to 313 K (Table 6).
FIGURE 5. Arrhenius plot.
3596 J. Org. Chem. Vol. 75, No. 11, 2010

Van Linn and Cook
JOCArticle
FIGURE 6. Eyring plot.
TABLE 8. Thermodynamic Dataa
compd X
substituent X
σþ
σF
ΔGq (kJ/mol)
ΔHqa (kJ/mol)
ΔSq a (J/K mol)
7a
7b
7c
7d
7e
7f
Et
-0.30
-0.28
-0.26
0.00
-0.07
0.11
0.15
0.4187
0.3908
0.3629
0.0000
0.0977
-0.1535
-0.2093
-1.1025
87.5
87.7
87.8
89.9c
89.4
90.8
91.2
96.3
146.7d
147.0d
147.2d
150.2b
149.4d
150.4d
150.4d
156.9d
200d
200d
200d
199b
200d
200d
200d
200d
iPr
tBu
H
F
Cl
Br
NO2
7g
7h
0.79
[cis] = 12.5 mM; [TFA] = 27.0 mM ; at 303.0 K. aIf ΔSq remains reasonably constant. bDetermined graphically from the Eyring plot.
cCalculated from ΔHq and ΔSq. dData were calculated based on the Hammett plot and assumed negligible change in ΔSq.
determined to be 150.2 kJ/mol and 199 J/K mol, respectively
changes in entropy would not affect ΔGq to a significant
degree. If this were true, ΔHq was calculated for the remain-
ing compounds and the data is shown in Table 8.
3
and the data is depicted in Table 8. Since both ΔHq and ΔSq
were determined, the Gibb’s free energy of activation (ΔGq)
was calculated to be 89.9 kJ/mol. It was important to realize
that the Hammett free energy relationship correlated rate
data to the free energy of the reaction. Since the Hammett
plot was constructed and ΔGq had been determined for
1-phenyltetrahydro-β-carboline 7d, ΔGq for the remaining
compounds studied in the Hammett analysis were deter-
mined, and the data is presented in Table 8. The difference in
ΔGq across the series of compounds was found to be 8.8 kJ/
mol. Presumably, ΔSq remained relatively constant through-
out the series of compounds studied in the Hammett analy-
sis. Due to the relatively small effect of ΔSq on ΔGq, small
Conclusions
Three potential mechanistic pathways have been consid-
ered for the cis to trans epimerization of 1,2,3-trisubstituted
1,2,3,4-tetrahydro-β-carbolines. One of these, the olefinic
pathway, had been ruled out by Czerwinski32 and in the
present study by deuterium-labeling experiments. Results
from epimerizations carried out in TFA-d indicated that no
deuterium was incorporated at the C-1 position, which
would be necessary if the olefinic pathway was to take place.
The remaining two mechanisms of epimerization, namely the
J. Org. Chem. Vol. 75, No. 11, 2010 3597

Van Linn and Cook
JOCArticle
carbocationic pathway and the retro-Pictet-Spengler pathway,
were compared on a kinetic basis.
cis Nb-benzyltetrahydro-β-carbolines into their more stable
trans diastereomers. This process has important implications
in regard to the total synthesis of indole alkaloids by internal
asymmetric induction.
A series of 1-(40-substituted)-phenyltetrahydro-β-carbo-
lines were synthesized by the Pictet-Spengler cyclization in
order to study the cis to trans epimerization mechanism.
In order to study this trend in more detail, the epimerization
reaction was followed by 1H NMR spectroscopy. The
observed pseudo-first-order rate constants were determined
graphically, which permitted a quantitative comparison of
the rates of epimerization of each of the 1-(40-substituted)-
phenyltetrahydro-β-carbolines studied (7a-h). The obser-
ved rate constants were employed to construct a Hammett
plot, which correlated to a line very well when σþ values were
utilized. A F value of -1.4 was observed for the series of
compounds studied and the negative slope was consistent
with a positively charged intermediate. The magnitude of the
F value was found to be considerably smaller than those
expected for a benzylic cation, however this effect was
attributed to electron delocalization from the neighboring
indole ring, thus stabilizing the proposed carbocationic
intermediate.
Experimental Section
General Procedure for the Preparation of Both Cis and Trans
Diastereomers via the Pictet-Spengler Reaction of Aromatic
Aldehydes under Acidic Conditions.26 L-Tryptophan ethyl ester
(13.5 g, 0.058 mol) was added to a 500-mL three-neck round-
bottom flask containing dry benzene (300 mL). The flask was
attached to a Dean-Stark trap topped by a reflux condenser for
azeotropic removal of water during the course of the reaction.
Trifluoroacetic acid (0.12 mol, 2.0 equiv) was added, and
this was followed by addition of the aldehyde in dry benzene
(0.070 mol, 1.2 equiv). The mixture was allowed to heat to reflux
for 4-8 h under argon until all of the tryptophan ethyl ester was
consumed as indicated by TLC (silica gel, hexanes/EtOAc, 3:1).
The reaction mixture was then cooled to rt, and the solvent was
removed under reduced pressure. The residue was dissolved in
EtOAc (300 mL) and washed with a solution of cold, saturated
NaHCO3 (3 ꢀ 300 mL) to remove TFA. The organic layer was
separated and dried (Na2SO4). Analysis by TLC (silica gel,
hexanes/EtOAc, 3:1) of the crude oil indicated the presence of
two major components along with some unreacted aldehyde.
The residue was chromatographed on silica gel (gradient elu-
tion, EtOAc/hexane =1:10, 2:10, 3:10, 4:10) to provide the pure
cis and trans diastereoisomers, respectively.
It was believed that a second protonation took place at the
Nb-nitrogen atom after formation of the carbocationic inter-
mediate, which resulted in a dicationic intermediate. The
observed first-order kinetics with respect to TFA (after
initial protonation of the substrate) was consistent with the
formation of the dicationic intermediate in the carbocationic
mechanism as this was determined to be the rate-determining
step. On the other hand, pseudo-first-order kinetics was
not expected in the retro-Pictet-Spengler process since the
formation of the iminium ion intermediate required the
Nb-nitrogen atom to be unprotonated.
Kinetic isotope experiments were devised and carried out
where small primary kinetic isotope effects of 1.2-1.3 were
observed for three separate tetrahydro-β-carbolines. The
determined KIEs were considerably smaller than those
reported by O’Connor31 for deprotonation at the indole-2
position in the Pictet-Spengler mechanism. This implied
that protonation at the indole-2 position was not involved in
the rate-determining step of the cis to trans epimerization
process. Therefore, the retro-Pictet-Spengler mechanism
was ruled out since the rate of epimerization was dependent
upon the concentration of TFA (first-order kinetics after
protonation of the substrate) and protonation at the indole-2
position was not involved in the rate-determining step.
In separate, chemical experiments, Nb-benzyltryptophan
ethyl ester was not observed when 1-(40-substituted)-phenyl-
tetrahydro-β-carbolines were epimerized with TFA in the
presence of water or under the ethanolic HCl conditions of
Hamaker.34 Presumably, the iminium ion intermediate did
not form in either case, and thus, the retro-Pictet-Spengler
process did not occur since neither of these two products was
observed by TLC or 1H NMR spectroscopy.
cis-1-Phenyl-1,2,3,4-tetrahydro-9H-β-carboline-3-carboxylic
Acid Ethyl Ester (5d). The general procedure was followed, and
the cis/trans diastereomers were separated by column chroma-
tography on silica gel. L-Tryptophan ethyl ester HCl (20.1 g,
0.075 mol), benzaldehyde (8.3 mL, 0.081 mol), and TFA (13.2
mL, 0.18 mol) were used to obtain the cis isomer (95% overall
3
yield, cis þ trans). 5d: mp 161-163 °C; [R]27 -8.89 (c 0.48,
D
1
CHCl3); H NMR (500 MHz, CDCl3) δ 7.62 (d, J = 7.7 Hz,
1H), 7.54 (s, 1H), 7.47-7.41 (m, 5H), 7.27-7.24 (m, 1H),
7.23-7.17 (m, 2H), 5.31 (s, 1H), 4.38-4.29 (m, 2H), 4.02 (dd,
J = 11.2, 4.2 Hz, 1H), 3.30 (ddd, J = 15.1, 4.2, 1.8 Hz, 1H),
3.11-3.06 (m, 1H), 2.68 (br s, 1H), 1.41 (t, J = 7.1 Hz, 3H); 13
C
NMR (125 MHz, CDCl3): δ 173.2, 141.1, 136.6, 135.1, 129.4,
129.13, 129.10, 127.6, 122.4, 120.1, 118.7, 111.4, 109.4, 61.7,
59.2, 57.4, 26.1, 14.7; MS (EI) m/e (rel intensity) 320 (Mþ, 69),
247 (65), 218 (100), 169 (42), 144 (36), 77 (45). Anal. Calcd for
C20H20N2O2: C, 74.98; H, 6.29; N, 8.74. Found: C, 75.21; H,
6.40; N, 8.66. This material was employed in a later step.
cis-1-(4-Isopropylphenyl)-1,2,3,4-tetrahydro-9H-β-carboline-
3-carboxylic Acid Ethyl Ester (5b). The general procedure was
followed, and the cis/trans diastereomers were separated by
column chromatography on silica gel. L-Tryptophan ethyl
ester HCl (20.1 g, 0.074 mol), 4-isopropylbenzaldehyde (13.6
mL, 0.090 mol), and TFA (13.2 mL, 0.18 mol) were used to
3
obtain the cis isomer (91% overall yield, cis þ trans). 5b: mp
195-197 °C; [R]27 -15.56 (c 1.02, CHCl3); 1H NMR (500
D
MHz, CDCl3) δ 7.63-7.61 (m, 1H), 7.58 (s. 1H), 7.37 (d, J = 8.2
Hz, 2H), 7.30 (d, J = 8.1 Hz, 2H), 7.25-7.23 (m, 1H), 7.22-7.17
(m, 2H), 5.26 (s, 1H), 4.37-4.30 (m, 2H), 4.00 (dd, J = 11.2, 4.2
Hz, 1H), 3.29 (dd, J = 14.2, 4.2 Hz, 1H), 3.10-3.05 (m, 1H),
3.00 (p, J = 6.9 Hz, 1H), 2.60 (br s, 1H), 1.40, (t, J = 7.2 Hz,
3H), 1.34 (d, J = 6.9 Hz, 6H); 13C NMR (125 MHz, CDCl3) δ
173.2, 149.8, 138.5, 136.6, 135.4, 129.1, 128.9, 127.6, 127.5,
127.3, 122.3, 120.0, 118.7, 111.4, 109.3, 61.6, 58.8, 57.5, 34.4,
26.2, 24.5, 24.4, 14.7; MS (EI) m/e (rel intensity) 362 (Mþ, 94),
289 (87), 261 (54), 218 (100), 169 (32), 144 (33). Anal. Calcd for
C23H26N2O2: C, 76.21; H, 7.23; N, 7.73. Found: C, 76.35; H,
7.35; N, 7.61. This material was employed in a later step.
In conclusion, a Hammett study alone was insufficient to
distinguish between a carbocation-mediated pathway and a
retro-Pictet-Spengler pathway in the 1-(40-substituted)-
phenyl-1,2,3,4-tetrahydro-β-carboline series. However, ana-
lysis of both rate data and chemical experiments proved to be
inconsistent with the retro-Pictet-Spengler mechanism, and
it was thus ruled out. On the other hand, the data were found
to be consistent with a carbocationic mechanism for the
C-1/N-2 bond scission process in the isomerization of
3598 J. Org. Chem. Vol. 75, No. 11, 2010

Van Linn and Cook
JOCArticle
General Procedure for the Alkylation of the Nb-H Tetrahydro-
β-carbolines.26 Hunig’s base (N,N-diisopropylethylamine, DI-
PEA; 87.0 mmol, 15 equiv) and benzyl bromide (7.0 mmol,
1.2 equiv) were added to a solution of tetrahydro-β-carboline
(5a-e,g,i, 8a-e,g; 5.8 mmol, 1 equiv) in dry acetonitrile
(10 mL). This mixture was cooled to 0 °C and allowed to come
to rt overnight. The solvent was removed under reduced pressure,
and the residue was dissolved in cold EtOAc (10 mL) to remove the
salt of the Hunig’s base which had precipitated. The solution was
filtered, and the filtrate was dried (Na2SO4) and concentrated in
vacuo to yield a brown oil. Analysis of the crude material by TLC
indicated the presence of a major component along with some
unreacted benzyl bromide and starting material. The residue was
purified on a silica gel column via flash chromatography (hexane/
EtOAc 7:1) to yield the pure Nb-benzyltetrahydro-β-carboline
isomer, respectively, as an oil which was then triturated with
hexanes to form a pale yellow solid or white solid.
cis-2-Benzyl-1-(4-ethylphenyl)-1,2,3,4-tetrahydro-9H-β-car-
boline-3-carboxylic Acid Ethyl Ester (7a). The modified general
procedure was followed. The cis Nb-H analogue 5a (0.50 g, 1.1
mmol), DIPEA (3.3 mL, 18.9 mmol), and benzyl bromide (0.2
mL, 1.5 mmol) were used to obtain the cis Nb-benzyl isomer
(69%). 7a: [R]27D -86.49 (c 0.38, CHCl3); 1H NMR (500 MHz,
CDCl3) δ 7.59 (d, J = 7.8 Hz, 1H), 7.39-7.24 (m, 9H), 7.20 (d,
J = 8.1, 2H), 7.18-7.16 (m, 2H), 4.99 (s, 1H), 4.13 (d, J = 14.8
Hz, 1H), 3.98 (d, J = 14.8 Hz, 1H), 3.87 (dd, J = 7.9, 3.8 Hz,
1H), 3.86-3.75 (m, 2H), 3.43 (dd, J = 15.6, 6.3 Hz, 1H), 3.11
(dd, J = 15.2, 4.9 Hz, 1H), 2.69 (dd, J = 15.2, 7.6 Hz, 2H), 1.29
(t, J = 7.6 Hz, 3H), 1.15 (t, J = 7.1 Hz, 3H); 13C NMR (125
MHz, CDCl3): δ 173.8, 144.5, 138.8, 137.8, 136.8, 134.1, 129.8,
128.5, 128.4, 127.44, 127.37, 122.2, 119.9, 118.8, 111.2, 107.6,
62.0, 61.5, 61.1, 57.2, 29.0, 24.6, 16.0, 14.4; MS (EI) m/e (rel
intensity) 438 (Mþ, 31), 365 (100), 248 (30), 218 (54). Anal. Calcd
for C29H30N2O2: C, 79.42; H, 6.89; N, 6.39. Found: C, 79.51; H,
6.92; N, 6.35.
4.12 (d, J = 14.7 Hz, 1H), 4.01 (d, J = 14.7 Hz, 1H), 3.88 (dd,
J = 6.5, 5.2 Hz, 1H), 3.86-3.81 (m, 1H), 3.72-3.67 (m, 1H),
3.44 (dd, J = 15.6, 6.6 Hz, 1H), 3.11 (dd, J = 14.9, 4.4 Hz, 1H),
1.36 (s, 9H), 1.12 (t, J = 7.1 Hz, 3H); 13C NMR (125 MHz,
CDCl3) δ 173.8, 151.2, 139.0, 137.3, 136.7, 133.9, 129.6, 129.5,
128.5, 127.4, 125.7, 122.1, 119.9, 118.8, 111.2, 107.8, 61.7, 61.1,
61.0, 57.5, 35.0, 31.8, 24.2, 14.4; MS (EI) m/e (rel intensity): 466
(Mþ, 25), 393 (90), 275 (26), 218 (100). Anal. Calcd for
C31H34N2O2: C, 79.79; H, 7.34; N, 6.00. Found: C, 79.85; H,
7.53; N, 5.93.
Procedure for the 1H NMR Kinetics Experiments. The 1,2,3-
trisubstituted 1,2,3,4-tetrahydro-β-carboline (7a-h; 1.25 ꢀ 10-5
mol, 12.5 mM final concentration) was dissolved into CDCl3
(800 μL) and then transferred to an NMR tube. The proton
spectrum was taken, and then the tube was ejected. The TFA
(200 μL of a 135 mM solution in CDCl3, 27.0 mM final
concetration) was quickly added to the tube and the tube injected
into the spectrometer with an internal temperature of 303.0 K. The
sample was locked and shimmed before starting the kinetic
experiment. A series of 50 spectra were taken over the course of
48 min for all compounds except for the 4-nitro compound (7h),
where 50 spectra were taken over an 11 h period. The resulting 2D
data set was then phased manually and then divided into 50 equal
1
slices, each representing one H spectrum. Each spectrum was
integrated with the trans isomer calibrated to one proton. The ratio
of cis to trans isomers was then determined, which allowed for the
calculation of the cis isomer concentration. The concentration
data was then plotted against time. A first-order exponential decay
was realized, and the pseudo-first-order rate constant was deter-
mined graphically.
Acknowledgment. We greatly acknowledge Professors
Graham Moran, Guilherme Indig, and Alan Schwabacher for
€
helpful discussions as well as Dr. F. Holger Forsterling for
technical support. This work was supported in part by NIMH
MH046851 and the Lynde and Harry Bradley Foundation.
cis-2-Benzyl-1-(4-tert-butylphenyl)-1,2,3,4-tetrahydro-9H-β-
carboline-3-carboxylic Acid Ethyl Ester (7c). The modified gen-
eral procedure was followed. The cis Nb-H analogue 5c (0.52 g,
1.4 mmol), DIPEA (3.6 mL, 20.6 mmol), and benzyl bromide
(0.3 mL, 2.1 mmol) were used to obtain the cis Nb-benzyl isomer
(61%). 7c: mp 155-156 °C; [R]27D þ48.57 (c 0.14, CHCl3); 1H
NMR (500 MHz, CDCl3) δ 7.61 (d, J = 7.8 Hz, 1H), 7.41-7.32
(m, 7H), 7.29-7.25 (m, 4H), 7.20-7.15 (m, 2H), 5.02 (s, 1H),
Supporting Information Available: Supplemental figures,
complete experimental procedures, synthetic procedures, and
1
compound characterization data as well as H and 13C NMR
spectra for all compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
J. Org. Chem. Vol. 75, No. 11, 2010 3599