Studies directed toward the synthesis of cryptoheptine

Article abstract of DOI:10.1021/np990412p

Synthesis of 5,10-dihydro-10-methylindolo[3,2-b][1]benzazepin-12(11H)- one (2), an isomer of the reported structure for cryptoheptine (1), is presented. Attempts to convert 2 to 1 led to 10-methylindolo[3,2-b][1]- benzazepin-12-one (10), an oxidation prod

Full text of DOI:10.1021/np990412p

J . Nat. Prod. 2000, 63, 643-645
643
Stu d ies Dir ected tow a r d th e Syn th esis of Cr yp toh ep tin e
Pingsheng Zhang† and Donald E. Bierer*
Shaman Pharmaceuticals, 213 East Grand Avenue, South San Francisco, California 94080-4812
Received August 18, 1999
Synthesis of 5,10-dihydro-10-methylindolo[3,2-b][1]benzazepin-12(11H)-one (2), an isomer of the reported
structure for cryptoheptine (1), is presented. Attempts to convert 2 to 1 led to 10-methylindolo[3,2-b][1]-
benzazepin-12-one (10), an oxidation product of 2 and presumably 1. These results highlight the potential
instability of cryptoheptine and cast doubt on its proposed structure.
Sch em e 1a
Cryptolepis sanguinolenta (Lindl.) Schlechter, a member
of the Asclepiadaceae family and Periplocoideae subfamily,
is a shrub indigenous to tropical West Africa with a long
history of ethnomedical use.1 In several West African
nations, root decoctions have been used to treat a variety
of illnesses, including hepatitis,2 malaria,3 and diabetes.1,4
These indigenous uses have led to an extensive investiga-
tion of this plant, resulting in the isolation of a variety of
indole-containing alkaloids.5 Most of these alkaloids feature
the indoloquinoline ring system. One exception to this
commonality is cryptoheptine (1), which presents an in-
teresting indolobenzazepine structure.6 Our interest in
searching for antidiabetic compounds from ethnobotanical
plant sources, and the fascinating structure proposed for
cryptoheptine, led us to pursue a total synthesis of this
compound. This report describes the results of our study.
a
Reagents: (a) cat. piperidine, toluene, rt, 78%; (b) 10% HCl, THF, reflux,
84%; (c) (i) NaH, DMF, -10 °C, (ii) CH3I, 83%; (d) H2, 10% Pd/C, toluene;
(e) p-TsOH, toluene, Dean-Stark, 44% over two steps.
yield. Next, regiospecific introduction of the indole methyl
group was accomplished using a NaH/MeI protocol. Hy-
drogenation of 8 (10% Pd/C, toluene) gave intermediate 9,
which was quickly treated with para-toluenesulfonic acid
under Dean-Stark conditions to give the target keto isomer
of cryptoheptine, 2, in a 44% two-step yield.
Resu lts a n d Discu ssion
The proposed structure of cryptoheptine (1) contains a
triply unsaturated azepine ring and an acidic benzylic
proton at the 5a position.6 This structure can exist in a
number of different tautomeric forms, with indolobenz-
azepinones 2 and 3 being two reasonable (apparent)
tautomers of 1. Recognizing that a direct synthesis of 1
might prove difficult, we chose ketone 2 as our initial
synthetic target.
Our synthesis began with known 1-acetyl-3-oxoindole
(4).1 Condensation of 4 with glyoxal 5, which was prepared
by selenium dioxide oxidation of 2-nitroacetophenone, gave
hydroxyketone 6 in 78% yield (Scheme 1). Dehydration of
6 with concomitant removal of the acetyl protecting group
in a refluxing solution of 10% HCl provided 7 in an 84%
Cryptoheptine isomer 2 was prepared as a yellow solid
melting at 260 °C and having a molecular ion peak at m/z
262.1109. Elemental analysis confirmed the molecular
formula of 2 as C17H14N2O. An absorbance at 1628 cm-1
in the infrared spectrum and a downfield peak at δ 184.5
in the 13C NMR spectrum indicated the presence of a
carbonyl functionality. Three singlets were observed in the
1H NMR spectrum. An exchangeable proton was observed
at δ 6.86, corresponding to the NH proton of the azepine
ring. A two-proton singlet was observed at δ 3.88, and a
three-proton singlet was observed at δ 3.74, corresponding
to the methylene protons of the azepine ring and the indole
methyl group, respectively. As expected, the 2D COSY
spectrum revealed two aromatic four-spin systems, each
of which were correlated to their adjacent rings using
HMQC and HMBC data. Comparisons of the NMR assign-
* To whom correspondence should be addressed: Bayer Corporation,
Department of Chemistry, 400 Morgan Lane, West Haven, CT 06516-4175.
Tel: (203) 812-3897. Fax: (203) 812-2650. E-mail: donald.bierer.b@bayer.com.
† Current address: Roche Carolina, Inc., 6173 East Old Marion Highway,
Florence, South Carolina 29506.
10.1021/np990412p CCC: $19.00
© 2000 American Chemical Society and American Society of Pharmacognosy
Published on Web 04/06/2000

644 J ournal of Natural Products, 2000, Vol. 63, No. 5
Zhang and Bierer
Ta ble 1. NMR Spectral Data (δ) of 2a and Comparison to
Cryptoheptine (1)b,c
ably stable isomer (apparent tautomer) of 1 is oxidized, and
the fact that the chromatography solvent used to isolate
cryptoheptine degrades 2 to 10, highlights the potential
instability of cryptoheptine and casts doubt on its proposed
structure.
isomer 2
1H
cryptoheptine (1)
atom no.
13C
1H
13C
1
2
3
4
7.94, dd (8.0, 0.8)
6.86, dd (7.6, 7.6)
7.38, dd (7.6, 7.6)
7.10, d (8.0)
131.6
117.5
132.6
118.2
144.6
7.96, d (8.1)
118.8
120.6
126.5
117.9
125.5
7.24, dd (8.1, 6.6)
7.45, dd (7.7, 6.6)
7.92, d (7.7)
Exp er im en ta l Section
Gen er a l Exp er im en ta l P r oced u r es. Anhydrous DMF
and toluene were obtained from Aldrich. Moisture- and air-
sensitive reactions were performed under a nitrogen atmo-
sphere. Analytical TLC was performed on E. Merck silica gel
60 F254 precoated plates (250 µm thickness) and analyzed
using UV light. Flash chromatography was performed on E.
Merck silica gel 60 (230-400 mesh) using nitrogen pressure.
1H and 13C NMR were recorded at 400 and 100 MHz,
respectively, with NMR shifts being expressed in ppm (δ)
downfield from TMS. NMR coupling constants (J ) are reported
in hertz. Mass spectrometry was performed on a Kratos MS
50 spectrometer. Combustion microanalysis was performed at
the University of California, Berkeley. Melting points are
uncorrected.
4a
5-NH
5a
5b
6
7
8
9
9a
10-CH3
10a
11
12
12a
6.82, s, 1H
118.0
119.6d
116.1
119.3
122.1
109.8
136.4
29.6
3.81, s, 1H
65.5
120.5
125.2
125.5
129.4
115.9
135.5
43.4
136.9
135.5
152.1
124.4
7.53, d (8.0)
8.95, d (8.0)
7.13, dd (7.6, 7.6)
7.29, dd (8.0, 7.6)
7.32, d (8.4)
7.65, dd (8.0, 6.7)
7.75, dd (8.5, 6.7)
7.72. d (8.5)
3.74, s, 3H
3.88, s, 2H
4.27, s, 3H
119.5d
40.4
8.75, s, 1H
9.87, s, 1H
184.5
123.1
a
Recorded in CDCl3 at 400 MHz for 1H and at 100 MHz for
2′-Nitr op h en ylglyoxa l (5). A mixture of selenium dioxide
(16.1 g, 145.0 mmol), dioxane (75 mL), and water (5 mL) was
stirred at 60 °C until dissolution occurred. 2′-Nitroacetophe-
none (20 g, 120.8 mmol) was then added in one portion. The
mixture was refluxed for 4 h and stirred at room temperature
overnight. The solid was filtered and washed with CH2Cl2. The
filtrate was diluted with CH2Cl2, washed with brine, and dried
over MgSO4. The crude product was purified by flash chro-
matography, eluting with ethyl acetate-hexane (1:2 to 1:1),
affording 4.5 g (22.5%) of unreacted 2′-nitroacetophenone and
a yellowish oil. The oil was distilled under vacuum to give 12.4
g (74%) of 5 as a thick oil: bp 145 °C/10 Torr (lit.7 124-125.5
13C. Literature data from ref 6 recorded in CDCl3 at 600 MHz.
b
13C values are rounded for comparison purposes. c The numbering
d
system used in ref 6 for cryptoheptine is used here. Assignments
may be reversed.
Ta ble 2. 1H COSY and HMBC Correlations for Isomer 2a
proton
COSY
H-2
H-1, H-3
H-2, H-4
H-3
HMBC
1
2
3
4
C-3, C-4a, C-12
C-4, C-12a
C-1, C-4a
C-2, C-12a
C-4
1
°C/3.5 Torr); H NMR (CDCl3) δ 9.52 (1H, s), 8.23 (1H, dd, J
5-NH
) 8.4, 1.2), 7.86 (1H, ddd, J ) 8.4, 7.2, 1.2), 7.78 (1H, ddd, J
) 8.4, 8.4, 1.2), 7.60 (1H, dd, J ) 7.6, 0.8); 13C NMR (CDCl3)
δ 189.1, 186.5, 135.1, 132.8, 130.8, 130.1, 124.1.
6
7
8
9
H-7
C-8, C-9a
C-5b, C-9
C-6, C-9a
C-5b, C-7
C-9a, C-10a
C-5a, C-10a, C-12, C-12a
H-6, H-8
H-7, H-9
H-8
1-Acet yl-2-[2-(2-n it r op h en yl)-2-oxo-1-h yd r oxyet h yl]-
1,2-d ih yd r oin d ole-3-on e (6). Piperidine (20 drops) was added
to a solution of 1-acetyl-3-oxoindole8 (4) (6.5 g, 37.1 mmol) and
5 (6.5 g, 36.3 mmol) in 20 mL of toluene. The mixture was
stirred overnight, with a precipitate being formed as the
reaction proceeded. The solid was filtered, washed with toluene
and diethyl ether, and then dried under vacuum to give 10.1
g (78%) of 6: mp 206 °C (dec); 1H NMR (DMSO-d6) δ 8.10 (1H,
d, J ) 8.4), 7.95 (1H, ddd, J ) 7.6, 7.6, 1.2), 7.77-7.68 (3H,
m), 7.58 (1H, d, J ) 7.6), 7.20 (1H, dd, J ) 7.6, 7.6), 6.55 (1H,
very broad), 5.17 (3H, s, br), 2.42 (3H, s, br); 13C NMR (DMSO-
d6) δ 204.2, 196.4, 168.7, 146.9, 136.9, 135.0, 134.7, 131.1,
129.5, 124.8, 123.5, 123.2, 122.9, 75.3, 69.4, 24.6; FABMS m/z
355 [M+1]+; anal. C 60.85%, H 3.85%, N 7.72%, calcd for
10-NCH3
11
a
Recorded in CDCl3 at 400 MHz for 1H and at 100 MHz for
13C.
ments for 2 with those described for cryptoheptine (1) are
made in Table 1.
We then attempted to convert 2 to 1 under a variety of
basic and acidic conditions. Treatment of 2 with NaH,
potassium ethoxide in EtOH, or potassium tert-butoxide
in tert-butanol gave a product having a molecular ion peak
at m/z 260.0956. NMR analysis and elemental analysis
confirmed that the product upon treatment of 2 with base
was oxidation product 10. The reaction of 2 with potassium
tert-butoxide was a very fast reaction, with the complete
conversion of 2 to 10 occurring within a matter of minutes.
Treatment of 2 with a solution of HCl in THF also gave
10. Interested in how labile tautomer 2 was to oxidation,
we then treated 2 with a solution of CHCl3-MeOH-35%
NH3 (90:10:1), which was the chromatography eluant used
in the isolation of cryptoheptine.6 As the reaction was
monitored by TLC, a new spot began to form after a few
minutes, and the reaction went to completion after stirring
overnight at room temperature. As anticipated, the product
from this experiment was again oxidation product 10.
We have synthesized ketone 2, an apparent tautomeric
form of the proposed structure of cryptoheptine (1), in 18%
overall yield and six steps from 2′-nitroacetophenone and
1-acetyl-3-oxoindole.1 Ketone 2 is readily oxidized under
acidic and basic conditions to ketone 10. Enol 3 or crypto-
heptine (1) were not observed under the experimental
conditions studied. The relative ease with which a presum-
C
18H14N2O6, C 61.01%, H 3.98%, N 7.91%.
2-[2-(2-Nit r op h e n yl)-2-oxoe t h ylid ie n e ]-1,2-d ih yd r o-
in d ole-3-on e (7). A 10% solution of HCl (50 mL) was added
at room temperature to a solution of oxoindole 6 (5.0 g, 14.2
mmol) in 500 mL of THF, and the mixture was refluxed for 4
h. The deep red solution was concentrated, and the residue
was treated with saturated NaHCO3 solution and then ex-
tracted with CH2Cl2. The CH2Cl2 solution was washed with
brine, dried over MgSO4, filtered, and then concentrated to
give the crude product. Purification of the crude product by
flash chromatography, eluting with CH2Cl2, afforded 3.5 g
(84%) of 7 as a deep red solid: mp 187-188 °C (lit.9 194 °C);
1H NMR (DMSO-d6) δ 11.26 (1H, s), 8.03 (1H, dd, J ) 7.6,
1.6), 7.92 (1H, dd, J ) 7.6, 1.6), 7.84 (1H, ddd, J ) 7.6, 7.6,
1.2), 7.78 (1H, ddd, J ) 7.6, 7.6, 1.6), 7.65-7.56 (2H, m), 7.30
(1H, d, J ) 8.0), 7.05 (1H, ddd, J ) 7.6, 7.6, 0.8), 6.40 (1H, s);
13C NMR (DMSO-d6) δ 190.1, 188.2, 154.0, 147.5, 143.3, 137.7,
134.9, 133.4, 132.0, 128.8, 124.8, 124.2, 122.2, 118.8, 113.5,
95.4; EIMS m/z 294 [M]+, 160 (100).
1-Meth yl-2-[2-(2-n itr op h en yl)-2-oxoeth ylid ien e]-1,2-d i-
h yd r o-in d ole-3-on e (8). Sodium hydride (0.72 g, 60% in oil,
18.12 mmol) was added to a solution of oxoindole 7 (4.44 g,

Synthesis of Cryptoheptine
J ournal of Natural Products, 2000, Vol. 63, No. 5 645
15.1 mmol) in 80 mL of DMF at -10 °C. The mixture was
stirred for 15 min, then CH3I was added. After stirring the
mixture at -10 °C for 2 h and then at room temperature for
2 h, the reaction mixture was quenched with water and then
extracted with CH2Cl2. The CH2Cl2 solution was washed with
brine, dried over MgSO4, filtered, and then concentrated.
Purification of the crude product by column chromatography,
eluting with ethyl acetate-hexane (1:2), afforded 3.85 g (83%)
8.28-8.20 (2H, m), 7.87 (1H, ddd, J ) 8.4, 8.4, 1.6), 7.71 (1H,
ddd, J ) 8.0, 8.0, 1.2), 7.61 (1H, ddd, J ) 8.0, 8.0, 0.8), 7.25
(1H, dd, J ) 7.6, 7.6), 7.14 (1H, d, J ) 8.4), 6.95 (1H, s), 3.57
(3H, s); 13C NMR (CDCl3) δ 182.4, 153.2, 147.9, 144.9, 142.6,
136.1, 133.6, 133.1, 132.7, 130.6, 130.1, 124.3, 123.4, 121.8,
113.9, 108.6, 29.1; HREIMS m/z 260.0957 [M]+, calcd for
C17H12N2O 260.0950; anal. C 77.30%, H 4.96%, N 10.46%, calcd
for C17H12N2O‚0.25H2O, C 77.11%, H 4.76%, N 10.58%.
1
of 8 as a deep red solid: mp 160-161 °C; H NMR (CDCl3) δ
Ack n ow led gm en t. The authors wish to thank ZhiJ un Ye
and Nigel Parkinson for their assistance in obtaining NMR
and mass spectral data, and Professor Henry Rapoport at the
University of California, Berkeley, for valuable consultation.
We also wish to thank and honor the traditional healers, local
scientists, communities, botanists, physicians, government
agencies, and nongovernmental agencies in Nigeria, Guinea,
Cameroon, Tanzania, Ghana, and Congo who contributed to
diabetes research at Shaman Pharmaceuticals.
8.07 (1H, d, J ) 8.0), 7.74 (1H, dd, J ) 8.0, 8.0), 7.70-7.55
(4H, m), 7.10-7.00 (2H, m), 6.30 (1H, s), 3.72 (3H, s); 13C NMR
(CDCl3) δ 190.1, 187.8, 155.5, 146.7, 143.0, 138.2, 137.4, 133.8,
130.6, 128.3, 125.2, 124.4, 122.3, 120.1, 110.1, 99.0, 33.7; anal.
C 66.23%, H 4.04%, N 9.06%, calcd for C17H12N2O4, C 66.23%,
H 3.92%, N 9.09%.
5,10-Dih yd r o-10-m e t h ylin d olo[3,2-b][1]b e n za ze p in -
12(11H)-on e (2). A mixture of oxoindole 8 (3.3 g, 10.71 mmol)
and Pd/C (10%, 1.0 g) in 500 mL of toluene was stirred at room
temperature under H2 (balloon) for 4 h. The balloon was
removed, and p-toluenesulfonic acid (300 mg, 1.75 mmol) was
added. The mixture was refluxed for 30 min, using a Dean-
Stark trap to remove the water formed during the reaction.
The mixture was then cooled to room temperature, the catalyst
was removed by filtration over Celite, and then the filtrate
was concentrated. Purification of the crude product by column
chromatography, eluting with CH2Cl2, afforded 1.25 g (44%)
of 2 as a yellow solid: mp 260 °C (dec); see Table 1 for 1H and
13C NMR data; IR (film) cm-1 3303, 1628, 1612, 1597, 1473,
1379, 1300, 1221, 742; HREIMS m/z 262.1109 [M]+, calcd for
C17H14N2O 262.1106; anal. C 77.88%, H 5.55%, N 10.72%, calcd
for C17H14N2O, C 77.84%, H 5.38%, N 10.68%.
Refer en ces a n d Notes
(1) Bierer, D. E.; Fort, D. M.; Mendez, C. D.; Luo, J .; Imbach, P. A.;
Dubenko, L. G.; J olad, S. D.; Gerber, R. E.; Litvak, J .; Lu, Q. Zhang,
P.; Reed, M. J .; Waldeck, N.; Bruening, R. C.; Noamesi, B. K.; Hector,
R. F.; Carlson, T. J .; King, S. R. J . Med. Chem. 1998, 41, 894-901,
and references therein.
(2) Diniz, M. A.; Silva, O.; Paulo, M. A.; Gomes, E. T. The Biodiversity of
African Plants; van der Maesen, L. J . G., van der Burgt, X. M., van
der Medenbach de Rooy, J . M., Eds.; Kluwer Academic Publishers:
Dordrecht, The Netherlands: 1996; pp 727-731.
(3) Boye, G. L. Antimalarial Action of Cryptolepis sanguinolenta Extract.
Proceedings of the International Symposium on East-West Medicine,
Oct 10-11, 1989, Seoul, Korea; 1990; pp 243-251.
(4) Luo, J .; Fort, D. M.; Carlson, T. J .; Noamesi, B.; nii-Amon-Kotei, D.;
King, S. R.; Tsai, J .; Quan, J .; Hobensack, C.; Lapresca, P.; Waldeck,
N.; Mendez, C. D.; J olad, S. D.; Bierer, D. E.; Reaven, G. M. Diabetic
Med. 1998, 15, 367-374.
(5) This subject has been reviewed by Go¨rlitzer. See: Go¨rlitzer, K.;
Ventzke-Nue, K. Pharmazie 1998, 53, 19-23. Also: Go¨rlitzer, K. Abh.
Braunschweigischen Wissenschaftlichen Gessellschaft 1997, 48, 7-36.
(6) Paulo, A.; Gomes, E. T.; Houghton, P. J . J . Nat. Prod. 1995, 58, 1485-
1491.
10-Meth ylin d olo[3,2-b][1]ben za zep in -12-on e (10). So-
dium ethoxide (52 mg, 0.762 mmol) was added at room
temperature to a solution of ketone 2 (100 mg, 0.381 mmol)
in 10 mL of ethanol, and the mixture was stirred for 3 h. The
solvent was removed by rotary evaporation, and the residue
was quenched with aqueous NH4Cl solution. The mixture was
extracted with CH2Cl2. The CH2Cl2 solution was washed with
brine, dried over MgSO4, filtered, and then concentrated to
give the crude product. The crude product was washed with
ethyl ether to give 91 mg (92%) of ketone 10 as a orange solid:
(7) Sato, T.; Ohta, M. Bull. Chem. Soc. J pn. 1955, 28, 480-481.
(8) See Supporting Information of ref 1 for details on the preparation of
4.
(9) Bond, C. C.; Hooper, M. J . Chem. Soc. 1969, 2453-2460.
1
mp >250 °C; H NMR (CDCl3) δ 8.75 (1H, dd, J ) 8.0, 1.2),
NP990412P