Complementary routes to both enantiomers of pipecolic acid and 4,5-dihydroxypipecolic acid derivatives

Article abstract of DOI:10.1016/j.tetlet.2007.12.097

Complementary new routes to both enantiomers of N-protected pipecolic acid and the corresponding 4,5-dihydroxylated derivatives are developed, which involve stereo-divergent allylation of a chiral N-allylimine and ring-closing metathesis as key steps.

Full text of DOI:10.1016/j.tetlet.2007.12.097

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 1365–1369
Complementary routes to both enantiomers of pipecolic
acid and 4,5-dihydroxypipecolic acid derivatives
Shital K. Chattopadhyay *, Titas Biswas, Tanmoy Biswas
Department of Chemistry, University of Kalyani, Kalyani 741 235, West Bengal, India
Received 24 September 2007; revised 12 December 2007; accepted 18 December 2007
Available online 14 January 2008
Abstract
Complementary new routes to both enantiomers of N-protected pipecolic acid and the corresponding 4,5-dihydroxylated derivatives
are developed, which involve stereo-divergent allylation of a chiral N-allylimine and ring-closing metathesis as key steps.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Imines; Allylation; Ring-closing metathesis; Amino acids
Pipecolic acid 1 (Fig. 1), a naturally occurring1 but non-
proteinogenic amino acid, has found widespread utility as a
component of several biologically active secondary meta-
bolites,2a,b as a proline mimic and b-turn inducer in many
designed peptides and synthetic drug candidates,2c–e as a
building block in organic synthesis,2f as an enzyme inhi-
bitor,2g and is receiving current attention3 for application
as an organo-catalyst. These applications have stimulated
considerable interest4 in the synthesis of pipecolic acid5
and its derivatives.6 However, only a few general routes
to both the enantiomers of pipecolic acid are reported.7
Herein, we report a stereo-divergent route to N-protected
R- and S-pipecolic acid from readily available8 R-2,3-O-
cyclohexylideneglyceraldehyde (2) based on our efforts to
develop stereocontrolled allylation of N-allylimine 3
derived from 2.
Allylation of a-chiral imines has been extensively stud-
ied9 as a route for the preparation of homoallylic amines.
A range of allylmetal reagents (wherein, the metal compo-
nent is either boron, magnesium, zinc, copper, tin, silicon,
indium or aluminum) have been shown to be effective for
this purpose. However, the more common allylmagnesium
or allylzinc reagents have found most applications as they
can be used without any additive. We opted to use these
in our current efforts towards allylation of 3.
Thus, imine 3 (Scheme 1) was prepared by straightfor-
ward dehydrative condensation of aldehyde 2 with allyl-
˚
amine in the presence of molecular sieves (4 A).
Compound 3 was then reacted with allylmagnesium bro-
mide under optimised conditions to give a mixture of the
homoallylic amines 4 (62%) and 5 (16%), which were
cleanly separated by silica gel chromatography. On the
other hand, treatment of imine 3 with allylzinc bromide
(prepared in situ from allyl bromide and zinc powder),
under optimised conditions, led to the formation of the
anti-isomer 5 as the major product (47%) together with 4
(19%).
O
O
N
H
1
CO2H
X
2, X = O
3, X = NCH2CH=CH2
Fig. 1.
*
Corresponding author. Tel.: +91 33 25828750x306; fax: +91 33
25828282.
E-mail address: skchatto@yahoo.com (S. K. Chattopadhyay).
The complementary mode of these addition reactions is
of advantage. However, the predominance of one isomer
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.12.097

1366
S. K. Chattopadhyay et al. / Tetrahedron Letters 49 (2008) 1365–1369
Ref. 8
ii
+
i
O
O
O
3
O
D-Mannitol
2
or iii
HN
HN
5
4
˚
Scheme 1. Reagents and conditions: (i) Allylamine, CH2Cl2, powdered molecular sieves (4 A), 0 °C, 24 h, 95%; (ii) allylmagnesium bromide (1.0 M
solution in THF, 2 equiv), Et2O, À30 °C, 12 h, 4 (62%), 5 (16%); (iii) allylzinc bromide (5 equiv), THF, 0 °C to rt, 12 h, 4 (19%), 5 (47%).
over the other is hard to interpret by classical conforma-
tional models since the nature of diastereoselection in imine
addition is known10 to be dependent on several factors.
The configuration of the new stereogenic centre in each
of the homoallylic amines 4 and 5 was therefore assigned
from the following synthetic work. The major product
from the allylmagnesium bromide reaction was treated
with HCl (6 N) to prepare amino diol 6 (Scheme 2) which,
without purification, was treated with Boc2O to form
N-tethered diene 7 in good yield. In continuation of our
earlier work11 on ring-closing metathesis12 (RCM) medi-
ated synthesis of heterocyclic amino acids,13 we treated 7
with Grubbs’ first generation catalyst, benzylidene bis-
tricyclohexylphosphinoruthenium(IV) dichloride (8), to
provide the unsaturated piperidine derivative 9 in good
yield.14 Hydrogenation of the latter followed by oxidative
cleavage of the 1,2-diol unit under the Sharpless protocol15
led to N-Boc-pipecolic acid 10 as a colourless crystalline
solid, mp 127–128 °C. Comparison of the measured [a]D
values, +63 (c 0.06, AcOH), +43 (c 1.2, MeOH), with the
literature values16 for (S)-N-Boc-pipecolic acid (À67.2 in
AcOH, À45.1 in MeOH) indicated the product to be of
R-configuration and of ꢀ93% optical purity. Conse-
quently, the configuration of the starting homoallylic
amine 4 was assigned as syn. Similarly, the major product
from the allylzinc bromide reaction, 5, was N-tosylated
leading to 11 and then subjected to RCM with catalyst 8.
The resulting piperidine derivative 12 was obtained as a
colourless solid, mp 110 °C, [a]D À71 (c, 0.13 in CHCl3).
Deprotection of the cyclohexylidene moiety in 12 leading
to diol 13, followed by hydrogenation and oxidative cleav-
age of the 1,2-diol unit then led to the formation of (S)-N-
tosylpipecolic acid 14, [a]D À42 (c, 0.17 in CHCl3), in an
overall yield of 46% over five steps from 5.
In an alternative approach, we prepared the homoallyl
alcohol 15 (Scheme 3) from aldehyde 2 following a known
procedure.8 Conversion of alcohol 15 to the corresponding
inverted amine 18 was then effected through azide 17
which, in turn, was obtained from mesylate 16 under the
conventional conditions. Reduction of the azide moiety
in 17 with LiAlH4 followed by treatment of the resulting
amine 18 with p-toluenesulfonyl chloride smoothly led to
the corresponding tosylamide 19. The latter on N-allyl-
ation with allyl bromide in the presence of sodium hydride
afforded the desired N-tethered diene 20 in good yield.
Ring-closing metathesis of compound 20 in the presence
of catalyst 8 smoothly led to the unsaturated piperidine
derivative 21. Repetition of the sequence of events adopted
for the conversion 12?14 on compound 21, that is, depro-
tection of the cyclohexylidene unit leading to 22, saturation
of the double bond to form 23 followed by oxidative cleav-
age of the 1,2-diol unit gave (R)-N-tosylpipecolic acid 24,
[a]D +40 (c, 0.13 in CHCl3), in an overall yield of 41%
over five steps from 20. The optical purity of each of the
OH
HO
OH
HO
i
iv
v
iii
4
RN
N
CO2H
BocN
Boc
6, R = H
ii
10
9
7, R = Boc
OH
O
O
O
O
iii
HO
iv
v
i
vi
5
TsN
TsN
TsN
12
N
CO2H
Ts
11
13
14
Scheme 2. Reagents and conditions: (i) HCl (6 N), THF, rt, 5 h, 88% (6), 91% (13): (ii) Boc2O, MeOH, NaHCO3, rt, 12 h, 77%; (iii) 8 (5 mol %), CH2Cl2,
rt, 7 h, 91% (9), 89% (12); (iv) Pd–C (10%), EtOAc, rt, 7 h, quantitative; (v) NaIO4 (4 equiv), RuCl3 (2.5 mol %), CCl4, CH3CN, H2O, rt, 2 h, 76% (10),
73% (14); (vi) TsCl, Et3N, CH2Cl2, rt, 24 h, 91%.

S. K. Chattopadhyay et al. / Tetrahedron Letters 49 (2008) 1365–1369
1367
O
O
O
O
O
O
v
iii
i
2
OR
R
NTsR
17, R = N3
15, R = H
19, R = H
ii
iv
vi
18, R = NH2
16, R = OMs
20, R = allyl
OH
O
OH
HO
O
HO
vii
viii
x
ix
TsN
TsN
TsN
CO2H
N
Ts
21
23
22
24
Scheme 3. Reagents and conditions: (i) Allylmagnesium iodide, Et2O, À50 °C to rt, 18 h, 63%; (ii) MsCl, Et3N, CH2Cl2, 0 °C to rt, 24 h, 87%; (iii) NaN3,
DMF, 80 °C, 36 h, 71%; (iv) LiAlH4, Et2O, 0 °C to rt, 8 h, 94%; (v) TsCl, pyridine, CH2Cl2, rt, 18 h, 81%; (vi) NaH, allyl bromide, THF–DMSO (5:1),
0 °C to rt, 16 h, 90%; (vii) 8 (5 mol %), CH2Cl2, rt, 1.5 h, 92%; (viii) HCl (6 N), THF, rt, 5 h, 76%; (ix) Pd–C (10%), EtOAc, rt, 6 h, 93%; (x) NaIO4
(4 equiv), RuCl3 (2.5 mol %), CCl4, CH3CN, H2O, rt, 3 h, 71%.
enantiomeric N-tosylpipecolic acids 14 and 24 was deter-
mined by chiral HPLC studies, which indicated the enan-
tiomeric excess values to be 96% and 93%, respectively.16
Various hydroxylated pyridine derivatives possess
diverse biological activities and therefore have received
considerable attention from synthetic organic chemists.17
Many hydroxylated pipecolic acid derivatives have also
been prepared in this regard.18 We therefore investigated
the conversion of the unsaturated piperidine derivative 12
to 4,5-dihydroxypipecolic acid derivatives. Thus, OsO4
mediated dihydroxylation of compound 12 (Scheme 4) pro-
vided an almost 1:1 mixture of the stereoisomeric diols 25
and 26 (stereochemistry not determined at this stage),
which were separated by column chromatography, com-
pound 26 eluting first. The hydroxyl groups in compound
25 were then protected as benzyl ethers to form 27. Cleav-
age of the cyclohexylidene unit in 27 leading to diol 29
followed by oxidative cleavage using the RuCl3–NaIO4
combination led to the formation of the pipecolic acid
derivative 30 with concomitant conversion of the benzyl
groups to the corresponding benzoates. The dibenzoate
derivative 30 was then hydrolysed to the dihydroxypipe-
colic acid derivative 31. The stereochemistry of the C-4
and C-5 centres indicated in compound 25, and in the
OR
RO
OR
RO
O
O
i
+
O
O
N
O
O
H
N
Ts
N
H
Ts
H
Ts
12
26, R = H
28, R = Bn
25, R = H
27, R = Bn
ii
ii
OBn
N
OCOPh
OH
BnO
BnO
PhOCO
HO
HO
v
iv
iii
27
OH
OH
CO2H
CO2H
N
N
H
Ts
Ts
Ts
29
31
30
OBn
OCOPh
OH
PhOCO
v
iv
iii
28
OH
OH
N
CO2H
CO2H
N
N
H
Ts
Ts
Ts
32
34
33
Scheme 4. Reagents and conditions: (i) OsO4, NMMO, acetone–H2O (4:1), rt, 12 h, 25 (46%), 26, 38%; (ii) NaH (3 equiv), BnBr (3 equiv), THF–DMSO
(10:1), 0 °C to rt, 18 h, 27 (67%), 28 (66%); (iii) HCl (6 N), THF, rt, 5 h, 29 (81%), 32 (82%); (iv) NaIO4 (4 equiv), RuCl3 (2.5 mol %), CCl4–CH3CN–H2O
(1:1:1.5), rt, 2 h, 30 (53%), 33 (59%); (v) KOH (1 M, 10 equiv), MeOH, rt, 10 h, 31 (71%), 34 (68%).

1368
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1
pound 30 including homonuclear decoupling experiments.
1
In the H NMR spectrum of compound 30 the H-5 signal
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and two isomeric 4,5-dihydroxypipecolic acid derivatives
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Acknowledgements
We are thankful to DST (Grant No. SR/S1-OC/51/
2005), Government of India, for financial assistance and
CSIR for fellowships to two of us.
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14. All new compounds reported here gave satisfactory spectroscopic
and/or analytical data. Data for 31: Mp: 197–198 °C. [a]D +54 (c 0.1,
MeOH). IR (KBr): 3347, 1710, 1328, 1159 cmÀ1. 1H NMR (600 MHz,
DMSO-d6): d 7.64 (2H, d, J = 8.1), 7.36 (2H, d, J = 7.8), 4.88 (1H, d,
J = 4.0, D2O exchangeable), 4.32 (1H, d, J = 7.2), 3.67 (1H, quin,
J = 1.8), 3.30 (1H, dd, J = 12.6, 5.6), 3.23 (1H, t, J = 11.0), 3.10 (1H,
ddd, J = 11.0, 5.4, 2.4), 2.34 (3H, s), 2.22 (1H, ddd, J = 14.4, 3.6, 1.8),
1.65 (1H, ddd, J = 13.8, 7.2, 2.4). 13C NMR (75 MHz, DMSO-d6): d
171.7 (s), 142.8 (s), 137.8 (s), 129.6 (d), 128.7 (d), 66.8 (d), 65.7 (d),
50.7 (d), 42.0 (t), 33.1 (t), 20.9 (q). HRMS (TOF MS ES+): obsd
338.0679 (M+Na); calcd 338.0674. Data for 34: Mp: 189–190 °C [a]D
+96 (c 0.12, MeOH). IR (KBr): 3418, 1346, 1727, 1324, 1159 cmÀ1
.
1H NMR (600 MHz, DMSO-d6): d 7.64 (2H, d, J = 7.8), 7.32 (2H, d,
J = 8.1), 4.78 (1H, br s, D2O exchangeable) 4.62 (1H, br s, D2O
exchangeable), 4.37 (1H, d, J = 6.0), 3.65 (1H, br s), 3.63–3.60 (1H,
m), 3.37 (1H, dt, J = 11.0, 3.6), 3.30 (1H, dd, J = 13.8, 1.8), 2.33 (3H,
s), 1.88 (1H, dt, J = 12.6, 6.6), 1.77 (1H, dt, J = 12.6, 3.0). 13C NMR
(75 MHz, CDCl3): d 172.0 (s), 142.7 (s), 137.2 (s), 129.3 (d), 127.1 (d),
66.0 (d), 65.7 (d), 54.8 (d), 47.9 (t), 29.0 (t), 21.0 (q). HRMS (TOF MS
ES+): obsd 338.0670 (M+Na); calcd 338.0674.
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1369
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19. These data will be disclosed in a full account of this work.