Stereoselective synthesis of (-)-galantinic acid

Article abstract of DOI:10.1055/s-0030-1258431

An efficient and highly concise synthesis of (-)-galantinic acid has been achieved using an asymmetric allylation reaction of Garner's aldehyde. Georg Thieme Verlag Stuttgart - New York.

Full text of DOI:10.1055/s-0030-1258431

PAPER
901
Stereoselective Synthesis of (–)-Galantinic Acid
S
A
tereoselective
S
y
b
nthesis of (–)
h
-Galantinic
A
i
cid shek Dubey, Anand Harbindu, Pradeep Kumar*
Division of Organic Chemistry, National Chemical Laboratory, Pune 411008, India
Fax +91(20)25902050; E-mail: pk.tripathi@ncl.res.in
Received 28 December 2010; revised 19 January 2011
As a part of our research programme aimed at developing
Abstract: An efficient and highly concise synthesis of (–)-galantin-
ic acid has been achieved using an asymmetric allylation reaction of
Garner’s aldehyde.
4
new synthetic methodologies and their application to-
5
6
wards the synthesis of lactones and amino alcohols, we
became interested in devising a new route to (–)-galantin-
ic acid based on a diastereoselective iterative allylation
protocol.
Key words: chiral pool, amino alcohol, iterative asymmetric allyla-
tion, oxidation, stereoselective synthesis
In our synthetic approach towards galantinic acid (1),
Garner’s aldehyde was used as the starting material
(
–)-Galantinic acid (1), a non-proteinogenic e-amino acid,
(
(
Scheme 1). Commercially available Garner’s aldehyde
4) was subjected to asymmetric allylation using an ally-
is a component of the peptide antibiotic galantin I (3), iso-
lated from the culture broth of Bacillus puluvifaciens.1
The originally proposed structure of galantinic acid 2 was
later proved to be incorrect and revised to 1 by Ohfune and
7
lating reagent prepared from (R,R)-5 (Figure 2) and allyl-
magnesium chloride at –78 °C to give homoallylic
alcohol, syn 6a and anti 6b in excellent yield with diaste-
reomeric ratio 19:1; 6a and 6b were easily separated by
column chromatography. Our next aim was to synthesize
the 1,3-anti alcohol 10a. Towards this end, the hydroxy
group of homoallylic alcohol 6a was first protected as the
tert-butyldimethylsilyl ether (6a → 7) followed by oxida-
2
a
co-workers, who also reported its first total synthesis
2
b
(Figure 1).
Due to its potent biological activity and unique structure
with an array of functionalities, (–)-galantinic acid has
been a synthetic target of considerable interest, and, there-
fore, a great deal of effort has been devoted towards its
8
tive cleavage of the double bond by using sodium perio-
3
synthesis. The synthetic approaches described so far for
date, 2,6-lutidine, and osmium tetroxide to furnish the
aldehyde 8 in excellent yield. With aldehyde 8 in hand, the
stage was now set to generate another hydroxy center via
1
involve either chiral pool materials, particularly
3
a–e
3f,g
serine, and mannitol, asymmetric catalysis, or chiral
induction to establish one or more of stereogenic
9
iterative allylation. Accordingly compound 8 was sub-
3
h–j
centers present in the molecule. Most of these methods
suffer either from a large number of steps, overall low
yields, or low stereoselectivity. Therefore, a practical,
concise and expeditious high yielding synthesis of this tar-
get molecule is still desirable.
jected to asymmetric allylation using the allylating re-
agent derived from (S,S)-5 and allylmagnesium chloride.7
However, the diastereoselectivity obtained was only 8:1
in favor of the anti-isomer. Therefore, in order to improve
the selectivity we next attempted at the asymmetric ally-
OH
OH
2
H N
O
O
HOOC
OH
HO
OH
NH2
originally proposed
structure
revised structure
galantinic acid
galantinic acid
(
1)
(2)
H2N
OH OH
O
O
OH
OH
Me
NH
H
H
N
H2N
N
N
N
O
O
H
H
H
H
NH2
OH
O
O
N
N
NH2
N
N
H
NH2
HO
H
O
galantin I
3)
(
Figure 1 Structures of galantinic acid (1), galantin I (3), and originally proposed structure of galantinic acid 2
SYNTHESIS 2011, No. 6, pp 0901–0904
x
x
.
x
x
.2
0
1
1
Advanced online publication: 10.02.2011
DOI: 10.1055/s-0030-1258431; Art ID: T57410SS
Georg Thieme Verlag Stuttgart · New York
©

9
02
A. Dubey et al.
PAPER
OH
OH
CHO
NBoc
(a)
O
O
+
O
NBoc
NBoc
6b
4
6a
19:1
OTBS
OTBS
OTBS OH
OTBS OH
(
b)
(c) (i)
(c) (ii)
6
1
a
O
+
O
O
O
O
NBoc
NBoc
NBoc
NBoc
7
8
10a
13:1
10b
OTBS OTBS
OTBS OTBS
OH OH
(
d)
(f)
(
e)
COOH
COOH
0a
HO
O
O
NBoc
NBoc
NH2
11
12
1
Scheme 1 Reagents and conditions: (a) (R,R)-5, allylmagnesium chloride, Et O, –78 °C, 93%; (b) TBSCl, imidazole, CH Cl , 0 °C to r.t.,
2
2
2
9
5%; (c) (i) NaIO , 2,6-lutidine, OsO , dioxane–H O (3:1), r.t., 90%; (ii) (+)-Ipc BCH CH=CH 9, –78 °C, 86%; (d) TBSCl, imidazole, CH Cl ,
4 4 2 2 2 2 2 2
0
°C to r.t., 90%; (e) RuCl ·3 H O, NaIO , CCl –MeCN–H O (2:2:3), r.t., 80%; (f) (i) TFA, CH Cl , r.t., 25 min; (ii) Dowex 50Wx4 (elution
3
2
4
4
2
2
2
with aq 1 M NH ), 75%.
3
lation reaction using (+)-allyldiisopinocampheylborane In conclusion, we have accomplished a very concise and
1
0
(
9) (Figure 2). To our delight, the reaction gave good di- practical synthesis of (–)-galantinic acid using a double
astereoselectivity (10a/10b, 13:1) and good yields. Both asymmetric allylation in an overall 37% yield starting
diastereomers were separated nicely by column chroma- from Garner’s aldehyde as a chiral pool starting material.
tography.
All reactions were carried out under argon or N in oven-dried
2
glassware using standard gas-light syringes, cannulas and septa.
Melting points are uncorrected. Solvents and reagents were purified
and dried by standard methods prior to use. Optical rotations were
Ti
Cl
O
Ph
O
B
measured at r.t. at the sodium D line on a Jasco-181 digital polarim-
1
O
eter. IR spectra were recorded on an FT-IR instrument. H NMR
Ph
spectra were recorded at 200, 400, and 500 MHz relative to CDCl
3
Ph
1
3
as internal standard and C NMR spectra were recorded at 50, 100,
Ph
2
O
and 125 MHz relative to CDCl . Column chromatography was per-
3
formed on silica gel (100–200 and 230–400 mesh) using an eluent
mixture of petroleum ether (PE) and EtOAc. Microanalytical data
were obtained using a Carlo–Erba CHNS-O-EA 1108 elemental an-
alyzer.
(
R,R)-5
9
Figure 2 Structures of (R,R)-5 and allylating borane reagent 9
Having synthesized the desired anti-1,3-diol framework, tert-Butyl (S)-4-[(S)-1-Hydroxybut-3-enyl)-2,2-dimethyloxazo-
lidine-3-carboxylate (6a)
A 0.8 M soln of allylmagnesium chloride in THF (5.6 mL, 4.5
our next task was to oxidize the olefin to a carboxylic
group followed by further synthetic manipulation to
achieve the synthesis of target compound 1. To this end,
the newly formed hydroxy group in 10a was first protect-
mmol) was added dropwise over 10 min at 0 °C under argon to a
5
soln of (R,R)-(h -cyclopentadienyl)chloro[(4R,trans)-2,2-dimeth-
yl-a,a,a¢,a¢-tetraphenyl-1,3-dioxolane-4,5-dimethanolato]titanium
ed as the tert-butyldimethylsilyl ether to give 11 in excel-
(
5; 3.06 g, 5.0 mmol) in Et O (60 mL). After stirring at 0 °C for 1 h,
2
lent yield. Subsequent oxidative cleavage of the olefin the mixture was cooled to –78 °C, and a soln of tert-butyl (S)-4-
using ruthenium(III) chloride and sodium periodate af- formyl-2,2-dimethyloxazolidine-3-carboxylate (4; 917 mg, 4.0
mmol) in Et O (5 mL) was added over 5 min. Stirring was continued
forded the desired acid 12. Finally, global deprotection of
all acid-labile protecting groups with trifluoroacetic acid
followed by treatment with Dowex 50Wx4 (eluent, aq 1
2
at –78 °C for 3 h. The mixture was then treated with aq 45% NH4F
soln (20 mL), stirred at r.t. for 12 h, filtered through Celite, and ex-
tracted with Et O (2 × 50 mL). The combined organic phases were
washed with brine, dried (Na SO ), and concentrated under reduced
2
M NH ) furnished (–)-galantinic acid (1) in 75% yield.
3
2
4
The physical and spectroscopic data of 1 were in complete
pressure; d.r. greater than 19:1 (syn/anti) (integration of the carbon
25
agreement with those described in literature {[a] –29.7 signals of the newly formed chiral center). These two diastereomers
D
2
b
25
(
c 0.5, H O); Lit. [a] –29.4 (c 0.5, H O)]. The overall were separated by chromatography (silica gel, gradient 20–25%
2
D
2
EtOAc–PE) to give homoallylic alcohol 6a (1.0 g, 93%) as a color-
yield of the target compound 1 was found to be 37% in
seven steps. Thus our synthesis of 1 proved to be efficient
in terms of overall yield in comparison to the literature
methods reported to date.
less syrupy liquid.
a]D25 –17.2 (c 0.95, CHCl ) [Lit. [a] –17.0 (c 0.95, CHCl )].
7
25
[
3
D
3
–
1
IR (neat): 3420, 2980, 2800, 1705, 1653, 1380, 1360 cm .
Synthesis 2011, No. 6, 901–904 © Thieme Stuttgart · New York

PAPER
Stereoselective Synthesis of (–)-Galantinic Acid
903
1
H NMR (400 MHz, CDCl ): d = 1.45 (s, 6 H), 1.49 (s, 9 H), 2.21–
EtOAc–PE) to give anti-homoallylic alcohol 10a (0.85 g, 86%) as
an oily liquid.
3
2
.49 (m, 2 H), 2.85 (br s, 1 H), 3.50–4.09 (m, 4 H), 5.05–5.21 (m, 2
H), 5.69–5.92 (m, 1 H).
25
[
a]D –30.4 (c 0.5, CHCl3).
1
3
C NMR (100 MHz, CDCl ): d = 26.2, 28.3, 36.2, 65.3, 71.3, 72.0,
IR (neat): 3503, 2930, 1702, 1653, 1388, 1256, 1093 cm–1.
3
7
9.4, 99.1, 117.6, 133.4, 155.6.
1
H NMR (500 MHz, CDCl ): d = 0.05 (s, 6 H), 0.88 (s, 9 H), 1.25
s, 6 H), 1.45–1.61 (m, 11 H), 2.17–2.27 (m, 2 H), 3.79–4.46 (m, 5
MS(ESI): m/z = 272.20 (M + H)+.
3
(
Anal. Calcd for C H NO : C, 61.97; H, 9.29; N, 5.16. Found: C,
H), 5.06–5.16 (m, 2 H), 5.71–5.93 (m, 1 H).
14
25
4
6
1.88; H, 9.35; N, 5.11.
13
C NMR (125 MHz, CDCl ): d = –4.4, 26.1, 26.4, 29.9, 37.9, 41.7,
3
6
0.3, 63.2, 71.2, 74.3, 79.7, 97.9, 115.5, 138.9, 151.7.
tert-Butyl (S)-4-[(S)-1-(tert-Butyldimethylsiloxy)but-3-enyl]-
,2-dimethyloxazolidine-3-carboxylate (7)
To a stirred soln of alcohol 6a (1.0 g, 3.68 mmol) in CH Cl (10 mL)
MS (ESI): m/z = 452.32 (M + Na)+.
2
2
2
Anal. Calcd for C H NO Si: C, 61.50; H, 10.09; N, 3.26. Found:
C, 61.39; H, 10.11; N, 3.29.
2
2
43
5
was added imidazole (0.5 g, 7.37 mmol). To this soln TBSCl (0.832
g, 5.52 mmol) was added at 0 °C and the mixture was stirred at r.t.
for 6 h (TLC monitoring). The reaction was quenched by addition
of sat. aq NH Cl soln and allowed to stir for 10 min. The aqueous
layer was extracted with CH Cl (3 × 25 mL). The combined organ-
ic layers were washed with brine, dried (Na SO ), and concentrated
under reduced pressure. The crude material was purified by flash
column chromatography (silica gel, PE–EtOAc, 98:2) to give TBS
ether 7 (1.35 g, 95%) as an oily liquid.
tert-Butyl (S)-4-[(1S,3R)-1,3-Bis(tert-butyldimethylsiloxy)hex-
5-enyl]-2,2-dimethyloxazolidine-3-carboxylate (11)
To a stirred soln of alcohol 10a (500 mg, 1.16 mmol) in CH Cl (15
mL) was added imidazole (157 mg, 2.32 mmol). To this soln was
added TBSCl (262 mg, 1.74 mmol) at 0 °C and the mixture was
stirred at r.t. for 4 h. The reaction was quenched by the addition of
sat. aq NH Cl soln, and allowed to stir for 10 min. The aqueous lay-
er was extracted with CH Cl (3 × 25 mL). The combined organic
layers were washed with brine, dried (Na SO ), and concentrated
4
2
2
2
2
2
4
4
25
[
a]D –22.2 (c 1, CHCl3).
2
2
–
1
2
4
IR (neat): 2999, 1670, 1653, 1380, 1360 cm .
under reduced pressure. The crude material was purified by flash
column chromatography (silica gel, PE–EtOAc, 98:2) to give TBS
ether 11 (0.57 g, 90%) as an oily liquid.
1
H NMR (200 MHz, CDCl ): d = 0.04 (s, 6 H), 0.87 (s, 9 H), 1.49–
3
1
.63 (m, 15 H), 2.03–2.31 (m, 2 H), 3.81–3.97 (m, 2 H), 4.08–4.22
(
1
m, 2 H), 5.01–5.09 (m, 2 H), 5.69–5.9 (m, 1 H).
25
[
a]D –33.3 (c 1.1, CHCl3).
3
C NMR (50 MHz, CDCl ): d = –4.5, 22.6, 25.8, 26.7, 28.6, 35.8,
IR (neat): 2956, 2930, 2858, 1696, 1653, 1383, 1255, 1095 cm–1.
3
6
0.3, 63.2, 71.2, 79.9, 94.9, 116.7, 136.4, 152.2.
1
H NMR (200 MHz, CDCl ): d = 0.03 (s, 6 H), 0.06 (s, 6 H), 0.88
s, 9 H), 0.92 (s, 9 H), 1.12–1.63 (m, 17 H), 2.20–2.30 (m, 2 H),
MS (ESI): m/z = 408.55 (M + Na)+.
3
(
Anal. Calcd for C H NO Si: C, 62.29; H, 10.19; N, 3.63. Found:
C, 62.20; H, 10.11; N, 3.69.
3.67–3.96 (m, 3 H), 4.06–4.30 (m, 2 H), 5.00–5.08 (m, 2 H), 5.70–
5.91 (m, 1 H).
2
0
39
4
1
3
C NMR (50 MHz, CDCl ): d = –4.6, –4.4, –3,7, –3.6, 25.8, 26.0,
3
tert-Butyl (S)-4-[(1S,3R)-1-(tert-Butyldimethylsiloxy)-3-hydro-
xyhex-5-enyl]-2,2-dimethyloxazolidine-3-carboxylate (10a)
2
1
6.6, 26.7, 28.4, 28.7, 40.2, 43.3, 60.9, 62.7, 63.1, 68.9, 79.9, 94.9,
16.9, 135.3, 152.6.
To a soln of 7 (1 g, 2.70 mmol) in dioxane–H O (3:1, 27 mL) were
2
+
+
MS (ESI): m/z = 544.80 (M + H) , 566.78 (M + Na) .
added 2,6-lutidine (0.63 mL, 5.4 mmol), 2.5% OsO in t-BuOH
4
(
1.65 mg, 0.016 mmol), and NaIO (2.31 g, 10.8 mmol). The mix-
4
Anal. Calcd for C H NO Si : C, 61.83; H, 10.56; N, 2.58. Found:
C, 62.75; H, 10.41; N, 2.49.
2
8
57
5
2
ture was stirred at 25 °C (TLC monitoring). When the reaction was
complete, H O (50 mL) and CH Cl (50 mL) were added. The or-
2
2
2
ganic layer was separated and the aqueous layer was extracted with
CH Cl (3 × 50 mL). The combined organic layers were washed
(
3S,5S)-5-[(S)-3-(tert-Butoxycarbonyl)-2,2-dimethyloxazolidin-
2
2
4-yl]-3,5-bis(tert-butyldimethylsiloxy)pentanoic Acid (12)
Compound 11 (300 mg, 0.551 mmol) was dissolved in CCl4–
MeCN–H O (2:2:3, 14 mL), NaIO (235 mg, 1.10 mmol) and
with brine and dried (Na SO ). The organic layer was concentrated
2
4
to give aldehyde 8 (0.89 g, 90%) as a pale yellow oil, which was
used as such for the next step without purification.
2
4
RuCl ·3 H O (10 mg, 0.027 mmol) were then added. The resulting
3
2
mixture was stirred vigorously at r.t. for 3 h. The mixture was ex-
tracted with CH Cl (3 × 30 mL). The combined organic extracts
(
+)-B-Methoxydiisopinocampheylborane (1.45 g, 4.59 mmol) was
dissolved in Et O (20 mL) under argon and cooled to –78 °C. To this
soln was added 1.0 M allylmagnesium bromide in Et O (4.5 mL, 4.4
mmol) via syringe. The resulting mixture was stirred at –78 °C for
1
appeared. The solvent was removed via rotary evaporator and
pumped under high vacuum for 1 h. The white residue was then dis-
solved in toluene (30 mL) and cooled to –78 °C. The aldehyde 8
2
2
2
were dried (Na SO ) and concentrated. Column chromatography
2
4
2
(
silica gel, PE–EtOAc, 8:2) of the crude product provided 12 (248
mg, 80%) as a brown-colored syrupy liquid.
5 min and warmed to r.t. over 1 h, at this time a white precipitate
25
[
a]D –29.2 (c 1.2, CHCl3).
–
1
IR (neat): 3373, 2930, 2858, 1708, 1384, 1256, 1098 cm .
1H NMR (400 MHz, CDCl
): d = 0.02 (s, 6 H), 0.04 (s, 6 H), 0.78
(
0.89 g, 2.29 mmol) in toluene (10 mL) was cooled to –78 °C and
3
transferred to the chiral allylborane soln via cannula over a 20-min
period. The mixture was stirred for 6 h and quenched with 1.0 M
NaOH (10 mL) at –78 °C. Aq 30% H O soln (10 mL) was added
and the mixture stirred at r.t. overnight. The mixture was extracted
with Et O (3 × 25 mL). The organic extracts were combined,
washed with sat. NaHCO and NaCl soln, dried (Na SO ), and con-
centrated; d.r. greater than 13:1 (anti/syn) (integration of the carbon
signals of the newly formed chiral center). These 2 diastereomers
were separated by chromatography (silica gel, gradient 5–10%
(s, 9 H), 0.81 (s, 9 H), 1.18–1.63 (m, 17 H), 2.34–2.70 (m, 2 H),
3.78–3.98 (m, 2 H), 4.02–4.27 (m, 3 H).
13
2
2
C NMR (100 MHz, CDCl ): d = –5.1, –4.6, –3.7, 17.8, 25.6, 26.6,
3
2
1
8.4, 28.7, 29.7, 38.5, 44.2, 60.7, 63.1, 67.1, 68.7, 80.3, 95.0, 152.6,
77.2.
2
3
2
4
+
+
MS (ESI): m/z = 562.80 (M + H) , 584.78 (M + Na) .
Anal. Calcd for C H NO Si : C, 57.71; H, 9.87; N, 2.49; Found:
2
7
55
7
2
C, 57.61; H, 9.81; N, 2.42.
Synthesis 2011, No. 6, 901–904 © Thieme Stuttgart · New York

9
04
A. Dubey et al.
PAPER
2007. (h) Raghavan, S.; Ramakrishna Reddy, S. J. Org.
(
3S,5S,6S)-6-Amino-3,5,7-trihydroxyheptanoic Acid [(–)-Gal-
antinic Acid, 1]
Chem. 2003, 68, 5754. (i) Pandey, S. K.; Kandula, S. R.;
Kumar, P. Tetrahedron Lett. 2004, 45, 5877. (j) Dubey, A.;
Kauloorkar, S. H.; Kumar, P. Tetrahedron 2010, 66, 3159.
(4) (a) Jha, V.; Kondekar, N. B.; Kumar, P. Org. Lett. 2010, 12,
2762. (b) Kumar, P.; Gupta, P.; Naidu, S. V. Chem. Eur. J.
2006, 12, 1397.
(5) (a) Kumar, P.; Naidu, S. V.; Gupta, P. J. Org. Chem. 2005,
70, 2843. (b) Kumar, P.; Naidu, S. V. J. Org. Chem. 2005,
70, 4207. (c) Kumar, P.; Gupta, P.; Naidu, S. V. Chem. Eur.
J. 2006, 12, 1397. (d) Kumar, P.; Naidu, S. V. J. Org. Chem.
2006, 71, 3935. (e) Gupta, P.; Kumar, P. Eur. J. Org. Chem.
A soln of 12 (200 mg, 0.36 mmol) in CH Cl (2 mL) and TFA (0.5
mL) was stirred at r.t. for 25 min. The mixture was concentrated in
vacuo to give an oily residue. The residue was passed through a col-
umn of Dowex 50Wx4 (100–200) ion exchange resin (H O, then aq
2
2
2
1
M NH ) to give 1 which was recrystallized (H O–MeOH) to give
3
2
2
b
pure 1 (52 mg, 75%) as a colorless solid; mp 128–130 °C (Lit.
25–130 °C).
1
25
2b
25
[
a]D –29.7 (c, 0.5, H O) [Lit. [a]D –29.4)].
2
1
H NMR (500 MHz, CDCl ): d = 1.51–1.83 (m, 2 H), 2.37 (dd,
3
J = 5.8, 13.6 Hz, 1 H), 2.49 (dd, J = 6.5, 13.8 Hz, 1 H), 3.13 (dt,
J = 7.0, 12.2 Hz, 1 H), 3.61 (q, J = 8.32, 14.91 Hz, 1 H), 3.91–4.21
2008, 1195. (f) Kumar, P.; Pandey, M.; Gupta, P.; Naidu, S.
V.; Dhavale, D. D. Eur. J. Org. Chem. 2010, 6993.
6) (a) Fernandes, R. A.; Kumar, P. Eur. J. Org. Chem. 2000,
3447. (b) Kumar, P.; Bodas, M. S. J. Org. Chem. 2005, 70,
(
m, 3 H).
(
MS (ESI): m/z = 194 (M + H)+.
3
3
60. (c) Dubey, A.; Kumar, P. Tetrahedron Lett. 2009, 50,
425. (d) Dubey, A.; Puranik, V. G.; Kumar, P. Org. Biomol.
Acknowledgment
Chem. 2010, 8, 5074; and references cited therein.
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Streit, P.; Schwarzenbach, F. J. Am. Chem. Soc. 1992, 114,
(
(
A.D. and A.H. thank CSIR New Delhi for senior research fel-
lowships. Financial support from DST, New Delhi (grant No. SR/
S1/OC-44/2009) is gratefully acknowledged.
2321.
8) (a) Pappo, R.; Alen, D. S. Jr.; Lemieux, R. U.; Johnson, W.
S. J. Org. Chem. 1956, 21, 478. (b) Wensheng, Y.; Yan, M.;
Ying, K.; Zhengmao, H.; Zhendong, J. Org. Lett. 2004, 6,
Reference and Notes:
3217.
(
(
(
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