Enantioselective addition of organolithium reagents to 3,4-dihydroisoquinoline in the presence of (-)-sparteine as an external ligand. Application for the synthesis of isoquinoline alkaloids

Article abstract of DOI:10.1016/S0957-4166(01)00233-6

Three isoquinoline alkaloids, (-)-salsolidine 2, (+)-carnegine 6 and (-)-1-phenyl-2-methyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline 7, were obtained in high yield and with 17-46% e.e. by the enantioselective additions of organolithium reagents to dihydroisoquinolines 1 and 5, in the presence of (-)-sparteine as a chiral ligand.

Full text of DOI:10.1016/S0957-4166(01)00233-6

TETRAHEDRON:
ASYMMETRY
Pergamon
Tetrahedron: Asymmetry 12 (2001) 1435–1440
Enantioselective addition of organolithium reagents to
3,4-dihydroisoquinoline in the presence of (−)-sparteine as an
external ligand. Application for the synthesis of isoquinoline
alkaloids
Maria Chrzanowska* and Joanna Sokołowska†
Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
Received 18 April 2001; accepted 22 May 2001
Abstract—Three isoquinoline alkaloids, (−)-salsolidine 2, (+)-carnegine 6 and (−)-1-phenyl-2-methyl-6,7-dimethoxy-1,2,3,4-tetra-
hydroisoquinoline 7, were obtained in high yield and with 17–46% e.e. by the enantioselective additions of organolithium reagents
to dihydroisoquinolines 1 and 5, in the presence of (−)-sparteine as a chiral ligand. © 2001 Elsevier Science Ltd. All rights
reserved.
1. Introduction
tioselectivities were observed even when the chiral
promoter was used in catalytic amounts. The use of
(−)-sparteine was particularly beneficial in n-butyl-
lithium and phenyllithium additions to N-aryl-
aldimines. Itsuno et al.5 described the enantioselective
addition of organolithium compounds to N-silyl, N-
alumino, and N-boryl imines. The best results were
obtained with benzaldehyde N-di-iso-butylalumino
imine (74% e.e.) when the (−)-sparteine–BuLi complex
was formed before addition to the imine. North et al.6
studied (−)-sparteine induced addition of organolithium
reagents to a,b-unsaturated N-arylaldimines, achieving
e.e.s of 70–88%.
The nucleophilic 1,2-addition of organometallic
reagents to imine CꢀN double bonds is a valuable
method for the synthesis of secondary amines. In the
last decade enantioselective additions to prochiral
imines in the presence of chiral catalysts/ligands have
been developed as a new strategy for the synthesis of
optically active amines, including alkaloids. The
stereoselective addition of C-nucleophiles to the CꢀN
bond has also been the subject of review articles.1–3 The
addition, in comparision with that of a carbonyl group,
has been limited by the poor electrophilicity of the
azomethine carbon atom. However, the reaction can be
considerably accelerated by the use of chiral ligands/
catalysts. A variety of compounds have been tested3 as
external chiral controllers of the enantioselectivity. The
influence of (−)-sparteine on the reactions of organo-
lithium compounds (RLi; R=Me, n-Bu, Ph, vinyl) with
various aldimines has been reported by Denmark,4
Itsuno5 and North.6
The lupine alkaloid, (−)-sparteine, is an inexpensive and
commercially available, enantiomerically pure diamine
used as a chiral bidentate ligand in many types of
synthetic reaction. It has been used for several aspects
of asymmetric syntheses such as deprotonation,7–9 the
polymerization of methacrylate esters,10 the intra-11 and
intermolecular12 carbolithiation of alkenes and the
enantioselective addition of organolithium and Grig-
nard reagents to carbonyl compounds.13
Denmark et al.4 were the first to successfully use (−)-
sparteine in such reactions (72–91% e.e.). High enan-
(−)-Sparteine has been used as an external ligand in the
addition reactions of different types of imine but has
not been applied to addition reactions of cyclic imines.
In connection with our study on the asymmetric synthe-
sis of isoquinoline alkaloids,14 we planned to investigate
this approach for the enantioselective synthesis of salso-
lidine 2, a simple isoquinoline alkaloid. In a similar
reaction, reported previously,15 the addition of methyl-
* Corresponding author. Fax: 48 61 8658008; e-mail: brozda
@main.amu.edu.pl
Graduated student; current adress: Karol Marcinkowski University
of Medical Sciences, Department of Biochemistry and Molecular
Biology, Swiecickiego 6, 60-780 Poznan, Poland.
†
0957-4166/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0957-4166(01)00233-6

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M. Chrzanowska, J. Sokołowska / Tetrahedron: Asymmetry 12 (2001) 1435–1440
lithium to 3,4-dihydroisoquinoline 1 was carried out in
the presence of (4S,5S)-5-(2-methoxyphenoxymethyl)-4-
(4-methylthiophenyl)-2-phenyl-2-oxazoline to give (R)-
(+)-salsolidine 2 in 41% e.e.
of chiral ligand/catalyst, the reaction temperature and
the type of solvent, on the enantioselectivity. The
results are shown in Tables 1–3. According to the
previous results,5 the (−)-sparteine–organolithium com-
plex (1:1) was first generated at −76°C for 30 min
before a solution of imine 1 was added, also at low
temperature.
Herein, we describe the results of the addition of vari-
ous organolithium reagents to 6,7-dimethoxy-3,4-dihy-
droisoquinoline
(−)-sparteine applied as an external chiral ligand and/or
catalyst (Scheme 1).
1 performed in the presence of
The enantiomeric excess (e.e.) was established by HPLC
analysis using a Chiracel OD-H column (a comparison
was made of the retention times of each enantiomer
with that of a racemic sample and, in a few cases with
the literature data17) and/or from the relative integra-
tion of 1H NMR spectra of the product run in the
presence of the chiral shift reagent, TADDOL.18 The
specific rotation measurements were used for determi-
nation of the sign of the optical rotation from which
the absolute configuration of the product was deduced.
2. Results and discussion
We carried out the addition of methyl-, n-butyl- and
phenyllithium to 6,7-dimethoxy-3,4-dihydroisoquino-
line 1 in the presence of (−)-sparteine. In order to
optimize the reaction conditions we investigated the
influence of a number of factors, including the amount
Scheme 1.
Table 1. Addition of methyllithium to imine 1
Entry
(−)-Sparteine
Solvent
Temperature
Time (h)
Yield (%)
Config.a
% e.e. HPLC
% e.e. NMR
(mol equiv.)
(°C)
1
2
3
4
5
6
7
8
1
0.2
1
0.2
0.2
0.2
0.2
0.2
Toluene
Toluene
Toluene
Toluene
Et2O
THF
THF
(i-Pr)2O
−76“−15
−76“−15
−20
−20
−20
−20
−76
−20
20
20
2
2
2
2
2
2
77
50
75
85
86
90
63
81
(S)-(−)
(S)-(−)
(S)-(−)
(S)-(−)
(S)-(−)
30.4
4.0
31.6
2.9
15.0
14.0b
12.0
0
14.0
14.0
13.6
2.0c
3.1c
15.1
2.2c
12.9
(S)-(−)
a Confirmed by comparing the specific rotation with a literature value.16
b [h]D −7.8 (0.51, EtOH).
c Racemic within error of the method.
Table 2. Addition of n-butyllithium to imine 1
Entry
(−)-Sparteine (mol equiv.)
Solvent
Temperature (°C) Time (h)
Yield (%)
[h]D (CHCl3)
% e.e. HPLC
1
2
3
4
1
0.2
1
Toluene
Toluene
Et2O
−76
−76
−76“−15
−76
2
3
20
2
77
90
49
63
−20.3
−12.3
−4.6
0
46.5
28.0
9.6
0
1
THF

M. Chrzanowska, J. Sokołowska / Tetrahedron: Asymmetry 12 (2001) 1435–1440
Table 3. Addition of phenyllithium to imine 1
1437
Entry
(−)-Sparteine
Solvent
Temperature
Time (h)
Yield (%)
Config.a
% e.e. HPLC
% e.e. NMR
(mol equiv.)
(°C)
1
2
3
4
5
1.0
1.4
1.0
1.4
1.0
Toluene
THF
THF
Et2O
Et2O
−76
−76
−76
−76“−15
−76
2.5
1
1.5
20
2
34
64
79
31
37
(R)-(+)
17.0
2.1c
5.0
–
–
3.0
–
2.2c
17.0b
(R)-(+)
17.0
a Confirmed by comparing the specific rotation with a literature value.17,22
b [h]D +3.1 (0.96, CHCl3).
c Racemic within error of the method.
As evidenced in Table 1, in additions of methyllithium
to imine 1, the best enantioselectivity was obtained for
the reaction run in toluene in the presence of one molar
equivalent of (−)-sparteine, at −76°C for the first 2 h
and at −15°C for the next 18 h (entry 1). Salsolidine 2
in levorotatory (S)-form16 (31.6% e.e. by HPLC) was
isolated after column chromatography purification fol-
lowed by (−)-sparteine. Results obtained for reactions
run in THF (Table 1, entries 6, 7) suggested the forma-
tion of ( )-2.
rized in Table 3 (Scheme 2). The same e.e. of 17% was
obtained regardless of the solvent used, toluene or
diethyl ether (Table 3, entries 1, 5). The levorotatory
enantiomer of compound 4, [h]D −20.0 (0.50, CHCl3),
was obtained for the first time in 1991 by Wiegrebe22
from ( )-4 by resolution of diastereomeric salts with
(−)-diacetone-2-ketogulonic acid hydrate. The (R)-
absolute configuration of the dextrorotatory enan-
tiomer 4 (prepared from 1-phenyl-6,7-dimethoxy-3,4-
dihydroisoquinoline using the asymmetric transfer
hydrogenation procedure of Noyori et al.17) was estab-
lished in 1996. Thus, the (R)-configuration of (+)-4
could be assigned on the basis of the sign of the specific
rotation and the retention time of the predominant
enantiomer in HPLC analysis.17
Table 2 shows the results of the addition reactions of
n-butyllithium to imine 1. The addition of the pre-
formed (−)-sparteine–n-butyllithium complex to imine 1
proceeded faster than in the reaction with methyl-
lithium. The best result was achieved when the reaction
was completed in toluene at −76°C for 2 h with
equimolar (−)-sparteine (Table 2, entry 1). Pure, iso-
lated 1-butyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquino-
line 3 was shown to be levorotatory: [h]D −20.3 (0.73,
CHCl3), obtained in 46.5% e.e. (by HPLC). To the best
of our knowledge this compound has been reported in
the literature only as the racemate.19
To improve the enantioselectivity of the additions, a
series of experiments were completed using 6,7-
dimethoxy-3,4-dihydroisoquinolinium methiodide 5 as
a substrate. It was hoped that the iminium salt 5 would
be more reactive toward nucleophilic addition than
imine 1. The results are shown in Tables 4 and 5 and
Scheme 3.
On the basis of a similarity of the CD spectra20 of
amines 2 and 3 and those reported for simple 1-alkyl-
and 1-benzyltetrahydroisoquinoline alkaloids,21 as well
as analysis of the retention times for the (S)-enan-
tiomers in HPLC analysis, we assigned (S)-configura-
tion at the C(1) stereogenic center to compound 3.
In the addition of methyllithium to methiodide 5 (R)-
(+)-carnegine 6,23 another simple isoquinoline alkaloid,
was formed. The enantioselectivity of these reactions
was estimated by comparing the specific rotation with a
literature value for the natural (S)-enantiomer ([h]D
−24.9 (4.45, EtOH))16 and was disappointingly low
(Table 4, entry 1, [h]D +1.7 (1.0, EtOH), 7% e.e.).
The results for the addition of phenyllithium to the
3,4-dihydroisoquinoline
dimethoxy-1,2,3,4-tetrahydroisoquinoline 4 are summa-
1
leading to 1-phenyl-6,7-
The enantioselectivity of the phenyllithium addition
reactions to the iminium salt 5 (Table 5) were even
Scheme 2.

1438
M. Chrzanowska, J. Sokołowska / Tetrahedron: Asymmetry 12 (2001) 1435–1440
Table 4. Addition of methyllithium to iminium salt 5
Entry
(−)-Sparteine (mol equiv.)
Solvent
Temperature (°C)
Time (h)
Yield (%)
Config.a
1
2
3
4
1
1
1
1
Et2O
Et2O
Toluene
Toluene
−76“−10
−76
−76“−10
−76
24
2.5
24
24
67b
59
76
(R)-(+)
(R)-(+)
(R)-(+)
(R)-(+)
76
a Confirmed by comparing the specific rotation with a literature value.23
b [h]D +1.7 (1.0, EtOH).
Table 5. Addition of phenyllithium to iminium salt 5
Entry
(−)-Sparteine (mol equiv.)
Solvent
Temperature (°C)
Time (h)
Yield (%)
Config.a
1
2
3
1
0.2
0.2
Et2O
Et2O
Toluene
−76“−10
−76
−76
24
24
24
90
52
52
(R)-(−)
(R)-(−)
(R)-(−)
a Confirmed by comparing the specific rotation with a literature value.22
Scheme 3.
lower than those obtained with methyllithium. The
absolute configuration of the product, 1-phenyl-2-
methyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquinoline 7,
can be suggested to be (R), knowing that the N-methyl-
ation of 1-phenylisoquinoline derivatives is accompa-
nied by a reversal in the sign of the specific rotation.24
Compound 7 has been reported so far as a synthetic
product in enantiopure22 and racemic form25 and has
also recently been isolated as a racemate from the
leaves of Adhatoda vasica Nees.26
butyllithium to imine 1, the (S)-enantiomer was
formed, while phenyllithium addition resulted in forma-
tion of the (R)-enantiomer. Addition reactions to
iminium salt 5 led in all cases to the formation of
(R)-enantiomers. The lower enantioselectivity of reac-
tions with isoquinolinium salt 5 relative to that of imine
1 could be a consequence of steric hindrance at the
prochiral carbon atom caused by the N-methyl
substituent.
Although the enantioselectivity of the above reported
reactions is moderate, the additions of organolithium
reagents to dihydroisoquinoline derivatives 1 and 5 is a
convenient method for the synthesis of optically active
isoquinoline alkaloids: (−)-salsolidine 2, (+)-carnegine 6
and (−)-1-phenyl-2-methyl-6,7-dimethoxy-1,2,3,4-tetra-
hydroisoquinoline 7, as well as optically active iso-
quinoline derivatives 3 and 4. The method presented
can afford a variety of isoquinoline derivatives in one-
step and although in the work presented here (−)-
sparteine was not as effective as in the case of acyclic
imines, the search for a ligand of choice is still highly
desirable.
3. Conclusion
The enantioselective additions of organolithium com-
pounds to the dihydroisoquinoline derivatives 1 and 5,
performed in the presence of (−)-sparteine as an exter-
nal chiral ligand, resulted in the formation of several
optically active 1-substituted 1,2,3,4-tetrahydro-
isoquinoline derivatives. The enantioselectivity of the
addition reaction of the preformed (−)-sparteine–n-
buthyllithium complex to dihydroisoquinoline 1 proved
higher than that obtained when methyl- or phenyl-
lithium was used. In the additions of methyl- and

M. Chrzanowska, J. Sokołowska / Tetrahedron: Asymmetry 12 (2001) 1435–1440
1439
4. Experimental
[hexane/propan-2-ol=9:1; 0.5 mL/min; (S)-isomer
(major) 24.2 min, (R)-isomer 31.9 min].
4.1. General
(−)-3·HCl mp 198–200°C, ( )3·HCl mp 199–202°C
(MeOH–HCl) [lit.19 ( )-3·HCl mp 206–207°C]; 1H
NMR l (CDCl3): 0.94 (t, J=6.8 Hz, 3H, CH3), 1.25–
1.50 (m, 4H, CH2CH2CH2CH3), 1.64–1.86 (m, 2H,
CH2CH2CH2CH3), 1.72 (broad s, 1H, NH, disappeared
with D2O), 2.63–2.80 (m, 2H, ArCH2CH2N), 2.90–3.00
Melting points: determined on a Koffler block and are
not corrected. IR spectra: Perkin Elmer 180 in KBr
pellets. NMR spectra: Varian Gemini 300, in CDCl3,
with TMS as internal standard. Mass spectra (EI):
instrument AM D402. Specific rotation: Perkin Elmer
polarimeter 242B at 20°C. Merck Kieselgel 60 (70–230
mesh) was used for column chromatography and
Merck DC-Alufolien Kieselgel 60254 for TLC. Analyti-
cal HPLC: Waters HPLC system with Mallinkrodt-
Baker Chiracel OD-H column. (−)-Sparteine was
purchased from the Aldrich Chemical Co., dried over
KOH and distilled under reduced pressure. MeLi, n-
BuLi and PhLi were purchased from the Aldrich Chem-
ical Co.
(m,
1H,
ArCH2CH2N),
3.18–3.27
(m,
1H,
ArCH2CH2N), 3.85 (s, 3H, OCH3), 3.86 (s, 3H, OCH3),
3.87–3.92 (m, 1H, C(1)-H), 6.57 (s, 1H, ArH), 6.62 (s,
1H, ArH); MS m/z (%): 249 (M+, 5.6), 248 (6.3), 192
(100), 176 (23.3), 148 (7.2), 131 (4.3).
4.1.3. Addition of phenyllithium to 6,7-dimethoxy-3,4-
dihydroisoquinoline 1. Synthesis of (+)-1-phenyl-6,7-
dimethoxy-1,2,3,4-tetrahydroisoquinoline 4. The reaction
was carried out in the same way as for MeLi and
n-BuLi addition on 1 mmol scale. PhLi [1.8 M solution
in cyclohexane/ether (7:3) 1.3 mL] was used.
4.1.1. Addition of methyllithium to 6,7-dimethoxy-3,4-
dihydroisoquinoline 1. Synthesis of (−)-salsolidine 2. (−)-
Sparteine (234 mg, 1 mmol) was dissolved in dry
toluene (10 mL) under an argon atmosphere and the
mixture cooled to −76°C. MeLi (1.6 M solution in
Et2O, 2 mL) was added and the solution was stirred for
30 min at the same temperature. A solution of 6,7-
Pure amine
(dichloromethane/methanol,
4
after column chromatography
200:1“100:1) was
obtained as a solid; mp 131–133°C (methanol–propan-
2-ol) [lit.22 mp 132°C], [h]D +3.1 (0.96, CHCl3) {lit.22
[h]D −20.0 (0.50, CHCl3)}; 17% e.e. by HPLC [hexane/
propan-2-ol=85:15; 0.5 mL/min; (S)-isomer 26.7 min,
(R)-isomer (major) 37.6 min]. Spectral data were identi-
cal to those reported for the racemic compound.22,25
dimethoxy-3,4-dihydroisoquinoline 127 (191 mg,
1
mmol) in toluene (10 mL) was introduced dropwise.
Stirring was continued for 2 h at −76°C and the reac-
tion mixture was allowed to warm to −15°C and stirred
at this temperature for a further 18 h. The reaction was
quenched with 20% aqueous NH4Cl at low tempera-
ture. When the mixture reached room temperature, the
phases were separated and the aqueous was extracted
with diethyl ether until the Dragendorff test was nega-
tive. The combined organic extracts were dried and the
solvents removed under reduced pressure yielding an oil
(268 mg). The crude amine 2 was purified by column
chromatography (dichloromethane/methanol, 50:1).
Pure salsolidine 2 (77%) obtained was identical to ( )-2
in terms of spectral data28 as well as TLC comparison.
30.4% e.e. by HPLC [hexane/propan-2-ol=4:1; 0.5 mL/
min; (S) isomer (major) 22.4 min, (R) isomer 29.1 min].
4.1.4. Addition of methyllithium to 6,7-dimethoxy-3,4-
dihydroisoquinolinium methiodide 5. Synthesis of (+)-
carnegine 6. The reaction was carried out similarly to
the above on 1 mmol sale. To the preformed (−)-
sparteine–methyllithium complex,
a suspension of
iminium salt 527 in the solvent (see Table 4) was added.
Work-up was the same as for amine 2. Pure (+)-
carnegine 6 was obtained after column chromatography
(dichloromethane; dichloromethane/methanol, 100:1“
50:1) as an oil. 6·HCl mp 214–218°C (MeOH–HCl)
[lit.29 mp 209–211°C (EtOH)].
Spectral data of (+)-carnegine 6 obtained were identical
When the aqueous solution remaining from extraction
of salsolidine 2 was rendered strongly alkaline (pH=ꢀ
13) with 25% aqueous KOH and extracted with diethyl
ether, dried and evaporated, an additional amount of
amines (98 mg) was obtained which consisted mainly of
(−)-sparteine. Some (−)-sparteine was also recovered
to those reported in the literature.28,29
4.1.5. Addition of phenyllithium to 6,7-dimethoxy-3,4-
dihydroisoquinolinium methiodide 5. Synthesis of (−)-1-
phenyl-2-methyl-6,7-dimethoxy-1,2,3,4-tetrahydroisoquin-
oline 7. The reaction was carried out as in the case of
carnegine 6. PhLi [1.8 M solution in cyclohexane/ether
(7:3), 1.2 mL] was used for formation of the complex
with (−)-sparteine. Pure amine 7 was eluted from a
chromatographic column with dichloromethane. Mp
90–92°C (ether–hexane); lit. mp 85–86°C,22 74–76°C
(hexane).25 The product had identical spectral data to
those reported in the literature.22,26
from
column
chromatography
purification
(dichloromethane/methanol, 20:1“10:1).
4.1.2. Addition of n-butyllithium to 6,7-dimethoxy-3,4-
dihydroisoquinoline 1. Synthesis of (−)-1-n-butyl-6,7-
dimethoxy-1,2,3,4-tetrahydroisoquinoline 3. The reaction
was carried out in the same way as in the case of
methyllithium addition, on 1 mmol scale. n-Butyl-
lithium (1.6 M solution in hexane, 1.4 mL) was used.
Acknowledgements
Pure amine
3
after column chromatography
(dichloromethane/methanol, 100:1“50:1) was obtained
as an oil. [h]D −20.3 (0.73, CHCl3); 46.5% e.e. by HPLC
This work was financially supported by KBN grant No.
3 T09A 027 17.

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15. Bro´zda, D.; Chrzanowska, M.; Gluszynska, A.; Rozwad-
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