Access to new cytotoxic bisindole alkaloids by a modified Borch reductive amination process

Article abstract of DOI:10.3987/COM-00-9001

A slight modification of the Borch reductive amination method (delayed addition of NaBH₃CN) was applied to an indole aldehyde compound, analog of the natural alkaloid, goniomitine. This reaction led to a series of new cytotoxic bisindole alkaloids with a 1,2,3,4-tetrahydroquinoline bridge.

Full text of DOI:10.3987/COM-00-9001

HETEROCYCLES, Vol. 53, No.11, 2000, pp. 2353 - 2356, Received, 7th July, 2000
ACCESS TO NEW CYTOTOXIC BISINDOLE ALKALOIDS BY A
MODIFIED BORCH REDUCTIVE AMINATION PROCESS
a
b
a
b
Guy Lewin * , Corinne Schaeffer , Reynald Hocquemiller , Edgar Jacoby ,
b
b
b
Stéphane Léonce , Alain Pierré , and Ghanem Atassi
Laboratoire de Pharmacognosie, Faculté de Pharmacie, av. J.B. Clément, 92296
a
Châtenay-Malabry Cedex France (e-mail: guy.lewin@cep.u-psud.fr). Institut de
b
Recherches Servier, 11 rue des Moulineaux, 92150 Suresnes, France
Abstract- A slight modification of the Borch reductive amination method (delayed
addition of NaBH CN) was applied to an indole aldehyde compound, analog of the
3
natural alkaloid, goniomitine. This reaction led to a series of new cytotoxic bis-
indole alkaloids with a 1,2,3,4-tetrahydroquinoline bridge.
In a previous publication, we reported on a slight modification of the Borch reductive amination method
1,2
which consists in delayed addition of NaBH CN.
In the case of reductive amination between an
3
enolizable aldehyde and aniline, this modification allows a one-pot synthesis of 2,3-disubstituted 1,2,3,4-
tetrahydroquinolines.
RCH2CH2NHC6H5
a)
aniline hydrochloride (5 eq)
RCH2CHO
MeOH, rt
(1 eq)
b)
R
R
N
H
a) NaBH3CN added immediately
b) NaBH3CN added after 20 min
This cyclization implies a Schiff base intermediate and results at the beginning from the addition of the
enamine form to the tautomeric Schiff base, as iminium salt. The application of this modified reductive
3
amination to the indole alkaloid (2), a semi-synthetic analog of natural goniomitine (3) derived from (-)-

4
vincadifformine (1), provides a series of new cytotoxic bisindole alkaloids which are described in this
paper.
OH
N
CHO
4 steps4
CO2CH3
N
N
H
H
HO
N
HN
N
H
CO2CH3
1
2
3
Reaction of the aldehyde (2) with aniline hydrochloride (5 eq, MeOH, 16 h, rt) afforded under the Borch
original conditions (immediate addition of NaBH CN) two main compounds (4) (23%) and (5) (28%)
3
while a delayed addition (20 min) of NaBH CN led only to 5 (45%). HR-EIMS of these two products
3
proved the expected reductive amination structure for 4 and the dimeric structure for 5. 2D homo- and
heteronuclear NMR experiments were in full agreement with the 2,3 disubstituted 1,2,3,4-
tetrahydroquinoline hypothesis and demonstrated that 5 was a pure compound and not a mixture of
5
diastereoisomers at C2 and C3.
7
6
8
5
4
NHC6H5
CO2CH3
R''
N
R'N
9
2
3
N
10
11
H
6
HO
N
R''
N
N
R
12
H
H
21
17
19
18
RN
3
4
15
14
R
R'
R"
5
6
7
8
9
1 0
1 1
OH
H
COCH
COCH
OCOCH
OH
OH
H
H
H
CO CH
2
3
3
3
3
3
3
CO CH
2
CO CH
2
3
3
COCH
H
CH
H
CO CH
3
2
CO CH
2
3
CO CH
3
2
CH OH
2
Numbering system of indole part according to ref. 8
In view of biological studies and structure activity relationships establishment, several derivatives were
prepared from 5. Reduction of the hydroxylamino functions was performed by aqueous TiCl in MeOH (6
3
6
eq, 20 h, rt) according to Murahashi's method and led in 38% yield to the triamine (6). Acetylation of 6
(pyridine, Ac O, 48 h, rt) furnished in 48 and 22% yields respectively 7, diacetylated on both indole
2
moieties, and the triacetylated compound (8). Acetylation of the hydroxylamino groups of 5 (pyridine,
Ac O, 3 h, rt) afforded in 68% yield the di-O-acetyl (9). A selective functionalization of 5 on the only
2
tetrahydroquinoline secondary amino group was accomplished by methylation (CH O/NaBH CN, AcOH,
2
3

2 h, rt) and provided in 64% yield the N-methyl derivative (10). Finally the LiAlH reduction of 5 (THF, 3
4
7
h, reflux) led to the tetrahydroxy compound (11) in 32% yield.
2
Stereochemistry of 5 at C2 and C3 - Previously, we had already observed that the relative stereochemistry
at C2 and C3 of the 1,2,3,4-tetrahydroquinoline was highly dependent on the starting aldehyde, a bulky
aldehyde yielding only the 2,3-trans disubstituted isomer because of steric hindrance of the substituents.
1
The very likely trans stereochemistry was inferred from H NMR experiments with the TiCl reduction
3
compound (6) (conclusions could not be drawn from 5 itself owing to the overlapping of H3, one H6 and
1
two other protons into a multiplet). H NMR spectrum of 6 displayed clearly both signals of H2 (td, J = 11
and 2.5 Hz) and H3 (ddd, J = 11.8, 11 and 5 Hz) at respectively δ 4.60 and 3.37 ppm. The observed J
H2-
= 11 Hz by irradiation experiments unambiguously proved the 2,3-trans relative configuration.
H3
Unfortunately, the amorphous state of compounds (5-11) did not allow determination of the absolute
configuration at C2 and C3 by X-Ray crystallography.
Biological results - The original bisindole structure of 5 and its derivatives led us to evaluate their
cytotoxicity (IC and cell cycle effect) on L1210 leukemia cells in culture compared to monomers (2) and
50
(4) (Table 1).
Table 1
IC (µM)
a
Compound
Effect on cell cycle
50
b
(% accumulation in G1)
---------------------------------------------------------------------------------------------------------------------
c
2
38
7.1
n s
4
61% at 50 µM
80% at 25 µM
no specificity
n s
5
2.7
3.1
6
7
> 100
24.8
> 100
8
55% at 100 µM
n s
9
1 0
1 1
5.3
3.4
79% at 25 µM
no specificity
a
Inhibition of L1210 cells proliferation measured by the microculture tetrazolium assay
b
c
46% of untreated cells are in the G1 phase
no studied
The most interesting compounds are 5 and 10 which induced a high and dose-dependent accumulation in
the G1-phase of the L1210 cell cycle. It is noteworthy that hydroxylamino group on the indole moieties
seems essential for the cytotoxicity with accumulation in the G1-phase but not the tetrahydroquinoline N-H
function. These observations could be related to probable occurence of intramolecular hydrogen bonds
between the carboxyl group and the hydroxylamino group of the two indole moieties. A computational
conformational analysis of 5 with Systematic Search within Sybyl v 6.2 confirmed these hydrogen bond
patterns in both 2S,3S and 2R,3R trans stereoisomers, which does not allow to specify the absolute

configuration at C2 and C3. Synthesis and biological study of analogs of 5 modified on the only
tetrahydroquinoline ring are in progress and will be the subject of a further article.
ACKNOWLEDGEMENTS
The authors would like to thank the J.P. Volland's laboratory (IdRS), especially P.-H. Lambert for mass
spectra measurements.
REFERENCES AND NOTES
1. R.F. Borch, M.D. Bernstein, and H.J.D.Durst, J. Am. Chem. Soc., 1971, 93, 2897.
2. G. Lewin and C. Schaeffer, Heterocycles, 1998, 48, 171.
3. L. Randriambola, J.-C. Quirion, C. Kan-Fan, and H.-P. Husson, Tetrahedron Lett., 1987, 28, 2123.
4. G. Lewin, C. Schaeffer, and P.-H. Lambert, J. Org. Chem., 1995, 60, 3282.
5. Selected data for compounds (4) and (5).
4: [(α) = -74° (c = 0.6, CHCl )]; UV (EtOH) λ nm (log ε) 220 (4.34), 234 (4.31), 248 (4.17), 322 (3.92); IR
D
3
max
-1
1
(CH Cl ) ν cm 3500-3200, 1715; H NMR (CDCl , 500 MHz), δ ppm: 0.76 (3 H-18), 3.82 (CO CH ), 4.00 (N-H), 4.53
2
2
3
2
3
(H-21), 4.87 (N-OH), 6.58, 6.65 and 7.14 (5 H of the aniline ring), 6.62 (H-17), 7.10 (H-10), 7.24 (H-11), 7.53 (H-12), 7.62
+
(H-9); HR-EIMS: M 445.2357, calcd for C
H N O 445.2365.
27 31 3 3
5: [(α) = -150° (c = 1.6, CHCl )]; UV (EtOH) λ
nm (log e) 224 (4.62), 250 (4.30), 320 (4.33); IR (CH Cl ) ν
2 2
D
3
max
-1
1
cm 3500-3250, 1715 (br); H NMR (CDCl , 500 MHz), δ ppm: tetrahydroquinoline part 2.75 (1 H-4), 3.36 (H-3), 3.50 (1
3
H-4), 4.33 (td, J = 11 and 2.5 Hz, H-2), 6.40 (H-8), 6.55 (H-6), 6.91 (H-5), 6.94 (H-7); indole part (both indole moieties were
not distinguished) 0.69 and 0.82 (6 H-18), 2.70 (2 H-3), 3.09 (dd, J = 15.6 and 11 Hz, 1 H-6), 3.32 (2 H-3), 3.35 (1 H-6),
3.55 and 3.89 (2 CO CH ), 4.46 and 4.59 (2 H-21), 6.40 and 6.45 (2 H-17), 7.05 (2 H-10), 7.22 (2 H-11), 7.51 and 7.58 (2
2
3
13
H-12), 7.71 (2 H-9); C NMR (CDCl , 125 MHz), δ ppm: tetrahydroquinoline part 38.5 (C-4), 41.6 (C-3), 61.2 (C-2),
3
117.7 (C-8), 120.3 (C-6), 130.8 (C-5), 132.8 (C-7); indole part (both indole moieties were not distinguished) 14.5 and 15.2 (2
C-18), 25.8 and 25.9 (2 C-14), 33.8 (C-6), 35.1 and 35.4 (2 C-19), 36.8 and 37.0 (2 C-15), 56.5 and 56.6 (2 CO CH ), 60.1
2
3
and 60.2 (2 C-3), 82.7 and 82.9 (2 C-21), 114.4 and 114.9 (2 C-9), 123.6 and 123.8 (2 C-12), 125.6 (2 C-10), 127.1 and
+
127.3 (2 C-11), 145.9 and 147.1 (2 C-17); HR-EIMS: M 795.4032, calcd for C
6. S.-I. Murahashi and Y. Kodera, Tetrahedron Lett., 1985, 26, 4633.
7. Selected data for compounds (6-11).
H
N O 795.3996.
48 53
5 6
1
6: [(α) = -124° (c = 1.3, CHCl )]; UV (EtOH) λ
nm 229, 253, 319; H NMR (CDCl , 500 MHz), δ ppm:
D
3
max
3
tetrahydroquinoline part 3.37 (ddd, J = 11.8, 11 and 5 Hz, H-3), 3.55 (dd, J = 16 and 11.8 Hz, 1 H-4), 4.60 (td, J = 11 and 2.5
+
Hz, H-2); indole part: 3.63 and 3.89 (2 CO CH ), 4.77 and 4.89 (2 H-21), 6.53 (2 H-17); HR-EIMS: M 763.4130, calcd for
2
3
C
H N O 763.4098.
48 53 5 4
-1
1
7: UV (EtOH) λ
nm 220, 230 (sh), 251, 320; IR (CH Cl ) ν cm 3380, 1720, 1645; H NMR (CDCl , 500
2 2 3
max
+
MHz), δ ppm: 2.24 and 2.33 (2 NCOCH ), 3.73 and 3.78 (2 CO CH ); HR-EIMS: M 847.4309, calcd for C
H N O
3
2
3
52 57 5 6
847.4315.
8: UV (EtOH) λ
-1
1
nm 223, 255 (sh), 320; IR (CH Cl ) ν cm 1720, 1645; H NMR (CDCl , 500 MHz), δ ppm:
max
2
2
3
+
2.12, 2.21 and 2.28 (3 NCOCH ), 3.67 and 3.74 (2 CO CH ); HR-EIMS: M 889.4414, calcd for C
H N O 889.4437.
3
2
3
54 59 5 7
-1
9: [(α) = +36° (c = 0.7, CHCl )]; UV (EtOH) λ
nm 220, 230 (sh), 254, 316; IR (CH Cl ) ν cm 3380, 1760,
2 2
D
3
max
1
1720; H NMR (CDCl , 500 MHz), δ ppm: 1.28 and 1.30 (2 OCOCH ), 3.82 and 3.87 (2 CO CH ), 4.62 and 4.77 (2 H-
3
3
2
3
+
21), 6.43 and 6.55 (2 H-17); HR-EIMS: M 879.4205, calcd for C
H N O 879.4170.
52 57 5 8
nm 218, 230 (sh), 320; IR (CH Cl ) ν cm 3400, 1715 (br);
2 2
-1
10: [(α) = -170° (c = 0.75, CHCl )]; UV (EtOH) λ
D
3
max
1
H NMR (CDCl , 500 MHz), δ ppm: 2.44 (N-CH ), 3.56 and 3.76 (2 CO CH ), 4.43 and 4.60 (2 H-21), 6.24 and 6.30 (2
3
3
2
3
+
H-17); EIMS: M 809.
1
11: [(α) = -22° (c = 0.9, CHCl )]; UV (EtOH) λ
nm 222, 250, 312, 322; H NMR (CDCl , 500 MHz), δ ppm:
3
D
3
max
tetrahydroquinoline part 6.34 (H-8), 6.58 (H-6), 6.94 (H-5 and H-7); indole part (both indole moieties were not distinguished)
0.63 and 0.73 (6 H-18), 4.39, 4.43 and 4.73 (2 CH OH), 5.11 and 5.61 (2 H-17), 6.91 and 7.12 (2 H-10), 7.12 and 7.23 (2 H-
2
+
11), 7.44 (2 H-12), 7.58 and 7.70 (2 H-9); HR-EIMS: M 739.4098, calcd for C
H N O 739.4105.
46 53 5 4
8. Numbering system proposed by J. Le Men and W.I.Taylor, Experientia, 1965, 21, 508.