Chemoenzymatic asymmetric synthesis of serotonin receptor agonist (R)-frovatriptan

Article abstract of DOI:10.1002/ejoc.201300114

A simple chemoenzymatic asymmetric route has been developed for the production of antimigraine agent (R)-Frovatriptan. Lipases and oxidoreductases have been identified as ideal biocatalysts for the production of enantiopure adequate synthetic intermediates under safe and environmentally friendly conditions. (S)-3-Hydroxy-2,3,4,9-tetrahydro-1H-carbazole-6-carbonitrile, an optimal building block for the synthesis of the drug, has been efficiently prepared through Candida antarctica lipase type B catalyzed acylation of its corresponding racemic mixture or alcohol dehydrogenase A mediated bioreduction of the corresponding ketone. The inversion of the chiral center has been identified as a key step, optimizing the process to avoid partial racemization. Finally, amine functionalization and nitrile hydrolysis have allowed the production of (R)-Frovatriptan in enantiomerically pure form. A simple chemoenzymatic preparation has been developed for the production of serotonin receptor agonist (R)-Frovatriptan starting from inexpensive starting materials. The enzymatic action towards ketone or alcohol intermediates is the key step for the asymmetric total synthesis of this drug. Copyright

Full text of DOI:10.1002/ejoc.201300114

Job/Unit: O30114
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Date: 06-05-13 17:26:30
Pages: 9
FULL PAPER
Chemoenzymatic Asymmetric Synthesis
A simple chemoenzymatic preparation has
been developed for the production of sero-
tonin receptor agonist (R)-Frovatriptan start-
ing from inexpensive starting materials. The
enzymatic action towards ketone or alcohol
intermediates is the key step for the asym-
metric total synthesis of this drug.
E. Busto, L. Martínez-Montero, V. Gotor,
V. Gotor-Fernández* ........................... 1–9
Chemoenzymatic Asymmetric Synthesis of
Serotonin Receptor Agonist (R)-Frovatriptan
Keywords: Synthetic methods / Asymmetric
synthesis
/ Nitrogen heterocycles / Bio-
catalysis / Serotonin receptors
0000, 0–0
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V. Gotor-Fernández et al.
FULL PAPER
DOI: 10.1002/ejoc.201300114
Chemoenzymatic Asymmetric Synthesis of Serotonin Receptor Agonist
(R)-Frovatriptan
Eduardo Busto,[a] Lía Martínez-Montero,[a] Vicente Gotor,[a] and
Vicente Gotor-Fernández*[a]
Keywords: Synthetic methods / Asymmetric synthesis / Nitrogen heterocycles / Biocatalysis / Serotonin receptors
A simple chemoenzymatic asymmetric route has been devel-
oped for the production of antimigraine agent (R)-Frovatrip-
tan. Lipases and oxidoreductases have been identified as
ideal biocatalysts for the production of enantiopure adequate
synthetic intermediates under safe and environmentally
friendly conditions. (S)-3-Hydroxy-2,3,4,9-tetrahydro-1H-
carbazole-6-carbonitrile, an optimal building block for the
synthesis of the drug, has been efficiently prepared through
Candida antarctica lipase type B catalyzed acylation of its
corresponding racemic mixture or alcohol dehydrogenase A
mediated bioreduction of the corresponding ketone. The in-
version of the chiral center has been identified as a key step,
optimizing the process to avoid partial racemization. Finally,
amine functionalization and nitrile hydrolysis have allowed
the production of (R)-Frovatriptan in enantiomerically pure
form.
Introduction
Triptans are a class of safe drugs that are clinically useful
for the treatment of migraine. They behave as agonists of
serotonin receptors (5-HT1B/1D) inducing the constriction
of the blood vessels, and then reducing the neurogenic in-
flammation in the brain and consequently the physiological
symptoms of migraine.[1] From the group of triptans, which
are all based on the tryptamine core, Frovatriptan (2a) is a Figure 1. Structures of tryptamine core (1), (R)-Frovatriptan (2a)
and (R)-Ramatroban (2b).
second-generation molecule that shows a very high affinity
for human 5-HT1B and 5-HT1D receptors. In addition, Fro-
vatriptan displays exceptional selectivity for the brain vas-
culature relative to traditional triptans, reducing the unde-
sired side effects to the coronary arteries and other periph-
eral vessels (Figure 1).[2]
triptan moiety through enantiomeric chromatographic sep-
arations,[4,5] or the formation of diastereomeric salts.[6] Un-
fortunately, overall yields are generally low owing to the
modest results achieved for the preparation of the optically
active intermediates.
Frovatriptan is especially indicated for the treatment of
menstrual migraine owing to its long deposition half life,
ca. 26 h, providing a sustained effect and reducing migraine
recurrence that often appears with first-generation triptans
(i.e. almotriptan and rizatriptan).[3] Frovatriptan is mar-
keted exclusively as the (R)-enantiomer because in vitro
testing in rabbit basilar arteries shows a higher activity for
the (R)-isomer than its antipode.[4] Different approaches
have been described for the preparation of the enantiopure
drug, the key step being the introduction of chirality in the
The excellent properties displayed by enzymes in terms
of enantioselectivity and reactivity make biocatalysis an at-
tractive methodology for the production of enantioenriched
compounds[7] under very mild and environmentally accept-
able reaction conditions.[8] In particular, the applicability
and versatility of biotransformations have been extensively
demonstrated for the manufacture of heterocycles[9] and
pharmaceutically active ingredients[10] with the highest
quality standards.[11] For instance, we have recently demon-
strated the versatility of lipases and alcohol dehydrogenases
(ADH) for the production of enantiopure (S)-2,3,4,9-tetra-
hydro-1H-carbazol-3-ol, which is an optimal building block
for the synthesis of (R)-Ramatroban (2b) used in the treat-
ment of allergic rhinitis and asthma.[12]
[a] Departamento de Química Orgánica e Inorgánica, Instituto
Universitario de Biotecnología de Asturias, Universidad de
Oviedo,
33006 Oviedo, Spain
Fax: +34-98 5103448
E-mail: vicgotfer@uniovi.es
Taking advantage of the possibilities that biocatalysis of-
fers for the synthesis of enantioenriched carbazoles,[11]
herein we present an original route for the preparation of
Homepage: www.uniovi.net/bioorganica
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300114.
2
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Chemoenzymatic Asymmetric Synthesis of (R)-Frovatriptan
enantiopure (R)-Frovatriptan. This approach is based on alcohol 6 with acetic acid. Treatment of the crude reaction
the synthesis of adequate racemic alcohol and prochiral with K2CO3 in a MeOH/H2O (1:1) solution afforded race-
ketone intermediates, optimizing the production of enantio- mic alcohol 6 in 69% yield.
pure alcohol precursors through kinetic resolution and
At this point, the kinetic resolution of racemic alcohol
asymmetrization processes, respectively. The selective modi- 6 was exhaustively tested by using a representative set of
fication of different functionalities will finally allow us to biocatalysts. Immobilized lipases were initially selected be-
synthesize desired (R)-Frovatriptan.
cause of their easy handling and simple isolation of the op-
tically active products. From this class of enzymes, Candida
antarctica lipase type A (CAL-A), Rhizomucor miehei lipase
(RM-IM), Burkholderia cepacia (formerly Pseudomonas
cepacia lipase) immobilized on ceramics (PSL-C I) and
Candida antarctica lipase type B (CAL-B) were selected for
an initial test of activity. Vinyl acetate (8), as activated acyl
donor, and tetrahydrofuran (THF) as solvent at 30 °C, were
fixed as standard reaction conditions for the acetylation
processes of racemic alcohols (Table 1).
Results and Discussion
In the course of this ongoing project involving the syn-
thesis of enantiopure (R)-Frovatriptan, we initially focused
our attention towards the synthesis of suitable synthetic
precursors for the target compound. In this particular case,
the electron-withdrawing amide group linked to the indole
ring increases the difficulty of the synthetic approach rela-
tive to the one leading to Ramatroban, because the Fischer
indolization reaction promoted by ZnCl2 that was success-
fully used in the synthesis of 2b did not proceed to any
extent. We thus turned our attention to find suitable reac-
tion conditions for the production of 6-cyano-2,3,4,9-tetra-
hydro-1H-carbazol-3-ol [(Ϯ)-6], a very useful intermediate
for the production of enantioenriched Frovatriptan, starting
with the production of 4-hydroxycyclohexanone (4), which
is often prepared by using toxic chromium(VI) reagents.[13]
Owing to the toxicity of chromium(VI) salts, 1,4-cyclohex-
anediol (3) was oxidized by using the safe alcohol oxidant
Ca(ClO)2.[14] The reaction was performed under very mild
reaction conditions by using an AcOH/H2O mixture at
0 °C, to isolate 4 in 88% yield and without observing the
formation of any side products (Scheme 1). Then, the
Table 1. Kinetic resolution of alcohol (Ϯ)-6 with different lipases
by using 3 equiv. of vinyl acetate (8), a 1:1 (w/w) enzyme/substrate
ratio in THF (0.1 m) at 30 °C and 250 rpm.
Entry Enzyme
t [h]
eep [%][b] ees [%][c] c [%][d]
E[e]
1
2
CAL-A
RM-IM 21
PSL-C I
PSL-C I
CAL-B
21
95
Ͼ99
95
99
99 (43)
51
31
35
24
51
50
60
Ͼ200
Ͼ200
Ͼ200
Ͼ200
3
2
8
9
Ͼ99
98
Ͼ99 (46) 50
4[a]
5
[a] Half the amount of enzyme was used. [b] Enantiomeric excess
of the product determined by HPLC. Isolated yields in parentheses.
[c] Enantiomeric excess of the substrate determined by HPLC. Iso-
lated yields in parentheses. [d] c = ees/(ees +ees). [e] E = ln[(1–c)(1–
eep)]/ln[(1–c)(1+eep)].[16]
Because racemic alcohol 6 is a quite hindered substrate,
Fischer indolization of 4 with 4-cyanophenylhydrazine Candida antarctica lipase A was attempted first as the bio-
hydrochloride (5) was attempted to obtain racemic alcohol catalyst owing to the good results obtained with bulky sub-
6. The presence of the electron-withdrawing nitrile group in strates,[15] however moderate enantioselectivity was
the phenyl hydrazine turns the Fischer condensation into a achieved after 21 h (Table 1, Entry 1). On the other hand,
complicated goal. In spite of this, we were able to find opti- RM-IM lipase showed excellent enantiopreference although
mal conditions for the reaction by using p-toluenesulfonic with a moderate reaction rate (Table 1, Entry 2). Next, PSL-
acid as catalyst in acetic acid heated to reflux. The analysis C, a very versatile catalyst for the kinetic resolution of race-
of the reaction crude mixture revealed a clean mixture of mic alcohols, but far from optimal in the resolution of the
alcohol 6 and the corresponding acetate 7, which is sponta- Ramatroban alcohol precursor,[12] was tested allowing the
neously formed through acid catalyzed esterification of recovery of (S)-alcohol (Ͼ99% ee) and (R)-acetate (95% ee)
Scheme 1. (a) Chemical synthesis of racemic alcohol (Ϯ)-6. (b) Lipase-catalyzed acetylation of (Ϯ)-6 by using vinyl acetate (8) as acyl
donor and THF as solvent.
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V. Gotor-Fernández et al.
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after a very short reaction time (Table 1, Entry 3). To try to details), and formation of reaction by-products owing to
obtain both products in enantiopure form a lower catalytic the PPh3-mediated reduction of the nitrile group. Therefore,
loading of PSL-C I (ratio enzyme/substrate 1:2 w/w) was we turned our attention to an alternative procedure for the
attempted, which lead to a lower reaction rate with im- production of amine (R)-12. Alcohol (S)-6 was mesylated
proved enantiomeric excess (Table 1, Entry 4). Finally, in CH2Cl2, giving (S)-10 in good yield and now possessing
CAL-B was used as a biocatalyst, which resulted in both a good leaving group for SN2 displacement. Next, (S)-10
substrate and product being isolated in enantiopure form was treated with sodium azide in dimethyl sulfoxide
after 9 h (Table 1, Entry 5), which is similar to the results (DMSO) at 50 °C, to afford azide (R)-11 with complete in-
found with Ramatroban.[12]
version of the chiral center (see Supporting Information for
Because of the inherent yield limitations of kinetic reso- details). The chemical reduction of the azide group by using
lution processes, the bioreduction of 9 was envisaged to try a palladium-catalyzed hydrogenation led to enantiopure
to obtain higher isolated yields for alcohol (S)-6, which is amine (R)-12 in quantitative yield following simple fil-
a valuable intermediate in the synthesis of (R)-Frovatriptan. tration purification (Scheme 3).
For that reason bioreduction processes were considered, fo-
cusing on the identification of a selective ADH to obtain
the (S)-alcohol in enantiopure form. Then racemic alcohol
6
was chemically oxidized by using Dess–Martin
periodinane to afford ketone 9 in good yield (Scheme 2).
Scheme 2. Chemical oxidation of racemic alcohol 6 and bioreduc-
tion of ketone 9 by using ADH-A in aqueous media.
Scheme 3. Chemical synthesis of enantiopure amine (R)-12 from
(S)-6.
To search for optimal conditions for the bioreduction of
9, preliminary experiments on a milligram scale were per-
formed by using a series of commercial ADHs, such as
alcohol dehydrogenases from Lactobacillus brevis (ADH-
LB), Candida parapsilosis (ADH-CP) and Rhodococcus
ruber (ADH-A). Although ketone 9 was recovered unal-
tered when ADH-CP or ADH-LB were used as biocata-
lysts, complete conversion occurred with ADH-A, to grati-
fyingly give alcohol (S)-6 in enantiopure form. It should
be noted that ADH-A was also found to be an excellent
biocatalyst for the production of an (S)-alcohol precursor
of Ramatroban.[12] With scale-up of the process in mind the
bioreduction was studied on a 40 mg scale by using overex-
pressed cells of ADH-A in E. coli, to give enantiopure (S)-
alcohol in 98% isolated yield after 24 h at 30 °C. Enantio-
pure (S)-6 was obtained in 67% yield after two steps, which
is an improvement on the yield obtained in the CAL-B cata-
lyzed kinetic resolution (46%).
Trying to explore all the possibilities in the production
of (R)-Frovatriptan, the enzymatic aminolysis of (R)-12 was
also considered (Scheme 4). Then, the kinetic resolution
was attempted by using CAL-B, a versatile enzyme for the
development of aminolysis reactions,[18] which previously
was identified as the optimal catalyst for the resolution of
alcohol (Ϯ)-6.
At this point, and considering that amine (R)-12 is the
optimal intermediate for the synthesis of enantiopure
Frovatriptan, the transformation of alcohol (S)-6 into
amine (R)-12 was attempted. Based on our experience in
the synthesis of Ramatroban[12] the combination of the Mit-
sunobu inversion of the alcohol with diphenylphosphoryl
azide (DPPA) followed by the Staudinger reduction of the
obtained azide 11 with PPh3 was attempted for the prepara-
tion of enantiopure (R)-12.[17] Unfortunately, the nitrile
Scheme 4. Lipase-mediated kinetic resolution of racemic amine 12
in THF at 30 °C.
Ethyl methoxyacetate 13 was employed, which is a very
substituent leads to two main drawbacks: extensive racemi- versatile non-activated ester for enzymatic aminolysis pro-
zation in the inversion step (see Supporting Information for cesses,[19] and THF as solvent at 30 °C. Aliquots were regu-
4
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Chemoenzymatic Asymmetric Synthesis of (R)-Frovatriptan
larly taken from the reaction flask, converting in situ the
unreacted amine into carbamate 15 to measure the enantio-
meric excess of the remaining amine (see Exp. Sect.). After
confirming that no reaction was observed in the absence of
the enzyme, very low reactivity and moderate enantio-
selectivity was detected after 24 h in the presence of CAL-
B (Table 2, Entry 1). On the other hand, PSL-C I formed
(R)-amide 14 with good enantiopreference and 35% conver-
sion after 3 h, although longer reaction times did not pro-
vide higher conversion values (Table 2, Entry 2).
Table 2. Lipase-catalyzed kinetic resolution of (Ϯ)-12 with different
lipases (ratio enzyme:substrate 1:1 w/w) by using 3 equiv. of ethyl
methoxyacetate 13 in dry THF (0.1 m) at 30 °C and 250 rpm.
Entry Enzyme
t [h]
eep [%][a]
ees [%][b] c [%][c] E[d]
1
2
CAL-B
PSL-C I
24
3
97
94
7
51
7
35
37
112
[a] Enantiomeric excess of the product determined by HPLC. [b] En-
antiomeric excess of the substrate determined by HPLC. [c] c = ees/
(ees +ees). [d] E = ln[(1–c)(1–eep)]/ln[(1–c)(1+eep)].[16]
Scheme 5. Chemical synthesis of (R)-Frovatriptan from (R)-12.
Conclusions
In spite of the good enantiopreference (E = 112) ob-
tained in the resolution of the racemic amine, the (R)-amide
was isolated with only 94% ee and a moderate conversion
value (35%). These modest results dissuaded us from using
the enantioenriched amide for the total synthesis of (R)-
Frovatriptan. Therefore, the chemical synthesis of enantio-
pure amine (R)-12 from alcohol (S)-6 was selected as the
best option to perform the final synthesis of (R)-2a. First,
amine (R)-12 was selectively monobenzylated with the com-
bination of sodium cyanoborohydride (NaBH3CN) and
benzaldehyde in MeOH at 0 °C, adding acetic acid and
formaldehyde to the reaction medium to afford tertiary
amine (R)-16 in good yield (Scheme 5). Next, the hydrolysis
of the nitrile group was performed in phosphoric acid (85%
in H2O) at 110 °C, to afford enantiopure (R)-17 in moderate
yield. Finally, the benzyl group was removed through Pd/
C-catalyzed hydrogenation, leading to Frovatriptan in high
purity and in enantiopure form. The absolute configuration
of Frovatriptan (R)-2a was determined from its optical ro-
tation value relative to that described in the literature:
[α]2D0 = +57.0 (c = 0.5, MeOH) {ref.[20] [α]2D0 = +60.0 (c =
0.5, MeOH)}.
In summary, we have developed a new chemoenzymatic
strategy for the production of enantiopure (R)-Frovatrip-
tan, a very useful triptan for the treatment of migraine
headache. Based on the importance of developing asymmet-
ric methods for the production of chiral drugs,[21] lipases
and alcohol dehydrogenases have been successfully applied.
The action of Candida antarctica lipase type B (CAL-B)
and ADH-A from Rhodoccocus ruber provided optimal ac-
cess to (S)-3-hydroxy-2,3,4,9-tetrahydro-1H-carbazole-6-
carbonitrile, a very useful intermediate for the synthesis of
Frovatriptan. The useful alcohol intermediate was subjected
to SN2 inversion with sodium azide followed by reduction
of the azide to afford enantiopure (R)-amine. Later, selec-
tive functionalization of the obtained amine allowed the
preparation of enantiopure (R)-Frovatriptan in 10 steps and
7% overall yield from inexpensive 1,4-cyclohexanediol. This
route overcomes limitations in the laborious traditional
stereoselective synthesis of Frovatriptan enantiomers rely-
ing on chromatographic separations or the formation of
diastereomeric salts.
Although in some cases the experimental procedures
have been described for enantiopure products, the syntheses
of racemic compounds have been also studied to develop
Experimental Section
analytical methods for chiral HPLC separations of the General Methods: Chemical reagents were purchased from different
commercial sources and used without further purification. Solvents
were distilled from an adequate desiccant under nitrogen. Alcohol
dehydrogenase from Candida parapsilosis (ADH CP, 30 U/mL), was
obtained from Codexis Inc. The authors thank Prof. Wolfgang
Kroutil for the generous gift of ADH-A over expressed in E. coli
BL21 cells: Cells were grown in LB-amp medium (1 L, 100 mg/
L) at 30 °C and 250 rpm for 24 h. Then, an additional amount of
ampicillin (50 mg/L), ZnCl2 (100 mg/L) and isopropyl thiogalacto-
side (IPTG, 450 mg/L) were added and shaken for 24 h at 20 °C.
The cells were centrifuged and the pellet was washed and sus-
pended in phosphate buffer (pH 7.5, 50 mm), frozen with liquid
products to assure that no racemization occurs along the
synthetic route. Unfortunately, we were not able to find
suitable conditions for the separation of Frovatriptan
enantiomers by using chiral columns and normal phase
HPLC techniques. For that reason, both racemic and
enantiopure Frovatriptan were transformed into the corre-
sponding benzylated derivative 17 (Scheme 5). HPLC
analyses clearly demonstrate that no racemization occurred
in the synthesis of Frovatriptan along the global synthesis
(see Supporting Information for details).
Eur. J. Org. Chem. 0000, 0–0
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V. Gotor-Fernández et al.
Chiralcel OJ-H n-hexane/2-propanol (80:20), 0.8 mL/min, 30 °C, tR
FULL PAPER
nitrogen and lyophilized. The obtained cells were stored at 4 °C.
Candida antarctica lipase type B (CAL-B, Novozyme 435, 7300 (R) = 15.1 min, tR (S) = 17.7 min.
PLU/g) and Rhizomucor miehei lipase (RM-IM, 150 IUN/g) were
(؎)-6-Cyano-2,3,4,9-tetrahydro-1H-carbazole-3-yl Acetate (7):
a gift from Novozymes. Pseudomonas cepacia lipase currently
known as Burkhloderia cepacia lipase (PSL-C I, 1638 U/g solid)
was acquired from Sigma–Aldrich. Candida antarctica lipase type
A (CAL-A, 2.6 U/mg solid) was purchased from Codexis Inc. Flash
chromatography was performed by using silica gel 60 (230–
240 mesh). Melting points were taken on samples in open capillary
tubes. IR spectra were recorded with NaCl plates or KBr pellets.
1H, 13C NMR, DEPT and 2D NMR experiments (COSY, HMBC
and HSQC) were obtained by using different spectrometers (1H,
300.13 and 13C, 75.5 MHz) or (1H, 400.13 and 13C, 100.6 MHz).
HRMS experiments were carried out by ESI+ or ESI– experiments.
Measurement of the optical rotation was done at 590 nm. HPLC
analyses were carried out at 210 nm by using a Chiralcel OD col-
umn (25 cmϫ 4.6 mm I.D.), Chiralcel OJ-H (25 cmϫ 4.6 mm
I.D.), Chiralpak IC (25 cmϫ 4.0 mm I.D.).
Et3N (42 μL, 0.30 mmol), 4-dimethylaminopyridine (DMAP, 3 mg,
0.03 mmol) and acetic anhydride (19 μL, 0.20 mmol) were success-
ively added under inert atmosphere to a solution of racemic 3-
hydroxy-2,3,4,9-tetrahydro-1H-carbazole-6-carbonitrile (6, 23 mg,
0.11 mmol) in CH2Cl2 (1 mL). The reaction was stirred for 1.5 h at
room temp. and after this time the solvent was removed under re-
duced pressure, obtaining a crude mixture that was purified by
flash chromatography on silica gel (eluent: EtOAc/hexane = 40:60)
to afford 7 as a white solid (27 mg, 0.11 mmol, 97%). Rf (40%
EtOAc/hexane): 0.25, m.p. 169–171 °C (dec.). IR (KBr): ν = 3403,
˜
1
2211, 1707, 1655, 1265 cm–1. H NMR (300.13 MHz, CDCl3): δ =
2.08 (s, 3 H, 7-H), 2.08–2.18 (m, 2 H, 3-H), 2.79–2.97 (m, 3 H, 8-
H, 4-H), 3.06–3.12 (m, 1 H, 8-H), 5.33 (qu, J = 5.9 Hz, 1 H, 5-H),
7.32–7.39 (m, 2 H, 14-H, 15-H), 7.76 (s, 1 H, 11-H), 8.34 (br. s, 1
H, N-H) ppm. 13C NMR (75.5 MHz, CDCl3): δ = 20.1 (CH2, C-
3), 21.3 (CH3, C-7), 26.6 (CH2, C-8), 27.3 (CH2, C-4), 69.2 (CH,
C-5), 102.1 (C, C-13), 107.8 (C, C-9), 111.2 (CH, C-15), 120.8 (C,
C-10), 123.0 (CH, C-11), 124.5 (CH, C-14), 127.4 (C, C-2), 135.1
(C, C-16), 137.9 (C, C-12), 170.7 (C, C-6) ppm. HRMS (ESI+):
calcd. for (C15H14N2NaO2)+ [M + Na]+ 277.0947; found 277.0942.
HPLC: Chiralcel OJ-H n-hexane/2-propanol (80:20), 0.8 mL/min,
30 °C, tR (S) = 19.0 min, tR (R) = 21.4 min.
4-Hydroxycyclohexanone (4): A suspension of Ca(ClO)2 (65%,
654 mg, 3.0 mmol) in H2O (14 mL) was added dropwise to a solu-
tion of 1,4-cyclohexanediol (3, 696 mg, 6.0 mmol) in AcOH
(12 mL) containing a few crystals of KBr at 0 °C. The resulting
suspension was stirred at 0 °C for 3 h, and after this time the sol-
vent was evaporated under reduced pressure. The crude was puri-
fied by flash chromatography on silica gel (eluent: EtOAc/hexane
= 70:30) to afford 4 as a colorless oil (601 mg, 5.26 mmol, 88%).
CAL-B Catalyzed Kinetic Resolution (؎)-3-Hydroxy-2,3,4,9-tetra-
hydro-1H-carbazole-6-carbonitrile (6): Vinyl acetate (8, 1.56 mL,
R (70% EtOAc/hexane): 0.32. IR (NaCl): ν = 3405, 2943, 1707,
˜
f
1426, 1247, 1084 cm–1. H NMR (300.13 MHz, CDCl3): δ = 1.88– 16.95 mmol) was added under an inert atmosphere to a suspension
1
2.09 (m, 4 H, 3-H), 2.24–2.33 (m, 2 H, 2-H), 2.48 (br. s, 1 H, O- containing racemic alcohol 6 (1.20 g, 5.65 mmol) and CAL-B
H), 2.54–2.63 (m, 2 H, 2-H), 4.13–4.20 (m, 1 H, 4-H) ppm. 13C
NMR (75.5 MHz, CDCl3): δ = 33.5 (2CH2, C-3), 37.0 (2CH2, C-
2), 66.9 (CH, C-4), 211.4 (C, C-1) ppm. HRMS (ESI+): calcd. for
(C6H10NaO2)+ [M + Na]+ 137.0573; found 137.0580.
(1.20 g) in dry THF (57 mL). The reaction was shaken at 30 °C
and 250 rpm until the optical purity of alcohol (S)-6 was Ͼ99%
(9 h). Then the reaction was stopped and the enzyme filtered and
washed with MeOH (3ϫ 15 mL). The solvent was removed by dis-
tillation under reduced pressure and the crude mixture was purified
by flash chromatography on silica gel (eluent: EtOAc/hexane =
30:70 to 70:30) to afford (R)-(+)-7 (614 mg, 43% and 99% ee)
[α]2D0 = +21.9 (c = 1, CH2Cl2)] and (S)-(–)-6 (554 mg, 46% isolated
yield and Ͼ99% ee) [α]2D0 = –16.8 (c = 0.5, MeOH)].
(؎)-3-Hydroxy-2,3,4,9-tetrahydro-1H-carbazole-6-carbonitrile (6):
p-Toluenesulfonic acid monohydrate (165 mg, 0.87 mmol) was
added to a suspension of 4-hydroxycyclohexanone (4, 500 mg,
4.38 mmol) and 4-cyanophenylhydrazine hydrochloride (5, 738 mg,
4.38 mmol) in AcOH (29 mL) and the mixture was stirred for 6 h
at 120 °C. Then, the reaction was quenched with an aqueous satu-
rated solution of NaHCO3 (100 mL), and the suspension extracted
with EtOAc (3ϫ 100 mL). Organic phases were combined, dried
with Na2SO4, and the solvent evaporated under reduced pressure.
The crude mixture containing a mixture of alcohol 6 and acetate 7
was dissolved in a MeOH/H2O solution (1:1 v/v, 40 mL) and
K2CO3 (3.60 g, 26.0 mmol) was added, stirring the solution at
45 °C for 3 h. After this time, MeOH was removed under reduced
pressure, and the aqueous phase extracted with EtOAc (3ϫ
20 mL). Organic phases were combined, the solvent was evaporated
under reduced pressure and the crude was purified by flash
chromatography on silica gel (eluent: EtOAc/hexane = 50:50 to
70:30), to afford 6 as a white solid (637 mg, 3.00 mmol, 69%). Rf
3-Oxo-2,3,4,9-tetrahydro-1H-carbazole-6-carbonitrile (9): Dess–
Martin reagent (149 mg, 0.34 mmol) was added under an inert at-
mosphere to
a solution of racemic alcohol (Ϯ)-6 (50 mg,
0.23 mmol) in MeCN (2.5 mL). The mixture was stirred at room
temp. for 1 h, before quenching the reaction with an aqueous satu-
rated solution of Na2S2O3/NaHCO3 (1:1, 5 mL). The mixture was
extracted with EtOAc (3ϫ 5 mL), and the organic layers were com-
bined, dried with Na2SO4 and the solvent evaporated under re-
duced pressure. The resulting crude mixture was purified by flash
chromatography on silica gel (eluent: EtOAc/hexane = 50:50) to
afford 9 as a white solid (33 mg, 0.16 mmol, 68%). Rf (50% EtOAc/
hexane): 0.27, m.p. 212–214 °C. IR (KBr): ν = 3342, 2213, 1689,
˜
1477, 1261, 1176 cm–1. 1H NMR (300.13 MHz, [D6]DMSO): δ =
2.73 (t, J = 7.0 Hz, 2 H, 3-H), 3.15 (t, 3J = 6.8 Hz, 2 H, 4-H), 3.54
(s, 2 H, 6-H), 7.40 (d, J = 8.4 Hz, 1 H, 13-H), 7.47 (d, J = 8.4 Hz,
1 H, 12-H), 7.90 (s, 1 H, 9-H), 11.58 (br. s, 1 H, N-H) ppm. 13C
NMR (75.5 MHz, [D6]DMSO): δ = 21.8 (CH2, C-3), 35.9 (CH2,
C-6), 38.0 (CH2, C-4), 100.5 (C, C-11), 106.9 (C, C-7), 112.0 (CH,
C-13), 120.8 (C, C-8), 123.0 (CH, C-9), 123.7 (CH, C-12), 126.6
(C, C-2), 135.8 (C, C-14), 138.3 (C, C-10), 208.5 (C, C-5) ppm.
HRMS (ESI+): calcd. for (C13H11N2O)+ [M + H]+ 211.0866; found
211.0867.
(60% EtOAc/hexane): 0.18, m.p. 229–231 °C (dec.). IR (KBr): ν
˜
1
= 3401, 3295, 2920, 2214, 1621, 1479, 1322, 1047 cm–1. H NMR
(300.13 MHz, CD3OD): δ = 1.92–2.15 (m, 2 H, 3-H), 2.60–2.67 (m,
1 H, 6-H), 2.80–2.96 (m, 2 H, 4-H), 3.01–3.08 (m, 1 H, 6-H), 4.17–
4.21 (m, 1 H, 5-H), 7.29 (d, J = 8.4 Hz, 1 H, 13-H), 7.36 (d, J =
8.4 Hz, 1 H, 12-H), 7.76 (s, 1 H, 9-H) ppm. 13C NMR (75.5 MHz,
CD3OD): δ = 21.8 (CH2, C-3), 30.9 (CH2, C-6), 32.3 (CH2, C-4),
68.5 (CH, C-5), 102.0 (C, C-11), 109.0 (C, C-7), 112.8 (CH, C-13),
122.5 (C, C-8), 123.9 (CH, C-9), 124.8 (CH, C-12), 129.3 (C-2),
137.9 (C, C-14), 140.3 (C, C-10) ppm. HRMS (ESI+): calcd. for
(C13H12N2NaO)+ [M + Na]+ 235.0842; found 235.0834. HPLC:
Bioreduction of 3-Oxo-2,3,4,9-tetrahydro-1H-carbazole-6-carbo-
nitrile (9) with Lyophilized ADH-A: 2-Propanol (309 μL), NADH
6
www.eurjoc.org
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Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O30114
/KAP1
Date: 06-05-13 17:26:30
Pages: 9
Chemoenzymatic Asymmetric Synthesis of (R)-Frovatriptan
(2.9 mg) and ketone 9 (43 mg, 0.20 mmol) were successively added
to a solution containing rehydrated E. Coli/ADH-A cells (41 mg)
in TRIS-HCl Buffer (50 mm, pH 7.5, 4.7 mL). The reaction was
shaken for 24 h at 30 °C and 250 rpm. Then, the suspension was
extracted with EtOAc (3ϫ 5 mL), and the organic layers were col-
lected, dried with Na2SO4, and the solvent removed by distillation
(184 mg, 0.87 mmol, 99%). Rf (1% NH3/MeOH): 0.27, m.p. 221–
223 °C (dec.). IR (KBr): ν = 3210, 2919, 2211, 1513, 1475,
˜
1
1261 cm–1. H NMR (300.13 MHz, CD3OD): δ = 1.74–1.87 (m, 1
H, 3-H), 2.10–2.16 (m, 1 H, 3-H), 2.42–2.50 (m, 1 H, 6-H), 2.85–
2.90 (m, 2 H, 4-H), 2.99–3.06 (m, 1 H, 6-H), 3.18–3.27 (m, 1 H, 5-
H), 7.31 (d, J = 8.3 Hz, 1 H, 13-H), 7.38 (d, J = 8.3 Hz, 1 H, 12-
H), 7.77 (s, 1 H, 9-H) ppm. 13C NMR (75.5 MHz, CD3OD): δ =
22.7 (CH2, C-3), 31.1 (CH2, C-6), 33.0 (CH2, C-4), 49.3 (CH, C-
5), 102.0 (C, C-11), 109.8 (C, C-7), 112.8 (CH, C-13), 122.5 (C, C-
8), 123.9 (CH, C-9), 124.8 (CH, C-12), 129.1 (C, C-2), 138.0 (C, C-
14), 140.3 (C, C-10) ppm. HRMS (ESI+): calcd. for (C13H14N3)+
[M + H]+ 212.1182; found 212.1174. [α]2D0 = +22.0 (c = 0.25,
MeOH) for 99% ee.
to afford (S)-9 as a white solid (42.5 mg, 0.20 mmol, 98%). [α]2D0
–16.8 (c = 0.5, MeOH) in Ͼ99% ee.
=
(S)-6-Cyano-2,3,4,9-tetrahydro-1H-carbazole-3-yl Methanesulfonate
(10): Et3N (670 μL, 4.80 mmol), DMAP (29 mg, 0.24 mmol) and
methanesulfonyl chloride (371 μL, 4.80 mmol) were successively
added under an inert atmosphere to a solution of alcohol (S)-6
(520 mg, 2.45 mmol) in CH2Cl2 (25 mL). The reaction was stirred
at room temp. for 1 h, and then the solvent was evaporated under
reduced pressure. The crude mixture was purified by flash
chromatography on silica gel (eluent: EtOAc/hexane = 50:50 to
(؎)-N-(6-Cyano-2,3,4,9-tetrahydro-1H-carbazol-3-yl)-2-methoxy-
acetamide (14): Et3N (11 μL, 0.08 mmol) and methoxyacetyl chlor-
ide (8 μL, 0.08 mmol) were successively added under an inert atmo-
100:0) to afford (S)-10 as a white solid (597 mg, 2.06 mmol, 84%). sphere to a solution of racemic amine 12 (15 mg, 0.07 mmol) in
R (60% EtOAc/hexane): 0.37, m.p. 171–173 °C (dec.). IR (KBr): ν
CH2Cl2 (1 mL) and the resulting solution stirred for 2 h at room
temp. After this time the solvent was removed under reduced pres-
˜
f
= 3376, 2213, 1627, 1477, 1342, 1324, 1170 cm–1. 1H NMR
(300.13 MHz, [D6]DMSO): δ = 2.17–2.27 (m, 2 H, 3-H), 2.87–2.98 sure and the crude mixture was purified by flash chromatography
(m, 3 H, 4-H, 7-H), 3.13–3.20 (m, 1 H, 7-H), 3.25 (s, 3 H, 6-H), on silica gel (eluent: EtOAc) to afford amide 14 as a white solid
5.20–5.22 (m, 1 H, 5-H), 7.38 (d, J = 8.4 Hz, 1 H, 14-H), 7.44 (d,
(15 mg, 0.05 mmol, 76%). Rf (100% EtOAc): 0.28, m.p. 219–
J = 8.4 Hz, 1 H, 13-H), 7.94 (s, 1 H, 10-H), 11.48 (br. s, 1 H, N- 221 °C. IR (KBr): ν = 3335, 2219, 1651, 1535, 1474 cm–1. 1H NMR
˜
H) ppm. 13C NMR (75.5 MHz, [D6]DMSO): δ = 19.3 (CH2, C-3),
27.5 (CH2, C-7), 27.8 (CH, C-4), 37.8 (CH3, C-6), 77.5 (CH, C-5), (m, 1 H, 9-H), 2.82–2.93 (m, 3 H, 4-H, 9-H), 3.31 (s, 3 H, 8-H),
(400.13 MHz, [D6]DMSO): δ = 1.89–1.95 (m, 2 H, 3-H), 2.57–2.62
100.3 (C, C-12), 105.6 (C, C-8), 111.9 (CH, C-14), 120.9 (C, C-9),
122.8 (CH, C-10), 123.5 (CH, C-13), 126.8 (C, C-2), 136.0 (C, C-
3.82 (s, 2 H, 7-H), 4.13 (br. s, 1 H, 5-H), 7.34 (d, J = 8.0 Hz, 1 H,
16-H), 7.40 (d, J = 8.0 Hz, 1 H, 15-H), 7.82–7.86 (m, 2 H, 12-H,
15), 137.9 (C, C-11) ppm. HRMS (ESI+): calcd. for N-H), 11.38 (br. s,1 H, N-H) ppm. 13C NMR (100.6 MHz, [D6]-
(C14H14N2NaO3S)+ [M + Na]+ 313.0617; found 313.0619. [α]2D0
=
DMSO): δ = 21.3 (CH2, C-3), 26.7 (CH2, C-9), 28.3 (CH2, C-4),
44.6 (CH, C-5), 58.6 (CH3, C-8), 71.5 (CH2, C-7), 100.1 (C, C-14),
107.8 (C, C-10), 111.8 (CH, C-16), 121.0 (C, C-11), 122.7 (CH, C-
12), 123.3 (CH, C-15), 126.9 (C, C-2), 136.5 (C, C-17), 138.0 (C,
C-13), 168.4 (C, C-6) ppm. HRMS (ESI+): calcd. for
(C16H17N3NaO2)+ [M + Na]+ 306.1213; found 306.1205. HPLC:
Chiralcel OJ-H n-hexane/2-propanol (70:30), 0.8 mL/min, 30 °C, tR
(S) = 10.2 min, tR (R) = 15.0 min.
+25.6 (c = 0.25, MeOH) for Ͼ99% ee. HPLC: Chiralcel OD n-
hexane/2-propanol (70:30), 0.8 mL/min, 30 °C, tR (S) = 10.4 min,
tR (R) = 16.4 min.
(R)-3-Azido-2,3,4,9-tetrahydro-1H-carbazole-6-carbonitrile
(11):
Sodium azide (449 mg, 6.90 mmol) was added to a solution of the
methanesulfonate (S)-10 (400 mg, 1.38 mmol) in DMSO (14 mL).
The solution was stirred for 8 h at 50 °C, and after this time the
reaction was quenched with H2O (50 mL) and extracted with
EtOAc (3ϫ 50 mL). The organic phases were combined and dried
with Na2SO4, and the solvent removed under reduced pressure. The
crude mixture was purified by flash chromatography on silica gel
(eluent: EtOAc/hexane = 20:80 to 30:70) to afford (R)-11 as a white
solid (208 mg, 0.88 mmol, 63%). Rf (20% EtOAc/hexane): 0.20,
(؎)-Benzyl 6-Cyano-2,3,4,9-tetrahydro-1H-carbazol-3-ylcarbamate
(15): Et3N (11 μL, 0.08 mmol) and benzyl chloroformate (12 μL,
0.08 mmol) were successively added under an inert atmosphere to
a solution of racemic amine 12 (15 mg, 0.07 mmol) in CH2Cl2
(1 mL). The solution was stirred at room temp. for 2 h and then,
the solvent was removed under reduced pressure, and the crude
m.p. 142–144 °C. IR (KBr): ν = 3318, 2218, 2099, 1624, 1474, mixture was purified by flash chromatography on silica gel (eluent:
˜
1252 cm–1. 1H NMR (300.13 MHz, CDCl3): δ = 2.07–2.27 (m, 2
H, 3-H), 2.78–3.15 (m, 4 H, 4-H, 6-H), 4.01–4.09 (m, 1 H, 5-H),
7.35 (d, J = 8.4 Hz, 1 H, 13-H), 7.41 (d, J = 8.4 Hz, 1 H, 12-H),
7.81 (s, 1 H, 9-H), 8.16 (br. s, 1 H) ppm. 13C NMR (75.5 MHz,
EtOAc/hexane = 50:50) to afford 15 as a white gum (18 mg,
0.05 mmol, 65%). R (50% EtOAc/hexane): 0.27. IR (NaCl): ν =
˜
f
3321, 2930, 2218, 1698, 1526, 1475 cm–1. H NMR (300.13 MHz,
1
CDCl3): δ = 1.95–2.16 (m, 2 H, 3-H), 2.58–2.66 (m, 1 H, 12-H),
CDCl3): δ = 20.5 (CH2, C-3), 26.4 (CH2, C-6), 27.5 (CH2, C-4), 2.80–2.93 (m, 2 H, 4-H), 3.07–3.14 (m, 1 H, 12-H), 4.16–4.23 (m,
56.6 (CH, C-5), 102.1 (C, C-11), 107.5 (C, C-7), 111.3 (CH, C-13), 1 H, 5-H), 4.98 (d, J = 7.1 Hz, 1 H, N-H), 5.14 (s, 2 H, 7-H), 7.28–
120.8 (C, C-8), 122.9 (CH, C-9), 124.5 (CH, C-12), 127.2 (C, C-2), 7.39 (m, 7 H, 9-H, 10-H, 11-H, 18-H, 19-H), 7.75 (s, 1 H, 15-H),
135.1 (C, C-14), 137.9 (C, C-10) ppm. HRMS (ESI+): calcd. for
(C13H11N5Na)+ [M + Na]+ 260.0907; found 260.0900. [α]2D0 = –24.0
(c = 0.5, MeOH) for 99% ee. HPLC: Chiralcel OJ-H n-hexane/2-
propanol (70:30), 0.8 mL/min, 30 °C, tR (R) = 9.8 min, tR (S) =
11.9 min.
8.32 (br. s, 1 H, N-H) ppm. 13C NMR (75.5 MHz, CDCl3): δ =
20.6 (CH2, C-3), 27.6 (CH2, C-12), 28.2 (CH2, C-4), 46.6 (CH, C-
5), 66.7 (CH2, C-7), 102.1 (C, C-17), 108.3 (C, C-13), 111.2 (CH,
C-19), 120.8 (C, C-14), 123.0 (CH, C-15), 124.5 (CH, C-18), 127.3
(C, C-2), 128.1 (3CH, C-9, C-11), 128.5 (2CH, C-10), 135.1 (C, C-
8), 136.2 (C, C-20), 137.9 (C, C-16), 155.7 (C, C-6) ppm. HRMS
(ESI+): calcd. for (C21H19N3NaO2)+ [M + Na]+ 368.1369; found
368.1364. HPLC: Chiralcel OJ-H n-hexane/2-propanol (70:30),
0.8 mL/min, 30 °C, tR (S) = 22.9 min, tR (R) = 26.3 min.
(R)-3-Amino-2,3,4,9-tetrahydro-1H-carbazole-6-carbonitrile
(12):
MeOH (9.0 mL) was carefully added to a suspension containing
(R)-11 (208 mg, 0.88 mmol) and Pd/C (10% w/w, 75 mg) and a hy-
drogen balloon. The resulting suspension was stirred at room tem-
perature for 1 h and after this time the reaction was stopped and
the mixture was filtered through Celite. The solvent was evaporated
under reduced pressure to afford amine (R)-12 as a white solid
Lipase-Catalyzed Kinetic Resolution of (؎)-3-Amino-2,3,4,9-tetra-
hydro-1H-carbazole-6-carbonitrile (12): Ethyl methoxyacetate (13,
59 μL, 0.50 mmol) was added to a suspension of racemic amine 12
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
7

Job/Unit: O30114
/KAP1
Date: 06-05-13 17:26:30
Pages: 9
V. Gotor-Fernández et al.
FULL PAPER
(21 mg, 0.10 mmol) and corresponding enzyme (21 mg) in dry THF
(1 mL). The reaction was shaken at 30 °C and 250 rpm, and regular
aliquots were taken. The aliquots were dissolved in CH2Cl2
(0.5 mL), K2CO3 (20 mg) and benzyl chloroformate (1 drop) were
successively added to form the amine derivative for enantiomeric
excess measurements. The suspension was shaken at room temp.
for 5 min at 250 rpm, dried with Na2SO4 and the solvent removed
with a nitrogen flow. The crude mixture was dissolved in hexane/
2-propanol 90:10 (1 mL) filtered and injected into the HPLC appa-
ratus.
140.7 (C, C-16), 174.5 (C, C-17) ppm. HRMS (ESI+): calcd. for
(C21H24N3O)+ [M + H]+ 334.1914; found 334.1908. [α]2D0 = +118 (c
= 0.5, MeOH) for 99% ee. HPLC: Chiralpak IC n-hexane/2-prop-
anol (70:30), 0.8 mL/min, 30 °C, tR (R) = 21.2 min, tR (S) =
23.8 min.
(R)-3-(Methylamino)-2,3,4,9-tetrahydro-1H-carbazole-6-carbox-
amide (2a), Frovatriptan: MeOH (11.0 mL) was carefully added to
a suspension containing (R)-17 (120 mg, 0.36 mmol) and Pd/C
(10% w/w, 97 mg) under a H2 atmosphere. The resulting suspension
was stirred at room temperature for 1 h and after this time the
reaction was stopped and filtered through Celite. The solvent was
evaporated under reduced pressure to afford amine (R)-2a as a
white solid (62 mg, 0.25 mmol, 71%). Rf (3% NH3/MeOH): 0.18,
(R)-3-[Benzyl(methyl)amino]-2, 3,4,9-tetrahydro-1H-
carbazole-6-carbonitrile (16): Benzaldehyde (111 μL, 1.11 mmol)
was added to a solution of (R)-12 (180 mg, 0.85 mmol) in MeOH
(4.5 mL) and stirred for 2 h at room temperature. After this time,
the reaction was cooled to 0 °C and NaBH3CN (160 mg,
2.55 mmol) was added portion wise for 1 h, then acetic acid (99 μL,
1.70 mmol) was added dropwise and the mixture was stirred at 0 °C
for 5 h. Next, a formaldehyde solution (37% w/v in H2O, 104 μL,
1.18 mmol) was added and the reaction was stirred at 0 °C for an
additional 5 h. Finally, the reaction was quenched with an aqueous
saturated solution of Na2CO3 (20 mL), extracted with EtOAc (3ϫ
20 mL). The organic phases were combined, dried with Na2SO4
and the solvent removed under reduced pressure to obtain a crude
mixture that was purified by flash chromatography on silica gel
(eluent: EtOAc/hexane = 50:50 to 70:30) to afford (R)-16 as a col-
orless liquid (194 mg, 0.61 mmol, 72%). Rf (50% EtOAc/hexane):
m.p. 140–141 °C. IR (KBr): ν = 3402, 3362, 2922, 1651, 1593, 1471,
˜
1
1384 cm–1. H NMR (300.13 MHz, CD3OD): δ = 1.65–1.78 (m, 1
H, 3-H), 2.13–2.17 (m, 1 H, 3-H), 2.40–2.46 (m, 1 H, 7-H), 2.46 (s,
3 H, 6-H), 2.77–2.86 (m, 2 H, 4-H, 5-H), 3.03–3.10 (m, 1 H, 7-H),
3
7.28 (d, JHH = 8.3 Hz, 1 H, 14-H), 7.62 (dd, J = 8.5, J = 1.8 Hz,
1 H, 13-H), 8.02 (d, J = 1.8 Hz, 1 H, 10-H) ppm. 13C NMR
(75.5 MHz, CD3OD): δ = 22.8 (CH2, C-3), 28.3 (CH2, C-7), 30.0
(CH2, C-4), 33.9 (CH3, C-6), 57.4 (CH, C-5), 109.7 (C, C-8), 111.5
(CH, C-14), 119.1 (CH, C-10), 121.7 (CH, C-13), 125.0 (C, C-9),
128.9 (C, C-2), 137.0 (C, C-15), 140.6 (C, C-11), 174.5 (C, C-
12) ppm. HRMS (ESI+): calcd. for (C14H18N3O)+ [M + H]+
244.1444; found 244.1445. [α]2D0 = +57.0 (c = 0.5, MeOH) for (R)-
1 in 99% ee [ref. [α]2D0 = +60.0 (c = 0.5, MeOH) for (R)-1].[20]
0.17. IR (NaCl): ν = 3327, 2926, 2844, 2217, 1624, 1473, 1338 cm–1.
˜
Synthesis of (R)-17 from (R)-2a. HPLC Measurement of the ee for
Frovatriptan: Benzaldehyde (9 μL, 0.09 mmol) was added to a solu-
tion of (R)-2a (15 mg, 0.06 mmol) in MeOH (1 mL) and the solu-
tion was stirred at room temp. for 2 h. After this time, the solution
was cooled to 0 °C and NaBH3CN (11 mg, 0.18 mmol) was added,
and the reaction stirred again at 0 °C for 1 h. Then, the reaction
was quenched with an aqueous saturated solution of Na2CO3
(5 mL) and extracted with EtOAc (3ϫ 5 mL). The organic phases
were combined, dried with Na2SO4, the solvent was evaporated
under reduced pressure and the crude mixture was purified by flash
chromatography on silica gel (eluent: MeOH/EtOAc = 5:95) to af-
ford (R)-17 as a white solid (14 mg, 0.04 mmol, 70%) without ob-
serving racemization (see Supporting Information for details).
1H NMR (400.13 MHz, CD3OD): δ = 1.78–1.89 (m, 1 H, 3-H),
2.20–2.24 (m, 1 H, 3-H), 2.32 (s, 3 H, 6-H), 2.67–3.00 (m, 5 H, 4-
H, 5-H, 12-H), AB system (δA = 3.67, δB = 3.71, JAB = 13.0 Hz, 2
H, 7-H), 7.22–7.38 (m, 7 H, 9-H, 10-H, 11-H, 18-H, 19-H), 7.74 (s,
1 H, 15-H) ppm. 13C NMR (100.6 MHz, CD3OD): δ = 23.3 (CH2,
C-3), 24.1 (CH2, C-12), 26.7 (CH2, C-4), 38.3 (CH3, C-6), 59.3
(CH2, C-7), 61.2 (CH, C-5), 102.0 (C, C-17), 110.3 (C, C-13), 112.8
(CH, C-19), 122.5 (C, C-14), 123.9 (CH, C-15), 124.8 (CH, C-18),
128.5 (CH, C-11), 129.3 (C, C-2), 129.6 (2CH, C-9), 130.8 (2CH,
C-10), 138.1 (C, C-8), 140.3 (2C, C-16, C-20) ppm. HRMS (ESI+):
calcd. for (C21H22N3)+ [M + H]+ 316.1808; found 316.1814. [α]2D0
= +163.2 (c = 0.4, MeOH) for 99% ee. HPLC: Chiralpak IC n-
hexane/2-propanol (90:10), 0.8 mL/min, 30 °C, tR (R) = 16.1 min,
tR (S) = 17.8 min.
Supporting Information (see footnote on the first page of this arti-
cle): Conditions for chiral separations, copies of chromatograms
(R)-3-[Benzyl(methyl)amino]-2,3,4,9-tetrahydro-1H-carbazole-6-
carboxamide (17): A solution of (R)-16 (100 mg, 0.32 mmol) in
H3PO4 (85% in H2O, 10 mL) was stirred at 110 °C for 5 h. After
this time the reaction was quenched with an aqueous saturated
solution of Na2CO3 (40 mL, until pH 8 was reached), and extracted
with EtOAc (4ϫ 40 mL). The organic phases were combined, dried
with Na2SO4 and the solvent removed by distillation under reduced
pressure to give a crude mixture that was purified by flash
chromatography on silica gel (eluent: MeOH/EtOAc = 5:95) to af-
ford (R)-17 as a white solid (65 mg, 0.19 mmol, 61 %). Rf (5 %
1
and H, 13C and DEPT NMR spectra, along with 2D NMR spec-
tra for Frovatriptan are included in the Supporting Information of
this article.
Acknowledgments
The authors thank Novo Nordisk Co. for the generous gift of the
CAL-B (Novozyme 435) and also Prof. W. Kroutil for ADH-A
overexpressed in E. coli cells. Financial support of this work by the
Spanish Ministerio de Economía y Competividad (MEC) through
the CTQ-2011-24237 project is grateful acknowledged.
MeOH/EtOAc): 0.18, m.p. 101–103 °C. IR (KBr): ν = 3406, 1654,
˜
1596, 1473, 1374 cm–1. 1H NMR (300.13 MHz, CD3OD): δ = 1.83–
1.97 (m, 1 H, 3-H), 2.25–2.29 (m, 1 H, 3-H), 2.37 (s, 3 H, 6-H),
2.74–2.92 (m, 3 H, 4-H, 12-H), 3.00–3.04 (m, 2 H, 5-H, 12-H), AB
system (δA = 3.72: δB = 3.78, JAB = 12.8 Hz, 2 H, 7-H), 7.27–7.42
(m, 6 H, 9-H, 10-H, 11-H, 19-H), 7.62 (dd, J = 8.4, JЈ = 1.6 Hz, 1
H, 18-H), 8.05 (d, J = 1.6 Hz, 1 H, 15-H) ppm. 13C NMR
(75.5 MHz, CD3OD): δ = 23.6 (CH2, C-3), 24.2 (CH2, C-12), 26.9
(CH2, C-4), 38.4 (CH3, C-6), 59.4 (CH2, C-7), 61.5 (CH, C-5),
110.4 (C, C-13), 111.5 (CH, C-19), 119.1 (CH, C-15), 121.7 (CH,
C-18), 125.0 (C, C-14), 128.6 (CH, C-11), 129.1 (C, C-2), 129.7
(2CH, C-9), 130.8 (2CH, C-10), 136.9 (C, C-8), 140.2 (C, C-20),
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Received: January 22, 2013
Published Online: ᭿
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