Synthesis of bioactive 2-aza-analogues of ipecac and alangium alkaloids

Article abstract of DOI:10.1002/cmdc.201000230

License to kill: Substitution of the methine carbon C2 in the ipecac or alangium alkaloids by nitrogen yields functional mimetics with low micromolar to high naonomolar IC₅₀ values against Trypanosoma brucei, the parasite that causes African tr

Full text of DOI:10.1002/cmdc.201000230

MED
DOI: 10.1002/cmdc.201000230
Synthesis of Bioactive 2-Aza-Analogues of Ipecac and Alangium Alkaloids
Michael Kçlzer,[a] Kerstin Weitzel,[b] H. Ulrich Gçringer,[b] Eckhard Thines,[c] and Till Opatz*[a]
Polycyclic alkaloids with potent eukaryotic cytotoxicity have
been isolated from the tropical shrubs Cephaelis ipecacuanha
and Alangium lamarckii.[1,2] Their skeleton features an invariable
benzoquinolizidine core, which is linked to a bi- or tricyclic
unit by a methylene bridge (Figure 1).
their therapeutic window is small, and safer alternatives are
needed. Herein, we describe a short and versatile synthesis of
functional emetine mimetics, which facilitates the establish-
ment of structure–activity relationships.[11–13]
Replacement of methine carbon C2 in the ABC-ring system
of the ipecac and alangium alkaloids by nitrogen should yield
analogues resembling their natural counterparts with respect
to their preferred conformation and charge distribution at
physiological pH. At neutral pH, the diethylenetriamine portion
of the aza-analogues will be protonated at both termini, that
is, at the same positions as the natural products.[14] Further-
more, the possibility of attaching different DE- or DEF-ring sys-
tems to the aza-analogous ABC-ring building block in a CꢀN
bond formation should permit the preparation of a broad vari-
ety of analogues in a combinatorial search for drugs with fa-
vorable properties (Scheme 1).
Scheme 1. Retrosynthetic analysis of 2-azaemetine.
Figure 1. Structures of some ipecac and alangium alkaloids.
For the synthesis of the crucial pyrazinoisoquinoline building
block 6, (S)-2-aminobutyric acid (8) was nosylated and coupled
to homoveratrylamine. N-Allylation of the sulfonamide fol-
lowed by dihydroxylation and periodate cleavage gave a reac-
tive aldehyde. Upon treatment with POCl3, the aldehyde con-
verted to the protected tricycle 10 in a Speckamp cyclization
with virtually complete diastereoselectivity.[15,16] Denosylation
with thiolate[17] and reduction of amide 11 resulted in the sec-
ondary amine 6. Surprisingly, all attempts to link 6 to a suitable
DE-ring system by means of an N-alkylation or a reductive ami-
nation were unsuccessful. In contrast, reaction with N-chloro-
acetyl homoveratrylamine followed by a Bischler–Napieralski
cyclization produces the desired dihydroisoquinoline 13, which
is the 2-aza-analogue of O-methylpsychotrine. Catalytic hydro-
genation of the imine resulted in a 2:1 mixture of 2-azaeme-
tine (5) and 2-azaisoemetine (14), while reduction with
NaCNBH3 gave a 7:1 selectivity in favor of the natural diaste-
reomer (Scheme 2).[18,19] Attempts to prepare either amine 5 or
14 by enantioselective reduction of imine 13 failed, presuma-
bly due to the chelating properties of reactant and/or product.
Due to their striking bioactivity,[3–5] the ipecac alkaloid emet-
ine (1) and its synthetic derivative dehydroemetine have been
used for the treatment of protozoal infections, such as amebia-
sis, trypanosomiasis and bilharziasis, which pose a constant
threat to inhabitants of tropical areas.[6,7] In addition, emetine
(1) has recently become an attractive anticancer and antiviral
target.[8–10] However, these alkaloids have severe side effects;
[a] Dr. M. Kçlzer, Prof. Dr. T. Opatz
Institute for Organic Chemistry, University of Mainz
Duesbergweg 10-14, 55128 Mainz (Germany)
Fax: (+49)6131-3922338
E-mail: opatz@uni-mainz.de
[b] K. Weitzel, Prof. Dr. H. U. Gçringer
Genetics, Darmstadt University of Technology
Schnittspahnstrasse 10, 64287 Darmstadt (Germany)
[c] Dr. E. Thines
Institute for Biotechnology and Drug Research (IBWF)
Erwin Schrçdinger-Str. 56, 67663 Kaiserslautern (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cmdc.201000230.
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ChemMedChem 2010, 5, 1456 – 1464

Scheme 3. Synthesis of 2-azaisoemetine 14 with a diasteromeric ratio (d.r.)
of >10:1. Reagents and conditions: a) EtO2CCOCl, Et2O, 08C!258C, 1.75 h;
b) POCl3, toluene/EtOH (10:1), reflux, 3 h; c) H2, Pd/C, EtOH, 1 bar, 12 h;
d) LiOH, THF/H2O (3:1), 45 min; e) Fmoc-OSu, NaHCO3, H2O/1,4-dioxane (1:1),
45 min, 39% (five steps); f) 11, DIC, DMAP, CH2Cl2, 08C!258C, 1.5 h;
g) Et2NH, MeCN, 45 min, 69% (two steps); h) LiAlH4, THF, microwave, 658C,
15 min, 97%. Fmoc-OSu, N-(9-fluorenylmethoxycarbonyloxy) succinimide;
Fmoc, 9-fluorenylmethoxycarbonyl; DIC, N,N’-diisopropylcarbodiimide;
DMAP, 4-(dimethylamino)pyridine.
Boc-protected N-chloroacetyltryptamine 19 was used for the
N-alkylation of 6. After deprotection and Bischler–Napieralski-
cyclization, reduction of the resulting dihydrocarboline with
NaCNBH3 gave a 2.1:1 mixture of 21 and its C1’-epimer 22 (2-
azadeoxyisotubulosine; Scheme 4).[24]
After separation of the diastereomeric pairs 5/14 and 21/22
by preparative HPLC, biological evaluation of the pure com-
pounds was performed using different cellular test systems.
The antiprotozoal activity was assayed using noninfective
(insect stage) and infective (blood stage) African trypanosomes
(Trypanosoma brucei brucei, strain Lister 427). The cytotoxicity
against mammalian cells was determined using L-1210 and
colo 320 cell lines. The results are summarized in Figure 2 and
Table 1.
Scheme 2. Preparation of 2-azaemetine 5. Reagents and conditions: a) 2-
NO2C6H4SO2Cl, aq NaOH, CH2Cl2, RT, 12 h, then reflux, 3 h; b) EDC·HCl, homo-
veratrylamine, 1-hydroxybenzotriazole, THF, 3.5 h; c) allyl bromide, K2CO3,
BnNEt3Cl, MeCN, RT, 2.5 h, then 508C, 2 h, 63% (three steps); d) K2OsO2(OH)4,
(DHQD)2-PHAL, K3[Fe(CN)6], tBuOH/H2O (1:1), 08C, 3.5 h; e) NaIO4, THF/H2O
(1:2), 1.5 h; f) POCl3, CH2Cl2, 14 h, 79% (three steps); g) 2-mercaptoethanol,
DBU, DMF, 508C, 2 h, 97%; h) LiAlH4, THF, 508C, 2 h, 89%; i) EtNiPr2, DMF,
908C, 14 h; j) POCl3, toluene, reflux, 45 min, 49% (two steps); k) NaCNBH3,
AcOH, EtOH, 608C, 2 h, 76% (product ratio: 5/14; 7:1); l) H2, Pd/C, THF,
20 bar, 4 d, 56% (product ratio: 5/14; 2:1). EDC, 1-ethyl-3-[3-dimethylamino-
propyl]carbodiimide; (DHQD)2-PHAL, hydroquinidine 1,4-phthalazinediyl
diether; DBU, 1,8-diazabicyclo[5.4.0]undec-7-ene.
While compounds 5 and 21 were about six to sevenfold less
potent than emetine (1), they were about tenfold more active
than their C1’-epimers in the T. b. brucei assay. The same holds
true for the cytotoxicity, which roughly parallels the antitrypa-
nosomal activity.[25,26] However, compared to emetine, which
was used as a positive control, the 2-aza-analogue 5 exhibits
an improved potency/toxicity ratio (Table 1). In all tested cases,
infective stage trypanosomes were about twice as sensitive
than noninfective parasites; this is probably due to the differ-
ent surface structure of the parasite at the noninfective stage.
Importantly, onset of growth arrest coincides with the develop-
ment of a severe and characteristic morphological alteration in
which the normally elongated parasite cell bodies retract from
the anterior end, thereby generating a rounded, tadpole-like
cell shape. Identical morphological characteristics were also
identified with 5- and 21-treated Leishmania tarentolae cells,[27]
indicating that the same, or at least a similar, growth inhibition
For the preparation of 14, the Fmoc-protected tetrahydro-
isoquinoline-1-carboxylic acid rac-16 obtained in five steps
from homoveratrylamine was used as the coupling component
for the northern half of the compound.[20–22] Acylation of amine
11 was achieved according to a modified Steglich protocol.[23]
After removal of the Fmoc group, double reduction yielded 14
with high diastereoselectivity. Apparently, the configurationally
labile C1’ had epimerized to the thermodynamically favored
form during the last steps (Scheme 3).
The preparation of 2-azadeoxytubulosine 21 was achieved
according to the same sequence used for the preparation of 5.
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Table 1. Cytotoxicity against African trypanosomes and mammalian
cancer cell lines.[a]
Compd
LC50 (T. b. brucei) [mm]
LC50 [mm]
Colo 320
noninfective
infective
L-1210
0.04
1
5
14
21
22
23
0.03
0.18
2.1
0.015
0.08
1.0
0.1
2
5
0.3
5
0.4
2
0.3
5
0.2
0.1
2.0
>1.3
1.0
nd[b]
>25
>25
[a] All values determined at least in triplicate. Standard deviations vary
between 3–5%. [b] Not determined.
The assignment of the configuration of the newly formed
stereocenter at C1’ in compounds 5, 14, 21, and 22 was per-
formed based on one-dimensional NMR and NOESY data. In
each diastereomeric pair, one compound showed a significant
upfield shift of both the methoxy group in position 10 and the
hydrogen in position 11, indicative of an anisotropic shielding
by the E- or EF-ring system. This has also been observed for
the natural series.[1,24,29,30] While azaemetines 5 and 14 behaved
in a similar manner to their natural counterparts with respect
to the chemical shift, line shape, and coupling pattern of the
H-1’ resonances, nuclear Overhauser effect (NOE) data indicat-
ed an unusual behavior for compounds 21 and 22. As in the
natural series, one of the compounds (21) showed the down-
field shifts in the A-ring observed for the iso-series, but failed
to show the characteristic triplet for the H1’ signal. However,
its diastereomer 22 gave the doublet, which is characteristic
for tubulosine (3), but did not show any anisotropic shielding
by the A-ring. NOESY spectra recorded in CDCl3 revealed a re-
markable and unexpected proximity of the indole NH proton
to the axial proton in position 11b for 21 and 22. This is in
contrast to the corresponding data for compounds 5 and 14,
as well as to the crystal structure of tubulosine (3),[31] in which
the DEF-ring system points towards the opposite face of the
ABC tricycle. Thus, NOE contacts and coupling constants of
H1’, as well as the anisotropic shielding, are interchanged
when switching form the natural to the 2-aza-series. A possible
explanation for the unusual conformational preference of the
alangium-alkaloid-derived 2-aza-analogues could be the forma-
tion of a hydrogen bond between the indole NH and N2.[32,33]
Based on the full set of NOE data and molecular modeling ex-
periments, the configuration of 21 and 22 can be unambigu-
ously assigned; the results are in accordance with their biologi-
cal activity (Figure 3).[12,25]
Scheme 4. Synthesis of 2-azadeoxytubulosine 21 and the C1’-epimer 22
with a diasteromeric ratio (d.r.) of 2.1:1. Reagents and conditions:
a) ClCH2COCl, Et3N, CH2Cl2, 08C, 45 min; b) (Boc)2O, DMAP, CH2Cl2, 08C!
258C, 45 min, 75% (two steps); c) 6, EtNiPr2, BaO, MeCN, microwave, 1008C,
45 min, 81%; d) TFA, Me2S, 08C!258C, 12 h; e) POCl3, toluene, reflux, 1 h;
f) NaCNBH3, AcOH, EtOH/THF (50:3), 12 h 51% (three steps). Boc, tert-butoxy-
carbonyl; TFA, trifluoroacetic acid.
Figure 2. Antitrypanosomal activity against the insect stage of T. b. brucei
(strain Lister 427). 1, &; 5, ~; 14, &; 21, ^; 22, *.
To investigate the relevance of the southern half of the mol-
ecules for their bioactivity, the truncated compound 23 was
prepared by condensation of Fmoc-l-pipecolic acid with ABC
building block 11 followed by deprotection and reduction
(Scheme 5).
mechanism operates in different protozoan parasites. Remarka-
bly, the morphotype induced by compounds 5 and 21 resem-
bles the tadpole-like phenotype of a transdominant trypano-
some mutant reported by Lamb et al.[28] Thus, the two com-
pounds can be used as small-molecule modulators or “chemi-
cal genetic” tools to illuminate trypanosome morphogenetics.
Again, epimerization of the newly introduced stereocenter
was observed, which in this case led to a nearly equimolar mix-
ture of C1’-epimers. Remarkably, this mixture was neither
active against trypanosomes nor cytotoxic. Thus it can be as-
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Experimental Section
(S)-2-[Allyl-(2-nitrobenzenesulfonyl)amino]-N-[2-
(3,4-dimethoxyphenyl)ethyl]butyramide (9)
Compound 8 (500 mg, 3.6 mmol) and NaOH (427 mg,
10.7 mmol, 3 equiv) were dissolved in distilled H2O
(8 mL). After 5 min, a solution of 2-nitrobenzenesulfonyl
chloride (867 mg, 3.9 mmol, 1.1 equiv) in CH2Cl2 (5 mL)
was added slowly (20 min) with vigorous stirring. After
12 h stirring at RT, the reaction mixture was heated to
reflux for a further 3 h. After separation of the organic
layer, the aqueous phase was acidified to pH 2 by addi-
tion of 7% H2SO4 and extracted with CH2Cl2 (3ꢁ5 mL).
The combined organic layers were washed with brine
(2ꢁ10 mL), dried (Na2SO4), filtered and concentrated in
vacuo. (S)-2-(2-Nitrobenzenesulfonylamino)butyric acid
was obtained as a yellow oil that crystallized into color-
less needles (740 mg, 2.6 mmol, 71%): Rf =0.45 (EtOAc+
1% AcOH); mp: 132–1338C; [a]2D3 =ꢀ164.6 (c=1, CHCl3);
1H NMR (300 MHz, CDCl3): d=7.90–7.85 (m, 1H, H3’),
7.68–7.63 (m, 1H, H6’), 7.56–7.50 (m, 2H, H4’, H5’), 6.52
(d, J=8.0 Hz, 1H, SO2NH), 3.78 (dt, Jd =5.1 Hz, Jt =7.9 Hz,
1H, NCHCO), 1.73 (mc, 1H, CH3CH2-a), 1.65 (mc, 1H,
CH3CH2-b), 0.74 ppm (t, J=7.4 Hz, 3H, CH3CH2); 13C NMR
(100.6 MHz, [D6]DMSO): d=172.4 (CO), 147.3 (C2’), 133.9
(C4’), 133.4 (C1’), 132.4 (C5’), 129.9 (C6’), 124.1 (C3’), 57.2
(NCHCO), 25.2 (CH3CH2), 10.0 ppm (CH3CH2); IR (KBr): n˜ =
3486, 3333, 2932, 1712, 1632, 1535,1453, 1360, 1169,
1125, 1065, 789, 731 cmꢀ1; MS (ESI): m/z (%): 311.1 (100)
[M+Na]+; HRMS (ESI): m/z [M+Na]+ calcd for
C10H12N2O6S: 311.0308, found: 311.0299; Anal. calcd for
C10H12N2O6S: C 41.66, H 4.20, N 9.72, S 11.12, found: C
41.66, H 4.24, N 9.78, S 11.19.
Figure 3. Energy-minimized (MD/MM2) models of 2-azaemetine (5), 2-azaisoemetine (14),
2-azadeoxytubulosine (21) and 2-azadeoxyisotubulosine (22) showing relevant NOE
contacts.
The butyric acid intermediate (2.24 g, 7.8 mmol), HOBt·H2O (1.31 g,
8.6 mmol, 1.1 equiv) and homoveratrylamine (1.55 g, 8.6 mmol,
1.1 equiv) were suspended in dry THF (30 mL). Under stirring,
EDC·HCl (1.64 g, 8.6 mmol, 1.1 equiv)[34] was added and the mixture
was stirred for 5 h. The reaction mixture was concentrated in
vacuo, and the yellow oily residue was partitioned between EtOAc
(100 mL) and 1n HCl (100 mL). After the addition of further EtOAc
(100 mL), the aqueous phase was removed. The organic layer was
washed with 1n HCl (3ꢁ35 mL) and saturated aq NaHCO3 (3ꢁ
35 mL), dried (Na2SO4), filtered and concentrated in vacuo. (S)-N-[2-
(3,4-Dimethoxyphenyl)ethyl]-2-(2-nitrobenzenesulfonylamino)butyr-
amide was obtained as a viscous yellow oil that solidified to a
yellow glass (3.31 g, 7.3 mmol, 94%): Rf =0.44 (EtOAc+3% AcOH);
[a]2D2 =ꢀ59.3 (c=0.5, CHCl3); 1H NMR (400 MHz, CDCl3): d=8.10–
8.05 (m, 1H, H6’), 7.89–7.86 (m, 1H, H3’), 7.74 (mc, 2H, H4’, H5’),
6.80 (d, J=7.9 Hz, 1H, H5’’), 6.70 (dd, J=10.0, 2.0, Hz, 2H, H2’’,
H6’’), 6.40 (br t, J=5.7 Hz, 1H, NHCO), 3.86 (s, 3H, OCH3), 3.85 (s,
3H, OCH3), 3.79 (dt, Jd =3.3, Jt =7.6 Hz, 1H, NCHCO), 3.46 (mc, 1H,
ArCH2CH2-a), 3.36 (dtd, Jd =13.1, 5.6 Hz, Jt =7.2 Hz, 1H, ArCH2CH2-b),
2.70 (mc, 2H, ArCH2), 1.76 (mc, 1H, CH3CH2-a), 1.64 (ddd, J=14.8,
11.1, 3.7 Hz, 1H, CH3CH2-b), 0.77 ppm (t, J=7.4 Hz, 3H, CH3CH2);
13C NMR (100.6 MHz, CDCl3): d=170.2 (CO), 149.3 (C3’’), 148.0 (2C,
C2’, C4’’), 134.1 (C4’), 133.5 (C1’), 133.0 (C5’), 131.0 (2C, C6’,C1’’),
125.7 (C3’), 120.9 (C6’’), 112.1 (C2’’), 111.7 (C5’’), 59.5 (NCHCO), 56.1
(OCH3), 56.0 (OCH3), 41.0 (NCH2CH2), 35.4 (ArCH2), 26.3 (CH3CH2),
9.6 ppm (CH3CH2); IR (KBr): n˜ =3342, 3156, 1665, 1543, 1517, 1467,
1367, 1265, 1236, 1160, 1142, 1027 cmꢀ1; MS (ESI): m/z (%): 490.2
(28) [M+K]+, 474.2 (100) [M+Na]+, 469.2 (21) [M+NH4]+, 452.2 (97)
[M+H]+; HRMS (ESI): m/z [M+H]+ calcd for C20H25N3O7S: 452.1491,
Scheme 5. Synthesis of the truncated analogue 23 with a diasteromeric ratio
(d.r.) of 1.1:1. Reagents and conditions: a) Fmoc-l-pipecolic acid (structure
shown), DIC, DMAP, CH2Cl2, 08C!258C, 85 min; b) Et2NH, MeCN, 45 min;
c) LiAlH4, THF, 558C, 2 h, 65% (three steps).
sumed that the E-ring of the ipecac alkaloids and the EF-ring
system of the alangium alkaloids are required for their bio-
activity.
In conclusion, the presented 2-aza-analogues show the char-
acteristic structure–activity relationships of the natural series.
Although they are less active than their natural counterparts,
mimetic 5 shows an improved potency/toxicity ratio compared
with the benchmark compound emetine (1). The modular as-
sembly of the presented mimetics allowed, for the first time,
the determination of the role of the aromatic ring(s) in the
southern half of the scaffold, and this information will simplify
the identification of further key elements of the pharmaco-
phore.
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found: 452.1508; Anal. calcd for C20H25N3O7S: C 53.20, H 5.58, N
9.31, S 7.10, found: C 53.07, H 5.58, N 9.21, S 7.23.
mixture of diastereomers A and B): d=8.02 (ddd, J=5.7, 4.1,
1.7 Hz, 1H, H6’), 7.73 (mc, 2H, H4’, H5’), 7.63 (ddd, J=7.4, 5.7,
1.7 Hz, 1H, H3’), 6.83 (dd, J=8.0, 3.8 Hz, 1H, H5’’), 6.79–6.73 (m,
2H, H2’’, H6’’), 6.60 (br t, J=5.1 Hz, 1H, NHCOA), 6.36 (br t, J=
5.6 Hz, 1H, NHCOB), 4.16 (t, J=7.6 Hz, 1H, NCHCOB), 4.12 (t, J=
7.8 Hz, 1H, NCHCOA), 3.88 (s, 3H, OCH3), 3.87 (s, 3H, OCH3), 3.75
(ddt, Jd =14.9, 3.7 Hz, Jt =7.4 Hz, 1H, CHOH), 3.69–3.56 (m, 3H,
NCH2-a, HOCH2-a, ArCH2CH2-a), 3.55–3.46 (m, 2H, HOCH2-b,
ArCH2CH2-b), 3.32 (dd, J=15.5, 3.3 Hz, 1H, NCH2-aA), 3.21 (dd, J=
15.7, 7.9 Hz, 1H, NCH2-bB), 2.80 (t, J=6.4 Hz, 2H, ArCH2), 2.47 (br s,
2H, OHA), 2.16 (br s, 2H, OHB), 1.99 (mc, 1H, CH3CH2-a), 1.65 ppm
(mc, 1H, CH3CH2-b); 13C NMR, HSQC, HMBC (125.8 MHz, CDCl3): d=
171 (NCO), 149.19 (C3’’B), 149.18 (C3’’A), 148.15 (C2’A), 148.11 (C2’B),
147.91 (C4’’B), 147.89 (C4’’A), 134.3 (C4’A), 134.2 (C4’B) 132.8 (C1’B),
132.5 (C1’A), 132.1 (C5’B), 132.0 (C5’A), 131.5 (C6’A), 131.3 (C6’B),
131.1 (C1’’A), 131.0 (C1’’B), 124.48 (C3’B), 124.43 (C3’A), 120.93 (C6’’B),
120.88 (C6’’A), 112.1 (C2’’), 111.66 (C5’’A), 111.61 (C5’’B), 71.7
(CHOHA), 70.0 (CHOHB), 64.6 (CH2OHB), 63.5 (CH2OHA), 61.79
Using an adapted protocol by Albanese et al.,[35] the butyramide in-
termediate (2.91 g, 6.44 mmol) was dissolved in dry MeCN (30 mL),
and finely powdered K2CO3 (1.78 g, 12.89 mmol), allyl bromide
(1.68 mL, 19.33 mmol) and benzyltriethylammonium chloride
(147 mg, 10 mol%) were added. The mixture was stirred for 2.5 h
and then warmed to 508C until TLC indicated complete conversion
(120 min). The yellow reaction mixture was partitioned between
Et2O (30 mL) and H2O (20 mL). The organic layer was washed with
H2O (15 mL) and the combined aqueous layers were extracted into
EtOAc (3ꢁ20 mL). The combined organic layers were washed with
H2O (3ꢁ15 mL), dried (Na2SO4), filtered and concentrated in vacuo.
Compound 9 was obtained as a yellow oil of sufficient purity
(3.00 g, 6.1 mmol, 95%): Rf =0.35 (cyclohexane/EtOAc, 1:1); [a]D22
=
ꢀ32.5 (c=0.5, CDCl3); 1H NMR (500 MHz, CDCl3): d=8.03 (dd, J=
7.7, 1.5 Hz, 1H, H6’), 7.69 (mc, 2H, H4’, H5’), 7.61 (dd, J=7.6,
1.5 Hz, 1H, H3’), 6.81 (d, J=8.1 Hz, 1H, H5’’), 6.73 (m, 2H, H2’’,
H6’’), 6.50 (br t, J=5.5 Hz, 1H, NHCO), 5.64 (ddt, Jt =6.5, Jd =17.0,
10.1 Hz, 1H, =CH), 5.17 (ddd, J=17.0, 2.5, 1.4 Hz, 1H, =CH2-syn),
5.04 (ddd, J=10.1, 2.5, 1.4 Hz, 1H, =CH2-anti), 4.13 (t, J=7.5 Hz,
1H, NCHCO), 4.09 (ddt, Jd =16.2, 6.5, Jt =1.4 Hz, 1H, All-CH2-a), 3.91
(ddt, Jd =16.2, 6.5, Jt =1.4 Hz, 1H, All-CH2-b), 3.87 (s, 3H, OCH3),
3.86 (s, 1H, OCH3), 3.44 (pseudo-q, J=~7 Hz, 2H, ArCH2CH2), 2.73
B
B
(NCHCO), 56.1 (OCH3), 56.0 (OCH3), 48.8 (NCH2 ), 47.7 (NCH2 ), 41.3
A
B
B
(ArCH2CH2), 35.1 (ArCH2 ), 35.0 (ArCH2 ), 22.4 (CH3CH2 ), 21.9
A
A
B
(CH3CH2 ), 10.6 (CH3CH2 ), 10.2 ppm (CH3CH2 ); IR (NaCl): n˜ =3355,
3094, 2939, 2838, 1657, 1591, 1544, 1516, 1466, 1373, 1263, 1237,
1159, 1028, 852, 736 cmꢀ1; MS (ESI): m/z (%): 548.17 (100) [M+Na]+;
HRMS (ESI): m/z [M+Na]+ calcd for C23H31N3O9S: 548.1679, found:
548.1691; Anal. calcd for C23H31N3O9S: C 52.56, H 6.06, N 7.98, S
6.21, found: C 52.63, H 6.06, N 7.95, S 5.96.
(br t, J=~7 Hz, 1H, ArCH2), 2.01 (d-pseudo-quin, Jd =14.8 Hz, Jquin
~7 Hz, 1H, CH3CH2-a), 1.61 (d-pseudo-quin, Jd =14.8 Hz, Jquin
=
=
~7 Hz, 1H, CH3CH2-b), 0.73 ppm (t, J=7.4 Hz, 3H, CH2CH3);
13C NMR (100.6 MHz, CDCl3): d=169.5 (CO), 149.2 (C3’’), 148.0 (C2’),
147.9 (C4’’), 133.9 (2C, allyl=CH, C4’), 133.8 (C1’), 131.9 (C5’), 131.6
(C6’), 131.3 (C1’’), 124.3 (C3’), 120.9 (C6’’), 118. 9 (allyl=CH2), 112.1
(C2’’), 111.7 (C5’’), 61.5 (NCHCO), 56.1 (OCH3), 56.0 (OCH3), 47.9
(allyl-CH2), 41.1 (NCH2CH2), 35.2 (ArCH2), 22.4 (CH3CH2), 10.7 ppm
(CH3CH2); IR (NaCl): n˜ =2937, 1675, 1545, 1516, 1466, 1372, 1263,
1237, 1158, 1028 cmꢀ1; MS (ESI): m/z (%): 514.2 (81) [M+Na]+,
492.2 (70) [M+H]+, 468.3 (100), 428.3 (78); HRMS (ESI): m/z [M+H]+
calcd for C23H29N3O7S: 492.1804, found: 492.1814; Anal. calcd for
C23H29N3O7S: C 56.20, H 5.95, N 8.55, S 6.52, found: C 56.31, H 5.95,
N 8.41, S 6.34.
The dihydroxylated intermediate (2.80 g, 5.33 mmol) was dissolved
in THF (20 mL). After addition of H2O (40 mL) and finely powdered
NaIO4 (1.48 g, 6.93 mmol), the mixture was stirred until TLC indicat-
ed complete conversion (90 min). The mixture was partitioned be-
tween H2O (10 mL) and EtOAc (20 mL), and the aqueous phase
was extracted with EtOAc (2ꢁ10 mL). The combined organic layers
were washed with H2O (15 mL) and brine (20 mL), dried (Na2SO4),
filtered and concentrated in vacuo. The sensitive aldehyde, (S)-2-
[N-(2-oxoethyl-(2-nitrobenzenesulfonyl)amino]-N-[2-(3,4-dimethoxy-
phenyl)ethyl]butyramide, was obtained as a pale yellow foam
(2.58 g, 5.22 mmol, 98%): Rf =0.53 (EtOAc); 1H NMR (300 MHz,
CDCl3): d=9.42 (s, 1H, CHO), 8.01 (dd, J=7.5, 1.8 Hz, 1H, H6’), 7.72
(mc, 2H, H4’, H5’), 7.65 (mc, 1H, H3’), 6.81 (d, J=8.7 Hz, 1H, H5’’),
6.74 (m, 2H, H2’, H6’), 6.41 (br t, J=5.6 Hz, NHCO), 4.33–4.09 (m,
3H, CH2CHO, NCHCO), 3.86 (s, 6H, OCH3), 3.45 (mc, 2H, ArCH2CH2),
2.74 (t, J=7.0 Hz, 1H, ArCH2), 1.83 (mc, 1H, CH3CH2-a), 1.51
(d-pseudo-quin, Jd =14.5 Hz, Jquin =7.3 Hz, 1H, CH3CH2-b), 0.73 ppm
(t, J=7.3 Hz, 3H, CH2CH3); 13C NMR (100.6 MHz, CDCl3): d=196.9
(CHO), 169.3 (NCO), 149.13 (C3’’), 147.9 (C4’’), 147.82 (C2’), 134.4
(C4’), 133.1 (C1’), 132.2 (C5’), 131.5 (C6’), 131.1 (C1’’), 124.5 (C3’),
120.8 (C6’’), 112.1 (C2’’), 111.6 (C5’’), 61.3 (NCHCO), 56.1 (OCH3), 56.0
(OCH3), 53.3 (CH2CHO), 41.0 (ArCH2CH2), 35.0 (ArCH2), 23.0 (CH3CH2),
10.2 ppm (CH3CH2); IR (NaCl): n˜ =3355, 3094, 2938, 2837, 1733,
1671, 1591, 1544, 1516, 1465, 1372, 1354, 1263, 1237, 1160,
1028 cmꢀ1. The product was of sufficient purity and slowly decom-
posed upon standing.
(3S,11bR)-3-Ethyl-9,10-dimethoxy-2-(2-nitrobenzenesulfon-
yl)-1,2,3,6,7,11b-hexahydro-pyrazino[2,1-a]-isoquinolin-4-
one (10)
According to an adapted protocol by Sharpless et al.,[36,37] N-allyl
sulfonamide 9 (8.80 g, 17.90 mmol) was dissolved in tBuOH (50 mL)
and added under stirring to a solution of finely powdered K2CO3
(7.43 g, 53.70 mmol) and freshly ground K3[Fe(CN)6] (13.56 g,
41.20 mmol) in H2O (50 mL). The mixture was cooled to 08C and
(DHQD)2-PHAL (139.50 mg, 1 mol%) and K2OsO2(OH)4 (26.40 mg,
0.4 mol%) were added. The mixture was stirred at 08C until TLC in-
dicated complete conversion. (12 h). The reaction was quenched
by the addition of solid Na2SO3 (9 g, 71.6 mmol) and stirred for
45 min at RT. The mixture was partitioned between EtOAc (45 mL)
and H2O (45 mL), and the aqueous layer was extracted with EtOAc
(20 mL). The combined organic layers were washed with 0.5n HCl
(25 mL), brine (25 mL), saturated aq NaHCO3 (25 mL), again with
brine (20 mL), dried (Na2SO4), filtered and concentrated in vacuo.
Purification by column chromatography (EtOAc) gave (2’RS,2S)-2-
[2,3-dihydroxypropyl-(2-nitrobenzenesulfonyl)amino]-N-[2-(3,4-di-
The aldehyde intermediate (2.41 g, 4.88 mmol) was dissolved in
dry CH2Cl2 (25 mL) and freshly distilled POCl3 (900 mL, 9.77 mmol)
was added.[16,38] The yellow mixture slowly turned brown and stir-
ring was continued until TLC indicated complete conversion (2 h).
Besides the main cyclization product (Rf =0.19; EtOAc/petroleum
ether/CH2Cl2, 2:2:3), a side product was detected (Rf =0.37; EtOAc/
petroleum ether/CH2Cl2, 2:2:3). Excess POCl3 and solvent were re-
moved by distillation (1258C). The residue was partitioned be-
tween EtOAc (15 mL) and saturated aq NaHCO3 (20 mL), and the
aqueous phase was extracted with EtOAc (3ꢁ10 mL). The organic
methoxyphenyl)ethyl]butyramide
a pale yellow foam (8.65 g,
16.47 mmol, 92%): Rf =0.18 (EtOAc); [a]2D2 =ꢀ35 (c=1, CHCl3);
1H NMR, COSY, TOCSY, NOESY, HSQC, HMBC (500 MHz, CDCl3; 2:1
1460
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemMedChem 2010, 5, 1456 – 1464

phase was separated and dried (Na2SO4), filtered and concentrated
in vacuo to give a dark yellow viscous oil. Purification by column
chromatography (silica; EtOAc/petroleum ether/CH2Cl2, 2:2:3) gave
compound 10 as a yellow foam (2.04 g, 4.3 mmol, 88%): ee >94%
7.5 Hz, 3H, CH3); 13C NMR, HSQC, HMBC (100.6 MHz, CDCl3): d=
169.5 (CO), 147.9 (C9), 147.8 (C10), 127.5 (C7a), 126.3 (C11a), 111.8
(C8), 107.9 (C11), 60.5 (C3), 57.0 (C11b), 56.2 (OCH3), 55.8 (OCH3),
50.1 (C1), 39.0 (C6), 28.4 (C7), 25.8 (CH3CH2), 10.2 ppm (CH3CH2); IR
(NaCl): n˜ =3457, 3316, 2961, 2933, 2854, 1630, 1515, 1463, 1436,
1360, 1319, 1292, 1258, 1227, 1113, 1009, 848, 771 cmꢀ1; MS (ESI):
m/z (%): 291.2 (100) [M+H]+; HRMS (ESI): m/z [M+H]+ calcd for
C16H22N2O3: 291.1709, found: 291.1701.
1
(see Supporting Information); [a]2D5 =ꢀ55.8 (c=1, CHCl3); H NMR,
COSY, HSQC, HMBC (400 MHz, CDCl3): d=7.97–7.93 (m, 1H, H6’),
7.57–7.70 (m, 3H, H3’, H4’, H5’), 6.67 (s, 1H, H11), 6.56 (s, 1H, H8),
4.86 (dd, J=7.8, 3.5 Hz, 1H, H11b), 4.73–4.77 (m, 1H, H6-a), 4.40 (t,
J=6.1 Hz, 1H, H3), 4.12 (dd, J=12.6, 3.7 Hz, 1H, H1-a), 3.87 (s, 3H,
OCH3), 3.84 (s, 3H, OCH3), 3.30 (dd, J=12.6, 8.10 Hz, 1H, H1-b),
2.87 (dt, Jt =12.9 Hz, Jd =2.2, 1H, H6-b), 2.80 (br, dt, Jt =11.8 Hz,
Jd =2.2 Hz, 1H, H7-a), 2.55–2.65 (m, 1H, H7-b), 2.00–2.10 (m, 2H,
CH3CH2), 0.93 ppm (t, J=7.5 Hz, 3H, CH3CH2); 13C NMR, HSQC,
HMBC (100.6 MHz, CDCl3): d=167.4 (CO), 148.5 (C9), 148.2 (C10),
148.05 (C2’), 134.1 (C4’), 132.3 (C1’), 131.9 (C5’), 131.0 (C6’), 127.80
(C7a), 124.5 (C3’), 123.8 (C11a), 111.72 (C8), 108.78 (C11), 61.0 (C3),
56.2 (OCH3), 56.0 (OCH3), 54.3 (C11b), 48.9 (C1), 39.7 (C6), 28.31
(C7), 26.9 (CH3CH2), 9.7 ppm (CH3CH2); IR (NaCl): n˜ =2936, 1660,
1542, 1519, 1465, 1438, 1362, 1266, 1229, 1168, 1138, 852,
740 cmꢀ1; MS (FD): m/z (%): 475.2 (100) [M]+; HRMS (ESI): m/z
[M+H]+ calcd for C22H25N3O7S: 476.1491, found: 476.1498; Anal.
calcd for C22H25N3O7S: C 55.57, H 5.30, N 8.84, S 6.74, found: C
55.59, H 5.26, N 8.73, S 6.84.
(3S,11bR)-3-Ethyl-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-
2H-pyrazino[2,1-a]isoquinoline (6)
The secondary amine 11 (2.0 g, 6.9 mmol) was dissolved in dry THF
(15 mL) and LiAlH4 (2m in THF, 17 mL, 34.5 mmol, 5 equiv) was
added.[40] The grey suspension was stirred for 2 h at 508C. After
cooling to RT, the reaction was quenched by the addition of 2n aq
NaOH (10 mL) at 08C, and the colorless precipitate (insoluble alu-
minum salts) was removed by filtration. The filter cake was washed
with EtOAc (25 mL) and the organic layer was washed with H2O
(15 mL). The combined aqueous layers were extracted with EtOAc
(3ꢁ20 mL). The combined organic layers were washed with brine
(20 mL), dried (Na2SO4), filtered and concentrated in vacuo. The re-
sulting yellow oil was purified by column chromatography (silica;
EtOAc/petroleum ether/Et2NH, 5:5:1) to give compound 6 as a
yellow oil (1.69 g, 6.1 mmol, 89%): Rf =0.22 (EtOAc/petroleum
ether/Et2NH, 5:5:1); [a]D23 =ꢀ51.3 (c=0.3, CDCl3); 1H NMR, COSY,
HSQC, HMBC (400 MHz, CDCl3): d=6.61 (s, 1H, H11), 6.59 (s, 1H,
H8), 3.84 (OCH3), 3.83 (OCH3), 3.54 (dd, J=11.6, 2.6 Hz, 1H, H1-a),
3.21–3.05 (m, 2H, H11b, H7-a), 2.98–2.90 (m, 2H, H4-a, H6-a), 2.88–
2.79 (m, 1H, H3), 2.74 (dd, J=11.6, 10.3 Hz, 1H, H1-b), 2.62 (dd, J=
16.0, 4.1 Hz, 1H, H7-b), 2.54 (td, Jt =11.5, Jd =4.1 Hz, 1H, H6-b),
2.10 (t, J=10.6 Hz, 1H, H4-b), 1.43 (mc, 2H, CH3CH2), 0.97 ppm (t,
J=7.5 Hz, 3H, CH3CH2); 13C NMR, HSQC, HMBC (100.6 MHz, CDCl3):
d=147.6 (C10), 147.1 (C9), 127.2, 127.1 (C7a, C11a), 111.8 (C8),
107.7 (C11), 62.5 (C11b), 61.3 (C4), 56.3 (OCH3), 56.3 (C3), 56.2
(OCH3), 52.3 (C6), 50.7 (C1), 29.0 (C7), 27.1 (CH3CH2), 10.3 ppm
(CH3CH2); IR (NaCl): n˜ =3457, 3317, 2962, 2933, 2854, 1630, 1515,
1463, 1436, 1360, 1318, 1291, 1257, 1226, 1113, 1009, 848,
771 cmꢀ1; MS (ESI): m/z (%): 277.2 (100) [M+H]+; HRMS (ESI): m/z
[M+H]+ calcd for C16H24N2O2: 277.1916, found: 277.1927.
The minor product was the corresponding dihydropyrazinone, 1-
[2-(3,4-dimethoxyphenyl)ethyl]-3-ethyl-4-(2-nitrobenzenesulfonyl)-
3,4-dihydro-1H-pyrazin-2-one, which was isolated as a yellow oil
(209 mg, 0.44 mmol, 9%): [a]2D2 =+298.9 (c=1, CHCl3); 1H NMR,
COSY, HSQC, HMBC (400 MHz, CDCl3): d=7.97–7.94 (m, 1H, H6’),
7.72–7.61 (m, 3H, H3’, H4’, H5’), 6.71 (d, J=8.1 Hz, 1H, H5’’), 6.64–
6.59 (m, 2H, H2’’, H6’’), 6.04 (dd, J=5.5 Hz, 1.7 Hz, 1H, H5), 5.68 (d,
J=5.5 Hz, 1H, H6), 4.49 (ddd, J=8.9, 6.0, 1.8 Hz, 1H, H3), 3.84 (s,
3H, OCH3), 3.83 (s, 3H, OCH3), 3.61 (t, J=7.5 Hz, 2H, NCH2), 2.61
(m, 2H, NCH2CH2), 1.66 (mc, 2H, CH3CH2), 0.98 ppm (t, J=7.4 Hz,
3H, CH3CH2); 13C NMR, HSQC, HMBC (100.6 MHz, CDCl3): d=164.2
(C2), 149.1 (C2’), 148.2 (C3’’), 147.9 (C4’’), 134.3 (C4’), 132 (C1’),
131.9 (C5’), 130.8 (C6’), 130.2 (C1’’), 124.5 (C3’), 120.9 (C6’’), 118.7
(C6), 111.9 (C2’’), 111.4 (C5’’), 106.6 (C5), 60.7 (C3), 56.03 (OCH3),
55.98 (OCH3), 47.6 (NCH2), 34.2 (NCH2CH2), 23.8 (CH3CH2), 10.0 ppm
(CH3CH2); IR (NaCl): n˜ =2985, 2933, 1679, 1591, 1545, 1516, 1456,
1411, 1371, 1264, 1238, 1181, 1142, 1029, 852, 745 cmꢀ1; MS (ESI):
m/z (%): 498.1 (100) [M+Na]+; HRMS (ESI): m/z [M+Na]+ calcd for
C22H25N3O7S: 498.1311, found: 498.1326.
2-Chloro-N-[2-(3,4-dimethoxyphenyl)ethyl]acetamide (12)
Homoveratrylamine (931 mL, 5.52 mmol) and N,N-dimethylaniline
(1.12 mL, 8.85 mmol, 1.6 equiv) were dissolved in dry CH2Cl2
(34 mL) and the resulting solution was cooled to 08C. Under stir-
(3S,11bR)-3-Ethyl-9,10-dimethoxy-1,2,3,6,7,11b-hexahydro-
pyrazino[2,1-a]isoquinolin-4-one (11)
Cyclized sulfonamide 10 (1.9 g, 4.0 mmol) was dissolved in dry
DMF (20 mL) under rigorous exclusion of air. Under stirring, 2-mer-
captoethanol (2.8 mL, 40 mmol, 10 equiv) and DBU (2.99 mL,
20 mmol, 5 equiv) were added.[17,39] The mixture was stirred at
508C until TLC indicated complete conversion (2 h). The solvent
was removed in vacuo and the remaining yellow oil was purified
by column chromatography (silica; EtOAc/petroleum ether/CH2Cl2,
8:5:1). The resulting viscous oil crystallized within several months
into fine yellow needles (1.13 g, 3.9 mmol, 97%): Rf =0.23 (EtOAc/
petroleum ether/Et2NH, 8:5:1); mp: 978C; [a]2D2 =ꢀ212.4 (c=0,5,
ring, a solution of chloroacetyl chloride (0.46 mL, 5.8 mmol,
1.05 equiv) in dry CH2Cl2 (15 mL) was added.[41] The mixture was
stirred until TLC indicated complete conversion (2 h). The mixture
was partitioned between 1n aq HCl (20 mL) and CH2Cl2 (15 mL),
and the organic layer was separated and washed with 1n aq HCl
(3ꢁ10 mL) and brine (15 mL). After drying (Na2SO4), the organic
layer was filtered and concentrated in vacuo to give 12 as a
green–grey solid (1.39 g, 5.41 mmol, 98%): Rf =0.49 (petroleum
1
ether/EtOAc, 1:1); mp: 958C; H NMR (400 MHz, CDCl3): d=6.82 (d,
J=8.1 Hz, 1H, H5), 6.76–6.69 (m, 2H, H2, H6), 6.64 (br s, 1H, NH),
4.03 (s, 2H, CH2Cl), 3.87 (s, 3H, OCH3), 3.86 (s, 3H, OCH3), 3.54 (td,
Jt =7.0 Hz, Jd =5.9 Hz, 2H, ArCH2CH2), 2.79 ppm (t, J=7.0 Hz, 2H,
ArCH2CH2); 13C NMR (100.6 MHz, CDCl3): d=166.0 (CO), 149.3 (C3),
148.0 (C4), 131.0 (C1), 120.8 (C6), 112.9 (C2), 111.7 (C5), 56.04
(OCH3), 55.98 (OCH3), 42.7 (CH2Cl), 41.2 (ArCH2CH2), 35.1 ppm
(ArCH2CH2); IR (NaCl): n˜ =3424, 3313, 3282, 3082, 3000, 2940, 1662,
1642, 1589, 1555, 1518, 1464, 1419, 1262, 1238, 1158, 1139, 1027,
1
CHCl3); H NMR, COSY, HSQC, HMBC (400 MHz, CDCl3): d=6.63 (s,
1H, H11), 6.57 (s, 1H, H8), 4.88 (ddd, J=12.5, 4.9, 2.2 Hz, 1H, H6-a),
4.74 (dd, J=10.4, 4.3 Hz, 1H, H11b), 3.86 (s, 1H, OCH3), 3.85 (s, 1H,
OCH3), 3.69 (dd, J=12.6, 4.6 Hz, 1H, H1-a), 3.39 (dd, J=8.0, 3.6 Hz,
1H, H3), 2.96–2.83 (m, 2H, H7-a, H1-b), 2.82–2.76 (m, 1H, H6-b),
2.62 (br dt, Jd =~15 Hz, Jt =~3, Hz, 1H, H7-b), 2.07 (mc, 1H,
CH3CH2-a), 1.82 (br s, NH), 1.74 (mc, 1H, CH3CH2-b), 1.01 ppm (t, J=
ChemMedChem 2010, 5, 1456 – 1464
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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MED
1037, 808 cmꢀ1; MS (ESI): m/z (%): 258.10 (100) [M+H]+; HRMS
(ESI): m/z [M+H]+ calcd for C12H16ClNO3: 258.0891, found:
258.0890.
H11b), 3.02–2.92 (m, 2H, H4-a, H6-a), 2.70–2.57 (m, 3H, H4’-a, H4’-b,
H7-b), 2.53–2.41 (m, 2H, H3, H6-b), 2.31 (t, J=10.6 Hz, 1H, H4-b),
2.21 (t, J=11.1 Hz, 1H, H1-b), 1.92 (tdd, Jt =7.1 Hz, Jd =14.7, 2.7 Hz,
1H, CH3CH2-a), 1.52 (d-pseudo-quin, Jd =15.0 Hz, Jquin =~7 Hz, 1H,
CH3CH2-b), 1.01 ppm (t, J=7.5 Hz, 3H, CH3CH2); 13C NMR, HSQC,
HMBC (100.6 MHz, CDCl3): d=164.9 (C1’), 151.1 (C6’), 147.7 (C9),
147.4 (C7’), 147.1 (C10), 131.5 (C4a’), 127.4 (C11a), 127.1 (C7a),
121.7 (C8a’), 111.9 (C8), 110.4 (C8’), 110.0 (C5’), 107.8 (C11), 62.3
(C3), 61.5 (C11b), 60.1 (C4), 57.0 (C1), 56.3 (OCH3), 56.1 (OCH3),
55.94 (OCH3), 55.91 (OCH3), 51.8 (C6), 47.3 (C3’), 28.9 (C7), 25.9
(C4’), 24.0 (CH3CH2), 10.3 ppm (CH3CH2); IR (ATR): n˜ =2925, 2853,
1727, 1605, 1512, 1462, 1353, 1268, 1210, 1154, 1029, 857, 809,
732 cmꢀ1; MS (ESI): m/z (%): 480.28 (100) [M+H]+; HRMS (ESI): m/z
[M+H]+ calcd for C28H37N3O4: 480.2862, found: 480.2849.
(3S,11bR)-2-(6,7-Dimethoxy-3,4-dihydro-isoquinolin-1-yl-
methyl)-3-ethyl-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-
pyrazino[2,1-a]isoquinoline (13)
Amine 6 (350 mg, 1.27 mmol) was dissolved in dry DMF (4 mL) and
the resulting solution was degassed by ultrasonication under
argon for 15 min. After addition of EtNiPr2 (572 mg, 4.43 mmol,
3.5 equiv) and stirring for 5 min at 08C, a solution of amide 12
(326 mg, 1.27 mmol, 1 equiv) in DMF (1 mL) was added. The mix-
ture was stirred for 14 h at 908C. When TLC indicated complete
conversion of the amine, the solvent was removed in vacuo. The
crude was codistilled with toluene repeated and dried in vacuo
prior to further purification. Column chromatography (silica;
EtOAc/EtOH, 5:1) gave (3S,11bR)-N-[2-(3,4-dimethoxyphenyl)ethyl]-
2-[3-ethyl-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-pyrazino[2,1-
a]isoquinolin-2-yl]acetamide) as a brown oil (439 mg, 0.88 mmol,
70%): Rf =0.18 (EtOAc/EtOH, 5:1); [a]2D0 =ꢀ2.8 (c=0.5, CHCl3);
1H NMR, COSY, TOCSY, NOESY, HSQC, HMBC (500 MHz, CDCl3): d=
7.48 (t, J=5.7 Hz, 1H, NH), 6.78–6.73 (m, 3H, H2’, H5’, H6’), 6.59 (s,
1H, H8), 6.45 (s, 1H, H11), 3.86 (s, 3H, OCH3-4’), 3.84 (s, 3H, OCH3-9),
3.83 (s, 3H, OCH3-3’), 3.74 (s, 3H, OCH3-10), 3.59 (mc, 2H,
ArCH2CH2), 3.37 (d, J=17.3 Hz, 1H, NCH2CO-a), 3.15 (dd, J=11.2,
2.7 Hz, 1H, H1-a), 3.06–3.01 (m, 2H, H7-a, H11b), 2.96–2.88 (m, 2H,
H6-a, H4-a), 2.85 (d, J=17.3 Hz, 1H, NCH2CO-b), 2.82 (t, J=6.9 Hz,
2H, ArCH2), 2.62 (dd, 15.8, 3.7 Hz, 1H, H7-b), 2.50–2.41 (m, 2H, H6-b,
H3), 2.35 (t, J=11 Hz, 1H, H1-b), 2.05 (dd, J=11.4, 10.2 Hz, 1H,
H4-b), 1.50 (d-pseudo-quin, Jd =14.7 Hz, Jquin =~7 Hz, 1H, CH3CH2-a),
2-Azaemetine (5) and 2-Azaisoemetine (14)
Procedure A: A solution of imine 13 (19.2 mg, 42 mmol) and
NaCNBH3 (6.5 mg, 0.104 mmol, 2.5 equiv) in dry THF (400 mL) was
treated with AcOH (7.2 mL, 0.126 mmol, 3 equiv) and dry EtOH
(123 mL, 2.1 mmol, 50 equiv). The mixture was first stirred at 608C
for 2 h, and then at RT for a further 3 h. The reaction was stopped
by addition of saturated aq citric acid (2 mL) to destroy borane
complexes. The mixture was heated to 1008C by microwave irradi-
ation (CEM Discover, air cooling, IR temperature control, maximum
power 150 W, maximum pressure 15 bar). Since TLC still indicated
incomplete hydrolysis, the procedure was repeated. Finally, the
mixture was stirred for 12 h at 608C and basified to pH 12–14 by
addition of 4n aq NaOH. The product was extracted with EtOAc
(3ꢁ10 mL), and the combined organic layers were washed with
brine, dried (Na2SO4), filtered and concentrated in vacuo. Purifica-
tion by flash chromatography (silica; EtOAc/EtOH, 1:1) gave the
light sensitive compound 5 (5/14, d.r.=7:1) as a yellow oil
(14.6 mg, 30 mmol, 76%): Rf =0.21 (EtOAc/EtOH, 1:1).
1.61 (d-pseudo-quin, Jd =14.7 Hz,
Jquin =~7 Hz, 1H, CH3CH2-b),
0.84 ppm (t, J=7.5 Hz, 3H, CH2CH3); 13C NMR, HSQC, HMBC
(100.6 MHz, CDCl3): d=171.5 (CO), 149.3 (C4’), 148 (C9), 147.9 (C3’),
147.4 (C10), 131.3 (C1’), 127.2 (C7a), 126.9 (C11a), 120.7 (C6’), 112.0,
111.9 (C2’, C5’), 111.5 (C8), 107.6 (C11), 61.54 (C11b), 61.45(C3),
60.11 (C4), 59.3 (C1), 57.3 (NCH2), 56.3 (OCH3), 56.00 (OCH3), 55.99
(OCH3), 55.9 (OCH3), 51.9 (C6), 40.1 (ArCH2CH2), 35.3 (ArCH2CH2),
29.0 (C7), 24.2 (CH3CH2), 10.1 ppm (CH3CH2); IR (NaCl): n˜ =3456,
3333, 3097, 2975, 2932, 2874, 1712, 1593, 1536, 1453, 1422, 1360,
1291, 1266, 1169, 1125, 1065, 1009, 905, 789, 743, 731, 654, 597,
560, 501 cmꢀ1; MS (ESI): m/z (%): 498.30 (100) [M+H]+; HRMS (ESI):
m/z [M+H]+ calcd for C28H39N3O5: 498.2962, found: 498.2955.
Procedure B: A mixture of amide 17 (11 mg, 22 mmol) and solid
LiAlH4 (8 mg, 220 mmol, 10 equiv) in dry THF (100 mL) was heated
to 658C for 15 min by microwave irradiation (CEM Discover, air
cooling, IR temperature control, maximum power 150 W, maximum
pressure 15 bar). After cooling to RT and pressure equilibration,
cold 4n aq NaOH (2 mL) was added under ice cooling. The mixture
was stirred for 30 min, diluted with THF (2 mL), and the insoluble
aluminum salts were filtered off. The filter cake was washed with
THF (10 mL) and EtOAc (15 mL). The aqueous phase was extracted
with EtOAc (2ꢁ5 mL). The combined organic layers (including THF)
were washed with brine (10 mL), dried (Na2SO4), filtered and con-
centrated in vacuo. Fast purification by column chromatography
(silica; EtOAc/EtOH, 1:1) gave compound 5 (5/14, d.r.=1:10) as a
yellowish oil (10.1 mg, 20.97 mmol, 97%).
The above intermediate (190 mg, 0.38 mmol) was dissolved in dry
toluene (8 mL) and treated with freshly distilled POCl3 (105 mL,
1.15 mmol, 3 equiv) under stirring.[38] The yellow reaction mixture
was heated to reflux under rigorous exclusion of moisture until
TLC indicated complete conversion (45 min). Most of the toluene
and the POCl3 were removed by distillation, and the remaining vis-
cous brown residue was suspended in warm dioxane (7.5 mL). The
suspension was ultrasonicated and ice (35 g) was added. After basi-
fication with 4n aq NaOH, the product was extracted with EtOAc
(6ꢁ10 mL). The combined organic layers were washed with brine
(20 mL), dried (Na2SO4), filtered and concentrated in vacuo. Fast
purification by column chromatography (silica; EtOAc/EtOH, 1:1)
gave imine 13 as a viscous brown oil (127.6 mg, 0.27 mmol, 70%):
Rf =0.17 (EtOAc/EtOH, 1:1); [a]2D5 =+8.3 (c=0.2, CDCl3); 1H NMR,
COSY, HSQC, HMBC (400 MHz, CDCl3): d=7.81 (s, 1H, H8’), 6.70 (s,
1H, H5’), 6.55 (s, 1H, H8), 6.34 (s, 1H, H11), 4.29 (d, J=13.0 Hz, 1H,
NCH2C=N-a), 3.92 (s, 3H, OCH3-6’), 3.90 (s, 3H, OCH3-7’), 3.81 (s, 3H,
OCH3-9), 3.71 (s, 3H, OCH3-10), 3.70 (s, 2H, CH2-3’), 3.27 (dd, J=
11.1, 2.4 Hz, 1H, H1-a), 3.18–3.03 (m, 3H, 1H, NCH2C=N-b, H7-a,
Procedure C: A solution of imine 13 (105 mg, 0.24 mmol) in THF
(8 mL) was treated with Pd/C (10%, 10 mg), and the suspension
was stirred for 28 h under H2. The mixture was filtered over Celite,
and the remainder was washed with a warm mixture of EtOAc
(200 mL) and EtOH (20 mL). The filtrate was concentrated in vacuo
to give the crude product as a brown oil. Purification by column
chromatography (silica; EtOAc/EtOH, 1:1) gave several fractions
(total weight 73.8 mg). One fraction (22.0 mg) contained the
epimer 5 and amine 6. Separation of both components was effect-
ed by preparative TLC (pyridine/toluene/Et2NH, 4:6:1; 5 Rf =0.43,
amine 6 Rf =0.25) to give pure 5 (6.4 mg) and 6 (8.9 mg). Another
fraction (34.0 mg) contained the epimeric product mixture (5 and
14) in a 1:1.8 ratio (NMR): Yield: 73.8 mg (after column chroma-
tography, 0.15 mmol, 70%, impure; after prep TLC and HPLC: 21%,
1462
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemMedChem 2010, 5, 1456 – 1464

see below); UV (diastereomeric mixture, MeCN): l (log e)=285
(3.44), 225 nm (sh, 3.87). The diastereomers were separated from
the 1:1.8 mixture by prep RP-HPLC (mobile Phase: MeCN/phos-
phate buffer 25 mm, pH 7, 60:40). From a portion of the product
(27.0 mg), five fractions were obtained and analyzed by NMR: Frac-
tion 1, 4.99 mg, pure 5; Fraction 2, 3.40 mg, 1:4.5 mixture of 5/14;
Fraction 3, 6.02 mg, pure 14; Fraction 4, 3.27 mg, pure 14; Fraction
5,3.91 mg, unidentified side products. The amount of pure 5 ob-
tained was 4.99 mg (5.58 mg total); the amount of pure 14 ob-
tained was 9.29 mg (12.0 mg total); the combined yield of 5 and
14 was 17.6 mg. The total yield from 13 was 21%.
ment uncertain; IR (ATR): n˜ =3432, 2927, 2823, 1665, 1610, 1515,
1463, 1355, 1331, 1256, 1225, 1160, 1114, 1028, 857, 773 cmꢀ1; MS
(ESI): m/z (%): 482.30 (100) [M+H]+, 483.31 (10) [M+2H]+; HRMS
(ESI): m/z [M+H]+ calcd for C28H40N3O4: 482.3019, found: 482.3010.
Supporting information, containing general methods, all remaining
experimental procedures, growth curves, microscopy images and
NMR spectra of all compounds, is available on the WWW under
http://dx.doi.org/10.1002/cmdc.201000230.
Acknowledgements
Procedure D: A solution of imine 13 (10 mg, 21 mmol) in dry THF
(0.4 mL) was treated with Pd/C (10%, 1 mg), and the suspension
was stirred for 4 d at RT under H2 pressure (20 bar). The mixture
was filtered over Celite, and the remainder was washed with a
warm mixture of EtOAc (20 mL) and EtOH (10 mL). The combined
filtrates were concentrated in vacuo and the crude product was
purified by column chromatography (silica; EtOAc/EtOH, 1:1). A
diastereomeric mixture of 5 and 14 (2:1) was obtained as a yellow
oil (5.9 mg, 12.3 mmol, 56%).
This work was supported by the Deutsche Forschungsgemein-
schaft (SFB 579, TO and HUG). We thank Heinz Kolshorn
(Johannes Gutenberg-University, Mainz, Germany) and Thomas
Hackl (University of Hamburg, Hamburg, Germany) for NMR spec-
troscopic analyses, Andreas Werle (Department of Biotechnology,
University of Kaiserslautern, Germany) for expert technical assis-
tance as well as Bernd Meyer (University of Hamburg, Hamburg,
Germany) and Michael Brecht (Technische Universitꢀt Darmstadt,
Germany) for helpful discussions. HUG is an International Scholar
of the Howard Hughes Medical Institute (HHMI).
(1’S,3S,11bR)-2-[(6,7-Dimethoxy-1,2,3,4-tetrahydroisoquinolin-1-
yl)methyl]-3-ethyl-9,10-dimethoxy-1,3,4,6,7,11b-hexahydro-2H-
pyrazino[2,1-a]isoquinoline (5): [a]2D7 =ꢀ3.7 (c=0.5, CDCl3);
1H NMR, COSY, TOCSY, NOESY, HSQC, HMBC (400 MHz, CDCl3): d=
6.74 (s, 1H, H11), 6.62 (s, 1H, H5’), 6.60, (s, 1H, H8), 6.60 (s, 1H,
H8’), 4.37 (br d, J=9.6 Hz, 1H, H1’), 3.86 (2ꢁOCH3), 3.85 (2ꢁOCH3),
3.66 (mc, 1H, H1-a), 3.34 (d, J=9.8 Hz, 1H, H11b), 3.21–3.06 (m,
4H, NCH2-a, CH2-3’, H7-a), 3.02–2.91 (m, 3H, H4-a, H6-a, H4’-a), 2.74
(t, J=4.2 Hz, H4’-b), 2.64 (br d, J=15.8 Hz, H7-b), 2.55–2.43 (m, 2H,
H6-b, H3), 2.35–2.29 (m, 3H, NCH2-b, H1-b, H4-b), 2.22 (t, J=
10.8 Hz, 1H, NH), 1.73 (tdd, Jt =15.3 Hz, Jd =7.5, 2.5 Hz, 1H,
CH3CH2-a), 1.34 (mc, 1H, CH3CH2-b), 0.92 ppm (t, J=7.5 Hz, 3H,
CH3CH2); 13C NMR, HSQC, HMBC (100.6 MHz, CDCl3): d=147.99
(C9*), 147.89 (C10*), 147.59 (C6’*), 147.49 (C7’*), 127.5 (C8a’*), 127.3
(C4a’*), 127.2 (C11a*), 127.0 (C7a*), 111.9 (C8, C5’), 109.7 (C8’), 108.3
(C11), 61.4 (11b), 61.0 (C3), 60.2 (C4), 57.08 (NCH2), 57.05 (C1), 56.6
(OCH3), 56.3 (OCH3), 56.01 (OCH3), 55.99 (OCH3), 51.6 (C6), 50.8
(C1’), 38.7 (C3’), 29.2 (C7), 28.6 (C4’), 24.4 (CH3CH2), 10.2 ppm
(CH3CH2); * assignment uncertain; IR (ATR): n˜ =3420, 2925, 2853,
1663, 1610, 1516, 1463, 1357, 1330, 1258, 1225, 1114, 1033, 859,
772 cmꢀ1; MS (ESI): m/z (%): 482.30 (100) [M+H]+, 483.31 (20)
[M+2H]+; HRMS (ESI): m/z [M+H]+ calcd for C28H40N3O4: 482.3019,
found: 482.3010.
Keywords: alkaloids · aza-analogues · cyclization · natural
products · trypanosomes
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Published online on June 23, 2010
1464
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ChemMedChem 2010, 5, 1456 – 1464