The fate of the tryptophan stereocenter in the synthesis of 7,10,16,16a-tetrahydro-11H-quinazolino[2',3':3,4]pyrazino[1,2-b]β-carboline-5,8-diones

Article abstract of DOI:10.1016/S0957-4166(00)00282-2

Condensation reactions of anthranilic acid with iminoethers 14-17 derived from tetracycles 9-13 to give the title hexacyclic compounds reflect a preferred trans-relationship for H(10)-H(16a) protons in C(7)-unsubstituted products and a cis-relatioship for

Full text of DOI:10.1016/S0957-4166(00)00282-2

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 11 (2000) 3515–3526
The fate of the tryptophan stereocenter in the synthesis of
7,10,16,16a-tetrahydro-11H-quinazolino[2%,3%:3,4]-
pyrazino[1,2-b]b-carboline-5,8-diones
Antonio Madrigal, Mercedes Grande and Carmen Avendan˜o*
Departamento de Qu´ımica Orga´nica y Farmace´utica, Facultad de Farmacia, Universidad Complutense,
28040 Madrid, Spain
Received 9 June 2000; accepted 18 July 2000
Abstract
Condensation reactions of anthranilic acid with iminoethers 14–17 derived from tetracycles 9–13 to give
the title hexacyclic compounds reflect a preferred trans-relationship for H(10)–H(16a) protons in
C(7)-unsubstituted products and a cis-relatioship for H(7)–H(16a) protons in C(10)-unsubstituted
analogs. This synthetic strategy is limited by the steric hindrance of the substituent at C(10). Theoretical
calculations are in agreement with the experimental results. The regioselectivity in favor of the linear
tetracyclic compound 11 with respect to 12 has also been confirmed. © 2000 Published by Elsevier Science
Ltd.
1. Introduction
We are currently studying the synthesis of hexacyclic compounds related to the fungal
metabolite N-acetylardeemin1 such as C and D. Given the activity of N-acetylardeemin as a
reversor of Pgp-mediated multidrug resistance (MDR), compounds C and D could be poten-
tially useful in combination with antitumor drugs to maintain the therapeutic intracellular
concentrations when the 170 kD membrane glycoprotein Pgp is overexpressed.2 In fact, some of
the analogs so far studied have shown an interesting reversal activity of Pgp-mediated
resistance.3
The synthesis of hexacycles C and D starts with the transformation of tryptophan methyl
esters to the tricyclic pyrroloindoles and b-carbolines, respectively, followed by N-acylation and
condensation with amino acids to the corresponding tetracyclic piperazine-2,5-diones A and B.
Although direct condensation of the C(1)–N(2) amide function of these compounds with
* Corresponding author. Tel: 34-91-394 18 21; fax: 34-91-394 18 22; e-mail: avendano@eucmax.sim.ucm.es
0957-4166/00/$ - see front matter © 2000 Published by Elsevier Science Ltd.
PII: S0957-4166(00)00282-2

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A. Madrigal et al. / Tetrahedron: Asymmetry 11 (2000) 3515–3526
anthranilic acid and thionyl chloride4 failed, iminoethers were efficient derivatives to activate the
reaction substrate. Alternatively, C and D have been obtained by N(2)-acylation with o-azido-
benzoyl chloride and KHMDS, followed by intramolecular aza-Wittig reaction.5,6 Epimerization
of the ‘tryptophan’ stereocenter [C(15a) in C and C(16a) in D] in the last step of the first
mentioned strategy or at the N-acylation in the second, was observed in some instances. The
lability of this proton, which is due to its a-position to an imino group, has also been described
in condensations of anthranilic acid with iminoethers derived from proline-containing di-
ketopiperazines to form fused pyrrolo–pyrazino–quinazoline compounds.7
The results with the pyrazino–pyrrolo–indoles A precursors of compounds C so far studied
indicated a lower stability for the isomers with the H(7) and H(15a) protons in a cis-relation-
ship.5 However, the results with pyrazino-b-carbolines precursors of D8 were more intriguing,
since epimerization of the C(16a)-stereocenter seemed to take place independently to the cis- or
trans-relationship between H(7) and H(16a)-protons.6
In order to explore the effect of the C(10)-methyl substituent we study here the fate of the
tryptophan stereocenter in the reaction sequence to C(10)-unsubstituted analogs 18 (Scheme 1).
Scheme 1.

A. Madrigal et al. / Tetrahedron: Asymmetry 11 (2000) 3515–3526
3517
On the other hand, since most of compounds D so far studied derived from a tetrahydro-b-
carboline with a 1,3-cis-stereochemistry,8 we investigate the chemical behavior of tetracycles 13
derived from a b-carboline with a 1,3-trans-relationship 4. Finally, we also study the regioselec-
tivity of the cyclization in dipeptide anhydrides derived from the tetrahydro-b-carboline 3, to
give linear 11 or angular-fused tetracycles 12.
2. Results and discussion
A modification of the Pictet–Spengler reaction between
L
-tryptophan methyl ester and
glyoxylic acid gave 19 (Scheme 2). Compounds 2 or 3 resulted from subsequent decarboxylation
of 1 in refluxing xylene or esterification in dry methanol–thionyl chloride. The expected
H(1)–H(3) cis-relationship of 1 was confirmed by NOE experiments in compound 3·HCl.
Scheme 2. Reagents and conditions: (i) HOC-CO2H, EtOAc, rt, 16 h; (ii) xylene, 200°C, 2 h; (iii) dry MeOH, SOCl2,
rt, 48 h, Et2O
Treatment of 2 with N-Boc-
L
- or
D
-alanine using EDC [1-ethyl-3-[3%-(dimethylamino)-
propyl]carbodiimide] as the coupling reagent gave compounds 5 or 6 which, after N-deprotec-
tion by acid, afforded 9 or 10, respectively (Scheme 3). Unlike 6-methyl analogs B, which
1
showed clearly distinguished by H NMR both s-E and s-Z rotamers at room temperature,8
peptides 5 and 6 showed a single NH signal because of their side chain free rotation. The crude
iminoethers 14 and 15 derived from 9 and 10 were condensed with anthranilic acid to give
enantiomers (−)-18 and (+)-18, respectively, as single reaction products. To check the possible
existence of the minor diastereomers in the crude reaction mixtures, the CHCl3 extracts were
submitted to a fast column-chromatography. The excess of anthranilic acid was first eliminated
with hexane, a mixture of the condensated products 18 and the starting iminoethers was later
eluted by using chloroform–ethyl acetate as solvents. The piperazinediones, formed by partial
hydrolysis of iminoethers, were finally recovered by eluting with methanol. The weight of the
two latter fractions roughly corresponded to that of the reagents. The yields of these cyclizations
are in the range of those previously observed with other substrates.6
The retention of the tryptophan stereocenter configuration in the condensation of 14 was
confirmed by NOE experiments, while its epimerization in the condensation of 15 is in
accordance to the physicochemical properties of the products, which are identical except for the
specific rotation values, which are opposite.

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A. Madrigal et al. / Tetrahedron: Asymmetry 11 (2000) 3515–3526
Scheme 3. Reagents and conditions: (i) Boc-L- or D-Ala, EDC/CH2Cl2, 16 h, rt; (ii) F3CCO2H/CH2Cl2, rt, 4 h; (iii)
10% NH4OH; (iv) Et3O+F4B−/CH2Cl2, Na2CO3, rt, 16 h;( v) anthranilic acid, 130°C, 2 h
These results clearly show that the H(7)–H(16a) cis-relationship is thermodynamically pre-
ferred, which is in agreement with its calculated lower heat of formation [54.0 Kcal/mol for
(+)-18 and 55.6 Kcal/mol for its 16a-epimer]10 and with the results found in 1,2,4-trisubstituted
pyrazino[2,1-b]quinazoline-3,6-diones.11
The tetrahydro-b-carboline 4, with the 1,3-stereocenters in a trans-relationship, was obtained
quantitatively by reductive deprotection of its previously described N-benzyl derivative, which is
the product of a Pictet–Spengler reaction between methyl N-benzyltryptophanate and cyclohex-
anecarbaldehyde.12 Steric interactions did not permit the subsequent N-acylation of 4 with
Boc-protected -Ala, but the reaction worked with Boc-Gly to give 8, which after deprotection
L
cyclized to 13. Its stereochemistry was confirmed by NOE experiments, but the insolubility of
this tetracyclic piperazinedione precluded its activation to the required iminoether 17 and its
acylation with o-azidobenzoyl chloride (Scheme 4).
As in the case of b-carboline 4, compound 3 could only be acylated with the less hindered
N-Boc-a-amino acid, to give 7 in moderate yield (Scheme 4). The subsequent deprotection
regioselectively gave the cyclized product 11, being the angular compound 12 much less favored.
Conversion of 11 into the iminoether 16, followed by condensation with anthranilic acid, gave
hexacycle 19 isolated as the only reaction product using the methodology described for
compounds 18 (Scheme 5).

A. Madrigal et al. / Tetrahedron: Asymmetry 11 (2000) 3515–3526
3519
Scheme 4. Reagents and conditions: (i) C6H11CHO/benzene, reflux, 18 h; (ii) H2/C-Pd, MeOH, rt, 1.5 h; (iii)
Boc-
-Ala or Boc-Gly, EDC, CH2Cl2, rt, 16 h; (iv) F3CCO2H, CH2Cl2, rt, 4 h; (v) 10% NH4OH; (vi) Et3O+F4B−/
L
CH2Cl2, Na2CO3, rt, 16 h
Scheme 5. Reagents and conditions: (i) K2CO3, Et2O; (ii) Boc-Gly, EDC/CH2Cl2, 16 h, rt; (iii) F3CCO2H/CH2Cl2, rt,
4 h; (iv) 10% NH4OH; (v) Et3O+F4B−/CH2Cl2, Na2CO3, rt, 16 h; (vi) anthranilic acid, 130°C, 2 h
According to NOE experiments, the H(10) and H(16a) protons in 19 are in a trans-relation-
ship, since after irradiation of the H(16a) proton weak enhancements of the singlet correspond-
ing to the OCH3 group and of the doublet at a higher field [H(7)ax proton] were observed.
This result was also in agreement with the calculated heats of formation for both
possible diastereoisomers [18.1 Kcal/mol for the trans-isomer and 19.3 Kcal/mol for the
cis-isomer].10

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A. Madrigal et al. / Tetrahedron: Asymmetry 11 (2000) 3515–3526
Although the trans stereochemistry could arise from epimerization of any of the two
1
stereocenters, we conclude that epimerization occurs at the C(16a) stereocenter following H
NMR spectroscopic evidence. Thus, if we compare the chemical shift values (ppm) of the H(1)
protons in 2 (3.88) with that of the H(1) proton in 3 (5.78); the H(1) protons in 5 (4.5 and 5.8
for axial and equatorial, respectively), with the H(1) proton in 7 (6.07); the H(6) protons in 9
(4.27 and 5.57 for axial and equatorial, respectively), with the H(6) proton in 11 (6.28); the H(6)
protons in 14 (4.14 and 5.54 for axial and equatorial, respectively), with the H(6) proton in 16
(6.34); and the H(10) protons in 18 (4.26 and 5.80 for axial and equatorial, respectively), with
the H(10) proton in 19 (6.54); it can be seen that a Dl :2 ppm is maintained for the proton a
to the methoxycarbonyl group in all derivatives in respect of the corresponding protons in the
unsubstituted analogs. Epimerization of the C(10) stereocenter would imply a much greater
chemical shift for this proton in 19, since it would suffer the anisotropic effect of the C(8)
carbonyl group. Furthermore, the observed NOE between H-16a and H-7ax (H-7eq is deshielded
by the anisotropic effect of the neighboring C(5) carbonyl11) also confirms the epimerization at
C(16a).
From these results we conclude that in C(10)-unsubstituted hexacycles the cis-relationship
between H(7) and H(16a)-protons is preferred, while a trans-relationship between H(10) and
H(16a)-protons is favored in the C(7)-unsubstituted compounds.
3. Experimental
All reagents were of commercial quality (Aldrich, Fluka, SDS, Probus) and were used as
received. Solvents (SDS, Scharlau) were dried and purified using standard techniques. Reactions
were monitored by thin layer chromatography, on aluminum plates coated with silica gel with
a fluorescent indicator (Macherey–Nagel Alugram Sil G/UV254). Separations by flash chrom-
atography were performed on silica gel (SDS 60 ACC, 230–400 mesh). Melting points were
measured on a Reichert 723 hot stage microsope, and are uncorrected. Infrared spectra were
recorded on a Perkin–Elmer Paragon 1000 FT-IR spectrophotometer, with solid compounds
compressed into KBr pellets and liquid compounds placed between two NaCl disks. NMR
1
spectra were obtained on a Bruker AC-250 spectrometer (250 or 300 MHz for H, 63 MHz for
13C), with CDCl3 or DMSO-d6 as solvents (Servicio de Espectroscop´ıa, Universidad Com-
plutense). When necessary, assignments were aided by DEPT, COSY and 13C–1H correlation
experiments. Exchangeable assignments are marked with the symbol (*). Optical rotations were
determined at 25°C on a 1 ml cell, using a Perkin–Elmer 240 polarimeter operating at the
emission wavelength of a sodium lamp; concentrations are given in g/100 ml. Elemental analyses
were determined by the Servicio de Microana´lisis, Universidad Complutense, on a Perkin–Elmer
2400 CHN microanalyzer.
3.1. (1R,3S)-3-Methoxycarbonyl-1,2,3,4-tetrahydro-i-carboline-1-carboxylic acid 1
To a stirrred solution of methyl thryptophanate (4 g, 18.35 mmol) in 20 ml of ethyl acetate
were added 1.687 g (18.35 mmol) of glyoxalic acid, and the mixture was maintained at room
temperature over 16 h. The precipitate formed was filtered and washed with ethyl acetate to give
4.02 g (84%) of 1.

A. Madrigal et al. / Tetrahedron: Asymmetry 11 (2000) 3515–3526
3521
1
Data for 1: mp: 143–144°C. IR (KBr): 3390 (NH), 2694 (NH), 1634 (COOH) cm−1. H NMR
(DMSO-d6) l: 10.68 (br s, 1H, H-9), 7.40 (m, 2H, H-5 and H-8), 6.98 (m, 2H, H-6 and H-7),
4.80 (m, 1H, H-1), 4.27 (m, 1H, H-3), 3.77 (s, 3H, OCH3), 3.02 (m, 2H, H-4ax and eq) ppm. 13
C
NMR (DMSO-d6) l: 174.1 (COOCH3), 166.8 (COOH), 135.9 (C-8a), 129.2 (C-9a), 125.9
(C-4b), 120.8 (C-7), 118.1 (C-6), 117.3 (C-5), 111.7 (C-8), 104.5 (C-4a), 62.5 (C-1), 59.7 (C-3),
55.4 (COOCH3), 18.5 (C-4) ppm. Anal. calcd for C14H14N2O4: C, 61.31; H, 5.14; N, 10.21.
Found: C, 61.58; H, 5.19; N, 10.05.
3.2. Methyl (3S)-1,2,3,4-tetrahydro-i-carboline-3-carboxylate 2
A solution of 2.2 g (8.02 mmol) of 1 in 20 ml of xilene, was heated at 200°C over 2 h. The
organic layer was evaporated to give 1.8 g (98%) of 2.
Data for 2: mp: 99–100°C. IR (KBr): 3310 (NH), 2800 (NH), 1728 (COOCH3) cm−1. 1H NMR
(CDCl3) l: 8.18 (s, 1H, H-9), 7.1–7.6 (m, 4H, H-5 to 8), 3.88 (m, 2H, H-1), 3.74 (m, 1H, H-3),
3.48 (s, 3H, OCH3), 3.0–3.2 (m, 2H, H-4ax and eq) ppm. 13C NMR (Cl3CD) l: 175.9
(COOCH3), 136.0 (C-8a), 132.2 (C-9a), 127.0 (C-4b), 121.8 (C-7), 119.6 (C-6), 117.9 (C-5), 110.9
(C-8), 107.5 (C-4a), 56.0 (C-3), 52.3 (COOCH3), 42.2 (C-1), 25.5 (C-4) ppm. Anal. calcd for
C13H14N2O2: C, 67.79; H, 6.13; N, 12.17. Found: C, 68.08; H, 6.29; N, 11.95.
3.3. Dimethyl (1R,3S)-1,2,3,4-tetrahydro-i-carboline-1,3-dicarboxylate·HCl 3
To a cooled (0°C), stirred solution of 1 (1 g, 3.64 mmol) in dry methanol was dropwise added
thionyl chloride 0.62 ml (8.5 mmol) over 30 min and the stirring was continued at room
temperature up to 48 h. To the reaction mixture was added 3 ml of ethyl eter and the
precipitated crystals were filtered to give 0.82 g (70%) of 3·HCl.
1
Data for 3: mp: 120–121°C. IR (KBr): 3638 (NH), 1741 and 1627 (2 COOCH3) cm−1. H
NMR (DMSO-d6) l: 11.11 (s, 1H, H-9), 7.48 (m, 2H, H-5 and H-8), 7.15 (t, 1H, H-6, J=7 Hz),
7.04 (t, 1H, H-7, J=7 Hz), 5.78 (s, 1H, H-1), 4.74 (dd, 1H, H-3, J=5 and 11 Hz), 3.94 and 3.85
(2s, 6H, (C-1)-COOCH3 and (C-3)-COOCH3), 3.28 (dd, 1H, H-4eq, J=5 and 16 Hz), 3.15 (m,
1H, H-4ax) ppm.13C NMR (DMSO-d6) l: 170.3 and 167.8 (2COOMe)*, 143.2 (C-8a), 138.6
(C-9a), 127.1 (C-4b), 124.5 (C-7), 121.3 (C-6), 120.0 (C-5), 113.8 (C-8), 108.1 (C-4a), 82.5 (C-1),
56.3 (C-3), 55.6 and 55.0 (OCH3)*, 23.5 (C-4) ppm. Anal. calcd for C15H17N2O4Cl: C, 55.48; H,
5.38; N, 8.63. Found: C, 55.75; H, 5.41; N, 8.55.
3.4. N-Acylation of tetrahydro-i-carbolines. General Method. Synthesis of compounds 5–8
N-Boc-glycine,
L
- or
D
-alanine (5.32 mmol) and EDC (4.04 mmol) were added to the
corresponding tetrahydro-b-carboline (2.7 mmol). The reaction was kept in the dark with an
anhydrous calcium chloride tube and was magnetically stirred for 24 h. After concentration to
dryness at low pressure, the residue was extracted with a mixture of chloroform (15 ml) and 1N
hydrochloric acid (16 ml). The separated organic phase was washed with 1N sodium bicarbonate
(15 ml), dried and concentrated. Purification of the residue by column chromatography with
dichloromethane as solvent, gave compounds 5–8.

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3.4.1. Methyl (3S,2%S)-1,2,3,4-tetrahydro-2-N%-tert-butoxycarbonylalanyl-i-carboline-3-carbox-
ylate 5
Starting from 2 (0.62 g, 2.7 mmol) dissolved in dichloromethane (1 ml) Boc- -alanine (1 g,
L
5.32 ml) and EDC (0.776 g, 4.04 mmol) a yield of 0.77 g (72%) of 5 was obtained.
Data for 5: mp: 267–268°C. IR (KBr): 3326 (NH), 1742 (COOCH3), 1699 (CONH), 1651
1
(OCONH) cm−1. H NMR (CDCl3) l: 8.94 (s, 1H, H-9), 7.48 (d, 1H, H-8, J=7 Hz), 7.37 (d,
1H, H-5, J=7 Hz), 7.12 (m, 2H, H-6 and 7), 5.85 (d, 1H, NH, J=5.5 Hz), 5.18 (d, 1H, J=16.9
Hz, H-1eq), 4.92 (m, 2H, H-3 and H-2%), 4.5 (d, 1H, H-1ax, J=16.8 Hz), 3.52 (s, 3H, OCH3),
3.38 (m, 1H, H-4eq), 2.82 (m, 1H, H-4ax) 1.45 (m, 12H, C(CH3)3 and (C-2%)-CH3) ppm. 13C
NMR (CDCl3) l: 173.9 and 169.5 (COOMe and C-1%)*, 154.8 (NH-COO), 134.5 (C-8a), 129.6
(C-9a), 126.4 (C-4b), 122.1 (C-6), 119.7 (C-7), 118.8 (C-5), 111.6 (C-8), 105.1 (C-4a), 82.8
(C(CH3)3), 52.5 (OCH3), 51.0 (C-2%), 48.0 (C-3), 40.3 (C-1), 28.2 (C(CH3)3), 22.6 (C-4), 15.5
(C-2%-CH3) ppm. Anal. calcd for C21H27N3O5: C, 62.83; H, 6.78; N, 10.47. Found: C, 62.90; H,
6.82; N, 10.26.
3.4.2. Methyl (3S,2%R)-1,2,3,4-tetrahydro- 2-N%-tert-butoxycarbonylalanyl-i-carboline-3-carbox-
ylate 6
Starting from 2 (1 g, 4.30 mmol) dissolved in dichloromethane (1 ml) Boc- -alanine (1.61 g,
D
8.58 ml) and EDC (1.258 g, 6.5 mmol) a yield of 1.21 g (70%) of 6 was obtained.
Data for 6: mp: 258–259°C. IR (KBr): 3340 (NH), 1757 (COOCH3), 1667 (CONH), 1647
1
(OCONH) cm−1. H NMR (CDCl3) l: 9.33 (9.12) (s, 1H, H-9), 7.46 (m, 1H, H-6), 7.30 (d, 1H,
H-5, J=7.5 Hz), 7.10 (m, 2H, H-6 and 7), 5.78 (m, 1H, NH), 5.30 (d, 1H, H-1, J=16.9 Hz),
5.20 (m, 1H, H-3), 4.45 (d, 1H, H-1, J=16.9 Hz), 4.73 (m, 1H, (C-2%)-CH), 3.54 (s, 3H, OCH3),
3.44 (m, 1H, H-3eq), 3.02 (m, 1H, H-3ax), 1.46 (s, 9H, C(CH3)3), 1.35 (d, 3H, (C-2%)-CH3,
J=6.7 Hz) ppm. 13C NMR (CDCl3) l: 174.0 and 171.4 (COOMe and C-1%)*, 155.4 (NH-COO),
136.7 (C-8a), 129.4 (C-9a), 126.6 (C-4b), 122.1 (C-6), 119.5 (C-7), 118.2 (C-5), 111.2 (C-8), 106.2
(C-4a), 78.0 (C(CH3)3), 53.0 (OCH3), 51.4 (C-2%), 47.2 (C-3), 41.5 (C-1), 28.5 (C(CH3)3), 23.0
(C-4) 16.0 (C-2%-CH3) ppm. Anal. calcd for C21H27N3O5: C, 62.83; H, 6.78; N, 10.47. Found: C,
62.97; H, 6.86; N, 10.39.
3.4.3. Dimethyl (1R,3S)-1,2,3,4-tetrahydro-2-N%-tert-butoxycarbonylglycinyl-i-carboline-1, 3-di-
carboxylate 7
From a solution of compound 3 (0.32 g, 0.98 mmol), liberated and extracted from 3·HCl by
treatment with aqueous K2CO3 in Et2O, in dichloromethane (1 ml), Boc-glycine (0.34 g, 1.94
mmol) and EDC (0.281 g, 1.47 mmol) a yield of 0.27 g (62%) of 7 was obtained.
Data for 7: mp: 204–206°C. IR (KBr): 3320 (NH), 1750 (COOCH3), 1690 (CONH), 1647
1
(OCONH) cm−1. H NMR (CDCl3) l: 8.62 (s, 1H, H-9), 7.43 (d, 1H, H-8, J=7.5 Hz), 7.36 (d,
1H, H-5, J=7.5 Hz), 7.11 (m, 2H, H-6 and 7), 6.07 (s, 1H, H-1), 5.99 (m, 1H, H-3), 5.57 (m,
1H, (C-2%)-NH), 4.62 (m, 2H, (C-2%)-CH2), 3.70 and 3.62 (2s, 6H, (C-3)-COOCH3 and (C-1)-
COOCH3)*, 3.20 (m, 2H, H-4), 1.41 (s, 9H, C(CH3)3) ppm. 13C NMR (CDCl3) l: 173.1 and
172.2 (C-3)-COOCH3 and (C-1)-COOCH3)*, 169.7 (C-1%), 156.1 (NH-COO), 136.6 (C-8a), 131.2
(C-9a), 126.10 (C-4b), 122.8 (C-6), 119.9 (C-7), 118.6 (C-5), 111.4 (C-8), 106.4 (C-4a), 84.3
(C(CH3)3), 53.2 y 52.4 (C-1 and C-3)*, 53.1 and 52.7 ((C-1)-COOCH3 and (C-3)-COOCH3)*,
45.3 (C-2%), 28.4 (C(CH3)3), 23.0 (C-4) ppm. Anal. calcd for C22H27N3O7: C, 59.32; H, 6.11; N,
9.43. Found: C, 59.96; H, 6.45; N, 9.21.

A. Madrigal et al. / Tetrahedron: Asymmetry 11 (2000) 3515–3526
3523
3.4.4. Methyl (1R,3S)-1-cyclohexyl-1,2,3,4-tetrahydro-2-N%-tert-butoxycarbonylglycinyl-
i-carboline-3-carboxylate 8
Starting from 4 (0.5 g, 1.6 mmol) dissolved in dichloromethane (1 ml) Boc-glycine (0.554 g,
3.16 mmol) and EDC (0.460 g, 2.4 mmol) a yield of 0.54 g (72%) of 8 was obtained.
Data for 8: mp: 267-268°C. IR (KBr): 3326 (NH), 1742 (COOCH3), 1699 (CONH), 1651
1
(OCONH) cm−1. H NMR (CDCl3) l: 8.13 (s, 1H, H-9), 7.47 (d, 1H, H-8, J=7.4 Hz), 7.3 (d,
1H, H-5, J=7.4 Hz), 7.12 (m, 2H, H-6 and 7), 5.46 (m, 1H, H-3), 5.19 (s, 1H, (C-2%)-NH), 4.25
(d, 1H, H-1, J=5 Hz), 4.01 (m, 1H, H-4eq), 3.76 (s, 3H, OCH3), 3.24 (m, 3H, H-4ax and
(C-2%)-CH2), 1.38 (s, 9H, C(CH3)3), 1–2 (m, 11H, C6H11) ppm. 13C NMR (CDCl3) l: 172.1
(C-3)-COOCH3, 170.6 (C-1%), 156.0 (NH-COO), 136.2 (C-8a), 132.8 (C-9a), 126.3 (C-4b), 122.1
(C-6), 119.8 (C-7), 118.3 (C-5), 111.3 (C-8), 108.5 (C-4a), 80.0 (C(CH3)3), 55.6 (C-3), 52.3
((C-3)-COOCH3), 42.8 (C-1), 42.5 (C-2%), 30.4 and 30.0 (C-1%%, C-2%% and C-6%%)*, 29.7 (C(CH3)3),
26.6, 26.4, y 26.4 (C3%%-5%%)*, 22.4 (C-4) ppm. Anal. calcd for C26H35N3O5: C, 66.50; H, 7.51; N,
8.95. Found: C, 66.90; H, 7.87; N, 8.44.
3.5. General Method. Synthesis of compounds 9–13
A solution (2.49 mmol) of the corresponding N-aminoacyl derivative in a 1/2 mixture of
trifluoroacetic acid/dichloromethane, was magnetically stirred for 4h at rt. After neutralization
with 10% ammonium hydroxide, the dichloromethane extracts were dried under anhydrous
sodium sulfate, filtered and evaporated, to give a residue that, after column chromatography on
silica gel (chloroform), gave the corresponding piperazinedione.
3.5.1. (3S,12aS)-3-Methyl-2,3,6,7,12,12a-hexahydropyrazino[1,2-b]i-carboline-1,4-dione 9
Starting from 5 (1 g, 2.49 mmol) dissolved in a 1/2 mixture of trifluoroacetic acid/
dichloromethane (4 ml) a yield of 0.6 g (90%) of 9 was obtained.
1
Data for 9: mp: 205–206°C. IR (KBr): 3388 (NH), 1649 and 1625 (CONH) cm−1. H NMR
(CDCl3) l: 8.37 (s, 1H, H-9), 7.47 (d, 1H, H-8, J=7.4 Hz), 7.32 (d, 1H, H-11, J=7.4 Hz), 7.12
(m, 2H, H-9 and 10), 6.42 (s, 1H, H-2), 5.57 (m, 1H, H-6eq), 4.27 (m, 3H, H-3, H-12a, and
H-6ax), 3.5 (dd, 1H, H-12eq, J=2.5 and 15 Hz), 2.94 (m, 1H, H-12ax), 1.54 (d, 3H, (C-3)-CH3,
J=7 Hz) ppm. 13C NMR (CDCl3) l: 167.1 (C-4), 165.8 (C-1), 136.0 (C-11a), 128.4 and 126.3
(C-7a and C-11b)*, 122.3 (C-10), 120.0 (C-9), 118.0 (C-8), 110.8 (C-11), 106.9 (C-6a), 54.5
(C-12a), 51.4 (C-3), 40.2 (C-6), 27.4 (C-12), 22.7 ((C-3)-CH3) ppm. Anal. calcd for C15H15N3O2:
C, 66.90; H, 5.61; N, 15.60. Found: C, 66.97; H, 5.72; N, 15.41.
3.5.2. (3R,12aS)-3-Methyl-2,3,6,7,12,12a-hexahydropyrazino[1,2-b]i-carboline-1,4-dione 10
Starting from 6 (0.5 g, 1.24 mmol) dissolved in a 1/2 mixture of trifluoroacetic acid/
dichloromethane (1 ml) a yield of 0.274 g (82%) of 10 was obtained.
Data for 10: mp: 217–218°C. IR (KBr): 3326 (NH), 1657 and 1620 (CONH) cm−1. H NMR
(CDCl3) l: 8.72 (s, 1H, H-9), 7.5 (d, 1H, H-8, J=7.5 Hz), 7.44 (d, 1H, H-11, J=7.4 Hz), 7.12
(m, 2H, H-9 and H-10), 6.59 (s, 1H, H-2), 5.59 (d, 1H, H-6eq, J=16.5 Hz), 4.14 (m, 3H, H-3,
H-12a, and H-6ax), 3.42 (dd, 1H, H-12eq, J=3.2 and 14.2 Hz), 2.91 (m, 1H, H-12ax), 1.57 (d,
3H, (C-3)-CH3, J=7 Hz) ppm. 13C-NMR (CDCl3) l: 167.6 (C-4), 166.0 (C-1), 136.0 (C-11a),
128.6 and 126.3 (C-7a and C-11b)*, 122.1 (C-10), 119.8 (C-9), 117.9 (C-8), 111.0 (C-11), 106.8
(C-6a), 57.0 (C-12a), 50.5 (C-3), 40.4 (C-6), 26.8 (C-12), 20.1 ((C-3)-CH3) ppm. Anal. calcd for
C15H15N3O2: C, 66.90; H, 5.61; N, 15.60. Found: C, 67.02; H, 5.87; N, 15.97.

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3.5.3. Methyl (6R,12aS)-1,4-dioxo-2,3,6,7,12,12a-hexahydropyrazino[1,2-a]i-carboline-
6-carboxylate 11
Starting from 7 (1 g, 2.24 mmol) dissolved in a 1/2 mixture of trifluoroacetic acid/
dichloromethane (2 ml) a yield of 0.42 g (60%) of 11 was obtained.
Data for 11: mp: 210–211°C. IR (KBr): 3252 (NH), 1731 (COOCH3) 1687 and 1658 (CONH)
1
cm−1. H NMR (CDCl3) l: 8.49 (s, 1H, H-9), 7.49 (d, 1H, J=7.7 Hz, H-8), 7.37 (d, 1H, J=7.7
Hz, H-11), 7.31 (s, 1H, H-2), 7.15 (m, 2H, H-9 and H-10), 6, 28 (s, 1H, H-6), 4.51 (dd, 1H,
J=3.9 and 11.68 Hz, H-12a), 4.2 (m, 2H, H-3), 3.81 (s, 3H, OCH3), 3.52 (dd, 1H, J=4.1 and
15.8 Hz, H-12eq), 2.95 (m, 1H, H-12ax). 13C NMR (CDCl3) l: 168.3 (COOCH3), 167.5 (C-4),
163.4 (C-1), 136.43 (C-11a), 126.1 (C-7a), 124.7 (C-11b), 123.2 (C-10), 120.3 (C-9), 118.7 (C-8),
11.4 (C-11), 108.6 (C-6a), 54.8 (C-12a), 53.4 (COOCH3), 44.8 (C-3), 26.7 (C-12). Anal. calcd for
C16H15N3O4: C, 61.34; H, 4.83; N, 13.41. Found: C, 61.47; H, 4.99; N, 13.17.
3.5.4. Methyl (6S,12bR)-1,4-dioxo-2,3,6,7,12,12b-hexahydropyrazino[2,1-a]i-carboline-6-
carboxylate 12
Starting from 7 (1 g, 2.24 mmol) dissolved in a 1/2 mixture of trifluoroacetic acid/
dichloromethane (2 ml) a yield of 0.105 g (15%) of 12 was obtained.
Data for 12: mp: 104–105°C. IR (KBr): 3344 (NH), 1739 (COOCH3), 1683 and 1662 (CONH)
1
cm−1. H NMR (CDCl3) l: 9.24 (s, 1H, H-12), 7.49 (d, 1H, H-11, J=7.9 Hz), 7.36 (d, 1H, H-8,
J=7.9 Hz), 7.16 (m, 2H, H-9 and H-10), 6.49 (s, 1H, H-2), 5.92 (d, 1H, H-6, J=6.3 Hz), 5.77
(s, 1H, H-12b), 4.11 (m, 2H, H-3), 3.65 (s, 3H, OCH3), 3.45 (d, 1H, H-7eq, J=16 Hz), 3.16 (dd,
1H, H-7ax, J=4.7 and 14.0 Hz) ppm. 13C NMR (CDCl3) l: 170.4 (COOCH3), 166.0 and 163.8
(C-1 and C-4), 136.5 (C-7b), 126.1 (C-7a), 125.7 and 123.2 (C-11a and C-7a)*, 122.1 (C-9), 119.6
(C-10), 118.3 (C-11), 111.4 (C-8), 107.1 (C-12a), 52.8 (COOCH3), 52.3 (C-6), 50.9 (C-12b), 45.0
(C-3), 22.2 (C-7) ppm. Anal. calcd for C16H15N3O4: C, 61.34; H, 4.83; N, 13.41. Found: C, 61.48;
H, 5.07; N, 13.27.
3.5.5. (6R,12aS)-6-Cyclohexyl-2,3,6,7,12,12a-hexahydropyrazino[1,2-b]i-carboline-1,4-dione 13
Starting from 8 (1 g, 2.2 mmol) dissolved in a 1/2 mixture of trifluoroacetic acid/
dichloromethane (2 ml) a yield of 0.632 g (85%) of 13 was obtained.
1
Data for 13: mp: 334–335°C. IR (KBr): 3284 (NH), 1697 and 1644 (CONH) cm−1. H NMR
(DMSO-d6) l: 10.88 (s, 1H, H-9), 8.30 (s, 1H, H-2), 7.42 (d, 1H, H-8, J=7.6 Hz), 7.32 (d, 1H,
H-11, J=7.9 Hz), 7.02 (m, 2H, H-9 and H-10), 4.37 (d, 1H, H-6, J=8.4 Hz), 4.30 (dd, 1H,
H-12a, J=4.5 and 11.7 Hz), 4.06 (d, 1H, H-3eq, J=17.7 Hz), 3.85 (dd, 1H, H-3ax, J=2.9 and
17.7 Hz), 3.14 (dd, 1H, H-12eq, J=4.6 and 15.4 Hz), 2.88 (dd, 1H, H-12ax, J=15.4 and 11.7
Hz), 1-2 (m, 11H, C6H11) ppm. 13C NMR (DMSO-d6) l: 168.7 (C-4), 165.3 (C-1), 137.7 (C-11a),
134.5 (C-7a), 127.8 (C-11b), 123.0 (C-10), 121.0 (C-9), 119.6 (C-8), 112.9 (C-11), 107.5 (C-6a),
55.0 (C-12a), 45.7 (C-3), 42.3 (C-6), 31.8 and 31.2 (C-1%, C-2%, and C-6%)*, 28.0, 27.5, and 27.3
(C-12, C-3%, C-4%, C-5%)* ppm. Anal. calcd for C15H15N3O2: C, 71.19; H, 6.87; N, 12.45. Found:
C, 71.42; H, 6.97; N, 12.27.
3.6. Activation of piperazinediones. General Method. Synthesis of iminoethers 14–16
To a solution of the starting piperazinedione (0.34 mmol) and triethyloxonium tetra-
fluoroborate (3 equiv.) in dry dichloromethane (10 ml) was added anhydrous sodiun carbonate
(5 equiv.). The suspension was stirred at room temperature under a stream of argon for 16 h,

A. Madrigal et al. / Tetrahedron: Asymmetry 11 (2000) 3515–3526
3525
and was then poured onto ice water. The combined organic layers were dried over sodium
sulphate and evaporated under reduced pressure. The residue was used in the next reaction
without further purification.
1
Data for 14: H NMR (CDCl3) l: 8.66 (s, 1H, H-7), 7.41 (d, 1H, H-8, J=7 Hz), 7.3 (d, 1H,
H-11, J=7 Hz), 7.10 (m, 2H, H-9 and 10), 5.54 (d, 1H, H-6eq, J=16.5 Hz), 4.14 (m, 5H, H-3,
H-12a, H-6ax and OCH2CH3), 3.07 (dd, 1H, H-12eq, J=6.7 and 19.5 Hz), 2.75 (m, 1H,
H-12ax), 1.47 (d, 3H, (C-3)-CH3, J=7.2 Hz), 1.35 (t, 3H, OCH2CH3, J=7 Hz) ppm.
1
Data for 15: H NMR (CDCl3) l: 8.74 (s, 1H, H-7), 7.44 (d, 1H, H-8, J=7.5 Hz), 7.29 (d,
1H, H-11, J=7.5 Hz), 7.10 (m, 2H, H-9 and H-10), 5.5 (d, 1H, H-6eq, J=16.5 Hz), 4.2 (m, 5H,
H-3, H-12a, H-6ax, and OCH2CH3), 3.28 (dd, 1H, H-12eq, J=4.0 and 14.8 Hz), 2.78 (m, 1H,
H-12ax), 1.02 (d, 3H, (C-3)-CH3, J=7 Hz), 1.02 (t, 3H, OCH2CH3, J=7 Hz) ppm.
1
Data for 16: H NMR (CDCl3) l: 8,69 (s, 1H, H-7), 7.5 (d, 1H, H-8, J=7.7 Hz), 7.34 (d, 1H,
H-11, J=7.7 Hz), 7.15 (m, 2H, H-9 and H-10), 6, 34 (s, 1H, H-6), 4.47 (m, 1H, H-12a), 4.25 (m,
2H, H-3), 4.17 (q, 2H, OCH2CH3, J=7 Hz), 3.78 (s, 3H, OCH3), 3.38 (dd, 1H, H-12eq, J=4.0
and 15.5 Hz), 2.81 (dd, 1H, H-12ax, J=15.5 and 12.0 Hz), 1.35 (t, 3H, OCH2CH3, J=7 Hz)
ppm.
3.7. Synthesis of 7,10,16,16a-tetrahydroquinazolino[2%,3%:3,4]pyrazino[1,2-b]i-carboline-5,8-
diones. General method. Synthesis of (+)-18, (−)-18, and 19
A mixture of the corresponding iminoether (0.88 mmol) and anthranilic acid (3 equiv.) was
melted at 130°C under stream of argon for 2 h. The melt was cooled and triturated with 20%
aqueous ammonium hydroxide (2 ml), and the mixture was extracted with chloroform. The
combined organic layers were dried over anhydrous sodium sulphate and evaporated under
reduced pressure. The residue was chromatographed on silica gel as previously described, and
the final purification was performed by a second column chromatography eluting with
dichloromethane.
Starting from 14 (0.1 g, 0.33 mmol) and anthranilic acid (0.138 g, 1.01 mmol) a yield of 0.03
g (24%) of (−)-18 was obtained.
Data for (−)-18: mp: 235–236°C. IR (KBr): 3297 (NH), 1660 and 1628 (CONH) and 1622
1
(CꢀN) cm−1. H NMR (CDCl3) l: 8.35 (s, 1H, H-11), 8.29 (d, 1H, H-4, J=7 Hz), 7.79 (t, 1H,
H-2, J=7 Hz), 7.7 (d, 2H, H-1, J=7 Hz), 7.54 (m, 2H, H-3 and 15), 7.37 (d, 1H, H-12, J=7.5
Hz), 7.20 (m, 2H, H-13 and 14), 5.8 (d, 1H, H-10eq, J=16.2 Hz), 5.47 (q, 1H, H-7, J=7 Hz),
5.0 (dd, 1H, H-16a, J=11.8 and 4.2 Hz), 4.26 (d, 1H, H-10ax, J=16.3 Hz), 3.67 (dd, 1H,
H-16eq, J=4.9 and 11.8 Hz), 3.18 (m, 1H, H-16ax), 1.64 (d, 3H, (C-7)-CH3, J=7 Hz) ppm. 13C
NMR (CDCl3) l: 166.4 y 160.8 (C-8 and C-5)*, 150.1 (C-16b), 147.5 and 136.3 (C-17a and
C-4a)*, 135.0 (C-4), 132.5 (C-15a), 129.1 (C-15b), 127.3 (C-1), 127.1 (C-2), 127.0 (C-3), 126.6
(C-11a), 122.6 (C-14), 120.2 (C-13), 118.11 (C-12), 111.3 (C-15), 107.3 (C-15b), 58.3 (C-16a),
52.0 (C-7), 38.8 (C-10), 29.8 (C-16), 20.3 ((C-10)-CH3) ppm. Anal. calcd for C22H18N4O2: C,
71.34; H, 4.90; N, 15.13. Found: C, 71.86; H, 5.01; N, 15.02. [h]2D5 −353 (c 0.07, CHCl3).
Starting from 15 (0.15 g, 0.5 mmol) and anthranilic acid (0.207 g, 1.5 mmol) a yield of 0.056
g (30%) of (+)-18 was obtained.
Data for (+)-18, with the exception of the specific rotation value, were identical to those of
compound (−)-18. [h]2D5 +350 (c 0.07, Cl3CH).
Starting from 16 (0.3 g, 0.88 mmol) and anthranilic acid (0.361 g, 2.64 mmol), a yield of 0.116
g (32%) of 19 was obtained.

3526
A. Madrigal et al. / Tetrahedron: Asymmetry 11 (2000) 3515–3526
Data for 19: mp: 253–254°C. IR (KBr): 3347 (NH), 1735 (COOCH3), 1685 and 1662 (CONH)
1
and 1598 (CꢀN) cm−1. H NMR (CDCl3) l: 8.43 (s, 1H, H-11), 8.30 (d, 1H, H-4, J=8 Hz), 7.77
(m, 2H, H-2 and H-1), 7.52 (m, 2H, H-3 and H-15), 7.39 (d, 1H, H-12, J=8 Hz), 7.21 (m, 2H,
H-13 and H-14), 6.54 (s, 1H, H-10), 5.20 (dd, 1H, H-16a, J=4 and 12 Hz), 4.52 and 5.10 (2d,
2H, H-7eq and H-7ax, J=19.5 Hz)*, 3.83 (s, 3H, OCH3), 3.64 (dd, 1H, H-16eq, J=4 and 15.5
Hz), 3.07 (m, 1H, H-16ax) ppm. 13C NMR (CDCl3) l: 168.2 (COOCH3), 163.2 and 160.7 (C-8
and C-5)*, 149.6 (C-16b), 147.4 and 132.1 (C-17a and C-4a)*, 135.1 (C-4), 132.1 (C-15a), 125.1
(C-15b), 127.5 (C-1), 127.3 (C-2), 126.9 (C-3), 126.0 (C-11a), 123.3 (C-14), 120.4 (C-13), 118.7
(C-12), 111.4 (C-15), 108.9 (C-15b), 56.4 (C-10), 53.6 (COOCH3), 52.2 (C-16a), 44.8 (C-7), 29.1
(C-16) ppm. Anal. calcd for C23H18N4O4: C, 66.66; H, 4.38; N, 13.52. Found: C, 66.88; H, 4.57;
N, 13.11. [h]2D5 +50 (c 0.1, CH3COCH3).
Acknowledgements
We thank CICYT for financial support of this research through grants SAF-94-0517 and
SAF-97-0143.
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