Total synthesis of (R)-(+)-salsolidine by hydride addition to (R)-N-tert-butanesulfinyl ketimine

Article abstract of DOI:10.1016/j.tetasy.2007.01.033

(R)-(+)-Salsolidine 1 of high enantiomeric purity was synthesized using the Pomeranz-Fritsch-Bobbitt methodology, in which the reduction (NaBH₄, DIBAL-H) of N-tert-butanesulfinyl ketimine, derived from 3,4-dimethoxyacetophenone, was the key ste

Full text of DOI:10.1016/j.tetasy.2007.01.033

Tetrahedron: Asymmetry 18 (2007) 557–561
Total synthesis of (R)-(+)-salsolidine by hydride addition
to (R)-N-tert-butanesulfinyl ketimine
Agnieszka Grajewska and Maria D. Rozwadowska*
Faculty of Chemistry, A. Mickiewicz University, ul. Grunwaldzka 6, 60-780 Poznan´, Poland
Received 19 January 2007; accepted 29 January 2007
Available online 27 February 2007
Abstract—(R)-(+)-Salsolidine 1 of high enantiomeric purity was synthesized using the Pomeranz–Fritsch–Bobbitt methodology, in which
the reduction (NaBH4, DIBAL-H) of N-tert-butanesulfinyl ketimine, derived from 3,4-dimethoxyacetophenone, was the key step. The
synthesis was completed in a sequence of reactions involving the removal of the chiral auxiliary, N-alkylation with 2,2-diethoxyethyl
bromide and final cyclization/hydrogenolysis.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
O
O
O
O
R
Nu
S
S
S
[H-]
R2M
∗
M
∗
∗
N
R
N
R1
R
HN
R1
R
Chiral non-racemic N-sulfinimines are versatile precursors
for the asymmetric synthesis of amines, providing easy
access to chiral building blocks for the construction of
nitrogen-containing natural products and biologically
active compounds. The increasingly significant role of
N-sulfinimines as substrates in asymmetric synthesis, in
particular that of sulfinimines developed by Davis et al.1
and Ellman et al.,2 has been reported in several review arti-
cles.1–3
N
RL
S
R1 R2
H
RS
R2
H
Figure 1.
Surprisingly, not many examples concerning the 1,2-addi-
tion of hydride ions to N-sulfinyl ketimines have been
reported in the literature,4–12 which is in significant con-
trast to a large number of reactions involving N-sulfinyl
aldimines and organometallic reagents.1–3
Among the many types of transformations of N-sulfin-
imines, the diastereoselective 1,2-addition of nucleophilic
reagents to the imine double bond to afford a-branched
chiral N-sulfinyl-protected amines is one of the most popu-
lar methods for the preparation of this class of amines. An
important factor of this chemistry is the possibility of con-
trolling the steric course of the addition to access the amine
with a desired configuration of the newly generated a-stereo-
genic center.
In the majority of experiments concerning the hydride
reduction of N-sulfinyl ketimines, efforts have been made
to access any of the diastereomeric reduction products
using the same enantiomer of either (R)- or (S)-sulfinimine.
One solution to this problem was the appropriate selection
of the reducing agents. In the reduction of ketimines, the
following pairs of reductors have been shown to give
products with a reversal of diastereselectivity: L-Selectride
versus NaBH4,5–7 L-Selectride versus DIBAL-H,8–10
L-Selectride versus 9BBN,10 catecholborane versus
LiBHEt3.11,12 Another possibility was to introduce an addi-
tional chelating agent to the reduction system9 or to vary
the reaction conditions.7–9
From a synthetic point of view, the synthesis of a-substi-
tuted amines with a tertiary a-carbon can be accomplished
in two ways: either by the addition of organometallic
reagents (preferably RMgX, RLi) to N-sulfinyl aldimines,
or by hydride reduction of the corresponding N-sulfinyl
ketimines (Fig. 1).
Another concept that allowed both diastereomeric amines
to be obtained in this type of synthesis was based on the
reversal of the order of introduction of the substituents
to C-a. Such an approach was applied by Ellman et al.13
*
Corresponding author. Tel.: +48 61 8291322; fax: +48 61 8658008;
e-mail: mdroz@amu.edu.pl
0957-4166/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2007.01.033

558
A. Grajewska, M. D. Rozwadowska / Tetrahedron: Asymmetry 18 (2007) 557–561
in the synthesis of bisabolene derivatives: one diastereo-
meric addition product was obtained in the reaction of
an N-sulfinyl aldimine with an organometallic reagent,
while the other by a hydride reduction of the corresponding
N-sulfinyl ketimine. However, this strategy can be applied
only when both types of reactions proceed via a transition
state in which both nucleophilic species are delivered from
the same face of the C@N double bond. Such a situation
exists in the additions of Grignard reagents to aldimines
on the one hand and NaBH4 or DIBAL-H reduction of
ketimines on the other. In both cases, a six-membered che-
lated cyclic transition state has been proposed (Fig. 1). In
this model, the metal is coordinated to the sulfinyl oxygen
while the sulfur substituent and the larger group on the
imine moiety are in pseudo-equatorial positions, thus forc-
ing the nucleophile to approach from the less hindered site,
which is that occupied by the larger substituent, securing a
high degree of diastereoselectivity.
formed the diastereoselective synthesis of (S)-(ꢀ)-salsol-
idine ent-1 of high enantiomeric purity (98% ee) applying
as the key step the addition of methyl magnesium bromide
to the corresponding (R)-tert-butanesulfinyl aldimine.14
Herein we report the synthesis of the enantiomeric
(R)-(+)-salsolidine 1 performed by reduction (DIBAL-H
or NaBH4) of (R)-tert-butanesulfinyl ketimine
2
(Scheme 2).
The synthesis was initiated by condensation of 3,4-dimeth-
oxyacetophenone with (R)-tert-butanesulfinyl amide in the
presence of Ti(IV) salts according to the Ellman proce-
dure.18 Depending on the reaction conditions and the ratio
of the reagents used (Table 1), oily ketimine 2,
[a]D = +22.2, isolated as a single isomer, was produced in
yields ranging from 0% with Ti(OiPr)4 (entry 1) to 79%
(entry 4) when in the reaction a mixture of sulfinamide/
ketone/Ti(OEt)4 at the ratio: 0.91/1/1.97 in THF was
heated at reflux for 14 h.
Herein we report the synthesis of (R)-(+)-salsolidine 1, per-
formed by the Pomeranz–Fritsch–Bobbitt methodology, in
which hydride addition to N-tert-butanesulfinyl ketimine
was the key step. This synthesis was undertaken as being
complementary to the previous one,14 in which Grignard
addition to N-tert-butanesulfinyl aldimine afforded the
enantiomeric (S)-(ꢀ)-salsolidine ent-1.
Ketimine 2 was then used in the key step of the synthesis, in
the hydride addition to the imine C@N double bond. Since
we were interested in obtaining access to the enantiomeric
a-substituted amine 3 from the same N-sulfinyl chiral
directing group as used in the Grignard addition,14 NaBH4
or DIBAL-H seemed to be the reagent of choice. To opti-
mize the reaction conditions, several experiments using
these reagents were carried out with the results shown in
Table 2.
2. Results and discussion
Our stereoselective modifications of the Pomeranz–Frit-
sch–Bobbitt synthesis of isoquinoline alkaloids performed
so far, involved as the crucial step, the enantioselective15,16
and diastereoselective17 addition of carbon nucleophiles to
a prochiral ‘Pomeranz–Fritsch imine’ and further elabora-
tion to the tetrahydroisoquinoline ring system by acid-cat-
alyzed cyclization/hydrogenolysis of the chiral addition
products (Scheme 1). In this way (S)-(ꢀ)-salsolidine and
(S)-(ꢀ)-carnegine with moderate enantiomeric excess
(46% and 36%)15,16 and both enantiomers of O-methyl-
bharatamine were prepared with satisfactory ee (73% and
88%).17
In most cases, sulfinamide 3 was prepared in satisfactory
yield (except for entry 4) and diastereoselectivity, irrespec-
tive of the reducing agent used, NaBH4 or DIBAL-H, tak-
ing into account a reasonable compromise between the
yield and diastereoselectivity. Moreover, in all cases, the
diastereomer characterized by a shorter retention time
(HPLC) was produced as the major isomer. Reduction
products from three experiments (entries 1, 2, and 5) were
combined and recrystallized from isopropyl ether/hexane
to afford sulfinamide 3 with 98:2 dr, mp 79–81 °C,
[a]D = ꢀ36.3 which was used in the next step of the synthe-
sis. Compound 3 could be also prepared in the one-pot
procedure reported by Ellman et al.,7,19 with a less
satisfactory diastereoselectivity (ca. 70:30 dr).
Recently, as a continuation of this study, being interested
in the synthetic potential of chiral N-sulfinylimines, we per-
CH3O OCH3
CH3O
CH3O
MeLi
CH3O
CH3O
CH3O
CH3O
N
NH
CH3
NR
CH3
chiral
ligand
CH3O OCH3
Pomeranz-Fritsch
amine
(S)-(-)-salsolidine (R=H)
(S)-(-)-carnegine (R=CH3)
Pomeranz-Fritsch imine
O
CH3
Bn
∗
N
O
CH3O OCH3
CH3O
CH3O
CH3O
CH3O
N
N
O
∗
∗
O-Methylbharatamine
Scheme 1.

A. Grajewska, M. D. Rozwadowska / Tetrahedron: Asymmetry 18 (2007) 557–561
559
CH3O
CH3O
CH3O
CH3O
CH3O
CH3O
O
S
[H]
Ti(OEt)4
THF
H
N
+
N
O
CH3
S
S
THF
H2N
2
3
4
CH
3 O
CH
3 O
HCl
EtO OEt
NH
CH3O
CH3O
CH3O
CH3O
CH3O
CH3O
BrCH2CH(OEt)2
CH2Cl2
TFA
NaBH4
.
NH
CH3
NH2 HCl
1
5
CH3
CH3
Scheme 2.
Table 1. Condensation of 3,4-dimethoxyacetophenone with (R)-t-butanesulfinamide
Entry
t-BuSONH2
(mmol)
Ketone
(mmol)
Ti(OR)4 (mmol)
THF (ml)
Time of reaction
at reflux (h)
Yield (%)
1
2
3
4
1
1.79
2
1
3.58
2
Ti(OiPr)4 (2.5)
Ti(OEt)4 (3.58)
Ti(OEt)4 (4)
3
7
8
2
4
13
10
14
0
24
28
79
0.91
1
Ti(OEt)4 (1.97)
Table 2. Reduction (DIBAL-H, NaBH4) of sulfinyl ketiminea
2
Entry
2 (mmol)
Reducting agent (mmol)
Temperature (°C)
Time (h)
Yield (%)
drb (%)
1
2
3
4
5
1
1
1
1
1
DIBAL-H (2.3)
DIBAL-H (2.3)
NaBH4 (4)
NaBH4 (4)
NaBH4 (4)
ꢀ22
0.45
1.5
120
24
89 (Crude)
87
90
58
95:5
98:2
93:7
92:8
96:4
ꢀ68
ꢀ40
ꢀ35 ! ꢀ17
ꢀ50c ! rt
24
83 (Crude)
a THF as solvent.
b After column chromatography. Evaluated by HPLC: Chiralcel OD-H, iPrOH/hexane 10:90, 0.5 ml/min. The retention time of the minor diastereoisomer
was the same as that of the starting ketimine 2.
c No reaction at ꢀ50 °C.
Upon treatment of sulfinamide 3 with hydrochloric acid at
0 °C, followed by stirring of the mixture at rt for 2h, the
primary amine 4 was isolated as a hydrochloride salt,
4ÆHCl, mp 202–203 °C, [a]D = +5.8 (lit.14 for ent-4ÆHCl
mp 201–203 °C, [a]D = ꢀ6.9), in 89% yield. The (R)-config-
uration of the new stereocenter in 3 and 4 was deduced
only at the end of the synthesis on the basis of the positive
sign of the specific rotation of the target alkaloid, (+)-sal-
solidine 1 for which the (R)-configuration had been previ-
ously established.20
of ketimine 2. The target alkaloid was obtained in 20%
overall yield and with 95.5% ee, in a five-step reaction
sequence. The result is similar to that reached in our previous
synthesis14 of the enantiomeric (S)-(ꢀ)-salsolidine ent-1 in
which methyl magnesium bromide was added to the corre-
sponding (R)-tert-butanesulfinyl aldimine. The synthesis
described here, and the previous one,14 are examples illus-
trating the opportunity to prepare the desired enantiomeric
addition product using the same chiral auxiliary, by choos-
ing the proper synthetic strategy, either the ‘Grignard’ or
‘hydride’ approach.
Amine 4 was then alkylated with bromoacetaldehyde
acetal/potassium carbonate in acetonitrile at reflux to
give the ‘Pomeranz–Fritsch amine’ 5, oil, [a]D = +28.0, in
moderate yield (58%).
4. Experimental
4.1. General
The tetrahydroisoquinoline nitrogen-containing ring was
closed by a one-pot two-step procedure involving sequen-
tial treatment of aminoacetal 5 with 6 M hydrochloric acid
for 20 h followed by NaBH4/TFA reduction to afford (+)-
salsolidine 1 in 58% yield with 95.5% ee.
Melting points: determined on a Koffler block and are not
corrected. IR spectra: Perkin–Elmer 180 in KBr pellets.
NMR spectra: Varian Gemini 300, with TMS as internal
standard. Mass spectra (EI): instrument AM D402. Spe-
cific rotation: Perkin–Elmer polarimeter 243B at 20 °C.
Analytical HPLC: Waters HPLC system with Mallin-
krodt–Baker Chiralcel OD-H column. Merck DC-Alufo-
lien Kieselgel 60254 were used for TLC. Merck Kieselgel
60 (70–230 mesh) were used for column chromatography.
3. Conclusion
The diastereoselectivity of the Pomeranz–Fritsch–Bobbitt
synthesis of isoquinoline alkaloid (R)-(+)-salsolidine 1,
was accomplished in the key step, by using a (R)-tert-but-
anesulfinyl chiral auxiliary attached to the nitrogen atom
THF was freshly distilled from LiAlH4. Ti(OiPr)4 was
purchased from Fluka. Ti(OEt)4, tert-butanesulfinamide,

560
A. Grajewska, M. D. Rozwadowska / Tetrahedron: Asymmetry 18 (2007) 557–561
NaBH4, and DIBAL-H were purchased from Aldrich and
used as received.
1H, CHCH3), 6.82–6.92 (m, 3H, ArH). 13C NMR (CDCl3):
22.70, 22.75, 53.84, 55.43, 55.99, 110.26, 111.38, 118.56,
136.61, 148.59, 149.10. EI MS m/z (%): 285 (M+, 0.6),
229 (6), 213 (2), 179 (2), 165 (100), 150 (5). Anal. Calcd
for C14H23NSO3: C, 58.92; H, 8.13; N, 4.91; S, 11.21.
Found: C, 58.94; H, 8.15; N, 4.88; S, 10.95.
4.2. (R)-(+)-N-[1-(3,4-Dimethoxyphenylethylidene)]-2-
methylpropanesulfinamide 2
To a solution of 3,4-dimethoxyacetophenone (180 mg,
1 mmol) and tert-butanesulfinamide (110 mg, 0.9 mmol)
in anhydrous THF (2 ml), Ti(OEt)4 (0.41 ml, 1.97 mmol,
technical grade, ꢁ20% Ti) was added. The mixture was
heated at 60–70 °C for 14 h under an argon atmosphere.
Water (5 ml) was added on rapid stirring, then the reaction
mixture was filtered through a pad of CeliteÒ, the filter
cake was washed with CH2Cl2 and the phases were sepa-
rated. The organic phase was washed with brine, dried over
Na2SO4, and the solvent was evaporated. The crude reac-
tion product was purified by silica gel column chromato-
graphy with CH2Cl2/MeOH (100:0.2). Yield: 79%; green
oil; [a]D = +22.2 (c 1.12, CHCl3). IR (KBr), m (cmꢀ1):
1567, 1517, 1272. 1H NMR (CDCl3): d 1.32 (s, 9H,
C(CH3)3), 2.74 (s, 3H, CH3C@N), 3.92 (s, 3H, OCH3),
3.94 (s, 3H, OCH3), 6.88 (d, J = 8.5 Hz, 1H, ArH), 7.46
(dd, J = 2 Hz, J = 8.5 Hz, 1H, ArH), 7.58 (d, J = 2 Hz,
1H, ArH). 13C NMR (CDCl3): 19.6, 22.5, 55.8, 55.9,
57.18, 109.6, 109.9, 121.3, 131.3, 148.6, 152.2, 175.4. EI
MS m/z (%): 283 (M+, 1), 267 (0.7), 227 (73), 179 (100),
164 (25). Anal. Calcd for C14H21NSO3: C, 58.97; H, 7.25;
N, 4.67; S, 11.57. Found: C, 59.34; H, 7.42; N, 4.95; S,
11.29.
4.4. (R)-(+)-1-(3,4-Dimethoxyphenyl)ethylamine
hydrochloride 4ÆHCl
Sulfinamide 3 (100 mg, 0.35 mmol) was dissolved in EtOH
(2 ml) and the solution was cooled to 0 °C. Concentrated
HCl (0.1 ml) was added and the reaction mixture was stir-
red at room temperature for 2 h then saturated K2CO3
(7 ml) was added and the reaction mixture was extracted
twice with ethyl acetate. The combined organic extracts
were dried over Na2SO4 and the solvent was evaporated.
To the crude reaction product 3% solution of HCl in
MeOH (2 ml) was added and after 1 h the solvents were
evaporated. The residue was washed a few times with ethyl
acetate and dried in air providing a white solid (68 mg, 89%
yield); mp 202–203 °C, [a]D = +5.8 (c 0.86, MeOH). Its
spectral characteristics corresponded to that of (S)-(ꢀ)-
enantiomer.14
4.5. (R)-(+)-N-(2,2-Diethoxyethyl)-1-(3,4-dimethoxy-
phenyl)ethylamine 5
To a solution of amine 4 (45 mg, 0,24 mmol) prepared from
the amine hydrochloride 4ÆHCl by treatment with 20%
NaOH, in MeCN (2 ml), anhydrous K2CO3 (56 mg,
0.4 mmol) was added followed by 2-bromo-1,1-diethoxy-
ethane (0.27 mmol, 0.04 ml). The reaction mixture was
heated under an argon atmosphere at 82 °C for 25 h. On
heating, two additional amounts of K2CO3 and 2-bromo-
1,1-diethoxyetane were added. After completion of the
reaction, the mixture was filtered off and the solvent evap-
orated providing 200 mg of an oil, which was purified by
silica gel column chromatography with CH2Cl2/MeOH
(100:0.6). Yield: 58%; [a]D = +28.0 (c 0.66, CHCl3). Its
spectral characteristics corresponded to that of (S)-(ꢀ)-
enantiomer.14
4.3. (R,R)-(ꢀ)-N-[1-(3,4-Dimethoxyphenylethyl)]-2-
methylpropanesulfinamide 3
4.3.1. Reduction with NaBH4. A solution of ketimine 2
(39 mg, 0.14 mmol) in 0.3 ml of THF was cooled to
ꢀ40 °C. Sodium borohydride (22 mg, 0.14 mmol) was then
added and the reaction mixture stirred at ꢀ40 °C for 120 h.
MeOH (2 ml) was added at ꢀ40 °C, followed by brine
(3 ml) at room temperature. The reaction mixture was
extracted three times with ethyl ether, and the combined
organic extracts were dried over Na2SO4. The solvent
was evaporated to give 36 mg of colorless oil. The crude
product was purified by silica gel column chromatography
with CH2Cl2/MeOH (100:0.25). Yield: 90%, dr: 93:7.
4.6. (R)-(+)-Salsolidine 1
4.3.2. Reduction with DIBAL-H. A solution of ketimine 2
(30 mg, 0.1 mmol) in THF (1 ml) under an argon atmo-
sphere was cooled to ꢀ68 °C and DIBAL-H (1 M solution
in hexane, 0.23 ml, 0.23 mmol in 0.5 ml of THF) was then
added. The reaction mixture was stirred at ꢀ68 °C for
1.5 h, then, when room temperature was reached, the sol-
vents were evaporated. The residue was treated with 10%
NaOH (3 ml) and extracted three times with ethyl ether.
The combined organic extracts were dried over Na2SO4
and the solvent was evaporated to provide 36 mg of color-
less oil. The crude product was purified by silica gel column
chromatography with CH2Cl2/MeOH (100:0.25). Yield:
87%, dr: 98:2. [a]D = ꢀ36.3 (c 0.32, CHCl3); mp 79–81 °C
(from iPr2O/hexane). IR (KBr), m (cmꢀ1): 2974, 1520,
A solution of aminoacetal 5 (40 mg, 0.13 mmol) and 5 M
aqueous HCl (1.3 ml) was stirred overnight at rt. The mix-
ture was basified with 20% aqueous NaOH and extracted
five times with dichloromethane. The combined organic
extracts were dried over Na2SO4 and evaporated to give
a solid (25 mg), which was dissolved in dichloromethane
(6 ml) and cooled to 0 °C. To this solution, NaBH4
(66 mg, 1,7 mmol) was added along with slow introduction
of trifluoroacetic acid (0.93 ml, 12.2 mmol) in dichloro-
methane (2 ml). The reaction mixture was stirred at rt for
24 h, then the solvent was removed under reduced pressure.
The resultant precipitate was dissolved in water (4 ml),
basified with 20% aqueous NaOH and extracted three
times with dichloromethane. The organic extract was dried
over Na2SO4 and evaporated to provide 25 mg of a yellow
oil which was purified by silica gel chromatography with
CH2Cl2/MeOH (100:0.6). Yield: 58%, ee: 95.5% (HPLC:
1
1266, 1063. H NMR (CDCl3): d 1.24 (s, 9H, C(CH3)3),
1.51 (d, J = 6.3 Hz, 3H, CH3CH), 3.37 (br s, 1H, NH),
3.88 (s, 3H, OCH3), 3.89 (s, 3H, OCH3), 4.47–4.53 (m,

A. Grajewska, M. D. Rozwadowska / Tetrahedron: Asymmetry 18 (2007) 557–561
561
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Acknowledgements
This work was supported by a research Grant from the
State Committee for Scientific Research for 2003–2006
(KBN Grant No. 4T09A 07824).
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