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Abstract
The thesis entitled “Stereoselective Synthesis of C6-Substituted Furanoid Sugar
Amino Acids, Synthesis and Conformational Studies of Peptidomimetics Containing
Sugar Amino Acids and the Total Synthesis of Conagenin” consists of three chapters:
CHAPTER I: Deals with the synthesis of C6-substituted furanoid sugar amino acids. This
chapter is divided into three parts: PART-A: deals with the stereoselective synthesis of
C6-substituted 3,4-dideoxy furanoid sugar amino acids; PART-B: concerns with the
stereoselctive synthesis of the various isomers of 3,4-dideoxy furanoid sugar amino acids
with methyl substitution at the C6 position; PART-C: describes the synthesis of furanoid
sugar amino acid with methyl substitution at the C6 position.
CHAPTER II: Describes the synthesis and conformational studies of peptidomimetics
containing sugar amino acids.
CHAPTER III: Describes the total synthesis of Conagenin.
CHAPTER I
PART-A: Stereoselective Synthesis of C6-Substituted 3,4-Dideoxy Furanoid Sugar
Amino Acids
Emulating the basic principles followed by nature to build its vast repertoire of
biomolecules, organic chemists are developing many novel multifunctional building
blocks to construct peptides that mimic the ordered secondary structures displayed by the
biopolymers and their functions. Rationally chosen monomeric units from the large
number of structurally diverse designed building blocks are woven together in specific
sequence by iterative synthetic methods leading to the development of novel
homo-polymers and hetero-polymers with architecturally beautiful 3D structures and
desirable properties. Sugar amino acids constitute an important class of such
polyfunctional scaffolds where the carboxyl, amino and hydroxyl termini provide an
excellent opportunity to organic chemists to create structural diversities akin to nature’s
molecular arsenal. The rigid furanoid or pyranoid rings of these molecules make them
ideal candidates as non-peptide scaffolds in peptidomimetic studies where they can easily
be incorporated by using their carboxyl and amino termini utilizing well-developed
solid-phase or solution-phase peptide synthesis methods. At the same time, it allows an
efficient exploitation of the structural diversities of carbohydrate molecules to create
i
Abstract
combinatorial library of sugar amino acid-based molecular frameworks predisposed to
fold into architecturally beautiful ordered structures which may also have interesting
properties. The protected/unprotected hydroxyl groups of sugar rings can also influence
the hydrophobic/hydrophilic nature of such molecular assemblies.
Inspired by those salient features of sugar amino acids, literature survey and
ultimately our thrust towards developing new methods in organic synthesis, prompted us
to endeavor towards the synthesis of C6-substituted furanoid sugar amino acids 1a-d
(Figure 1) in which presence of several chiral centers can gives rise to large number of
possible isomers.
BocHN
6
5
R
O
2
CO2Me
1a-d
a: R = Me, b: R = CH2Ph, c: R = CHMe2, d: R = CH2OBn.
Figure 1
Synthesis of C6-substituted sugar amino acids 1a-d comes under one general
scheme. Our synthesis was started with nucleophilic addition of an anion, generated from
3,4-O-isopropylidene-1,1-dibromobut-1-en-3,4-diol
2,
to
(S)-N,N-dibenzylamino
aldehyde 3a-d. The dibromo compound 2 was obtained quantitatively from
(R)-glyceraldehyde acetonide, which could be made by oxidative cleavage of 1,2:5,6-diO-isopropylidene-D-mannitol using NaIO4, following the reported procedure. Then
N,N-dibenzylaminoaldehydes 3a-d were also prepared accordingly as directed in
literature. The reaction was carried out at –78 ºC for 30 minutes and at room temperature
for another 30 minutes, then aldehydes 3a-d were added at –78 ºC and raised to room
temperature. The yields of the products 4a-d were 78-93% (Scheme 1).
Br
Bn2N
n
Br
O
O i) BuLi, THF, -78 ºC to rt, 1 h
ii) aldehyde 3a-d, -78 ºC to rt, 2 h
R
O
HO
i) H2, Pd(OH)2/C, MeOH/EtOAc
ii) Boc2O, Et3N, MeOH
O
2
4a-d
R
R
O
O
BocHN
CSA, MeOH, 0 ºC
OH
OH
BocHN
OH
OH
5a-d
6a-d
a: R = Me, b: R = CH2Ph, c: R = CHMe2, d: R = CH2OBn.
Scheme 1
ii
Abstract
Compounds 4a-d were subjected to hydrogenation in the presence of Pd(OH)2/C
in a mixture of MeOH and EtOAc to give saturated and debenzylated free amine that was
protected with Boc2O in the presence of Et3N in MeOH to get compounds 5a-d in
83-92% yield. Deprotection of acetonide group of compounds 5a-d with CSA in MeOH
at 0 ºC gave triol products 6a-d in 85-95% yield.
Selective sulfonylation of the primary hydroxyl group of 6a-d with 2,4,6triisopropylbenzenesulfonyl chloride in the presence of pyridine in CH2Cl2 gave sulfonate
intermediate that was treated with K2CO3 in MeOH to get the tetrahydrofuran frameworks
7a-d in 72-88% yield (Scheme 2).
i) TrisCl, Py, CH2Cl2
ii) K2CO3, MeOH
6a-d
OH
BocHN
i) SO3-Py complex, Et3N, CH2Cl2, DMSO
1a-d
ii) NaClO2, NaH2PO4.2H2O,
2-methyl-2-butene, tBuOH
iii) CH2N2, Et2O
O
R
7a-d
a: R = Me, b: R = CH2Ph, c: R = CHMe2, d: R = CH2OBn.
Scheme 2
Finally, a two-step oxidation process converted the primary hydroxyl group of
7a-d into an acid that was treated with an excess of diazomethane in ether to give the
target molecules 1a-d in 84-88% yield.
PART-B: Stereoselctive Synthesis of the Various Isomers of 3,4-Dideoxy Furanoid
Sugar Amino Acids with Methyl Substitution at the C6 Position
To demonstrate the versatility of the method described above, we employed it
successfully for the synthesis of six isomers of 3,4-dideoxy furanoid sugar amino acids
with methyl substitution at the C6 position of 1a, 8-12 (Figure 2).
BocHN
O
CO2Me
Me
BocHN
O
CO2Me
BocHN
O
Me
Me
1a
BocHN
O
CO2Me
8
BocHN
O
Me
Me
11
12
Figure 2
iii
O
Me
9
CO2Me
BocHN
10
CO2Me
CO2Me
Abstract
The starting materials for the synthesis of compounds 1a and 8 were L-alaninederived amino aldehyde 3a and compound 2. Synthesis of compound 1a was out lined in
Schemes 1 & 2. The isomer 8 was started from inverting the free hydroxyl group of 4a by
oxidation-reduction sequence (Scheme 3). Treatment of 4a with SO3-Py complex in the
presence of Et3N in a mixture of CH2Cl2 and DMSO at 0 ºC afforded unstable keto
intermediate that was immediately reduced diasteroselectively by using K-Selectride at
–78 ºC in THF to give compound 13 in 75% yield in two steps. Compound 13 was then
transformed into furanoid 3,4-dideoxy sugar amino acid (2R,5S,6S)-8 (48% overall yield
from 13) following the same procedure as described for the synthesis of 1a.
Bn2N
Me
Me
Bn2N
O
HO
i) SO3-Py, Et3N, DMSO, CH2Cl2
ii) K-Selectride, THF
O
HO
O
i) H2, Pd(OH)2/C, MeOH
ii) Boc2O, Et3N, MeOH
O
4a
13
Me
Me
O
CSA, MeOH, 0 ºC
O
BocHN
OH
OH
BocHN
i) TrisCl, Py, CH2Cl2
ii) Boc2O, Et3N, MeOH
OH
OH
15
14
OH
BocHN
O
Me
16
i) SO3-Py, Et3N, DMSO, CH2Cl2
ii) NaClO2, NaH2PO4.2H2O,
2-methyl-2-butene, tBuOH
iii) CH2N2, Et2O
8
Scheme 3
For the synthesis of compounds 9 and 10, L-alanine-derived N,N-dibenzylamino
aldehyde 3a was reacted with the Li-acetylide, generated from 17. Compound 17, an
enantiomer of 2, was prepared from L-ascorbic acid following the reported procedure.
The adduct 18, formed in 89% yield exclusively with no trace of other diastereomer, was
converted to 19 in 82% yield by the same oxidation-reduction sequence used for the
synthesis of 13. Compounds 18 and 19 were next transformed into the furanoid 3,4dideoxy sugar amino acids (2S,5R,6S)-9 (51% overall yield from 18) and (2S,5S,6S)-10
(52% overall yield from 19), respectively (Scheme 4), following the same procedure as
described for the synthesis of 1a.
iv
Abstract
Me
Bn2N
Br
O L-aldehyde (3a)
Br
O
i) Oxidation
ii) Reduction
O
HO
Bn2N
Me
O
HO
O
17
O
18
19
19
9
18
10
Scheme 4
Syntheses of isomers 11 and 12 were carried out by reacting D-alanine-derived
N,N-dibenzylamino aldehyde 20 with the Li-acetylide of 17 and 2 to give adducts 21 and
22 respectively, in 89% yield. These adducts 21 and 22 can be transformed into the
furanoid 3,4-dideoxy sugar amino acids (2S,5S,6R)-11 and (2R,5S,6R)-12 in 51% overall
yield, respectively (Scheme 5), following the same procedure as described for the
synthesis of 1a.
Me
Bn2N
17
D-alaninal 20
O
HO
O
21
Me
Bn2N
2
11
D-alaninal 20
12
O
HO
O
22
Scheme 5
PART-C: Synthesis of Furanoid Sugar Amino Acid with Methyl Substitution at the
C6 Position
Having synthesized successfully the 3,4-dideoxy furanoid sugar amino acids
derived from different α-amino acids and various isomers of the alanine-derived sugar
amino acids, we were motivated to focus our attention on the synthesis of hydroxylated
C6-substituted furanoid sugar amino acid 23 (Figure 3).
O
O
BocHN
O
CO2Me
Me
23
Figure 3
The synthesis of compound 23 was started from the partial hydrogenation of
alkyne group of 18 using Pd-C/BaSO4 as catalyst in hexane that gave the cis alkene 24 in
v
Abstract
79% yield (Scheme 6). Deprotection of acetonide moiety of 24 was carried out by treating
it with CSA in MeOH to give triol 25 in 94% yield. Primary hydroxyl group of 25 was
treated with TrisCl in the presence of pyridine in CH2Cl2 to give selective mono-sulfonate
intermediate that was converted to compound 26 in 77% yield by treating with K2CO3 in
MeOH. Treatment of 26 with catalytic amount OsO4 in the presence of NMO in a mixture
of acetone and H2O afforded syn hydroxylated compound 27 in 94% yield.
18
H2, Pd/BaSO4
hexane
O CSA, MeOH Bn2N
Bn2N
OH
OH
O
OH
OH
24
25
OH
HO
1. TrisCl, Py, CH2Cl2
2. K2CO3, MeOH
OH OsO4, NMO
acetone/H2O
Bn2N
O
OH
Bn2N
O
Me
Me
27
26
Scheme 6
Hydrogenation of the compound 27 in the presence of catalytic amount of
Pd(OH)2/C in MeOH resulted free amine that was protected using Boc2O in basic
condition to give compound 28 in 88% yield. Selective acetonide protection of two
secondary hydroxyl groups of 28 using 2,2-DMP in the presence of catalytic amount CSA
in acetone gave compound 29 in quantitative yield. Finally, oxidation of the primary
hydroxyl group of 29 using catalytic amount of TEMPO and tBuOCl in basic condition
converted it into an acid that was treated with an excess of CH2N2 in ether to give the
requisite product 23 in 81% yield (Scheme 7).
OH
HO
27
1. H2, Pd(OH)2/C, MeOH
2. Boc2O, Et3N, MeOH
OH
BocHN
O
2,2-DMP, CSA
acetone
Me
28
O
O
1. TEMPO, tBuOCl, H2O
2. CH2N2, Et2O
OH
BocHN
23
O
Me
29
Scheme 7
In conclusion, we have demonstrated the synthesis of different C6-substituted
sugar amino acids following the common strategy.
vi
Abstract
CHAPTER-II
Synthesis and Conformational Studies of Peptidomimetics Containing Sugar Amino
Acids
For discovering new peptide-based drugs, many structurally rigid nonpeptidic
molecular scaffolds have been designed. Insertion of these moieties in appropriate sites, a
common approach to restrict the conformational degrees of freedom in peptides, produces
the specific three-dimensional structures required for binding their receptors. Many of
these templates containing sugar amino acids have been used extensively as
conformationally constrained scaffolds in many peptidomimetic studies.
Herein we have undertaken the design of the peptidomimetic analogs of a known
antagonist of vasoactive intestinal peptide (VIP), which is a widely distributed naturally
occurring neuropeptide with wide ranging biological activities. The invitro studies
suggest that VIP acts as a growth factor and plays a dominant role in the sustained or
indefinite proliferation of cancer cells. The peptide sequence Leu1-Met2-Tyr3-Pro4-Thr5Tyr6-Leu7-Lys8 1 (Figure 1) is known to be one such VIP receptor binding inhibitor and
also has the potential to arrest the growth of malignant cells.
HO
OH
H
N
H2N
O
O
O
N
H
N
O
N
H
H
H
N
O
O
N
H
H
N
O
OH
O
SMe
OH
1
NH2
Figure 1
The design of the peptidomimetic analogs of 1 could be greatly facilitated by the
knowledge of its 3D-structure. So we carried out structural studies of the octapeptide 1
using various NMR techniques that established a well-defined turn structure involving
Tyr3-Pro4-Thr5-Tyr6 residues. Based on these structural studies, novel peptidomimetic
analogs 7, 7a, 8, 8a, 9, 9a, 10 and 11 were subsequently developed using various furanoid
sugar amino acids 2-6 (Figure 2).
vii
Abstract
H2N
5
CO2H
R O S
2
5
H2N
2
CO2H
S O R
2
3
OBn
BnO
H2N
2
5
CO2H
O
H2N
CO2H H2N
O
4
CO2H
O
5
6
Figure 2
Insertion of sugar amino acids (2S,5R)-2 and its enantiomer (2R,5S)-3 as dipeptide
isosters in place of the Tyr3-Pro4 segment of 1 led to the formation of the analogs 7 and
7a, respectively (Figure 3). Incorporation of sugar amino acid 4, racemic mixture of 2 and
3, into the octapeptide 1 by replacing Tyr3-Pro4 and Pro4-Thr5 gave diastereomeric
mixture of products 7, 7a and 8, 8a, respectively. These diastereomers were separated by
HPLC.
NH2
OH
SMe
O
H2N
H
N
N
H
O
H
N
2
5
O
O
N
H
O
O
H
N
OH
N
H
O
O
OH
7: (2S,5R)
7a: (2R,5S)
HO
H
N
H2N
O
O
H
N
N
H
5
O
O
O
H
N
2
H
N
N
H
O
O
OH
O
8 and 8a
SMe
OH
SMe
H
N
H2N
H
N
2
5
O
O
O
9: (2S,5R)
9a: (2R,5S)
HO
Figure 3
viii
O
N
H
OH
O
NH2
Abstract
The HPLC purified diastereomeric peptides, obtained here by insertion of 4 into 1
by replacing Tyr3-Pro4, were matched with the peptides 7 and 7a, obtained from insertion
of enantiomerically pure sugar amino acids 2 and 3. Further truncation of 1, replacing
three of its amino acids Tyr3-Pro4-Thr5 with 2, 4 and deleting one residue each from both
the termini, gave analog 9 and diastereomeric mixture of products 9 and 9a, respectively
(Figure 3). However these diastereomeric mixture of products 9 and 9a could not be
separated by HPLC.
We also synthesized analogs of the pentapeptide Met-Pro-Thr-Tyr-Leu-OH,
derived from octapeptide 1, replacing its Pro-Thr dipeptide segment with 5 to get the
peptide 10 (Figure 4). Furan sugar amino acid 6 has been used to replace Pro4-Thr5 in the
octapeptide 1 sequence to generate the more hydrophobic peptide analog 11.
SMe
H
N
H2N
H
N
O
O
O
N
H
O
OH
O
10
HO
HO
OBn
BnO
H
N
H2N
O
O
N
H
H
N
H
N
O
O
O
O
N
H
H
N
O
OH
O
11
SMe
OH
NH2
Figure 4
The synthesis of furanoid sugar amino acids, starting materials for the peptide
analogs of 1, was described in Scheme 1-3. The starting material for the synthesis of
Fmoc-protected 2 was furan carboxylic acid 13, which was prepared from L-glutamic
acid 12 in 66% yield, following the reported procedure. The compound 13 was converted
to alcohol 14 in 78% yield by treating with borane-dimethyl sulfide complex in THF at 0
ºC to room temperature. The primary hydroxyl group of 14 was protected as trityl ether
by using trityl chloride, pyridine and catalytic amount of DMAP to give 15 in 77% yield.
The lactone 15 was reduced with DIBAL-H in CH2Cl2 to afford 16 as a mixture of
diastereomers at anomeric position in 86% yield. Acetylation of lactol hydroxyl group of
ix
Abstract
16 using Ac2O, Et3N and catalytic amount of DMAP gave a glycosyl acetate compound
17 in 96% yield. Treatment of the compound 17 with trimethylsilyl cyanide in the
presence of BF3-Et2O afforded diastereomeric mixture of glycosyl cyanide alcohols 18 in
64% yield. Reduction of the cyanide group of 18 was carried out by refluxing with
LiAlH4 in ether to furnish the diastereomeric mixture of free amino alcohol that was
protected using FmocOSu to give 19 in 57% yield and the other isomer. Finally, the
primary hydroxyl group of 19 was oxidized to an acid Fmoc-2 in 67% yield using Jones’
reagent (Scheme 1).
NH2
HO2C
CO2H
NaNO2, H2SO4
O
H2O
O
12
O
OH
O
18
O
O
OH TrCl, Et3N
DMAP, CH2Cl2
O
14
OTr Ac2O, Et3N
DMAP, CH2Cl2
AcO
O
OTr TMSCN,BF3-Et2O
CH3CN
17
16
15
NC
BH3-DMS
THF, 0 ºC
13
OTr DIBAL-H, CH2Cl2
HO
-78 ºC
O
CO2H
1. LiAlH4, ether, reflux FmocHN
2. FmocOSu, CH2Cl2
OH
O
19
+
other isomer
Jones' reagent FmocHN
acetone, 0 ºC
O
CO2H
Fmoc-2
Scheme 1
Synthesis of the enantiomer of Fmoc-2, Fmoc-3, was started with the known
compound 20, which was prepared from D-glucose following the reported procedure.
Hydrogenation of 20 using Pd(OH)2/C as a catalyst in MeOH gave benzyl deprotected
compound 21 in 95% yield. Selective silylation of the primary hydroxyl group of 21
using TBDPSCl, Et3N, and catalytic amount of DMAP gave 22 in 96% yield. Treatment
of diol 22 with Ph3P/I2/imidazole in reflux toluene led to the formation of an olefinic
compound 23 in 85% yield. Olefin compound 23 was subjected to hydrogenation using
Pd(OH)2/C as catalyst in MeOH to furnish 24 in 93% yield. Compound 24 was
transformed into 25 in 85% yield by using TBAF in THF. Compound 25 was transformed
into the required Fmoc-protected product 3 following a three-step process-oxidation of
the primary hydroxyl to get the carboxylic acid 26, Boc-deprotection using TFA in
CH2Cl2, followed by Fmoc protection using FmocOSu to furnish Fmoc-3 in 68% yield in
three steps (Scheme 2). All the spectral data of Fmoc-3 are in an agreement with Fmoc-2
except rotation which is equal in magnitude but opposite in sign, indicating that both are
enantiomers.
x
Abstract
BnO
HO
OBn
BocHN
O
H2, Pd(OH)2/C
BocHN
OH
MeOH
20
Ph3P, imidazole
I2, toluene, reflux
HO
OH
TBDPSCl, Et3N
OH
DMAP, DMF
O
OTBDPS
O
OH
O
OTBDPS
O
22
H2, Pd(OH)2/C
BocHN
MeOH
23
BocHN
BocHN
21
BocHN
OH
OTBDPS
TBAF, THF
O
24
NaIO4, RuCl3.H2O BocHN
CH3CN:CCl4:H2O
(1:1:1.5)
O
1. TFA, CH2Cl2
FmocHN
CO2H 2. FmocOSu, aq. Na2CO3
26
25
O
CO2H
Fmoc-3
Scheme 2
Fmoc- 4, racemic mixture of Fmoc-2 and 3, can easily be prepared from the
Boc-protected compound 5 which was prepared from D-fructose following the reported
procedure. Hydrogenation of Boc-5 using Pd(OH)2/C as a catalyst in MeOH gave
saturated furanoid sugar amino acid Boc-4 in 91% yield. Finally, compound Boc-4 was
converted to Fmoc-4 in 79% yield in two steps (Scheme 3), following the same procedure
as described for the synthesis of Fmoc-3. The other sugar amino acid 6 was prepared from
D-mannitol following the reported procedure.
BocHN
O
Boc-5
CO2H
H2, Pd (OH)2/C
MeOH
BocHN
O
CO2H
1. TFA
2. FmocOSu,
aq. Na2CO3
FmocHN
Boc-4
O
CO2H
Fmoc-4
Scheme 3
The various sugar amino acids were then used to prepare our target
peptidomimetic analogs of 1. The peptides were prepared by solid phase peptide synthesis
method.
Conformational analysis of peptide 1, 7, 7a and 9 were carried out in DMSO-d6 by
extensive NMR studies including TOCSY and ROESY experiments and molecular
dynamics (MD) studies. Variable temperature studies were carried out to measure the
temperature coefficients of the amide proton chemical shifts (Δδ/ΔT), which provided
information about their involvements in intramolecular hydrogen bonds. The detailed
NMR studies in DMSO-d6 and subsequent constrained MD simulations revealed that the
peptide 1 has 10-membered type-II β-turn around Pro4-Thr5residue (Figure 5).
xi
Abstract
Figure 5: Stereo view of the 25 superimposed energy-minimized structure 1 sampled during 50
cycles of the 300 ps constrained MD simulations following the simulated annealing protocol: Hbonded region (left), full structure (right)
Peptide 7 shows the existence of a 10-membered hydrogen bonded turn structure
between ThrNH→MetCO, which mimics a regular β-turn structure, where as the rest of
the peptide backbone seems to take an extended conformation (Figure 6).
Figure 6: Stereo view of the 25 superimposed energy-minimized structure 7 sampled during 50
cycles of the 300 ps constrained MD simulations following the simulated annealing protocol: Hbonded region (left), full structure (right)
In compound 7a the observed β-turn conformation of the peptide is similar to that
of 7. MD calculations showed the existence of a 10-membered H-bond between
ThrNH→MetCO (Figure 7).
Figure 7: Stereo view of the 25 superimposed energy-minimized structure 7a sampled during 50
cycles of the 300 ps constrained MD simulations following the simulated annealing protocol: Hbonded region (left), full structure (right)
In compound 9, TyrNH shows small magnitude of Δδ/ΔT (–2.8 ppb/ºK),
indicating its participation in intramolecular hydrogen bonding. The MD calculations
showed the existence of a β-turn like structure having a 10-membered intramolecular
hydrogen bond (Figure 8).
Figure 8: Stereo view of the 25 superimposed energy-minimized structure 9 sampled during 50
cycles of the 300 ps constrained MD simulations following the simulated annealing protocol: Hbonded region (left), full structure (right)
xii
Abstract
In conclusion, the β-turn structure found in VIP antagonist 1 was successfully
reproduced by introducing the nonproteinogenic dipeptide isosters in the molecule,
resulting in the development of peptidomimetic analogs.
Of the various analogs tested, 8a showed good anticancer activity in the entire
human cancer cell lines tested except HeP-G2 (liver cancer). It was the most promising
analog in HeP-2 (laryngeal cancer), MiaPaCa.2 (pancreas), KB (oral) and ECV-304
(endothelial) cancer cell lines. In the ovarian cancer cell line PA-1, the smaller mimetic
analog 10 showed maximum anti-cancer activity at 1 nM concentration whereas analog
8a showed similar inhibition of proliferation at 100 nM concentration.
CHAPTER-III
The Total Synthesis of Conagenin
Conagenin (1), a low molecular weight immunomodulator, was isolated from the
fermentation broth of Streptomyces roseosporus by Ishizuka and co-workers. It stimulates
activated T cells, which produce lymphokines and generate antitumor effector cells. The
antitumor efficacies of adriamycin and mitomycin C against murine leukemias are also
enhanced by 1, making it a potential candidate for cancer chemotherapy.3 It has a densely
functionalised structure consisting of a (2R,3S,4R)-2,4-dihydroxy-3-methylpentanoic acid
moiety with three contiguous chiral centres, coupled to a (S)-α-methylserine possessing a
quaternary chiral centre.
Retrosynthetically, compound 1 can be obtained from the peptide coupling of two
fragments 2 and 3. For the construction of both the fragments, 3-methyl-2-butene-1-ol 4
was used as a common starting material. In our synthesis, we could utilize the successful
application, as a key step, of the method developed by our group for the synthesis of
2-methyl-1,3-diols via radical mediated regio and stereoselective ring opening of chiral
trisubstituted 2,3-epoxy alcohols at the more substituted center using Cp2TiCl.
OAc OAc
OH
OH
O
CO2H
H
N
CO2H
OH
2
+
OH
OH
4
1
BocHN
CO2Me
3
Scheme 1
xiii
Abstract
Starting material for the synthesis of fragment 2 was syn epoxy alcohol 8 which
was prepared from 4 in five steps. Benzylation of the hydroxyl group of 4 with BnBr,
NaH and catalytic amount of TBAI gave compound 5 in 94% yield. SeO2-mediated
allylic oxidation of compound 5 furnished aldehyde 6 in 63% yield. Grignard addition to
the aldehyde 6 with MeMgI gave racemic mixture 7, 7a in 84% yield. The racemic
mixture was separated by Sharpless kinetic resolution to afford pure allylic alcohol 7 in
48% yield that was treated with mCPBA to get the requisite syn allylic alcohol 8 in 40%
yield (Scheme 2).
4
SeO2, TBHP (70% aq. solution)
NaH, BnBr, TBAI (cat.) BnO
BnO
CHO
5
MeMgI, Et2O, 0 oC, 1 h
6
O
Sharpless kinetic
resolution
BnO
7 and 7a
7
+
BnO
OH
7b
O
O
mCPBA
BnO
7
BnO
+
BnO
OH
OH
OH
OH
8a
8
Scheme 2
The epoxy alcohol 8 on treatment with active titanium species Cp2TiCl in THF
furnished the required all syn product 9 in 84% yield almost exclusively as a single
isomer. The two secondary hydroxyl groups of 9 were protected with acetyls using Ac2O,
Et3N and catalytic amount of DMAP to give 10 in 88% yield. Debenzylation of
compound 10 using hydrogenation in the presence of catalytic amount of Pd/C in EtOAc
gave primary hydroxyl intermediate that was converted to an acid 2, in 81% yield in three
steps, using two step oxidation sequence (Scheme 3).
OH
8
Cp2TiCl
OAc OAc
OH
OBn
Ac2O, Et3N, DMAP
OBn
i) H2, 10% Pd/C, EtOAc, rt, 1h
ii) SO3-Py, Et3N, CH2Cl2, DMSO
iii) NaClO2, NaH2PO4.2H2O
2
10
9
Scheme 3
Fragment 3 was started from aldehyde 6 which was reduced to allylic alcohol 11
in 89% yield using NaBH4 in MeOH at 0 ºC (Scheme 4). Reaction of 12, prepared in 80%
yield by the Katsuki-Sharpless catalytic asymmetric epoxidation of 11 using (–)-DIPT,
xiv
Abstract
with trichloroacetonitrile in the presence of a catalytic amount of DBU gave
trichloroacetimidate 13 that underwent facile intramolecular SN2 opening of the epoxide
ring by Et2AlCl to furnish the oxazole 14 in 84% yield in two steps. Acid hydrolysis of
the oxazole ring 14 gave an amino diol that was protected in situ using Boc2O to furnish
15 in 92% yield. Compound 15 was subjected to hydrogenation using Pd/C as catalyst in
EtOAc to give triol 16 in 94% yield. Selective oxidative cleavage of 1,2-diol moiety of 16
using NaIO4 resulted in the formation of an aldehyde which was oxidized to an acid and
esterified using CH2N2 to give Boc-(S)-α-methylserine methyl ester 3 in 72% yield in
three steps.
6
HO
NaBH4, MeOH
OBn
O
D-(-)- DIPT, Ti (OiPr)4, HO
CH2Cl2
OBn
12
11
O
NH
Cl3C
CCl3CN, DBU
CH2Cl2
O
O
Et2AlCl, CH2Cl2
OBn
Cl3C
OBn
N
OH
14
13
OH
OH
BocHN
1M HCl, THF
then NaHCO3, Boc2O
OBn
OH
H2, 10% Pd/C
EtOAc
BocHN
OH
OH
16
15
i) NaIO4, CH2Cl2, aq. NaHCO3
3
ii) NaClO2, NaH2PO4.2H2O,
2-methyl-2-butene, tBuOH
iii) CH2N2, ether, 0 ºC
Scheme 4
The coupling of the fragments 2 and 3 was carried out using DCC, HOBt and
DMAP to furnish the ester 17 in 91% yield. The ester intermediate was treated with acid
followed by base treatment to get the protected conagenin 18 in 78% yield. Finally,
saponification of 18 furnished the target molecule 1 in 83% yield (Scheme 5). Our
synthetic conagenin showed a rotation value of [α]D = +50.2 (c 0.38, MeOH); lit. value:
[α]D = +55.4 (c 4.6, MeOH).
OAc OAc
2
+
3
DCC, HOBt
DMAP
OAc OAc
CO2Me
O
O
i) TFA
NHBoc ii) aq. NaHCO
3
O
H
N
CO2Me 1M K2CO3
MeOH
OH
1
18
17
Scheme 5
In conclusion, the total synthesis of this molecule not only provided an access to a
larger quantity necessary for further biological studies, the scheme developed here can
help to design and build more potent synthetic analogs.
xv