Carbohydrate Research 388 (2014) 8–18
Contents lists available at ScienceDirect
Carbohydrate Research journal homepage: www.elsevier.com/locate/carres
Synthesis of new b2,2-amino acids with carbohydrate side chains: impact on the synthesis of peptides Gangavaram V. M. Sharma ⇑, Karnekanti Rajender, Gattu Sridhar, Post Sai Reddy, Marumudi Kanakaraju Organic and Biomolecular Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India
a r t i c l e
i n f o
Article history: Received 9 October 2013 Received in revised form 24 January 2014 Accepted 30 January 2014 Available online 10 February 2014
a b s t r a c t The study describes the synthesis of new geminally disubstituted C-linked carbo-b2,2-amino acids (b2,2-Caas) with different carbohydrate side chains, and their use in the synthesis of b2,2-peptides. The study infers that the side chain has an influence on the synthesis of peptides and their conformational behaviour. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: b2,2-Amino acids b2,2-Peptides Geminally disubstituted Carbohydrate side chain
1. Introduction Peptide oligomers, synthesized from non-proteinogenic amino acids that generate diverse folding patterns stabilized by non-covalent interactions, are referred to as foldamers.1 The initial reports2 on peptides from b-amino acids triggered considerable research interest on b-peptides.3 However, reports on the peptides from b2-amino acids and geminally disubstituted b2,2- and b3,3-amino acids4 are scarce. Studies in the area of ‘foldamers’,1 by using C-linked carbo amino acids (Caas),5a non-natural amino acids with carbohydrate side chains, resulted in peptides with skeletal and structural diversity.5b–d Earlier, b2,2-peptides reported from b2,2-Caas with D-xylose side chain showed diverse conformations6 by structural analysis. These studies evidently indicated that the C-3 –OMe group of D-xylo furanoside side chain has participated in electrostatic interaction to support the conformational stability. The above observations prompted us to undertake a study on the synthesis of new b2,2-Caas 1, 2 and 3 (Fig. 1), besides, b2,2-Caa 4 (Fig. 1) with no –OMe group at C-3 position, to understand the impact of the –OMe group in the thus derived b2,2-peptides. 2. Results and discussion 2.1. Synthesis of Boc-(R)-b2,2-Caa-OMe 1 The Boc-(R)-b2,2-Caa-OMe (1) was prepared from the known aldehyde 57 prepared from diacetone glucose (GDA). Accordingly, ⇑ Corresponding author. Tel.: +91 40 2719 3154. E-mail address: [email protected] (G.V.M. Sharma). http://dx.doi.org/10.1016/j.carres.2014.01.026 0008-6215/Ó 2014 Elsevier Ltd. All rights reserved.
reaction of aldehyde 5 with 98% formaldehyde8 and 1M NaOH in THF/H2O solvent mixture (1:1) at 0 °C to room temperature for 16 h gave the 1,3-diol 6 in 58% yield (Scheme 1). The diol 6 on selective protection with TBSCl, imidazole and n-Bu2SnO at 20 °C for 1 h gave 7 (74%). Oxidation of alcohol 7 with IBX in EtOAc at reflux for 1 h furnished aldehyde 7a, which on further reaction with NaClO2 and H2O2 at 0 °C to room temperature for 5 h afforded acid 8. Reaction of 8 with CH2N2 (generated in situ) in ether at 0 °C to room temperature for 2 h furnished ester 9 in 59% yield (over 3 steps). Desilylation of 9 on treatment with TBAF in THF at 0 °C to room temperature for 3 h furnished alcohol 10 in 89% yield. Reaction of 10 with Tf2O and pyridine in CH2Cl2 at 0 °C to room temperature for 30 min gave triflate 10a, which on subsequent treatment with NaN3 in DMF at 0 °C to room temperature for 3 h furnished azide 11 in 76% yield. Finally, azide 11 on reaction with 10% Pd/C–H2 in MeOH at room temperature for 4 h afforded the amine 12, which on further reaction with (Boc)2O and Et3N in CH2Cl2 gave Boc-(R)-b2,2-Caa-OMe 1 in 85% yield. To ascertain stereochemistry in 1, it was first subjected to reduction with DIBAL-H in CH2Cl2 at 0 °C to room temperature for 1 h to afford the alcohol 13 (Scheme 1), which, on cyclization with NaH in THF at 0 °C to room temperature for 3 h gave the cyclic urethane derivative 14. The structure of 14 was established with the help of detailed 1D and 2D NMR experiments (DQFCOSY, NOESY, 600 MHz). The presence of the NOE correlation between H-2 and H-5 protons suggests their proximity with each other. The ‘R’ configuration at C-3 enables to fix the configuration at C4 in 14 as ‘S’. Hence, it was confirmed that the absolute configuration at C-4 in ester 1 is ‘R’.
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G. V. M. Sharma et al. / Carbohydrate Research 388 (2014) 8–18
MeO 2C
O
MeO2C
O
(R)
BocHN MeO
O
O
O
O
O
(S)
BocHN O
O 2
1
MeO 2C
OMe
(R)
BocHN
BocHN O
MeO2C
OMe
(S)
O
O 4
3
Figure 1. Structures of new b2,2-amino acids.
O HO
O
O
H
a
O
MeO
6
O
RO 2C
d, e
O
O
O
MeO
MeO2C
O
f
O
R MeO
O
10a R = OTf 11 R = N 3 (76%)
O
O
MeO 2C i, j
O
g, h
O
MeO
8R=H 9 R = Me (59%) (over 3 steps)
O
(74%)
HO 10 (89%)
m MeO 2C
c
O 7
O
MeO
7a
TBSO MeO
(58%)
TBSO
TBSO
O O
b
O
5 OHC
HO
O
HO MeO
HO 2C
O
O
BocHN
O
O
MeO
RHN O MeO 12 R = H 1 R = Boc (85%)
MeO 2C n
HO
O 1b O
O O
13 R = Boc (82%)
l
dipeptide
O
HOOCF3C.H2N
O
R HN MeO
o
O
MeO
k
O 1a
O HN MeO
O O
(S)
O 14
Scheme 1. Reagents and conditions: (a) 98% HCHO, THF/H2O, 1M NaOH, 0 °C–rt, 16 h; (b) TBSCI, imidazole, n-Bu2SnO, CH2Cl2, 20 °C, 1 h; (c) IBX, EtOAc, DMSO, reflex, 1 h; (d) NaClO2, 30% H2O2, t-BuOH/H2O (7:3), 0 °C–rt, 5 h; (e) CH2N2, ether, 0 °C–rt ,2 h; (f) TBAF, THF, 0 °C–rt, 3 h; (g) Tf2O, pyridine, CH2Cl2, 0 °C–rt, 30 min; (h) NaN3, DMF, 0 °C–rt 3 h; (i) 10% Pd/C–H2, MeOH, rt, 4 h; (j) (Boc)2O, Et3N, CH2Cl2, 0 °C–rt, 5 h; (k) DIBAL-H, CH2Cl2, 0 °C–rt, 1 h; (l) NaH, THF, 0 °C–rt, 3 h; (m) aq 4M NaOH, MeOH, 0 °C–rt, 1 h; (n) CF3COOH, CH2Cl2, 0 °C–rt, 2 h; (o) HOBt, EDCI, DIPEA, CH2Cl2, 0 °C–rt, 6 h.
Ester 1 was subjected to hydrolysis with 4M NaOH in MeOH to afford the acid 1a, while, 1 on reaction with CF3COOH in CH2Cl2 furnished the salt 1b (Scheme 1). Acid 1a was subjected to peptide coupling (EDCI, HOBt, DIPEA) with 1b in CH2Cl2, which, however, met with failure to give the expected dipeptide. The thus observed result may be attributed to the sugar pucker and presence of 1,2-acetonide and C-3 –OMe functionalities on the same side and the steric congestion due to 1,2-acetonide. Hence, it was proposed to synthesize new b2,2-Caas 2 and 3 from D-ribose. 2.1.1. Synthesis of Boc-(S)-b2,2-Caa-OMe 2 Accordingly, aldehyde 159 on reaction with 98% formaldehyde and 1M NaOH for 16 h afforded the 1,3-diol 16 in 44% yield (Scheme 2). Treatment of diol 16 with TBSCl, imidazole and n-Bu2SnO in CH2Cl2 for 1 h gave 17 (73%). Oxidation of alcohol 17 with IBX at reflux for 1 h furnished the aldehyde 17a, which on further oxidation with NaClO2 and H2O2 gave acid 18. Esterification of acid 18 with CH2N2 for 2 h afforded 19 in 54% yield (over 3 steps). Treatment of 19 with TBAF in THF, followed by reaction of 20 with Tf2O and pyridine in CH2Cl2 gave triflate 20a. Subsequently, 20a on reaction with NaN3 in DMF at 0 °C to room temperature for 3 h furnished azide 21 in 80% yield. Reduction of azide 21 with 10% Pd/C-H2 in MeOH and subsequent reaction of 21a with (Boc)2O and Et3N in CH2Cl2 for 5 h afforded Boc-(S)-b2,2-Caa-OMe 2 in 80% yield.
2.1.2. Synthesis of Boc-(R)-b2,2-Caa-OMe 3 Likewise, ester 3 was prepared by a similar strategy. Thus, reaction of 17 with Tf2O and pyridine gave triflate 22, which was subsequently converted into 23 in 77% yield. Azide 23 on hydrogenation (10% Pd/C, MeOH), followed by protection [(Boc)2O, Et3N] of the amine 23a in CH2Cl2 furnished 24 in 92% yield (over 2 steps). Desilylation (TBAF) of 24 in THF and oxidation (IBX) of the alcohol 25 in EtOAc at reflux afforded 25a. Oxidation of 25a with NaClO2 and H2O2 for 5 h and reaction of 26 with CH2N2 gave Boc-(R)-b2,2-Caa-OMe 3 in 53% yield (over 3 steps) (Scheme 3). Alcohol 25 was treated with NaH in THF at 0 °C to room temperature for 3 h to furnish the cyclic derivative 27 (72%). Stereospecific assignment for the epimeric centre in 27 was carried out with the help of coupling and NOE correlation from NOESY JH1/H2 0 and JH2/H3 6.0, which suggests furanoside to be of 3T2 geometry. Further, NOEs between NH/H2 and NH/H3 suggest epimeric centre to be ‘S’ in urethane 27. Hence, it was confirmed that the absolute configuration at C-4 in ester 3 is ‘R’. The acids 2a/3a and salts 2b/3b were independently subjected to peptide coupling, which, however met with failure to give dipeptides. Eventhough, the new b2,2-Caas 1, 2 and 3 could be successfully synthesized, their conversion to peptides was unsuccessful. This may be attributed to the steric congestion due to the quaternary carbon at C-4 and the configuration of carbohydrate side chain. Hence, it was proposed to prepare a new b2,2-Caa 4, having a
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G. V. M. Sharma et al. / Carbohydrate Research 388 (2014) 8–18
O O
OMe
H O
HO a
HO
O
OMe
O
O
15 RO 2C
O
O
O
O
O
OMe
MeO 2C
g
O
OMe
h
R O
18 R = H 19 R = Me (54%) (over 3 steps)
O
O
O 20a R = OTf 21 R = N 3 (80%) (over 2 steps)
20 (83%) HO 2C
i
O 17a
HO O
OMe
TBSO
17 (73%)
MeO 2C
f O
O
O
OMe
TBSO
OHC c
16 (44%)
d, e
OMe
TBSO
b
HO
O
O
OMe
BocHN MeO 2C
O
OMe
O 2a
O
RHN
k O
O
21a R = H 2 R = Boc (80%) (over 2 steps)
MeO 2C
O
dipeptide
OMe
HOOCF 3C.H 2N
j
O
2b
O
Scheme 2. Reagents and conditions: (a) 98% HCHO, THF/H2O, 1M NaOH, 0 °C–rt, 16 h; (b) TBSCI, imidazole, n-Bu2SnO, CH2Cl2, 20 °C, 1 h; (c) IBX, EtOAc, DMSO, reflex, 1 h; (d) NaClO2, 30% H2O2, t-BuOH/H2O (7:3), 0 °C–rt, 5 h; (e) CH2N2, ether, 0 °C–rt ,2 h; (f) TBAF, THF, 0 °C–rt, 3 h; (g) (i) Tf2O, pyridine, CH2Cl2, 0 °C–rt, 30 min; (ii) NaN3, DMF, 0 °C–rt 3 h; (h) (i) 10% Pd/C-H2, MeOH, rt, 4 h; (ii) (Boc)2O, Et3N, CH2Cl2, 0 °C–rt, 5 h; (i) aq 4M NaOH, MeOH, 0 °C–rt, 1 h; (j) CF3COOH, CH2Cl2, 0 °C–rt, 2 h; (k) HOBt, EDCI, DIPEA, CH2Cl2, 0 °C–rt, 6 h.
R
a
17
O
RHN
OMe
O
b
HO
OMe
O
O
OMe
d
BocHN
TBSO
TBSO
O
c
O
O
O
23a R = H 24 R = Boc (92%) (over 2 steps)
22 R = OTf 23 R = N 3 (77%) (over 2 steps)
HO 2C
g
O 25 (89%)
O
OMe
BocHN OHC
O
RO2C
OMe e, f
BocHN O
O
OMe
O
O
3a
BocHN
i
O
O
O
MeO 2C
25a
h
26 R = H 3 R = Me (53% over 3 steps)
g
O
O
dipeptide
OMe
HOOCF3CH2N O
O 25
O
O 3b
OMe
(S)
HN O
O 27 (72%)
Scheme 3. Reagents and conditions: (a) (i) Tf2O, pyridine, CH2Cl2, 0 °C–rt, 30 min; (ii) NaN3, DMF, 0 °C–rt 3 h; (b) (i) 10% Pd/C–H2, MeOH, rt, 4 h; (ii) (Boc)2O, Et3N, CH2Cl2, 0 °C–rt, 5 h; (c) TBAF, THF, 0 °C–rt, 3 h; (d) IBX, EtOAc, DMSO, reflex, 1 h; (e) NaClO2, 30% H2O2, t-BuOH/H2O (7:3), 0 °C–rt, 5 h; (f) CH2N2, ether, 0 °C–rt, 2 h; (g) NaH, THF, 0 °C–rt, 3 h; (g) aq 4N NaOH, MeOH, 0 °C–rt, 1 h; (h) CF3COOH, CH2Cl2, 0 °C–rt, 2 h; (i) HOBt, EDCI, DIPEA, CH2Cl2, 0 °C–rt, 6 h.
1,2-acetonide group and a C-3 deoxy carbon (Fig. 1), since the b2,2Caas with D-xylo furanoside side chain successfully demonstrated electrostatic interactions between C-3 –OMe and NH of amide bond to stabilize the conformations. 2.2. Synthesis of Boc-(S)-b2,2-Caa-OMe 4 The synthetic study was initiated from C-3 deoxy aldehyde 28.10 Accordingly, reaction of aldehyde 2810 with 98% formaldehyde and 1M NaOH in aq THF followed by silylation of diol 29 in CH2Cl2 at 20 °C for 1 h gave 30 in 76% yield (Scheme 4). Oxidation
of 30 with TEMPO11 and BAIB in CH2Cl2 and subsequent reaction of 30a with CH2N2 afforded ester 31 in 65% yield (over 2 steps). Desilylation of 31 with TBAF, followed by reaction of 32 with Tf2O and pyridine in CH2Cl2 furnished triflate 32a, which on subsequent reaction with NaN3 in DMF gave azide 33 in 78% yield. Reduction (10% Pd/C, MeOH) of azide 33 and subsequent reaction of 33a with (Boc)2O and Et3N at 0 °C to room temperature for 5 h afforded Boc(S)-b2,2-Caa-OMe 4 in 87% yield. Ester 4 on reduction with DIBAL-H in CH2Cl2, followed by cyclization of 34 with NaH in THF at 0 °C to room temperature for 3 h gave cyclic derivative 35 (70%). The couplings and NOE correlation
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G. V. M. Sharma et al. / Carbohydrate Research 388 (2014) 8–18
O HO
O
H
HO
O
a
O
O
O
29 (64%)
MeO2C e, f
O
O
O
g
O
HO
O
O
O
k
O 33a R=NH2 4 R=NHBoc (87%)
O
l
O 34 (73%)
m
O
S
N H
j
O
33 (78%)
O O
O (S)
R
32 R = H (76%) 32a R = OTf
O
O 30a R = H 31 R = OMe (65% over 2 steps)
MeO2C
h, i
O N3
RO
O
TBSO
O
30 (76%)
MeO 2C
O
BocHN
c, d
TBSO
HO 28
RO2C
O
b
O
O 35 (70%)
HO2C
O
MeO 2C
O
O
HOOCF3C.H 2N
BocHN O
27a
27b
O O
n dipeptide Scheme 4. Reagents and conditions: (a) 98% HCHO, THF/H2O, 1M NaOH, 0 °C–rt, 16 h; (b) TBSCI, imidazole, n-Bu2SnO, CH2Cl2, 20 °C, 1 h; (c) TEMPO, BAIB, CH2Cl2/H2O (1:1), 0 °C–rt ,2 h; (d) CH2N2, ether, 0 °C–rt ,2 h; (e) TBAF, THF, 0 °C–rt, 3 h; (f) Tf2O, pyridine, CH2Cl2, 0 °C–rt, 30 min; (g) NaN3, DMF, 0 °C–rt 3 h; (h) 10% Pd/C-H2, MeOH, rt, 4 h; (i) (Boc)2O, Et3N, CH2Cl2, 0 °C–rt, 5 h; (j) DIBAL-H, CH2Cl2, 0 °C–rt 1 h; (k) NaH, THF, 0 °C–rt, 1 h; (l) aq 4M NaOH, MeOH, 0 °C–rt, 1 h; (m) CF3COOH, CH2Cl2, 0 °C–rt, 2 h; (n) HOBt, EDCI, DIPEA, CH2Cl2, 0 °C–rt, 6 h.
O OH
N3
a
O
O
O
N O H
O
O O
O
33b
OMe
N3
O
N3 c
O O
O
OMe
O b
33
O
O
OMe
H 2N
O
O 36 (79%)
O O 33a
N3
a
N3
O
O
O
N O H
O
O
O
OMe
O
O
O
N O H
O
OH
O
O
O 36a
O
H 2N
b
O
O
N O H
O
c
OMe
36 O
O
O
O 36b
N3
O
O
O
O
O
N O H
N OH
N O H
O
O
O
O
O
O
OMe
O
O
37 (70%) Scheme 5. Reagents and conditions: (a) aq 4M NaOH, MeOH, 0 °C–rt, 1 h; (b) 10% Pd/C-H2, MeOH, rt, 4 h; (c) HOBt, EDCI, DIPEA, CH2Cl2, 0 °C–rt, 6 h.
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G. V. M. Sharma et al. / Carbohydrate Research 388 (2014) 8–18
the structure could not be assigned due to insufficient information. Thus, the study infers that the stereochemistry of the carbohydrate side chain and C-3–OMe has influence in peptide synthesis and conformational behaviour. 4. Experimental 4.1. General methods
Figure 2. CD spectrum of tetrapeptide 37 in MeOH solution at concentration of 0.2 mM.
from NOESY suggest the furanoside to be of 3T2 geometry, while, the coupling between H3/H5 suggests epimeric centre to be ‘S’ in compound 35. Hence, the absolute configuration at C-4 in 4 is confirmed as ‘S’. For the synthesis of peptides, 4 was subjected to base (aq 4M NaOH) hydrolysis to give acid 4a, while 4 on reaction with CF3COOH afforded the salt 4b (Scheme 4). However, coupling of 4a and 4b met with failure in giving the desired dipeptide, then it was proposed to synthesize peptides from azide12 33. Accordingly, 33 was subjected to base hydrolysis with aq 4M NaOH at 0 °C to room temperature for 1 h to give acid 33b, while, 33 on hydrogenation with 10% Pd-C in MeOH at room temperature for 4 h furnished 33a (Scheme 5). Coupling of acid 33b with amine 33a under standard peptide coupling conditions13 in the presence of EDCI, HOBt and DIPEA in CH2Cl2 at 0 °C to room temperature for 6 h afforded the dipeptide 36 in 79% yield. As described above, 36 on base (LiOH) hydrolysis furnished acid 36a, while, catalytic reduction of 36 with Pd-C in MeOH gave the amine 36b. Further, coupling of 36a with 36b afforded the tetrapeptide 37 in 70% yield. However, attempted synthesis of hexapeptide from 37 met with failure. Detailed NMR studies (see Supporting information) in CDCl3 solution on tetrapeptide 37 were carried out using several 2DNMR experiments (TOCSY, ROESY, HSQC and HMBC) to deduce the structure. 1H NMR spectrum showed that all the amide protons have chemical shifts (d) >7 ppm, suggesting their participation in H-bonding. Solvent titration studies (when upto 33% DMSO-d6 was added sequentially to the CDCl3 solution) showed a very small change in the amide proton chemical shifts (Dd) of 8.0 Hz or
Contents lists available at ScienceDirect
Carbohydrate Research journal homepage: www.elsevier.com/locate/carres
Synthesis of new b2,2-amino acids with carbohydrate side chains: impact on the synthesis of peptides Gangavaram V. M. Sharma ⇑, Karnekanti Rajender, Gattu Sridhar, Post Sai Reddy, Marumudi Kanakaraju Organic and Biomolecular Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500 007, India
a r t i c l e
i n f o
Article history: Received 9 October 2013 Received in revised form 24 January 2014 Accepted 30 January 2014 Available online 10 February 2014
a b s t r a c t The study describes the synthesis of new geminally disubstituted C-linked carbo-b2,2-amino acids (b2,2-Caas) with different carbohydrate side chains, and their use in the synthesis of b2,2-peptides. The study infers that the side chain has an influence on the synthesis of peptides and their conformational behaviour. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: b2,2-Amino acids b2,2-Peptides Geminally disubstituted Carbohydrate side chain
1. Introduction Peptide oligomers, synthesized from non-proteinogenic amino acids that generate diverse folding patterns stabilized by non-covalent interactions, are referred to as foldamers.1 The initial reports2 on peptides from b-amino acids triggered considerable research interest on b-peptides.3 However, reports on the peptides from b2-amino acids and geminally disubstituted b2,2- and b3,3-amino acids4 are scarce. Studies in the area of ‘foldamers’,1 by using C-linked carbo amino acids (Caas),5a non-natural amino acids with carbohydrate side chains, resulted in peptides with skeletal and structural diversity.5b–d Earlier, b2,2-peptides reported from b2,2-Caas with D-xylose side chain showed diverse conformations6 by structural analysis. These studies evidently indicated that the C-3 –OMe group of D-xylo furanoside side chain has participated in electrostatic interaction to support the conformational stability. The above observations prompted us to undertake a study on the synthesis of new b2,2-Caas 1, 2 and 3 (Fig. 1), besides, b2,2-Caa 4 (Fig. 1) with no –OMe group at C-3 position, to understand the impact of the –OMe group in the thus derived b2,2-peptides. 2. Results and discussion 2.1. Synthesis of Boc-(R)-b2,2-Caa-OMe 1 The Boc-(R)-b2,2-Caa-OMe (1) was prepared from the known aldehyde 57 prepared from diacetone glucose (GDA). Accordingly, ⇑ Corresponding author. Tel.: +91 40 2719 3154. E-mail address: [email protected] (G.V.M. Sharma). http://dx.doi.org/10.1016/j.carres.2014.01.026 0008-6215/Ó 2014 Elsevier Ltd. All rights reserved.
reaction of aldehyde 5 with 98% formaldehyde8 and 1M NaOH in THF/H2O solvent mixture (1:1) at 0 °C to room temperature for 16 h gave the 1,3-diol 6 in 58% yield (Scheme 1). The diol 6 on selective protection with TBSCl, imidazole and n-Bu2SnO at 20 °C for 1 h gave 7 (74%). Oxidation of alcohol 7 with IBX in EtOAc at reflux for 1 h furnished aldehyde 7a, which on further reaction with NaClO2 and H2O2 at 0 °C to room temperature for 5 h afforded acid 8. Reaction of 8 with CH2N2 (generated in situ) in ether at 0 °C to room temperature for 2 h furnished ester 9 in 59% yield (over 3 steps). Desilylation of 9 on treatment with TBAF in THF at 0 °C to room temperature for 3 h furnished alcohol 10 in 89% yield. Reaction of 10 with Tf2O and pyridine in CH2Cl2 at 0 °C to room temperature for 30 min gave triflate 10a, which on subsequent treatment with NaN3 in DMF at 0 °C to room temperature for 3 h furnished azide 11 in 76% yield. Finally, azide 11 on reaction with 10% Pd/C–H2 in MeOH at room temperature for 4 h afforded the amine 12, which on further reaction with (Boc)2O and Et3N in CH2Cl2 gave Boc-(R)-b2,2-Caa-OMe 1 in 85% yield. To ascertain stereochemistry in 1, it was first subjected to reduction with DIBAL-H in CH2Cl2 at 0 °C to room temperature for 1 h to afford the alcohol 13 (Scheme 1), which, on cyclization with NaH in THF at 0 °C to room temperature for 3 h gave the cyclic urethane derivative 14. The structure of 14 was established with the help of detailed 1D and 2D NMR experiments (DQFCOSY, NOESY, 600 MHz). The presence of the NOE correlation between H-2 and H-5 protons suggests their proximity with each other. The ‘R’ configuration at C-3 enables to fix the configuration at C4 in 14 as ‘S’. Hence, it was confirmed that the absolute configuration at C-4 in ester 1 is ‘R’.
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G. V. M. Sharma et al. / Carbohydrate Research 388 (2014) 8–18
MeO 2C
O
MeO2C
O
(R)
BocHN MeO
O
O
O
O
O
(S)
BocHN O
O 2
1
MeO 2C
OMe
(R)
BocHN
BocHN O
MeO2C
OMe
(S)
O
O 4
3
Figure 1. Structures of new b2,2-amino acids.
O HO
O
O
H
a
O
MeO
6
O
RO 2C
d, e
O
O
O
MeO
MeO2C
O
f
O
R MeO
O
10a R = OTf 11 R = N 3 (76%)
O
O
MeO 2C i, j
O
g, h
O
MeO
8R=H 9 R = Me (59%) (over 3 steps)
O
(74%)
HO 10 (89%)
m MeO 2C
c
O 7
O
MeO
7a
TBSO MeO
(58%)
TBSO
TBSO
O O
b
O
5 OHC
HO
O
HO MeO
HO 2C
O
O
BocHN
O
O
MeO
RHN O MeO 12 R = H 1 R = Boc (85%)
MeO 2C n
HO
O 1b O
O O
13 R = Boc (82%)
l
dipeptide
O
HOOCF3C.H2N
O
R HN MeO
o
O
MeO
k
O 1a
O HN MeO
O O
(S)
O 14
Scheme 1. Reagents and conditions: (a) 98% HCHO, THF/H2O, 1M NaOH, 0 °C–rt, 16 h; (b) TBSCI, imidazole, n-Bu2SnO, CH2Cl2, 20 °C, 1 h; (c) IBX, EtOAc, DMSO, reflex, 1 h; (d) NaClO2, 30% H2O2, t-BuOH/H2O (7:3), 0 °C–rt, 5 h; (e) CH2N2, ether, 0 °C–rt ,2 h; (f) TBAF, THF, 0 °C–rt, 3 h; (g) Tf2O, pyridine, CH2Cl2, 0 °C–rt, 30 min; (h) NaN3, DMF, 0 °C–rt 3 h; (i) 10% Pd/C–H2, MeOH, rt, 4 h; (j) (Boc)2O, Et3N, CH2Cl2, 0 °C–rt, 5 h; (k) DIBAL-H, CH2Cl2, 0 °C–rt, 1 h; (l) NaH, THF, 0 °C–rt, 3 h; (m) aq 4M NaOH, MeOH, 0 °C–rt, 1 h; (n) CF3COOH, CH2Cl2, 0 °C–rt, 2 h; (o) HOBt, EDCI, DIPEA, CH2Cl2, 0 °C–rt, 6 h.
Ester 1 was subjected to hydrolysis with 4M NaOH in MeOH to afford the acid 1a, while, 1 on reaction with CF3COOH in CH2Cl2 furnished the salt 1b (Scheme 1). Acid 1a was subjected to peptide coupling (EDCI, HOBt, DIPEA) with 1b in CH2Cl2, which, however, met with failure to give the expected dipeptide. The thus observed result may be attributed to the sugar pucker and presence of 1,2-acetonide and C-3 –OMe functionalities on the same side and the steric congestion due to 1,2-acetonide. Hence, it was proposed to synthesize new b2,2-Caas 2 and 3 from D-ribose. 2.1.1. Synthesis of Boc-(S)-b2,2-Caa-OMe 2 Accordingly, aldehyde 159 on reaction with 98% formaldehyde and 1M NaOH for 16 h afforded the 1,3-diol 16 in 44% yield (Scheme 2). Treatment of diol 16 with TBSCl, imidazole and n-Bu2SnO in CH2Cl2 for 1 h gave 17 (73%). Oxidation of alcohol 17 with IBX at reflux for 1 h furnished the aldehyde 17a, which on further oxidation with NaClO2 and H2O2 gave acid 18. Esterification of acid 18 with CH2N2 for 2 h afforded 19 in 54% yield (over 3 steps). Treatment of 19 with TBAF in THF, followed by reaction of 20 with Tf2O and pyridine in CH2Cl2 gave triflate 20a. Subsequently, 20a on reaction with NaN3 in DMF at 0 °C to room temperature for 3 h furnished azide 21 in 80% yield. Reduction of azide 21 with 10% Pd/C-H2 in MeOH and subsequent reaction of 21a with (Boc)2O and Et3N in CH2Cl2 for 5 h afforded Boc-(S)-b2,2-Caa-OMe 2 in 80% yield.
2.1.2. Synthesis of Boc-(R)-b2,2-Caa-OMe 3 Likewise, ester 3 was prepared by a similar strategy. Thus, reaction of 17 with Tf2O and pyridine gave triflate 22, which was subsequently converted into 23 in 77% yield. Azide 23 on hydrogenation (10% Pd/C, MeOH), followed by protection [(Boc)2O, Et3N] of the amine 23a in CH2Cl2 furnished 24 in 92% yield (over 2 steps). Desilylation (TBAF) of 24 in THF and oxidation (IBX) of the alcohol 25 in EtOAc at reflux afforded 25a. Oxidation of 25a with NaClO2 and H2O2 for 5 h and reaction of 26 with CH2N2 gave Boc-(R)-b2,2-Caa-OMe 3 in 53% yield (over 3 steps) (Scheme 3). Alcohol 25 was treated with NaH in THF at 0 °C to room temperature for 3 h to furnish the cyclic derivative 27 (72%). Stereospecific assignment for the epimeric centre in 27 was carried out with the help of coupling and NOE correlation from NOESY JH1/H2 0 and JH2/H3 6.0, which suggests furanoside to be of 3T2 geometry. Further, NOEs between NH/H2 and NH/H3 suggest epimeric centre to be ‘S’ in urethane 27. Hence, it was confirmed that the absolute configuration at C-4 in ester 3 is ‘R’. The acids 2a/3a and salts 2b/3b were independently subjected to peptide coupling, which, however met with failure to give dipeptides. Eventhough, the new b2,2-Caas 1, 2 and 3 could be successfully synthesized, their conversion to peptides was unsuccessful. This may be attributed to the steric congestion due to the quaternary carbon at C-4 and the configuration of carbohydrate side chain. Hence, it was proposed to prepare a new b2,2-Caa 4, having a
10
G. V. M. Sharma et al. / Carbohydrate Research 388 (2014) 8–18
O O
OMe
H O
HO a
HO
O
OMe
O
O
15 RO 2C
O
O
O
O
O
OMe
MeO 2C
g
O
OMe
h
R O
18 R = H 19 R = Me (54%) (over 3 steps)
O
O
O 20a R = OTf 21 R = N 3 (80%) (over 2 steps)
20 (83%) HO 2C
i
O 17a
HO O
OMe
TBSO
17 (73%)
MeO 2C
f O
O
O
OMe
TBSO
OHC c
16 (44%)
d, e
OMe
TBSO
b
HO
O
O
OMe
BocHN MeO 2C
O
OMe
O 2a
O
RHN
k O
O
21a R = H 2 R = Boc (80%) (over 2 steps)
MeO 2C
O
dipeptide
OMe
HOOCF 3C.H 2N
j
O
2b
O
Scheme 2. Reagents and conditions: (a) 98% HCHO, THF/H2O, 1M NaOH, 0 °C–rt, 16 h; (b) TBSCI, imidazole, n-Bu2SnO, CH2Cl2, 20 °C, 1 h; (c) IBX, EtOAc, DMSO, reflex, 1 h; (d) NaClO2, 30% H2O2, t-BuOH/H2O (7:3), 0 °C–rt, 5 h; (e) CH2N2, ether, 0 °C–rt ,2 h; (f) TBAF, THF, 0 °C–rt, 3 h; (g) (i) Tf2O, pyridine, CH2Cl2, 0 °C–rt, 30 min; (ii) NaN3, DMF, 0 °C–rt 3 h; (h) (i) 10% Pd/C-H2, MeOH, rt, 4 h; (ii) (Boc)2O, Et3N, CH2Cl2, 0 °C–rt, 5 h; (i) aq 4M NaOH, MeOH, 0 °C–rt, 1 h; (j) CF3COOH, CH2Cl2, 0 °C–rt, 2 h; (k) HOBt, EDCI, DIPEA, CH2Cl2, 0 °C–rt, 6 h.
R
a
17
O
RHN
OMe
O
b
HO
OMe
O
O
OMe
d
BocHN
TBSO
TBSO
O
c
O
O
O
23a R = H 24 R = Boc (92%) (over 2 steps)
22 R = OTf 23 R = N 3 (77%) (over 2 steps)
HO 2C
g
O 25 (89%)
O
OMe
BocHN OHC
O
RO2C
OMe e, f
BocHN O
O
OMe
O
O
3a
BocHN
i
O
O
O
MeO 2C
25a
h
26 R = H 3 R = Me (53% over 3 steps)
g
O
O
dipeptide
OMe
HOOCF3CH2N O
O 25
O
O 3b
OMe
(S)
HN O
O 27 (72%)
Scheme 3. Reagents and conditions: (a) (i) Tf2O, pyridine, CH2Cl2, 0 °C–rt, 30 min; (ii) NaN3, DMF, 0 °C–rt 3 h; (b) (i) 10% Pd/C–H2, MeOH, rt, 4 h; (ii) (Boc)2O, Et3N, CH2Cl2, 0 °C–rt, 5 h; (c) TBAF, THF, 0 °C–rt, 3 h; (d) IBX, EtOAc, DMSO, reflex, 1 h; (e) NaClO2, 30% H2O2, t-BuOH/H2O (7:3), 0 °C–rt, 5 h; (f) CH2N2, ether, 0 °C–rt, 2 h; (g) NaH, THF, 0 °C–rt, 3 h; (g) aq 4N NaOH, MeOH, 0 °C–rt, 1 h; (h) CF3COOH, CH2Cl2, 0 °C–rt, 2 h; (i) HOBt, EDCI, DIPEA, CH2Cl2, 0 °C–rt, 6 h.
1,2-acetonide group and a C-3 deoxy carbon (Fig. 1), since the b2,2Caas with D-xylo furanoside side chain successfully demonstrated electrostatic interactions between C-3 –OMe and NH of amide bond to stabilize the conformations. 2.2. Synthesis of Boc-(S)-b2,2-Caa-OMe 4 The synthetic study was initiated from C-3 deoxy aldehyde 28.10 Accordingly, reaction of aldehyde 2810 with 98% formaldehyde and 1M NaOH in aq THF followed by silylation of diol 29 in CH2Cl2 at 20 °C for 1 h gave 30 in 76% yield (Scheme 4). Oxidation
of 30 with TEMPO11 and BAIB in CH2Cl2 and subsequent reaction of 30a with CH2N2 afforded ester 31 in 65% yield (over 2 steps). Desilylation of 31 with TBAF, followed by reaction of 32 with Tf2O and pyridine in CH2Cl2 furnished triflate 32a, which on subsequent reaction with NaN3 in DMF gave azide 33 in 78% yield. Reduction (10% Pd/C, MeOH) of azide 33 and subsequent reaction of 33a with (Boc)2O and Et3N at 0 °C to room temperature for 5 h afforded Boc(S)-b2,2-Caa-OMe 4 in 87% yield. Ester 4 on reduction with DIBAL-H in CH2Cl2, followed by cyclization of 34 with NaH in THF at 0 °C to room temperature for 3 h gave cyclic derivative 35 (70%). The couplings and NOE correlation
11
G. V. M. Sharma et al. / Carbohydrate Research 388 (2014) 8–18
O HO
O
H
HO
O
a
O
O
O
29 (64%)
MeO2C e, f
O
O
O
g
O
HO
O
O
O
k
O 33a R=NH2 4 R=NHBoc (87%)
O
l
O 34 (73%)
m
O
S
N H
j
O
33 (78%)
O O
O (S)
R
32 R = H (76%) 32a R = OTf
O
O 30a R = H 31 R = OMe (65% over 2 steps)
MeO2C
h, i
O N3
RO
O
TBSO
O
30 (76%)
MeO 2C
O
BocHN
c, d
TBSO
HO 28
RO2C
O
b
O
O 35 (70%)
HO2C
O
MeO 2C
O
O
HOOCF3C.H 2N
BocHN O
27a
27b
O O
n dipeptide Scheme 4. Reagents and conditions: (a) 98% HCHO, THF/H2O, 1M NaOH, 0 °C–rt, 16 h; (b) TBSCI, imidazole, n-Bu2SnO, CH2Cl2, 20 °C, 1 h; (c) TEMPO, BAIB, CH2Cl2/H2O (1:1), 0 °C–rt ,2 h; (d) CH2N2, ether, 0 °C–rt ,2 h; (e) TBAF, THF, 0 °C–rt, 3 h; (f) Tf2O, pyridine, CH2Cl2, 0 °C–rt, 30 min; (g) NaN3, DMF, 0 °C–rt 3 h; (h) 10% Pd/C-H2, MeOH, rt, 4 h; (i) (Boc)2O, Et3N, CH2Cl2, 0 °C–rt, 5 h; (j) DIBAL-H, CH2Cl2, 0 °C–rt 1 h; (k) NaH, THF, 0 °C–rt, 1 h; (l) aq 4M NaOH, MeOH, 0 °C–rt, 1 h; (m) CF3COOH, CH2Cl2, 0 °C–rt, 2 h; (n) HOBt, EDCI, DIPEA, CH2Cl2, 0 °C–rt, 6 h.
O OH
N3
a
O
O
O
N O H
O
O O
O
33b
OMe
N3
O
N3 c
O O
O
OMe
O b
33
O
O
OMe
H 2N
O
O 36 (79%)
O O 33a
N3
a
N3
O
O
O
N O H
O
O
O
OMe
O
O
O
N O H
O
OH
O
O
O 36a
O
H 2N
b
O
O
N O H
O
c
OMe
36 O
O
O
O 36b
N3
O
O
O
O
O
N O H
N OH
N O H
O
O
O
O
O
O
OMe
O
O
37 (70%) Scheme 5. Reagents and conditions: (a) aq 4M NaOH, MeOH, 0 °C–rt, 1 h; (b) 10% Pd/C-H2, MeOH, rt, 4 h; (c) HOBt, EDCI, DIPEA, CH2Cl2, 0 °C–rt, 6 h.
12
G. V. M. Sharma et al. / Carbohydrate Research 388 (2014) 8–18
the structure could not be assigned due to insufficient information. Thus, the study infers that the stereochemistry of the carbohydrate side chain and C-3–OMe has influence in peptide synthesis and conformational behaviour. 4. Experimental 4.1. General methods
Figure 2. CD spectrum of tetrapeptide 37 in MeOH solution at concentration of 0.2 mM.
from NOESY suggest the furanoside to be of 3T2 geometry, while, the coupling between H3/H5 suggests epimeric centre to be ‘S’ in compound 35. Hence, the absolute configuration at C-4 in 4 is confirmed as ‘S’. For the synthesis of peptides, 4 was subjected to base (aq 4M NaOH) hydrolysis to give acid 4a, while 4 on reaction with CF3COOH afforded the salt 4b (Scheme 4). However, coupling of 4a and 4b met with failure in giving the desired dipeptide, then it was proposed to synthesize peptides from azide12 33. Accordingly, 33 was subjected to base hydrolysis with aq 4M NaOH at 0 °C to room temperature for 1 h to give acid 33b, while, 33 on hydrogenation with 10% Pd-C in MeOH at room temperature for 4 h furnished 33a (Scheme 5). Coupling of acid 33b with amine 33a under standard peptide coupling conditions13 in the presence of EDCI, HOBt and DIPEA in CH2Cl2 at 0 °C to room temperature for 6 h afforded the dipeptide 36 in 79% yield. As described above, 36 on base (LiOH) hydrolysis furnished acid 36a, while, catalytic reduction of 36 with Pd-C in MeOH gave the amine 36b. Further, coupling of 36a with 36b afforded the tetrapeptide 37 in 70% yield. However, attempted synthesis of hexapeptide from 37 met with failure. Detailed NMR studies (see Supporting information) in CDCl3 solution on tetrapeptide 37 were carried out using several 2DNMR experiments (TOCSY, ROESY, HSQC and HMBC) to deduce the structure. 1H NMR spectrum showed that all the amide protons have chemical shifts (d) >7 ppm, suggesting their participation in H-bonding. Solvent titration studies (when upto 33% DMSO-d6 was added sequentially to the CDCl3 solution) showed a very small change in the amide proton chemical shifts (Dd) of 8.0 Hz or
