l-Proline catalyzed direct diastereoselective aldol reactions: towards the synthesis of lyxo-(2S,3S,4S)-phytosphingosine

Article abstract of DOI:10.1016/j.tetasy.2007.08.018

l-Proline catalyzed direct diastereoselective aldol reactions of α-amino aldehydes with cyclic ketones have been described and utilized further for the stereoselective synthesis of the 2-amino-1,3,4-triol unit as the phytosphingosines base backbone. This

Full text of DOI:10.1016/j.tetasy.2007.08.018

Tetrahedron: Asymmetry 18 (2007) 1975–1980
L-Proline catalyzed direct diastereoselective aldol reactions:
towards the synthesis of lyxo-(2S,3S,4S)-phytosphingosine
Indresh Kumar and C. V. Rode*
Chemical Engineering and Process Development Division, National Chemical Laboratory, Dr. Homi Bhabha Road,
Pune 411 008, India
Received 20 July 2007; accepted 16 August 2007
Abstract—L-Proline catalyzed direct diastereoselective aldol reactions of a-amino aldehydes with cyclic ketones have been described and
utilized further for the stereoselective synthesis of the 2-amino-1,3,4-triol unit as the phytosphingosines base backbone. This leads to the
formal synthesis of (2S,3S,4S)-lyxo-phytosphingosine.
Ó 2007 Published by Elsevier Ltd.
1. Introduction
contain long-chain amino diol and triol bases, which are
essential membrane components and are involved in
several cellular events such as the regulation of cell growth,
differentiation, adhesion, neuronal repair and signal trans-
duction.9 Phytosphingosines consist of an aliphatic chain
with a 2-amino-1,3,4-triol head and are important mem-
bers of the sphingoids family (Fig. 1). Phytosphingosine
itself is a bioactive lipid and its glycolated derivatives
display promising antitumor and antivirus activities.10
Due to their biological significance, several synthetic routes
have been described in the literature:11 most of the them are
The asymmetric aldol reaction using ‘directed processes,1
for example, modified carbonyls’ and ‘direct processes,2
for example, unmodified carbonyls’, as carbanion equiva-
lents is one of the most powerful and efficient methods
for carbon–carbon bond formation. Due to its atom-
economy, the direct process for asymmetric aldol reactions
through an ‘enamine’ intermediate using L-proline or its
derivatives as organocatalysts, that mimic the ‘enamine
catalysis’ of class I aldolases,3 has grown remarkably in
the last few years.4 Further exploitation of direct organo-
catalytic aldol reactions, along with other enantioselective
transformations in asymmetric synthesis, has been
reviewed very recently.5 Enantiopure a-amino aldehydes
are suitable precursors as acceptors in aldol reactions and
valuable starting materials for the synthesis of biologically
important compounds.6 Recently, the diastereoselective
aldol reaction of different N,N0-dibenzyl-a-amino
aldehydes with ketones7 and with dioxanone8 catalyzed
by proline has been developed. However, the direct
diastereoselective aldol reaction of cyclic ketones with
a-amino aldehydes needs to be explored further.
O
OH
(S)
(S)
OMe
(S)
HO
OH
Cbz
HN
1
OH
OH
HO
HO
13
12
OH
NH2
NH2
erythro-(2S,3R)-sphingosine
lyxo-(2S,3S,4S)-phytosphingosine
2
3
Stereoselective syntheses of amino-polyols are of major
interest because of their existence in a variety of biologi-
cally active compounds and their utility as synthetic
precursors for heterocyclic compounds. Sphingolipids
OH
OH
HO
HO
13
13
OH
OH
NH2
NH2
ribo-(2S,3S,4R)-phytosphingosine xylo-(2R,3S,4S)-phytosphingosine
5
4
*
Corresponding author. Tel.: +91 20 25892349; fax: + 91 20 2590 2621/
2612; e-mail: cv.rode@ncl.res.in
Figure 1.
0957-4166/$ - see front matter Ó 2007 Published by Elsevier Ltd.
doi:10.1016/j.tetasy.2007.08.018

1976
I. Kumar, C. V. Rode / Tetrahedron: Asymmetry 18 (2007) 1975–1980
quite lengthy using carbohydrates and amino acids as start-
ing materials, while asymmetric routes are rather rare.12
Recently, Enders et al. reported a direct organocatalytic
route for the asymmetric synthesis of D-arabino- and
L-ribo-phytosphingosine.13
available (S)-serine and (S)-proline by following literature
procedures.15 The direct diastereoselective aldol reaction
of (S)-amino aldehydes was carried out with 2 mol equiv
of cyclic ketones as donor in the presence of 20 mol % of
(S)-proline at different temperatures in solvents CHCl3–
DMSO (3:1), and the results are summarized in Table 1.
High levels of diastereoselectivities were obtained when
the reactions were carried out at 20 °C for 48 h. A further
improvement in the diastereoselectivity (>98%) was
observed by carrying out the reactions at 5 °C for 60 h with
yields of up to 85%. The enhancement in the diastereoselec-
tivity at a lower temperature can be explained through
better facial selection due to higher rigidity in the transition
state. Similar observations were reported by Enders and
Grondal for the direct diastereoselective aldol reaction of
amino aldehydes 6 with dioxanone for the synthesis of
carbohydrates.8 They also established the matched/mis-
matched situation required for both proline catalysts and
a-branched chiral aldehydes through kinetic-resolution in
the transition states, which depends upon the steric requi-
rement of chiral aldehydes. This assumption was further
2. Results and discussion
In continuation of our programme on the L-proline cata-
lyzed direct inter- and intramolecular diastereoselective
aldol reaction for the synthesis of amino-polyols and
iminosugars,14 we herein report the proline catalyzed
diastereoselective aldol reaction of a-amino aldehydes with
cyclic ketones and a new organocatalytic route for the
synthesis of the sphingoid class of compounds, with the
quick synthesis of the (2S,3S,4S)-lyxo-phytosphingosine
backbone.
In our study, we first prepared the starting a-amino alde-
hydes 6 with a cyclic core structure from commercially
Table 1. L-Proline catalyzed direct diastereoselective aldol reaction of a-amino aldehydes with cyclic ketones
O
OH
O
L-proline
O
(20 mol%)
+
*R
*R
H
(CHCl3:DMSO)
(3:1)
n= 1, 2
n= 1, 2
7, anti:syn
6
6
R*
Ketonea
Temperature (°C)
Time (h)
Yieldb (%)
7, dr (anti:syn)c
O
O
a
20
48
78
81
83
78
84
81
86
83:17
N
N
N
N
Cbz
Cbz
Boc
Cbz
O
O
O
O
O
b
c
d
e
f
5
60
60
60
48
60
60
>96:4
5
>98:not observed
>98:not observed
88:12
O
O
5
20
5
N
N
N
Cbz
Cbz
Cbz
O
O
>95:5
g
5
>98:not observed
Abbreviations: Boc = tert-butyloxycarbonyl, Cbz = benzyloxycarbonyl.
a General reaction conditions: 1.0 mmol of aldehyde, 2.0 mmol of cyclic ketone, 20 mol % L-proline, solvents CHCl3–DMSO (3:1).
b Isolated yields of 7.
c Determined by weighing separately after flash chromatography on silica gel.

I. Kumar, C. V. Rode / Tetrahedron: Asymmetry 18 (2007) 1975–1980
1977
supported by direct Mannich reaction for the synthesis of
amino sugars and derivatives.16a The formation of anti-
aldol products 7 consistent with the proline catalyzed aldol
reactions,8,17 which can be explained through the Houk–
List model proposed for cyclic ketones,18 where the cyclic
enamine intermediate and an intermolecular hydrogen
bonding play the critical role (Fig. 2).
out by treating with K2CO3 (cat)/MeOH at rt and the
progress of this reaction was monitored by TLC. After
complete consumption of 9 within 2 h, this reaction was
acidified with 2 M-HCl solution and additionally stirred
for 2 h to deprotect the acetonide moiety. The solvent
was evaporated and the crude material was passed through
a small pad of column to give 1 as a white solid with 76%
yield, which had the (2S,3S,4S)-2-amino-1,3,4-triol unit of
lyxo-phytosphingosine 3. The resulting compound 1 can be
transformed into lyxo-phytosphingosine by converting the
ester moiety in to a long chain carbon unit by using a
similar method as reported by Lin and co-workers.19 Our
approach provides quick access to phytosphingosine
backbones, which completes the formal synthesis of 3
(Scheme 1). All the new compounds were fully charac-
terized by spectroscopic means.
O
O
N
N
O
H
O
O
O
H
O
H
O
H
N
N
Cbz
(Disfavored)
Cbz
(Favored)
OH
O
OH
O
3. Conclusion
O
O
>
In conclusion, we have developed direct diastereoselective
aldol reactions of different a-amino aldehydes with cyclic
ketones using L-proline as an organocatalyst. This was
further utilized for a quick organocatalytic route to the
synthesis of the (2S,3S,4S)-lyxo-phytosphingosines back-
bone. This approach is very flexible and can be applied
to the synthesis of other stereoisomers of the sphingoid
family, since the organocatalyst and amino-aldehydes are
easily available in both D and L-forms. Further study in this
direction is currently underway and will be presented in the
future.
N
N
Cbz
Cbz
(anti)
(syn)
Figure 2. Houk–List model for direct diastereoselective aldol reaction of
cyclic ketones with a-amino aldehydes catalyzed by L-proline.
Our next aim was to prepare the phytosphingosine base-
backbone, which was carried out with anti-aldol product
7d as almost pure diastereomer. This product 7d was
subjected to a Baeyer–Villiger oxidation by treating with
3 equiv of m-CPBA (55% activity) and 3 equiv of NaHCO3
in dry CH2Cl2 for 4 h at rt and by adding another portion
of the same amounts of m-CPBA and NaHCO3 with addi-
tional stirring for 8 h at the same temperature to give the
desired hydroxy-lactone compound 8 in 85% isolated yield.
The hydroxy group of 8 was then protected with MOMCl
(1.2 equiv) and diisopropylethyl amine (1.3 equiv) in dry
CH2Cl2 at rt overnight to provide MOM-protected com-
pound 9 in 87% yield. The delactonization of 9 was carried
4. Experimental
4.1. General methods
All reagents were used as supplied. The reactions involving
hygroscopic reagents were carried out under an argon
atmosphere using oven-dried glassware. THF was distilled
OH
O
OH
O
O
(i)
O
(ii)
O
6d
N
N
Cbz
Cbz
7d
8
OMOM
OMOM
O
O
OMe
(iii)
(v)
(iv)
O
O
N
N
OH
O
Cbz
9
Cbz
10
OH
OMOM
(S)
O
(S)
steps
(S)
HO
13
HO
OMe
OH
NH2
OH
HN
Cbz
lyxo-(2S,3S,4S)-Phytosphingosine
1
3
Scheme 1. Reagents and conditions: (i) Cyclopentanone (2 mol equiv), L-proline (20 mol %), CHCl3–DMSO (3:1), 5 °C, 60 h, 78%, dr > 98%; (ii) m-CPBA
(55% activity, 6 equiv), NaHCO3, (6 equiv), 12 h, rt, 85%; (iii) MOMCl (1.2 equiv), DIPEA (1.3 equiv), dry CH2Cl2, rt, overnight, 87%; (iv) K2CO3 (cat)/
MeOH, rt, 2 h; (v) 2 M-HCl solution, rt, 2 h, 76% in two steps.

1978
I. Kumar, C. V. Rode / Tetrahedron: Asymmetry 18 (2007) 1975–1980
from sodium–benzophenone ketyl prior to use. Reactions
were followed by TLC using 0.25 mm Merck silica gel
plates (60F-254). Optical rotation values were measured
using JASCO P-1020 digital polarimeter using Na light.
IR spectra were recorded on Perkin–Elmer FT-IR 16 PC
spectrometer. The NMR spectra were recorded on a
Bruker system (200 MHz for 1H and 75 MHz for 13C).
The chemical shifts are reported using the d (delta) scale
for 1H and 13C spectra. Choices of deuterated solvents
(CDCl3, D2O) are indicated below. LC–MS was recorded
using electrospray ionization technique. All the organic
extracts were dried over sodium sulfate and concentrated
under an aspirator vacuum at room temperature. Column
chromatography was performed using (100–200 and
230–400 mesh) silica gel obtained from M/s Spectrochem
India Ltd Room temperature is referred as rt.
[M+Na]+ 350.21. Anal. Calcd for C17H29NO5: C, 62.36;
H, 8.93; N, 4.28. Found: C, 62.32; H, 8.89; N, 4.31.
4.5. (S)-Benzyl 4-((R)-hydroxy-((S)-2-oxocyclopentyl)-
methyl)-2,2-dimethyloxazolidine-3-carboxylate 7d
25
Yield 78%, 7d, (anti): ½aꢀD ¼ ꢁ66:5 (c 1, CHCl3), 1H NMR
(200 MHz, CDCl3): d = 1.42–1.63 (m, 8H), 1.81–2.28 (m,
5H), 3.85–4.02, (m, 2H), 4.12–4.25 (m, 2H), 5.15 (s, 2H),
7.34 (m, 5H), 13C NMR (75 MHz, CDCl3): d = 20.22,
23.16, 25.51, 26.67, 37.97, 50.63, 59.59, 63.18, 66.77,
70.78, 94.43, 127.94, 128.11, 128.35, 136.01, 152.04,
216.23. LC–MS (ESI-TOF): m/z for C19H25NO5 [M+H]+
348.27, [M+Na]+ 370.26. Anal. Calcd for C19H25NO5: C,
65.69; H, 7.25; N, 4.03. Found: C, 65.62; H, 7.31; N, 4.09.
4.6. (S)-Benzyl 2-((R)-hydroxy-((S)-2-oxocyclopentyl)-
methyl)pyrrolidine-1-carboxylate 7f
4.2. General procedure for direct diastereoselective aldol
reaction of a-amino aldehydes with ketones
25
Yield 81%, 7f, (anti): ½aꢀD ¼ ꢁ99:5 (c 1, CHCl3), (syn):
25
To a stirred solution of a-amino aldehyde 6 (1 mmol) and
cyclic ketone (2 mmol) in solvent CHCl3–DMSO (3:1,
8 ml) was added 20 mol % (S)-proline at 5 °C and the reac-
tion was stirred further for 60 h at the same temperature,
followed by TLC. The reaction mixture was reduced in
vacuo. The resulting residue EtOAc (30 ml) was taken
and stirred with 10% NaHCO3 solution (10 ml). The
organic layer was separated and washed with brine
solution, dried over Na2SO4 and evaporated in vacuo.
The crude residue was purified by flash chromatography
on silica gel with hexane: ethyl acetate (8:2–7:3) to give
the aldol product in good yield and diastereoselectivity,
which was determined by weighing separately after
chromatographic purification.
½aꢀD ¼ þ18:5 (c 1, CHCl3),1H NMR (200 MHz, CDCl3):
for 7f (anti): d = 1.62–1.85 (m, 4H), 1.94–2.25 (m, 6H),
2.28–2.31, (m, 1H), 3,38–3.61 (m, 2H), 3.75–4.02 (m, 1H),
4.11–4.28 (m, 1H), 5.12 (dd, J = 11.6 Hz, 2H), 7.34 (s,
5H), 13C NMR (75 MHz, CDCl3): d = 20.46, 23.71,
24.45, 26.24, 37.99, 46.86, 50.72, 59.96, 66.32, 71.67,
127.49, 127.62, 128.20, 136.74, 154.79, 216.09. LC–MS
(ESI-TOF): m/z for C18H23NO4 [M+H]+ 318.01,
[M+Na]+ 339.98. Anal. Calcd for C18H23NO4: C, 68.12;
H, 7.30; N, 4.41. Found: C, 68.19; H, 7.25; N, 4.49.
4.7. (S)-Benzyl 2-((R)-hydroxy-((S)-2-oxocyclohexyl)-
methyl)pyrrolidine-1-carboxylate 7g
25
Yield 86%, 7g, (anti): ½aꢀD ¼ ꢁ90:9 (c 1, CHCl3), 1H NMR
4.3. (S)-Benzyl 4-((R)-hydroxy-((S)-2-oxocyclohexyl)-
methyl)-2,2-dimethyloxazolidine-3-carboxylate 7b
(200 MHz, CDCl3): for 7f (anti): d = 1.50–2.02 (m, 10H),
2.14 (m, 1H), 2.34, (m, 1H), 2.45 (m, 1H), 3.38 (m, 1H),
3.40–3.71 (m, 2H), 4.07 (m, 1H), 5.09 (s, 2H), 7.33 (s,
5H), 13C NMR (75 MHz, CDCl3): d = 23.80, 24.99,
25.65, 28.18, 31.87, 42.68, 46.94, 52.11, 59.70, 66.57,
72.91, 127.63, 127.77, 128.34, 136.98, 155.50, 215.68. LC–
MS (ESI-TOF): m/z for C19H25NO4 [M+H]+ 332.10,
[M+Na]+ 354.07. Anal. Calcd for C19H25NO4: C, 68.86;
H, 7.60; N, 4.23. Found: C, 68.71; H, 7.55; N, 4.29.
25
Yield 81%, 7b (anti, major) ½aꢀD ¼ ꢁ78:7 (c 1, CHCl3), 7b
25
(syn, minor) ½aꢀD ¼ þ10:8 (c 0.8, CHCl3). 1H NMR
(200 MHz, CDCl3) for 7b (anti): d = 1.42–1.62 (m, 8H),
1.72–2.05 (m, 5H), 2.27 (m, 1H), 2.43 (m, 1H), 3.32–3.48
(m, 1H), 3.89 (m, 1H), 4.10–4.25 (m, 2H), 4.90–5.20 (m,
N–CH2Ph, 2H), 7.34 (s, Ar, 5H). 13C NMR (75 MHz,
CDCl3): d = 22.86, 24.19, 25.20, 28.47, 32.97, 42.60,
50.63, 60.09, 64.87, 67.03, 73.40, 93.92, 127.76, 128.01,
128.35, 136.35, 153.70, 215.91. LC–MS (ESI-TOF): m/z
for C20H27NO5 [M+H]+ 362.12, [M+Na]+ 384.08. Anal.
Calcd for C20H27NO5: C, 66.46; H, 7.53; N, 3.88. Found:
C, 66.41; H, 7.58; N, 3.85.
4.8. (S)-Benzyl 4-((S)-hydroxy((S)-6-oxotetrahydro-2H-2-
pyran-2-yl)methyl)-2,2-dimethyloxazolidin-3-carboxylate 8
To a stirred solution of compound 7d (700 mg, 2.02 mmol)
in dry CH2Cl2 (30 ml) at 0 °C were added m-CPBA (55%
activity; 1.0 g, 6 mmol) and NaHCO3 (508 mg, 6 mmol),
and then stirred at room temperature for 4 h after which
was added another portion of the same amounts of
m-CPBA and NaHCO3 with stirring for an additional
8 h. After completion of the reaction, the excess m-CPBA
was quenched with aq Na2S2O3. The insoluble material
was removed by filtration and washed with CH2Cl2. The
aqueous phase was extracted with CH2Cl2 and the
combined extracts were washed with satd NaHCO3, brine
solutions and dried over Na2SO4 and concentrated under
reduced pressure, the resulting pasty mass was purified by
column chromatography (pet ether–EtOAc, 3:2) and gave
4.4. (S)-tert-Butyl 4-((R)-hydroxy((S)-2-oxocyclohexyl)-
methyl)-2,2-dimethoxazolidine-3-carboxylate 7c
25
1
Yield 83%, 7c (anti): ½aꢀD ¼ ꢁ119:7 (c 0.85, CHCl3), H
NMR (200 MHz, CDCl3): d = 1.38–1.42 (m, 14H), 1.56–
1.78 (m, 3H), 1.82–2.15 (m, 4H), 2.31–2.42 (m, 2H),
2.45–2.57 (m, 1H), 3.62–3.75 (m, 1H), 3.81–3.92 (m, 1H),
4.05–4.21 (m, 2H), 13C NMR (75 MHz, CDCl3):
d = 24.18, 25.68, 27.88, 28.31, 28.72, 33.24, 43.06, 50.87,
59.61, 64.89, 73.60, 80.29, 93.72, 153.23, 216.21. LC–MS
(ESI-TOF): m/z for C17H29NO5 [M+H]+ 328.22,

I. Kumar, C. V. Rode / Tetrahedron: Asymmetry 18 (2007) 1975–1980
1979
8 (625 mg, 85% yield) as a colourless pasty liquid.
References
25
1
½aꢀD ¼ 14:2 (c 0.70, CHCl3), H NMR (200 MHz, CDCl3):
d = 1.42–1.75 (m, 8H), 1.75–2.01 (m, 2H), 2.52 (m, 2H),
3.41–3.62 (m, 3H), 2.98–3.105 (m, 1H), 3.63–3.84 (m,
3H), 4.06 (m, 1H), 4.27 (m, 1H),5.13 (dd, J = 12.5, 2H),
7.35 (s, 25H). 13C NMR (75 MHz, CDCl3): d = 18.33,
22.83, 24.25, 26.33, 29.48, 60.13, 63.45, 67.10, 78.94,
81.29, 93.70, 128.13, 128.22, 128.51, 135.97, 152.59,
170.62. LC–MS (ESI-TOF): m/z C19H25NO6 [M+H]+
364.18, [M+Na]+ 386.01. Anal. Calcd for C19H25NO6: C,
62.80; H, 6.93; N, 3.85. Found: C, 62.77; H, 6.89; N, 3.91.
1. (a) Kim, B. M.; Williams, S. F.; Masamune, S. In Compre-
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Pergamon Press: Oxford, 1991; Vol. 2; (b) Marhrwald, R. In
Modern Aldol Reaction; Wiley-VCH: Weinheim, 2004; Vols.
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4.9. (S)-Benzyl-4-((S)-(methoxymethoxy)((S)-6-oxotetra-
hydro-2H-2-pyran-2-yl)methyl)-2,2-dimethyloxazolidin-
3-carboxylate 9
To a stirred solution of compound 8 (0.6 g, 1.65 mmol) and
diisopropylethyl amine (DIPEA) (0.27 g, 2.14 mmol) in dry
CH2Cl2 (8 ml) was added MOMCl (0.16 g, 1.98 mmol) at
0 °C. The resulting reaction mixture was stirred further
overnight at rt. After completion of the reaction, the sol-
vent was evaporated and the residue was chromatographed
over silica gel to give compound 9 (0.58 g, 87% yield) as a
25
1
colourless pasty liquid. ½aꢀD ¼ ꢁ5:85 (c 0.6, CHCl3), H
NMR (200 MHz, CDCl3): d = 1.41–1.65 (m, 10H), 2.28–
2.62 (m, 2H), 3.37 (s, 3H), 3.85–4.15 (m, 3H), 4.20–4.48
(m, 2H), 4.69 (dd, J=6.5 Hz, 2H), 5.15 (s, 2H), 7.35 (s,
5H). 13C NMR (75 MHz, CDCl3): d = 18.43, 22.93,
24.35, 25.96, 29.59, 56.42, 58.62, 63.56, 67.20, 79.05,
80.96, 93.80, 98.55, 128.23, 128.32, 128.61, 136.08,
152.54, 170.72. LC–MS (ESI-TOF): m/z C21H29NO7
[M+H]+ 408.05, [M+Na]+ 430.03. Anal. Calcd for
C21H29NO7: C, 61.90; H, 7.17; N, 3.44. Found: C, 61.97;
H, 7.14; N, 3.49.
5. (a) Gaunt, M. J.; Johansson, C. C. C.; McNally, A.; Vo, N. T.
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references cited therein.
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1210–1212.
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H., Eds.; Wiley-VCH: Weinheim, 2004; p 48, Chapter 1.4; (d)
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4.10. (5S,6S,7S)-Methyl 7-(benzyloxycarbonylamino)-5,8-
dihydroxy-6-(methoxymethoxy)octanoate 1
To a solution of compound 9 (0.5 g, 1.23 mmol) in MeOH
(10 ml), was added K2CO3 in a catalytic amount and stir-
red at rt. The reaction reached to completion in about
2 h, monitored by TLC. This reaction was quenched with
an excess of dilute HCl and additionally stirred for 2 h at
the same temperature. The solvent was evaporated under
a reduced pressure and passed through a small pad of col-
umn to give white solid compound 1 (0.37 g, 76% yield)
25
after two steps. ½aꢀD ¼ ꢁ6:6 (c 0.7, CHCl3), 1H NMR
(200 MHz, CDCl3/D2O): d = 1.45–1.78 (m, 4H), 2.15–
2.30 (m, 2H), 3.36 (s, 3H,) 3.68 (s, 3H), 3.75–4.02 (m,
2H), 4.10–4.45 (m, 3H), 4.65–4.75 (m, 2H), 5.18 (s, 2H),
7.38 (m, 5H). 13C NMR (75 MHz, CDCl3/D2O):
d = 18.33, 30.93, 33.78, 50.35, 53.35, 56.31, 63.45, 67.10,
68.99, 86.90. 98.44, 128.05, 128.22, 128.51, 135.97,
153.56, 173.55. Anal. Calcd for C19H29NO8: C, 57.13; H,
7.32; N, 3.51. Found: C, 57.18; H, 7.29; N, 3.59.
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Acknowledgement
Indresh Kumar thanks CSIR, New Delhi, for the award of
senior research fellowship.

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