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Synopsis
1
SYNOPSIS
This thesis entitled “Synthesis of some biological active compounds containing chiral
vicinal amino alcohols” is divided in to three chapters.
The first chapter deals with the “ Stereoselective synthesis of (-)-Cytoxazone, (+)5-epi-Cytoxazone and its analogue”.
The second chapter deals with the “ Synthesis of (3S, 4S)-3-methoxy-4methylamino pyrrolidine and (3S, 4R)-3-methoxy-4-methylamino pyrrolidine”.
The third chapter deals with the
“ Synthesis of protected (2R, 3R, 4S)-4,7-
diamino-2,3-dihydroxyheptanoic acid, a constituent of Callipeltins A and D”.
CHAPTER-I:
“ Stereoselective synthesis of (-)-Cytoxazone, (+)-5-epi-Cytoxazone
and its analogue”.
Enantiomerically pure substituted 2-oxazolidinones are important target molecules
in organic synthesis. Some of them act as antibiotics against highly resistant gram-positive
bacteria. In 1998 Osada etal, isolated a novel cytokine modulator from streptomyces
species.
Fig-I
OMe
OMe
HO
HO
HO
NH
O
H H
H H
H H
NH
O
NH
O
O
O
O
1
2
3
Synopsis
2
This new 4,5-disubstituted oxazolidinone was named as (-)-cytoxazone 1 (Fig.I). It
interferes with cytokine IL4; IL10 and IgG production by selective inhibition of signaling
pathway of Th2 cells, but not Th1 cells. Inhibition of Th2 dependent cytokine products
would be potent chemotherapeutic agents in the field of Immunotherapy.
Scheme-I
NH2
COOH
AcCl, MeOH
(Boc)2O, Et3N, THF
HO
NHBoc
OMe
4
DMF
5
NHBoc
OMe
O
MeO
O
HO
CH3I, K2CO3
NHBoc
OH
LiCl, NaBH4
EtOH, THF
DMSO, (COCl)2
CH2=CH-MgBr, THF
MeO
7
6
O
HN
NHBoc
O
O3, CH2Cl2
NaH, THF
OH
MeO
NaBH4, MeOH
MeO
9
8
O
HN
O
OH
MeO
2
Cytoxazone 1 is different from known immunomodulators such as FK 506 and
rapamycin in respect of structure and biological activity. Therefore 1 and its analogues
have been a new subject of synthetic studies for the development of a cytokine modulators.
Synopsis
3
Due to its potent biological activity, several syntheses of 1 and its isomers have been
described in the literature. Very recently we reported a stereoselective synthesis of 1 via
stereoselective Grignard addition of p-methoxyphenylmagnesium bromide to Nbenzylimine derived from (S)-2,3-O-isopropylidine glyceraldehyde followed by a single
step conversion of N-Boc amine diol to oxazolidinone. Herein we report a short and
stereoselective synthesis of (-)-cytoxazone 1, (+)-5-epi-cytoxazone 2 which is a C-5epimer of cytoxazone and its analogue 3.
The commercially available p-hydroxy-D-phenylglycine 4 was converted to N-Boc
methyl ester 5 using AcCl, MeOH followed by treatment with (Boc)2O and Et3N (schemeI). Treatment of 5 with methyl iodide, potassium carbonate in DMF gave 6.
Scheme-II
NHBoc
NHBoc
OH
O3, CH2Cl2
OH
MeO
NaBH4, MeOH
OH
MeO
TBSCl
imidazole, DCM
10
8
NHBoc
NHBoc
OTBS
OTBS
OH
MeO
TPP, DIAD
O
PNBA, THF
O2N
11
O
OH
HN
NaH, THF
O
OH
OH
MeO
13
TBAF, DCM
12
NHBoc
MeO
LiOH, THF-H2O
O
MeO
1
4
Synopsis
The next step in the sequence is to convert the ester to aldehyde followed by
stereoselective addition of vinylmagnesium bromide to get syn N-Boc amino alcohol based
on the well-established protocol. Thus the ester 6 was reduced to alcohol 7 under LiBH4
conditions. Swern oxidation of 7 followed by in-situ reaction of the resulting aldehyde
with vinylmagnesium bromide yielded the syn 1,2-amino alcohol 8. Intramolecular
cyclization of compound 8 in the presence of NaH in THF yielded the oxazolidinone 9.
Oxidative cleavage of the double bond with ozone followed by immediate reduction afford
the (+)-5-epi-cytoxazone 2 whose 1H,
13
C NMR spectral data are in agreement with the
assigned structure.
The synthesis of (-)-cytoxazone 1 from 8 was achieved as follows (scheme-II):
Oxidative cleavage of 8 with O3 followed by reduction with NaBH4 gave diol 10. Primary
hydroxyl in 10 was protected as TBS ether 11. When compound 11 was subjected to
mitsunobu reaction conditions gave 12 as a p-nitrobenzoic ester with inverted
configuration. Hydrolysis of ester and deprotection of TBS ether in one pot yielded 13
whose NMR and rotational values are in agreement with the reported compound.
Treatment of compound 13 with NaH in THF gave (-)-cytoxazone 1 whose spectral data
was in agreement with the reported compound.
Some compounds contain oxazolidinone unit shows antibacterial activity against
gram positive and gram-negative bacteria. Keeping this criterion in view antibacterial
testing was done for both compounds. Interestingly we observed that (+)-5-epi-cytoxazone
2 and (-)-cytoxazone 1 shows some antibacterial activity against gram-positive (G+ve)
Bacillus subtilis and gram-negative (Gr-ve) Escherichia coli. by paper disc method. The
activity has been compared with standard streptomycin disc (10 Mcg). (+)-5-epi-
Synopsis
5
Cytoxazone 2 shows somewhat more antibacterial activity compared to the (-)-cytoxazone
1 against gram positive and gram-negative bacteria.
S. No
Samples
Diametre of Inhibition zone
Basillus subtilis
Escherichia coli
1
(-)-Cytoxazone (100 Mcg)
12
14
2
(+)-5-epi-Cytoxazone (100 Mcg)
16
20
3
Streptomycin (10Mcg)
18
22
Scheme-III
COOH
NH2
COOMe
AcCl, MeOH
NHBoc
(Boc)20, TEA, THF
LiCl, NaBH4
EtOH, THF
15
14
OH
OH
NHBoc
NaH, THF
DMSO, (COCl)2
NHBoc
CH2=CH-MgBr, THF
17
16
OH
O3, CH2Cl2
O
HN
O
18
NaBH4, MeOH
O
HN
O
3
Keeping the above results in view similarly we synthesized the analogue of 5-epicytoxazone 3 from phenylalanine (scheme-III), which is a naturally available amino acid
Synopsis
6
and we evaluated antibacterial activity for this compound. This analogue also possessing
good antibacterial activity against the microorganism candida albicans and p.auraginosa.
Sl.
Samples
No
Diametre of inhibition zone
Candida albicans
p.auraginosa
1
(-)-Compound 3 (100 Mcg)
11
22
2
Streptomycin (10Mcg)
18
22
CHAPTER-II: “
Synthesis of (3S, 4S)-3-methoxy-4-methylamino pyrrolidine and
(3S, 4R)-3-methoxy-4-methyl amino pyrrolidine”.
Biologically active chiral and non-racemic pyrrolidines are commonly found in
subunits of natural and unnatural products. Okada et.al showed that a new series of
quinoline compounds e.g., 22 (Fig-II), bearing a 3-methoxy-4-amino chiral pyrrolidine
derivatives at C-7 showed higher in vivo and in vitro antibacterial activity against gram
+Ve and gram – Ve bacteria. The pyrrolidine skeleton possesses 3-methoxy-4-amino or 3methoxy-4-methyl amino groups both in syn and anti form. Tsuzuki also showed that
(3S,4S)-3-methoxy-4-methyl amino pyrrolidines attached to the naphthyridine ring at C-7
in AG-7352 19, a new type of antitumor agent, showed potent activity equal or superior to
those of cisplatin and etoposide.
Synopsis
7
Fig-II
O
MeHN
N
N
N
N
MeO
NH2
OMe
COOH RHN
S
N
N
R1
R1
O
COOH
F
MeHN
N
N
F
(S,S)
AG-7352 (+) 19
OMe
RHN
MeO
(S,R)
20 a R = Me, R1 = H
20 b R = R1 = Boc
22
21 a R = Me, R1 = H
21 b R = R1 = Boc
The amino hydroxy moiety in erythro form is also present in a pyrrolidine containing
balanol analogue, which is a very effective inhibitor of protein kinase C (PKC).
Scheme-IV
OH
D-Isoascorbic acid
COOEt
O
O
HO
OTs
NaN3, DMF
O
O
MeO
N3
MeO
MeI, NaH
DMF
O
O
OH
DCM
29
OTs
30
BocHN
OMe
EtOH, (Boc)2o
(Boc)2O, THF
OMe
N
Boc
31
OMe
Pd/C, H2
N
MeOH, PTSA
O
TsO
N3
TPP, MeOH
TsO
N3
28
MeO
N3
DCM
25
TsCl, Et3N,
N3
O
27
26
HO
TsCl, Et3N,
24
HO
OH
LAH, THF
O
23
O
HO
N
Boc
Boc
32
20 b
NaN3, DMF
8h
8
Synopsis
Hence these novel pyrrolidine moieties must certainly contribute to the activity of the
above compounds. Therefore we undertook the synthesis of these 3,4-disubstituted
pyrrolidines and developed a new general strategy for the preparation of both syn and anti
compounds 21a and 20 a. Although an approach for the synthesis of 21 a is known, it is
racemic and the chiral pyrrolidine 21a was obtained after resolution. Also two approaches
are known for 20a based on SN2 displacement reaction and chiral resolution from tartaric
acid. Herein we represent a short stereoselective and general approach for the synthesis of
both 20a and 21a starting from Isoascorbic acid and ascorbic acid respectively.
Our approach to the synthesis of trans (S,S)-pyrrolidine 20b is outlined in (Scheme
IV). D-Isoascorbic acid was converted into the ester 24 by a sequence of reactions reported
earlier. Ester functionality was reduced using LAH to give diol 25. The primary alcohol
was converted into its azide 27 via the tosylate 26 using TsCl-Py and NaN3 in DMF. The
azidoalcohol thus obtained was protected as the methylether using MeI/ NaH to give
compound 28. Under the acidic conditions (cat. amount of p-TSA in MeOH) compound 28
underwent isopropylidine cleavage to furnish the diol 29, which was converted into ditosyl
derivative 30 using TsCl/Et3N. Reaction of compound 30 in TPP/ MeOH at reflux resulted
in azide reduction and regioselective cyclization to give the pyrrolidine derivative, which
was subsequently treated with (Boc)2O to give 31. Next, the tosyl group in compound 31
underwent SN2 displacement with NaN3 in DMF to furnish azide 32, which on further
reduction with Pd/C, H2 and protection with (Boc)2O afforded compound 20b whose
spectral and analytical data were in agreement with the reported values. Compound 20b,
after N-methylation and Boc deprotection was coupled with the naphthyridine moiety to
give AG-7352, which has been reported in the literature.
Synopsis
9
Scheme-V
OH
COOEt
L-Ascorbic acid
O
O
TsCl, Et3N,
O
35
NaN3, DMF
O
MeI, NaH
DMF
O
O
TsO
TsCl, Et3N,
HO
OH
DCM
BocHN
OMe
Pd/ C, H2
N
Boc
42
OTs
TPP, MeOH
N
(Boc)2O, THF
Boc
40
39
N3
TsO
EtOH, (Boc)2o
NaN3, DMF
3 days
41
OMe
N
Boc
43
OMe
N3
MeO
N3
MeO
MeOH, PTSA
O
38
37
36
N3
MeO
N3
HO
O
DCM
O
34
OTs
HO
LAH, THF
O
33
OH
HO
BocMeN
OMe
MeI, NaH
DMF
N
Boc
21 b
Cis pyrrolidine derivative 21b was synthesized from L-ascorbic acid using the
similar strategy described in (Scheme-V). In this case conversion of compound 41 to its
azide 42 required heating reaction mixture for 3 days at 80oC, this may be due to the steric
hindrance of the ‘trans’ configuration in compound 41. On treatment with Pd/C, H2 the
azide group in 42 was reduced to the amine, and was subsequently, protected as its Boc
derivative 43. On treatment with NaH, MeI in DMF compound 43 was N-methylated to
give 21b whose spectral and analytical data were in agreement with the assigned structure.
Synopsis
10
Tsuzuki et.al reported that compound 21b, after Boc deprotection, was coupled with the
quinoline moiety, to give 22.
CHAPTER-III:
“Synthesis
of
protected
(2R,
3R,
4S)-4,7-diamino-2,3-
dihydroxyheptanoic acid, a constituent of Callipeltins A and D”.
Cyclic depsipeptides have emerged as very important classes of bioactive
compounds in marine natural products. Among these cyclic depsipeptides callipeltin A 44a
(Fig.III) shows anti-HIV, antifungal and cytotoxic activities against selected human
carcinoma cell lines. The isolation of callipeltin A 44a was reported by Minale et.al in
1996 from a shallow water sponge of the calipelta species and later by D’Auria and coworkers from Latruncula sp. It shows marked activity in cytotoxic assays against KB and
P388 cells and recently it was found that callipeltin A 44a is a selective and powerful
inhibitor of Na/ Ca cardiac exchange and a positive inotropic agent in guinea pig left atria.
It contains a number of novel amino acid residues: methoxy-tyrosine, (2R, 3R, 4S)-4amino-7-guanidino-2,3-dihydroxy heptanoic acid (AGDHE) and (3S,4R)-3,4-dimethyl-Lglutamine.
Recent reports have revealed that the side-chain attached to the macrocycle in
callepeltin A 44a is essential for anti-HIV activity and it is also present in callipeltin D
44b. The key residue in the side-chain is the novel amino acid (2R,3R,4S)-4-amino-7guanidino-2,3-dihydroxy heptanoic acid (AGDHE).
Synopsis
11
Fig.III
CONH 2
L-dlMe-Gln
2R, 3R, 4S AGDHE
O
OH
NH
OH
O
L-Thr
H
N
H 2N
N
H
N
H
NH
O
NH
N
H
L-Thr
O
OH
HN
N
H
O
O
L-Ala
CONH 2
D-Arg
O
NH
O
L-MeAla
HN
N
Me
O
O
NMe
N
H
O
MeO
L-Leu
O
OH
NH 2
L-MeGln
B-MeOTyr
OH
Callipeltin A 44a
CONH 2
L-dlMe-Gln
2R, 3R, 4S AGDHE
O
OH
NH
O
H
N
H 2N
N
H
N
H
NH
OH
L-Thr
NH 2
O
HO
O
L-Ala
HN
O
OH
Callipeltin D 44b
Two syntheses have been reported for this fragment to date and one of these was
from our lab starting from D-glucose. Recently Lipton et.al reported this fragment from
protected L-ornithine. Herein we report an efficient synthesis of AGDHE fragment starting
from readily available D-ribose.
Synopsis
12
D-Ribose 45 was converted into compound 46 via a reported procedure in one step
(Scheme-VI). Compound 46 underwent Swern oxidation and Wittig olefination to afford
48, whose NMR spectral data are in agreement with the literature. Reduction of 48 using
LAH gave compound 49, which was subsequently converted to azide 51.
Scheme-VI
D-RIBOSE
Acetone, MeOH
H2SO4 (cat)
O
O
HO
O
45
OMe
O
O
H
(COCl)2, DMSO
Et3N, -78oC, 30 min
O
46
OMe
O
47
O
Ph3P=CH-COOEt
O
EtO
DCM, 0oC to rt, 1 h
OMe
LiAlH4, THF
O
O
O
HO
0oC to rt, 6 h, 87 %
O
48
TsCl, DCM, Et3N
0oC -rt, 6 h
NaN3, DMF
O
O
O
49
OMe
O
TsO
OMe
90oC, 8 h
O
N3
O
50
OMe
O
51
Under acidic conditions compound 51 underwent isopropylidine cleavage to give
anomeric mixture of diol 52 (scheme-VII), which was protected as the dibenzyl ether 53.
Hydrolysis of the O-glycosidic bond of 53 afforded the corresponding lactol 54.
Synopsis
13
Scheme-VII
51
MeOH, Conc. HCl
O
N3
OMe
BnBr, NaH
HO
60oC, 4 h
DMF, 0oC-rt, 2 h
OH
O
N3
BnO
O
N3
HCl (cat), 60oC, 2 h
BnO
OBn
53
52
60% AcOH-H20
OMe
OH
OBn
54
Compound 54 when treated with LAH, resulted in opening of the lactol and
reduction of azide to the corresponding amine (scheme-VIII) to give 55.
Scheme-VIII
O
N3
BnO
OBn
OH
OBn
54
LAH, THF,
0oC,
30 min
15% NaOH, H20,
(Boc)20, 12 h
BocHN
OBoc
OH OBn
55
The reaction was quenched with 15% NaOH solution and immediately (Boc)2O
was added to convert the amine in to the N-Boc derivative. Interestingly, it was found that
the primary hydroxy was also protected as its Boc ester to give 55, selectively, in good
yields. Compound 55 was converted into azide derivative 56 in 90% yield by SN2
displacement of the intermediate mesylate. When compound 56 was treated with NaH/
BnBr in DMF for N-benzylation, it also resulted in O-Boc ester cleavage to give 57.
Synopsis
14
Scheme-IX
OBn
55
MsCl, Et3N, DCM
NaN3, DMF, 80oC, 8 h
NaH, BnBr, DMF
BocHN
OBoc
N3
OBn
OBn
Bn
N
0oC- rt, 6 h
Boc
56
TPP, Benzene-H2O
Et3N, (Boc)2O, 12 h
N
OBn O
Boc
Bn
OH
NH OBn
Boc
58
OBn
57
OBn
Bn
OH
N3
Jones reagent
OMe
N
Boc
NH OBn
Boc
59
Azido alcohol 57 was reduced to the corresponding amine using TPP/ BenzeneH2O and was immediately protected as the Boc derivative 58, []D25 = - 14.2 (c 0.8,
CHCl3), [lit []D25 = - 14.8 (c 0.7, CHCl3)], whose NMR spectral data were in agreement
with the literature. Compound 58 is already known to give methyl ester 59 in one step.