Synthesis of new β2,2-amino acids with carbohydrate side chains: impact on the synthesis of peptides

Carbohydrate Research 388 (2014) 8–18

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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 <0.13 ppm, confirming participation of all the amide protons in H-bonding and a distinct structure in the peptide 37. In addition, 3JNH–CbH are either >8.0 Hz or <5.1 Hz, again implying a value of dihedral angle [C(O)–N–Cb–Ca] ±120°. The ROESY spectra showed several intraresidual and sequential NOE correlations. However, only a few medium range NOEs with rather weak intensities were observed. Thus, despite the fact that signatures for a well-defined conformation were though observed, it was not possible to arrive at a definite structure for 37. The CD spectrum of the peptide 37 is as shown below in Figure 2. 3. Conclusion In conclusion, the present study reported the synthesis of four new b2,2-Caas 1–4. Attempted synthesis of peptides from 1, 2 and 4 met with failure to give the expected peptides. However, the azido ester 33 successfully gave the di- and tetrapeptides. The conformational analysis on the short azido peptide 37 revealed the presence of signatures for a well-defined conformation, while,

Solvents were dried over standard drying agents and freshly distilled prior to use. Chemicals were purchased and used without further purification. All column chromatographic separations were performed using silica gel (Acme’s, 60–120 mesh). Organic solutions were dried over anhydrous Na2SO4 and concentrated below 40 °C in vacuo. 1H NMR and 13C NMR spectra were measured with Bruker, Avance 300 MHz and Varian Unity Inova-400, 500 and 600 MHz spectrometer with tetramethylsilane as an internal standard for solutions in CDCl3. J values are given in Hertz. IR spectra were recorded on Perkin–Elmer IR-683, JASCO FT/IR-5300 spectrophotometer with NaCl and KBr optics. Optical rotations were measured with JASCO DIP 300 digital polarimeter. Mass spectra were recorded on BRUKER MAXIS and CEC-21-11013 or Finnigan Mat 1210 double focusing mass spectrometers, operating at a direct inlet system or LC/MSD Trap SL (Agilent Technologies). 4.1.1. ((3aR,6S,6aR)-6-Methoxy-2,2dimethyltetrahydrofuro[2,3-d][1,3]dioxole-5,5-diyl) dimethanol (6) To a stirred solution of 5 (14.0 g, 68.62 mmol) in THF/H2O (1:1, 140 mL), aq 98% formaldehyde (28 mL) and 1M NaOH (84 mL) were added and stirred at room temperature for 16 h. Solvent was evaporated and residue extracted with EtOAc (3  50 mL). Organic layers were washed with brine (40 mL), dried (Na2SO4) and evaporated. The residue was purified by column chromatography (60–120 mesh Silica gel, 40% EtOAc in pet. ether) to give 6 (8.11 g, 58%) as a white solid; mp 74–76 °C; ½a25 D = +132.4 (c 0.35, CHCl3); IR (KBr): m 3295, 2986, 2940, 1462, 1380, 1218, 1077, 1025, 934, 857, 640 cm1; 1H NMR (300 MHz, CDCl3): d 5.79 (d, 1H, J = 3.9 Hz, H1), 4.75 (t, 1H, J = 4.9 Hz, H2), 4.08 (d, 1H, J = 2.6 Hz, H3), 3.85–3.81 (m, 4H, 2  OCH2), 3.52 (s, 3H, OMe), 2.32 (br s, 1H, OH), 2.0 (br s, 1H, OH), 1.35 (s, 3H, Me), 1.24 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 113.2, 104.1, 86.4, 80.8, 78.0, 63.6, 62.5, 58.6, 26.3, 25.6; HRMS (ESI+): m/z calcd for C10H18O6 (M+Na)+ 257.1008, found 257.1011. 4.1.2. ((3aR,5R,6S,6aR)-5-((tert-Butyldimethylsilyloxy)methyl)6-methoxy-2,2-dimethyltetrahydrofuro[2,3-d][1,3]dioxol-5yl)methanol (7) To a stirred solution of 6 (5 g, 21.36 mmol) in CH2Cl2 (50 mL), imidazole (4.35 g, 64.10 mmol), n-Bu2SnO (cat) and TBSCl (3.20 g, 21.36 mmol) were added and stirred at 20 °C for 1 h. The reaction mixture was diluted with CH2Cl2 (60 mL), washed with water (60 mL), brine (60 mL) and dried (Na2SO4). Solvent was evaporated and purified the residue by column chromatography (60–120 mesh Silica gel, 6% EtOAc in pet. ether) to give 7 (3.15 g, 74%) as a colourless syrup; ½a25 D = +94.9 (c 0.35, CHCl3); IR (neat): m 3505, 2937, 2855, 1632, 1465, 1378, 1254, 1212, 1113, 842, 779, 671 cm1; 1 H NMR (300 MHz, CDCl3): d 5.75 (d, 1H, J = 3.7 Hz, H1), 4.93 (dd, 1H, J = 4.9, 3.7 Hz, H2), 4.89 (d, 1H, J = 4.9 Hz, H3), 4.82 (dd, 1H, J = 3.4, 6.0 Hz, OCH), 4.71 (dd, 1H, J = 4.9, 3.8 Hz, OCH), 3.90–3.75 (m, 2H, –OCH2), 3.50 (s, 3H, OMe), 2.34 (dd, 1H, J = 7.2, 6.4 Hz, OH), 1.60 (s, 3H, Me), 1.35 (s, 3H, Me), 0.90 (s, 9H, 3 Me), 0.1 (s, 6H, 2 Me); 13C NMR (75 MHz, CDCl3): d 113.7, 104.3, 86.9, 80.9, 78.8, 65.0, 63.1, 59.1, 26.8, 25.8, 25.6, 18.3, 5.4; HRMS (ESI+): m/z calcd for C16H32O6Si (M+Na)+ 371.1868, found 371.1862.

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4.1.3. (3aR,5R,6S,6aR)-Methyl 5-((tert-butyldimethylsilyloxy) methyl)-6-methoxy-2,2-dimethyltetrahydrofuro[2,3-d][1,3] dioxole-5-carboxylate (9) To a solution of alcohol 7 (2.50 g, 7.18 mmol) dissolved in ethyl acetate (15 mL), IBX (2.41 g, 8.62 mmol) followed by DMSO (4 mL) were added and heated at reflux for 1 h. The reaction mixture was filtered through a pad of celite and extracted with EtOAc (2  20 mL). The combined organic layers were washed with water (20 mL), brine (20 mL), dried (Na2SO4) and evaporated to give the aldehyde 7a, which was used as such for the next reaction. To a solution of the above aldehyde 7a in t-butanol/water (7:3, 12 mL) at 0 °C, NaClO2 (0.98 g, 10.83 mmol) and 30% aq H2O2 (4.1 mL, 35.88 mmol) were added and stirred at room temperature for 5 h. The reaction mixture was extracted with EtOAc (3  30 mL), dried (Na2SO4) and evaporated to afford crude carboxylic acid 8 as a light yellow syrup. To a solution of acid 8 in ether (15 mL) at 0 °C, a solution of CH2N2 in ether (100 mL) was added till the persistence of yellow colour in the reaction mixture. After 2 h, ether was evaporated and purified the residue by column chromatography (60–120 mesh Silica gel, 10% EtOAc in pet ether) to afford 9 (1.6 g, 59%, over 3 steps) as a colourless syrup; ½a25 D = +87.6 (c 0.35, CHCl3); IR (neat): m 3450, 2935, 2857, 1745, 1632, 1464, 1378, 1253, 1098, 1021, 840, 778, 667 cm1; 1H NMR (300 MHz, CDCl3): d 5.79 (d, 1H, J = 3.8 Hz, H1), 4.64 (dd, 1H, J = 4.9, 3.8 Hz, H2), 4.06 (d, 1H, J = 4.9 Hz, H3), 3.94 (d, 1H, J = 10.6 Hz, –OCH), 3.74 (s, 3H, COOMe), 3.71 (d, 1H, J = 10.9 Hz, –OCH), 3.48 (s, 3H, OMe), 1.61 (s, 3H, Me), 1.36 (s, 3H, Me), 0.90 (s, 9H, 3 Me), 0.08 (s, 3H, Me) 0.07 (s, 3H, Me); 13 C NMR (75 MHz, CDCl3): d 169.6, 114.7, 105.3, 89.7, 81.4, 79.8, 66.7, 59.6, 52.1, 29.6, 26.8, 25.8, 18.2, 5.4, 5.6; HRMS (ESI+): m/z calcd for C17H32O7Si (M+Na)+ 399.1810, found 399.1804. 4.1.4. (3aR,5R,6S,6aR)-Methyl 5-(hydroxymethyl)-6-methoxy2,2-dimethyltetrahydrofuro[2,3-d][1,3]dioxole-5-carboxylate (10) To a solution of 9 (1.20 g, 3.19 mmol) in THF (5 mL) at 0 °C, 1 M TBAF in THF (3.83 mL) was added and stirred at room temperature for 3 h. THF was removed under reduced pressure and purified the residue by column chromatography (60–120 mesh Silica gel, 20% EtOAc in pet ether) to give 10 (0.74 g, 89%) as a colourless syrup; ½a25 D = +68.4 (c 0.35, CHCl3); IR (neat): m 3489, 2985, 2943, 1742, 1635, 1444, 1380, 1213, 1165, 1088, 1028, 862, 772 cm1; 1H NMR (300 MHz, CDCl3): d 5.83 (d, 1H, J = 3.4 Hz, H1), 4.75 (t, 1H, J = 4.2 Hz, H2), 4.12 (d, 1H, J = 4.9 Hz, H3), 3.83 (s, 2H, OCH2), 3.79 (s, 3H, COOMe), 3.56 (s, 3H, OMe), 2.30 (br s, 1H, OH), 1.65 (s, 3H, Me), 1.35 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 169.9, 114.6, 104.7, 88.1, 81.6, 78.8, 65.9, 59.6, 52.4, 26.1, 25.4; HRMS (ESI+): m/z calcd for C11H18O7 (M+Na)+ 285.0958, found 285.0954. 4.1.5. (3aR,5R,6S,6aR)-Methyl 5-(azidomethyl)-6-methoxy-2,2dimethyltetrahydrofuro[2,3-d][1,3]dioxole-5-carboxylate (11) A solution of 10 (0.52 g, 1.98 mmol) and pyridine (0.3 mL, 3.9 mmol) in CH2Cl2 (14 mL) was treated with triflic anhydride (0.39 mL, 2.38 mmol) at 0 °C and stirred at room temperature for 30 min. The reaction mixture was diluted with CH2Cl2 (35 mL), washed with water (15 mL) and brine (15 mL). It was dried (Na2SO4) and evaporated to give triflate 10a as a solid, which was immediately used for further reaction. To a solution of triflate 10a in DMF (5 mL) at 0 °C, NaN3 (0.25 g, 3.95 mmol) was added and stirred at room temperature for 3 h. The reaction mixture was treated with water (10 mL) and extracted with ether (2  10 mL). Organic layers were washed with brine (10 mL), dried (Na2SO4), evaporated and purified the residue by column chromatography (60–120 mesh Silica gel, 12% EtOAc in pet. ether) to give 11 (0.43 g, 76%) as a white solid; mp 67–69 °C; ½a25 D = +112.1 (c 0.25, CHCl3); IR (KBr): m 3500, 3344, 2985, 2941,

13

2839, 2520, 2098, 1760, 1457, 1377, 1267, 1212, 1092, 1026, 969, 892, 743, 656 cm1; 1H NMR (300 MHz, CDCl3): d 5.89 (d, 1H, J = 3.4 Hz, H1), 4.77 (dd, 1H, J = 4.5, 3.4 Hz, H2), 4.11 (d, 1H, J = 4.9 Hz, H3), 3.80 (s, 3H, COOMe), 3.70 (m, 1H, CHN3), 3.53 (s, 3H, OMe), 3.47 (s, 1H, CHN3), 1.67 (s, 3H, Me), 1.37 (s, 3H, Me); 13 C NMR (75 MHz, CDCl3): d 169.1, 114.9, 87.8, 82.3, 78.7, 59.6, 56.0, 52.4, 26.2, 25.4; HRMS (ESI+): m/z calcd for C11H17N3O6 (M+Na)+ 310.1014, found 310.1010. 4.1.6. (3aR,5R,6S,6aR)-Methyl 5-((tert-butoxycarbonylamino) methyl)-6-methoxy-2,2-dimethyltetrahydrofuro[2,3-d][1,3] dioxole-5-carboxylate (1) To a solution of 11 (0.3 g, 1.04 mmol) in MeOH (1.5 mL), catalytic amount of Pd-C (10%) was added and stirred the reaction mixture under hydrogen atmosphere at room temperature for 4 h. It was filtered and washed with EtOAc (10 mL). The filtrate was evaporated under reduced pressure to furnish amine 12, which was immediately used for the next reaction without further purification. To a stirred solution of the above amine 12 and Et3N (0.32 mL, 2.3 mmol) in CH2Cl2 (11 mL), (Boc)2O (0.37 mL, 1.72 mmol) was added at 0 °C. After 5 h, water (15 mL) was added and extracted with CH2Cl2 (2  20 mL). Organic layer was washed with brine (15 mL), dried (Na2SO4), evaporated and purified the residue by column chromatography (60–120 mesh Silica gel, 15% EtOAc in pet ether) to furnish 1 (0.32 g, 85%) as a colourless syrup; ½a25 D = +48.6 (c 0.55, CHCl3); IR (neat): m 3396, 2980, 2939, 1744, 1715, 1512, 1455, 1370, 1244, 1167, 1097, 1018, 989, 864, 775 cm1; 1H NMR (300 MHz, CDCl3): d 5.77 (d, 1H, J = 3.6 Hz, H1), 4.85 (br s, 1H, NH), 4.67 (dd, 1H, J = 4.3, 3.9 Hz, H2), 3.80 (d, 1H, J = 4.7 Hz, H3), 3.77 (s, 3H, COOMe), 3.53 (s, 3H, OMe), 3.50 (m, 2H, –CH2), 1.61 (s, 3H, Me), 1.43 (s, 9H, 3 Me), 1.33 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 169.9, 155.6, 114.7, 104.0, 86.9, 82.9, 79.8, 78.9, 59.9, 52.4, 46.7, 28.3, 26.1, 25.2; HRMS (ESI+): m/z calcd for C16H27NO8 (M+Na)+ 384.1636, found 384.1630. 4.1.7. tert-Butyl((3aR,5S,6S,6aR)-5-(hydroxymethyl)-6-methoxy2,2-dimethyltetrahydrofuro[2,3-d][1,3]dioxol-5-yl) methylcarbamate (13) To a stirred solution of 1 (0.3 g, 0.81 mmol) in CH2Cl2 (6 mL), DIBAL-H (0.9 mL, 0.99 mmol, 20% solution in hexane) was added at 0 °C and stirred at the same temperature for 1 h. Methanol (0.2 mL) was added to the reaction mixture at 0 °C and stirred for 10 min. Sat aq solution of sodium potassium tartarate (0.5 mL) was added, stirred for 10 min and evaporated methanol. It was diluted with water (2 mL) and extracted with CH2Cl2 (2  3 mL). The combined organic layers were washed with brine (2 mL) and dried (Na2SO4). Solvent was evaporated and purified the residue by column chromatography (60–120 mesh Silica gel, 80% EtOAc in pet ether) to afford alcohol 13 (0.21 g, 82%) as a colourless syrup; ½a25 D = +28.3 (c 0.32, CHCl3); IR (KBr): m 3280, 3083, 2929, 1679, 1562, 1454, 1379, 1294, 1171, 1107, 1064, 1014, 865, 712 cm1; 1 H NMR (300 MHz, CDCl3: d 5.77 (d, 1H, J = 3.8, H1), 4.83 (br s, 1H, NH), 4.73 (t, 1H, J = 4.7 Hz, H2), 4.12 (m, 1H, H3), 3.82 (dd, 2H, J = 18.7, 5.9 Hz, CH2), 3.62–3.52 (m, 1H, CH), 2.41 (br s, 1H, OH), 1.62 (s, 6H, 2 Me), 1.46 (s, 9H, 3 Me); 13C NMR (75 MHz, CDCl3): d 156.5, 113.4, 103.9, 85.4, 82.2, 79.5, 77.9, 63.2, 59.1, 44.3, 29.6, 28.3, 26.4, 25.7; HRMS (ESI+): m/z calcd for C15H26NO7 (M+H)+ 356.1685, found 356.1672. 4.1.8. (3aR,5S,6S,6aR)-6-Methoxy-2,2-dimethyldihydro-3aHspiro[furo[2,3-d][1,3]dioxole-5,50 -[1,3]oxazinan]-20 -one (14) To an ice cooled suspension of NaH (0.06 g, 2.40 mmol) in THF (1.5 mL), a solution of 13 (0.2 g, 0.60 mmol) in THF (2 mL) was added dropwise at 0 °C and stirred at room temperature for 3 h. The reaction mixture was quenched with saturated aq NH4Cl

14

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(1 mL) and extracted with EtOAc (3 mL). The combined organic layers were washed with water (2 mL), brine (2 mL) and dried (Na2SO4). Solvent was evaporated and purified the residue by column chromatography (60–120 mesh Silica gel, 80% EtOAc in pet. ether) to afford 14 (0.12 g, 78%) as a white solid; mp 140–142 °C; ½a20 D = +53.6 (c 0.37, CHCl3); IR (KBr): m 3393, 3307, 2988, 2925, 2853, 1708, 1482, 1481, 1381, 1289, 1239, 1261, 1211, 1189, 1162, 1092, 1068, 1010, 889, 855, 768, 636, 589; 1H NMR (300 MHz, CDCl3: d 5.69 (d, 1H, J = 3.8 Hz, H1), 5.06 (d, 1H, J = 6.8 Hz, CH), 4.85 (d, 1H, J = 7.2 Hz, NH), 4.74-4.64 (m, 4H, CH, CH2, H2), 3.69 (s, 3H, OMe), 3.59 (d, 1H, J = 4.2 Hz, H3), 1.47 (s, 3H, Me), 1.30 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 154.1, 112.5, 105.4, 85.6, 83.1, 79.5, 68.0, 58.4, 48.0, 26.6, 25.6; ESIMS: 282 (M+Na)+. 4.1.9. ((3aS,6R,6aR)-6-Methoxy-2,2-dimethyltetrahydrofuro [3,4-d][1,3]dioxole-4,4-diyl)-dimethanol (16) To a stirred solution of 15 (4.95 g, 24.50 mmol) in a mixture of water (25 mL) and THF (25 mL) at 0 °C, aq 98% formaldehyde (10 mL) and 1M NaOH (50 mL) were added sequentially and stirred at room temperature for 16 h. Work up as described for 6 and purification of the residue by column chromatography (60–120 mesh Silica gel, 35% EtOAc in pet. ether) afforded 16 (2.50 g, 44%) as a white solid; mp 93–95 °C; ½a25 D = 154.8 (c 0.48, CHCl3); IR (neat): m 3294, 2492, 1724, 1601, 1447, 1381, 1211, 1092, 1040, 872 cm1; 1 H NMR (400 MHz, CDCl3): d 4.90 (s, 1H, H1), 4.79 (d, 1H, J = 6.0 Hz, H2), 4.60 (d, 1H, J = 6.0 Hz, H3), 3.71 (t, 2H, J = 11.3 Hz, –CH2), 3.56 (t, 2H, J = 11.3 Hz, –CH2), 3.41 (s, 3H, OMe), 3.23 (br s, 1H, OH), 2.21 (br s, 1H, OH), 1.48 (s, 3H, Me), 1.30 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 112.0, 109.8, 88.2, 85.6, 81.4, 63.8, 55.4, 26.4, 24.6; HRMS (ESI+): m/z calcd for C10H18O6 (M+Na)+ 257.1001, found 257.1002. 4.1.10. ((3aS,4S,6R,6aR)-4-((tert-Butyldimethylsilyloxy)methyl)6-methoxy-2,2-dimethyl-tetrahydrofuro[3,4-d][1,3]dioxol-4yl)methanol (17) To a stirred solution of 16 (2.40 g, 10.26 mmol) in CH2Cl2 (24 mL), imidazole (2.09 g, 30.77 mmol), n-Bu2SnO (cat) and TBSCl (1.62 g, 10.26 mmol) were added and stirred at 20 °C for 1 h. Work up as described for 7 and purification of the residue by column chromatography (60–120 mesh Silica gel, 5% EtOAc in pet. ether) furnished 17 (2.6 g, 73%) as a colourless syrup; ½a25 D = 77.6 (c 0.16, CHCl3); IR (neat): m 3467, 2933, 2858, 1633, 1467, 1378, 1253, 1211, 1167, 1086, 1015, 970, 839, 778, 675, 588, 511 cm1; 1H NMR (300 MHz, CDCl3): d 4.91 (s, 1H, H1), 4.74 (d, 1H, J = 6.0 Hz, H2), 4.62 (d, 1H, J = 5.7 Hz, H3), 3.85 (m, 2H, –CH2), 3.66 (m, 2H, –CH2), 3.40 (s, 3H, OMe), 1.59 (d, 1H, J = 19.3 Hz, OH), 1.46 (s, 3H, Me), 1.30 (s, 3H, Me), 0.91 (d, 9H, J = 7.2 Hz, 3 Me), 0.09 (s, 3H, Me), 0.06 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 112.0, 109.7, 91.0, 86.4, 81.6, 66.4, 63.8, 55.4, 26.2, 25.8, 24.6, 5.5, 5.7; HRMS (ESI+): m/z calcd for C16H32O6Si (M+Na)+ 371.1865, found 371.1480. 4.1.11. (3aS,4S,6R,6aR)-Methyl-4-((tert-butyldimethylsilyloxy) methyl)-6-methoxy-2,2-di- methyltetrahydrofuro[3,4-d][1,3] dioxole-4-carboxylate (19) A solution of alcohol 17 (0.86 g, 2.47 mmol) in EtOAc (5 mL) was treated with IBX (0.83 g, 2.96 mmol) followed by DMSO (1 mL) and heated at reflux for 1 h. Work up as described for 7a gave the aldehyde 17a, which was used as such for the next step. To a solution of the above aldehyde 17a in t-butanol/water (7:3, 15 mL) at 0 °C, NaClO2 (0.70 g, 7.76 mmol) and 30% aq H2O2 (2.93 mL, 25.87 mmol) were added and stirred at room temperature for 5 h. Work up as described for 8 afforded crude carboxylic acid 18 as a light yellow syrup. To a solution of acid 18 in ether (10 mL) at 0 °C, a solution of CH2N2 in ether (80 mL) was added till the persistence of yellow col-

our in the reaction mixture. After 2 h, ether was evaporated and purified the residue by column chromatography (60–120 mesh Silica gel, 6% EtOAc in pet. ether) to afford 19 (1.05 g, 54%, over 3 steps) as a colourless syrup; ½a25 D = 89.8 (c 0.09, CHCl3); IR (neat): m 2934, 2857, 1748, 1620, 1466, 1375, 1258, 1107, 1059, 839, 777 cm1; 1H NMR (300 MHz, CDCl3): d 5.24 (d, 1H, J = 5.7 Hz, H2), 4.80 (s, 1H, H1), 4.52 (d, 1H, J = 5.8 Hz, H3), 4.00 (d, 1H, J = 9.2 Hz, –CH), 3.80 (d, 1H, J = 9.3 Hz, –CH), 3.70 (s, 3H, COOMe), 3.30 (s, 3H, OMe), 1.43 (s, 3H, Me), 1.32 (s, 3H, Me), 0.86 (s, 9H, 3 Me), 0.04 (s, 3H, Me), 0.03 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 170.9, 111.2, 107.5, 87.7, 83.7, 79.6, 62.3, 54.2, 50.9, 25.1, 24.6, 23.7, 6.7; HRMS (ESI+): m/z calcd for C17H32N3O7Si (M+Na)+ 399.1815, found 399.1818. 4.1.12. (3aS,4S,6R,6aR)-Methyl 4-(hydroxymethyl)-6-methoxy2,2-dimethyltetrahydrofuro[3,4-d][1,3]dioxole-4-carboxylate (20) To a solution of 19 (0.90 g, 2.39 mmol) in THF (4 mL) at 0 °C, 1 M TBAF in THF (2.39 mL) was added and stirred at room temperature for 3 h. THF was removed under reduced pressure and purified the residue by column chromatography (60–120 mesh Silica gel, 25% EtOAc in pet. ether) to afford 20 (0.52 g, 83%) as a colourless syrup; ½a25 D = 115.3 (c 0.06, CHCl3); IR (neat): m 3468, 2992, 2940, 2851, 1742, 1653, 1445, 1377, 1267, 1213, 1163, 1105, 1036, 868 cm1; 1 H NMR (300 MHz, CDCl3): d 5.33 (d, 1H, J = 6.0 Hz, H2), 4.90 (s, 1H, H1), 4.53 (d, 1H, J = 6.0 Hz, H3), 3.89 (s, 2H, –CH2), 3.76 (s, 3H, COOMe), 3.72 (s, 3H, OMe), 1.99 (br s, 1H, OH), 1.48 (s, 3H, Me), 1.32 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 172.1, 112.9, 108.4, 88.1, 84.9, 84.8, 81.2, 64.3, 55.4, 52.5, 25.9, 24.5; HRMS (ESI+): m/z calcd for C11H18N3O7 (M+Na)+ 285.0950, found 285.0947. 4.1.13. (3aS,4S,6R,6aR)-Methyl 4-(azidomethyl)-6-methoxy-2,2dimethyltetrahydrofuro[3,4-d][1,3]dioxole-4-carboxylate (21) A solution of 20 (0.40 g, 1.53 mmol) and pyridine (0.23 mL, 1.53 mmol) in CH2Cl2 (4 mL) was treated with triflic anhydride (0.31 mL, 1.83 mmol) at 0 °C and stirred at room temperature for 30 min. Work up as described for 10a gave triflate 20a as a solid, which was used for further reaction. To a solution of triflate 20a in DMF (5 mL) at 0 °C, NaN3 (0.3 g, 4.58 mmol) was added and stirred at room temperature for 3 h. Work up as described for 11 and purification of the residue by column chromatography (60–120 mesh Silica gel, 5% EtOAc in pet. ether) furnished 21 (0.35 g, 80%) as a yellow coloured syrup; ½a25 D = 86.3 (c 0.83, CHCl3); IR (neat): m 2942, 2108, 1744, 1599, 1532, 1443, 1381, 1264, 1211, 1107, 1061, 868 cm1; 1H NMR (300 MHz, CDCl3): d 5.24 (d, 1H, J = 6.1 Hz, H2), 4.87 (s, 1H, H1), 4.54 (d, 1H, J = 5.7 Hz, H3), 3.78 (s, 3H, COOMe), 3.64 (q, 2H, J = 11.7 Hz, –CH2), 3.33 (s, 3H, OMe), 1.48 (s, 3H, Me), 1.34 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 171.3, 113.0, 108.7, 87.4, 84.9, 81.0, 55.4, 52.7, 52.6, 25.9, 24.6; HRMS (ESI+): m/z calcd for C11H17N3O6 (M+Na)+ 310.1015, found 310.1024. 4.1.14. (3aS,4S,6R,6aR)-Methyl 4-((tert-butoxycarbonylamino) methyl)-6-methoxy-2,2-dimethyl-tetrahydrofuro[3,4-d][1,3] dioxole-4-carboxylate (2) To a solution of 21 (0.30 g, 1.04 mmol) in MeOH (1 mL), catalytic amount of Pd-C (10%) was added and the reaction mixture stirred at room temperature under hydrogen atmosphere for 4 h. Work up as described for 12 furnished amine 21a, which was used for the next reaction without further purification. To a stirred solution of the above amine 21a and Et3N (0.44 mL, 3.14 mmol) in CH2Cl2 (4 mL), (Boc)2O (0.36 mL, 1.57 mmol) was added at 0 °C. After 5 h, work up as described for 1 and purification of the residue by column chromatography (60–120 mesh Silica gel, 6% EtOAc in pet ether) furnished 2 (0.30 g, 80%) as a colourless syr-

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up; ½a25 D = +12.3 (c 0.24, CHCl3); IR (neat): m 2924, 1723, 1644, 1514, 1370, 1240 cm1; 1H NMR (300 MHz, CDCl3): d 5.29 (d, 1H, J = 5.8 Hz, H2), 4.87 (s, 1H, H1), 4.74 (br s, 1H, NH), 4.50 (d, 1H, J = 5.8 Hz, H3), 3.83–3.76 (m, 1H, –CH), 3.71 (s, 3H, COOMe), 3.36–3.30 (m, 1H, –CH), 3.34 (s, 3H, OMe), 1.47 (s, 3H, Me) 1.42 (s, 9H, 3 Me), 1.32 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 171.9, 155.6, 112.9, 108.3, 87.1, 84.6, 81.4, 55.4, 52.4, 43.4, 28.3, 25.9, 24.6; HRMS (ESI+): m/z calcd for C16H27NO8 (M+Na)+ 384.1634, found 384.1643. 4.1.15. (((3aS,4S,6R,6aR)-4-(Azidomethyl)-6-methoxy-2,2dimethyltetrahydrofuro[3,4-d][1,3]dioxol-4-yl)methoxy)(tertbutyl)dimethylsilane (23) A solution of 17 (1.45 g, 1.29 mmol) and pyridine (0.55 mL, 2.58 mmol) in CH2Cl2 (10 mL) was treated with triflic anhydride (0.84 mL, 1.55 mmol) at 0 °C and stirred at room temperature for 30 min. Work up as described for 10a afforded triflate 22 as a solid, which was used immediately for further reaction. To a solution of the above triflate 22 in DMF (20 mL) at 0 °C, NaN3 (2.0 g, 12.48 mmol) was added and stirred at room temperature for 3 h. Work up as described for 11 and purification of the residue by column chromatography (60–120 mesh Silica gel, 4% EtOAc in pet. ether) gave 23 (1.20 g, 77%) as a light yellow coloured syrup; ½a25 D = 85.3 (c 0.22, CHCl3); IR (neat): m 3450, 2931, 2857, 2104, 1634, 1464, 1377, 1254, 1209, 1163, 1105, 1010, 973, 840, 776, 674, 508, cm1; 1H NMR (300 MHz, CDCl3): d 4.86 (s, 1H, H1), 4.58 (d, 1H, J = 5.8 Hz, H2), 4.43 (d, 1H, J = 6.0 Hz, H3), 3.80 (d, 1H, J = 10.4 Hz, –CH), 3.69 (d, 1H, J = 10.4 Hz, –CH), 3.53 (d, 1H, J = 12.1 Hz, –CH), 3.35 (1H, J = 12.1 Hz, –CH), 3.35 (s, 3H, OMe), 1.44 (s, 3H, Me), 1.28 (s, 3H, Me), 0.90 (s, 9H, 3 Me), 0.08 (s, 3H, Me), 0.08 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 112.5, 109.4, 89.1, 85.8, 81.9, 61.7, 55.3, 53.1, 29.7, 25.8, 24.7, 5.4, 5.6; (ESI+): m/z calcd for C16H31N3O5Si (M+Na)+ 396.1930, found 396.1922. 4.1.16. tert-Butyl((3aS,4S,6R,6aR)-4-((tert-butyldimethylsilyloxy) methyl)-6-methoxy-2,2-dimethyltetrahydrofuro[3,4-d][1,3] dioxol-4-yl)methylcarbamate (24) To a solution of 23 (1.50 g, 4.02 mmol) in MeOH (2 mL), cat Pd-C (10%) was added and stirred the reaction mixture under hydrogen atmosphere at room temperature for 4 h. Work up as described for 12 furnished the amine 23a, which was used for further reaction. To a stirred solution of the above amine 23a and Et3N (1.68 mL, 12.09 mmol) in CH2Cl2 (14 mL), (Boc)2O (1.4 mL, 6.04 mmol) was added at 0 °C. After 5 h, work up as described for 1 and purification of the residue by column chromatography (60–120 mesh Silica gel, 7% EtOAc in pet. ether) furnished 24 (1.65 g, 92%) as a white solid; mp 55–57 °C; ½a25 D = 73.0 (c 0.14, CHCl3); IR (neat): m 3438, 2932, 2859, 1721, 1505, 1464, 1368, 1250, 1171, 1092, 1013, 839, 779 cm1; 1H NMR (300 MHz, CDCl3): d 5.28 (br s, 1H, NH), 4.84 (s, 1H, H1), 4.62 (d, 1H J = 6.0 Hz, H2), 4.51 (d, 1H, J = 6.0 Hz, H3), 3.84 (d,1H, J = 10.6 Hz, –CH), 3.65 (d, 1H, J = 10.6 Hz, –CH), 3.44– 3.30 (m, 2H, –CH2), 3.35 (s, 3H, OMe), 1.53 (s, 3H, Me), 1.44 (s, 9H, 3 Me), 1.28 (s, 3H, Me), 0.91 (s, 9H, 3 Me), 0.08 (s, 6H, 2 Me); 13C NMR (75 MHz, CDCl3): d 111.2, 108.9, 90.2, 85.6, 80.9, 65.5, 63.0, 54.7, 25.5, 25.0, 23.9, 17.5, 6.3, 6.4; HRMS (ESI+): m/z calcd for C21H41NO7Si (M+Na)+ 470.2550, found 470.2531. 4.1.17. tert-Butyl((3aS,4S,6R,6aR)-4-(hydroxymethyl)-6methoxy-2,2-dimethyltetrahy-drofuro[3,4-d][1,3]dioxol-4yl)methylcarbamate (25) To a solution of 24 (1.5 g, 3.35 mmol) in THF (6 mL) at 0 °C, 1 M TBAF in THF (3.35 mL) was added and stirred at room temperature for 3 h. THF was removed under reduced pressure and purified the residue by column chromatography (60–120 mesh Silica gel, 7% EtOAc in pet. ether) to give 25 (0.95 g, 89%) as a white solid; mp

15

102–104 °C; ½a25 D = +2.3 (c 0.30, CHCl3); IR (KBr): m 3270, 3084, 2982, 2930, 1755, 1676, 1568, 1445, 1382, 1302, 1258, 1211, 1181, 1088, 1048, 1003, 970, 928, 905, 872, 833, 793, 741, 679, 596, 513, 461, 479 cm1; 1H NMR (300 MHz, CDCl3): d 5.02 (t, 1H, J = 6.0 Hz, NH), 4.82 (s, 1H, H1), 4.54 (d, 1H, J = 6.0 Hz, H2), 4.47 (d, 1H, J = 6.0 Hz, H3), 3.72 (m, 1H, –CH), 3.64 (m, 1H, –CH), 3.42 (m, 1H, –CH), 3 3.12 (m, 1H, –CH), 2.72 (m, 1H, OH), 1.42 (s, 3H, Me), 1.39 (s, 9H, 3 Me), 1.24 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 112.9, 109.3, 86.1, 82.2, 80.1, 61.6, 55.5, 43.9, 28.3, 26.0, 24.5; HRMS (ESI+): m/z calcd for C15H27NO7 (M+Na)+ 356.1685, found 356.1690. 4.1.18. (3aS,4R,6R,6aR)-Methyl 4-((tert-butoxycarbonylamino) methyl)-6-methoxy-2,2-dimethyltetrahydrofuro[3,4-d][1,3] dioxole-4-carboxylate (3) A solution of the alcohol 25 (0.75 g, 2.25 mmol) dissolved in EtOAc (6 mL), IBX (0.76 g, 2.2.70 mmol) followed by DMSO (1 mL) were added and heated at reflux for 1 h. Work up as described for 7a gave the aldehyde 25a, which was used as such for the next step. To a solution of the above aldehyde 25a in t-butanol/water (7:3, 8 mL) at 0 °C, NaClO2 (0.32 g, 3.53 mmol) and 30% aq H2O2 (1.34 mL, 11.78 mmol) were added and stirred at room temperature for 5 h. Work up as described for 8 afforded carboxylic acid 26 as a light yellow syrup. To a solution of acid 26 in ether (5 mL) at 0 °C, a solution of CH2N2 in ether (40 mL) was added till the persistence of yellow colour in the reaction mixture. After 2 h, ether was evaporated and purified the residue by column chromatography (60–120 mesh Silica gel, 4% EtOAc in pet ether) to afford 3 (0.45 g, 53%), (over 3 steps) as a white solid; mp 91–93 °C; ½a25 D = +21.8 (c 0.46, CHCl3); IR (KBr): m 3368, 2980, 2841, 1750, 1709, 1524, 1441, 1370, 1327, 1267, 1171, 1100, 1057, 1005, 976, 949, 922, 905, 860, 783, 754, 721, 683, 646, 612, 521, 507, 490 cm1; 1H NMR (500 MHz, CDCl3): d 5.17 (s, 1H, H1), 5.12 (br s, 1H, NH), 4.72 (d, 1H, J = 5.9 Hz, H2), 4.66 (d, 1H, J = 5.9 Hz, H3), 3.80 (s, 3H, COOMe), 3.76 (m, 1H, –CH), 3.45 (m, 1H, –CH), 3.43 (s, 3H, OMe), 1.43 (s, 9H, 3 Me), 1.42 (s, 3H, Me), 1.29 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 169.3, 155.7, 113.3, 109.9, 91.8, 85.2, 83.0, 56.0, 52.6, 45.9, 28.3, 25.8, 24.8; HRMS (ESI+): m/z calcd for C16H27NO8 (M+Na)+ 384.1634, found 384.1643. 4.1.19. (3aS,4S,6R,6aR)-6-Methoxy-2,2-dimethyldihydro-3aHspiro[furo[3,4-d][1,3]-dioxole-4,50 -[1,3]oxazinan]-20 -one (27) To an ice cooled suspension of NaH (0.03 g, 1.32 mmol) in THF (1 mL), a solution of 25 (0.33 g, 0.36 mmol) in THF (1 mL) was added dropwise at 0 °C and stirred at room temperature for 3 h. Work up as described for 14 and purification of the residue by column chromatography (60–120 mesh Silica gel, 80% EtOAc in pet. ether) afforded 27 (0.18 g, 72%) as a white solid; mp 175-177 °C; ½a20 D = +33.7 (c 0.38, CHCl3); IR (KBr): m 3390, 3310, 2990, 2850, 1480, 1280, 1261, 1211, 1190, 1164, 1094, 1069, 1012, 890, 770, 640; 1H NMR (300 MHz, CDCl3): d 6.96 (br s, 1H, NH), 5.93 (s, 1H, H1), 4.65 (d, 1H, m, H2), 4.32 (m, 1H, OCH), 4.14 (m, 1H, OH), 3.79 (s, 1H, H3), 3.56 (m, 2H, NCH), 3.46 (s, 3H, OMe), 1.52 (s, 3H, Me), 1.33 (s, 3H, Me); 13C NMR (125 MHz, CDCl3): d 153.4, 113.4, 108.7, 85.6, 82.0, 78.8, 69.4, 55.3, 48.7, 25.9, 24.5; ESIMS: 282 (M+Na)+. 4.1.20. ((3aR,6aR)-2,2-Dimethyltetrahydrofuro[2,3d][1,3]dioxole-5,5-diyl)dimethanol (29) To a stirred solution of 28 (13.46 g, 79.18 mmol) in a mixture of water (40 mL) and THF (40 mL) at 0 °C, aq 98% formaldehyde (32 mL) and 1M NaOH (96 mL) were added sequentially and stirred the reaction mixture at room temperature for 16 h. Work up as described for 6 and purification of the residue by column chromatog-

16

G. V. M. Sharma et al. / Carbohydrate Research 388 (2014) 8–18

raphy (60–120 mesh Silica gel, 40% EtOAc in pet. ether) gave 29 (9.14 g, 64%) as a white solid; mp 72–74 °C; ½a25 D = 25.6 (c 0.97, CHCl3); IR (KBr): m 3393, 2987, 2934, 1729, 1646, 1460, 1381, 1252, 1261, 1168, 1052, 897, 868, 843, 780 cm1; 1H NMR (300 MHz, CDCl3): d 5.81 (d, 1H, J = 3.8 Hz, H1), 4.76 (dd, 1H, J = 4.3, 5.3 Hz, H2), 3.87 (d, 1H, J = 11.5 Hz, OCH), 3.78–3.33 (m, 3H, OCH, OCH2), 2.16 (dd, 1H, J = 14.2, 6.0 Hz, H3), 1.94 (d, 1H, J = 14.2 Hz, H3), 1.54 (s, 3H, Me), 1.29 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 112.1, 106.3, 89.1, 81.2, 65.1, 64.7, 34.5, 26.7, 25.7; HRMS (ESI+): m/z calcd for C9H16O5 (M+Na)+ 227.08954, found 227.08960. 4.1.21. ((3aR,5R,6aR)-5-((tert-Butyldimethylsilyloxy)methyl)2,2-dimethyltetrahydrofuro[2,3-d][1,3]dioxol-5-yl)methanol (30) To a stirred solution of 29 (4.85 g, 23.76 mmol) in CH2Cl2 (84 mL), imidazole (4.23 g, 62.18 mmol), n-Bu2SnO (cat) and TBSCl (3.12 g, 20.71 mmol) were added and stirred at 20 °C for 1 h. Work up as described for 7 and purification of the residue by column chromatography (60–120 mesh Silica gel, 12% EtOAc in pet ether) afforded 30 (7.2 g, 76%) as colourless solid; mp 43–45 °C; ½a25 D = 13.9 (c 0.90, CHCl3); IR (neat): m 3481, 2953, 2933, 2858, 1643, 1467, 1379, 1254, 1215, 1168, 1101, 1025, 942, 840, 777 cm1; 1H NMR (500 MHz, CDCl3): d 5.82 (d, 1H, J = 3.4 Hz, H1), 4.77 (t, 1H, J = 4.9 Hz, H2), 3.77 (dd, 1H, , J = 11.2, 4.4 Hz, – OCH), 3.63–3.50 (m, 3H, –OCH, OCH2), 2.25 (dd, 1H, J = 7.3, 4.4 Hz, OH), 2.18 (dd, 1H, J = 14.1, 6.8 Hz, H3), 1.99 (d, 1H, J = 14.1 Hz, H3), 1.57 (s, 3H, Me), 1.32 (s, 3H, Me), 0.89 (s, 9H, 3 Me), 0.06 (s, 6H, 2 Me); 13C NMR (75 MHz, CDCl3): d 112.3, 106.3, 89.2, 81.5, 66.4, 65.0, 35.1, 27.2, 26.2, 25.8, 18.1, 5.5, 5.6; HRMS (ESI+): m/z calcd for C15H30O5Si (M+Na)+ 341.17499, found 341.17547. 4.1.22. (3aR,5S,6aR)-Methyl 5-((tertbutyldimethylsilyloxy)methyl-2,2-dimethyltetrahydrofuro[2,3d][1,3]dioxole-5-carboxylate (31) To a solution of 30 (0.81 g, 2.54) in 1:1 ratio of CH2Cl2/H2O (8.1 mL), TEMPO (0.14 g, 0.76 mmol) and BAIB (2.46 g, 7.63 mmol) were added at 0 °C and stirred at room temperature for 1.5 h. The reaction mixture was diluted with CHCl3 (2  15 mL), washed with satd aq hypo (15 mL), brine (10 mL) and dried (Na2SO4). Solvent was evaporated under reduced pressure to give acid 30a, which was directly used in the next step. To a solution of acid 30a in ether (10 mL) at 0 °C, a solution of CH2N2 in ether (30 mL) was added till the persistence of yellow colour in the reaction mixture. After 2 h, ether was evaporated and purified the residue by column chromatography (60–120 mesh Silica gel, 10% EtOAc in pet. ether) to afford 31 (0.57 g, 65%, over 2 steps) as a colourless oil; ½a25 D = 113.6 (c 0.23, CHCl3); IR (neat): m 2954, 2859, 1736, 1466, 1437, 1379, 1295, 1254, 1215, 1161, 1096, 1045, 1017, 943, 841, 778 cm1; 1H NMR (300 MHz, CDCl3,): d 5.77 (d, 1H, J = 3.6 Hz, H1), 4.66 (dd, 1H, J = 4.5, 3.6 Hz, H2), 3.76 (d, 1H, J = 10.4 Hz, –OCH), 3.75 (s, 3H, COOMe), 3.62 (d, 1H, J = 10.4 Hz, –OCH), 2.61 (d, 1H, J = 14.0 Hz, H3), 2.22 (dd, 1H, J = 14.0, 5.1 Hz, H3), 1.43 (s, 3H, Me), 1.27 (s, 3H, Me), 0.87 (s, 9H, 3 Me), 0.05 (s, 3H, Me) 0.04 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 172.7, 112.4, 106.6, 88.6, 80.4, 66.8, 52.2, 36.0, 25.9, 25.7, 18.2, 5.4, 5.6; HRMS (ESI+): m/z calcd for C16H30O6Si (M+Na)+ 369.17039, found 369.16971. 4.1.23. (3aR,5S,6aR)-Methyl 5-(hydroxymethyl)-2,2dimethyltetrahydrofuro[2,3-d][1,3]dioxole-5-carboxylate (32) To a solution of 31 (0.50 g, 1.45 mmol) in THF (2 mL) at 0 °C, 1 M TBAF in THF (1.45 mL) was added and stirred at room temperature for 3 h. THF was removed under reduced pressure and purified the residue by column chromatography (60–120 mesh Silica gel, 25% EtOAc in pet. ether) to give 32 (0.25 g, 76%) as a white solid; mp

105–107 °C; ½a25 D = 175.9 (c 0.72, CHCl3); IR (neat): m 3500, 2983, 2954, 2873, 1753, 1443, 1383, 1337, 1265, 1208, 1168, 1140, 1110, 1067, 1016, 982, 946, 915, 868, 841, 801, 745, 702 cm1; 1H NMR (300 MHz, CDCl3): d 5.82 (d, 1H, J = 3.3 Hz, H1), 4.69 (dd, 1H, J = 4.1, 3.3 Hz, H2), 3.77 (s, 3H, COOMe), 3.75 (d, 1H, J = 11.7 Hz, OCH), 3.52 (d, 1H, J = 11.7 Hz, OCH), 2.64 (d, 1H, J = 14.1 Hz, H3), 2.20 (dd, 1H, J = 14.1, 4.7 Hz, H3), 1.44 (s, 3H, Me), 1.28 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 172.5, 112.5, 106.5, 88.2, 80.2, 66.0, 52.3, 35.8, 25.6, 25.5; HRMS (ESI+): m/z calcd for C10H16O6 (M+Na)+ 255.08391, found 255.08350. 4.1.24. (3aR,5S,6aR)-Methyl 5-(azidomethyl)-2,2dimethyltetrahydrofuro[2,3-d][1,3]dioxole-5-carboxylate (33) A solution of 32 (1.61 g, 6.94 mmol) and pyridine (1.13 mL, 13.88 mmol) in CH2Cl2 (14 mL) was treated with triflic anhydride (1.37 mL, 13.88 mmol) at 0 °C and stirred at room temperature for 30 min. Work up as described for 10a gave triflate 32a as a solid, which was used for further reaction. To a solution of the above triflate 32a in DMF (10 mL) at 0 °C, NaN3 (1.36 g, 20.92 mmol) was added and stirred at room temperature for 3 h. Work up as described for 1 and purification of the residue by column chromatography (60–120 mesh Silica gel, 12% EtOAc in pet. ether) gave 33 (1.39 g, 78%) as a yellow oil; ½a25 D = 137.2 (c 0.72, CHCl3); IR (KBr): m 2987, 2953, 2106, 1758, 1734, 1439, 1379, 1254, 1217, 1161, 1113, 1044, 1016, 915, 842, 773 cm1; 1H NMR (300 MHz, CDCl3): d 5.87 (d, 1H, J = 3.2 Hz, H1), 4.71 (dd, 1H, J = 3.9, 3.2 Hz, H2), 3.79 (s, 3H, COOMe), 3.59 (d, 1H, J = 12.9 Hz, CHN3), 3.15 (d, 1H, J = 12.9 Hz, CH0 N3), 2.69 (d, 1H, J = 14.1 Hz, H3), 2.24 (dd, 1H, J = 14.1, 4.8 Hz, H3), 1.44 (s, 3H, Me), 1.28 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 172.0, 112.7, 106.5, 87.2, 80.1, 55.7, 52.6, 36.8, 25.6, 25.4; HRMS (ESI+): m/z calcd for C10H15N3O5 (M+Na)+ 280.09039, found 280.09021. 4.1.25. (3aR,5S,6aR)-Methyl 5-((tertbutoxycarbonylamino)methyl)-2,2dimethyltetrahydrofuro[2,3-d][1,3]dioxole-5-carboxylate (4) To a solution of 33 (1.30 g, 5.06 mmol) in MeOH (2 mL), catalytic amount of Pd-C (10%) was added and stirred at room temperature under hydrogen atmosphere for 4 h. Work up as described for 12 furnished amine 33a, which was used for the next reaction. To a stirred solution of the above amine 33a and Et3N (1.40 mL, 10.12 mmol) in CH2Cl2 (11 mL), (Boc)2O (1.39 mL, 6.07 mmol) was added at 0 °C. After 5 h, work up as described for 1 and purification of the residue by column chromatography (60–120 mesh Silica gel, 15% EtOAc in pet. ether) furnished 4 (1.39 g, 87%) as a white solid; mp 145-147 °C; ½a20 D = 26.7 (c 1.14, CHCl3); IR (neat): m 3374, 2979, 2924, 2852, 1714, 1513, 1437, 1367, 1248, 1218, 1162, 1117, 1021, 879, 854, 772, 669, 594 cm1; 1H NMR (500 MHz, CDCl3): d 5.84 (d, 1H, J = 3.2 Hz, H1), 4.95–4.85 (br s, 1H, NH), 4.72 (dd, 1H, J = 3.7, 3.2 Hz, H2), 3.76 (s, 3H, COOMe), 3.58 (dd, 1H, J = 13.8, 6.4 Hz, CHN), 3.30 (dd, 1H, J = 13.8, 5.1 Hz, CHN), 2.80 (d, 1H, J = 14.3 Hz, H3), 1.96 (dd, 1H, J = 14.3, 3.2 Hz, H3), 1.44 (s, 3H, Me), 1.42 (s, 9H, 3 Me), 1.28 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 112.6, 106.3, 87.0, 80.2, 79.7, 52.5, 46.9, 37.4, 29.7, 28.2, 25.7, 25.4; HRMS (ESI+): m/z calcd for C15H25NO7 (M+Na)+ 354.15232, found 354.15169. 4.1.26. tert-Butyl((3aR,5S,6aR)-5-(hydroxymethyl)-2,2dimethyltetrahydrofuro[2,3-d][1,3]dioxol-5yl)methylcarbamate (34) To a stirred solution of 4 (0.5 g, 1.58 mmol) in CH2Cl2 (2 mL), DIBAL-H (0.98 mL, 1.74 mmol, 20% solution in hexane) was added at 0 °C and stirred at the same temperature for 1 h. Work up as described for 13 and purification of the residue by column chromatography (60–120 mesh Silica gel, 25% EtOAc in pet ether) afforded alcohol 34 (0.35 g, 73%) as a colourless syrup; ½a20 D = 19.6 (c 0.30,

G. V. M. Sharma et al. / Carbohydrate Research 388 (2014) 8–18

CHCl3); IR (neat): m 3269, 3079, 2976, 2930, 2854, 1670, 1560, 1441, 1368, 1291, 1251, 1218, 1174, 1109, 1051, 985, 966, 897, 854, 722, 703 cm1; 1H NMR (500 MHz, CDCl3): d 5.80 (d, 1H, J = 3.3 Hz, H1), 4.97–4.89 (br s, 1H, NH), 4.78 (dd, 1H, J = 5.2, 3.3 Hz, H2), 3.73–3.60 (m, 2H, OCH2), 3.54 (dd, 1H, J = 14.1, 7.1 Hz, CHN), 3.35–3.24 (br s, 1H, OH), 2.91 (dd, 1H, J = 14.1, 4.2 Hz, CHN), 2.31 (d, 1H, J = 14.1 Hz, H3), 1.78 (dd, 1H, J = 14.1, 5.2 Hz, H3), 1.55 (s, 3H, Me), 1.44 (s, 9H, 3 Me), 1.29 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 157.3, 112.2, 106.1, 88.6, 83.3, 80.1, 63.7, 45.8, 36.0, 29.6, 28.5, 26.8, 25.7; HRMS (ESI+): m/z calcd for C14H25NO6 (M+Na)+ 326.15741, found 326.15651. 4.1.27. (3aR,5S,6aR)-2,2-Bimethyldihydro-3aH-spiro[furo[2,3d][1,3]dioxole-5,50 -[1,3]oxazinan]-20 -one (35) To an ice cooled (0 °C) suspension of NaH (0.03 g, 1.32 mmol) in THF (1 mL), a solution of 34 (0.1 g, 0.33 mmol) in THF (1 mL) was added dropwise at 0 °C and stirred at room temperature for 3 h. Work up as described for 14 and purification of the residue by column chromatography (60–120 mesh Silica gel, 85% EtOAc in pet. ether) afforded 35 (0.06 g, 70%) as a white solid; mp 163–165 °C; ½a20 D = 43.6 (c 0.41, CHCl3); IR (neat): m 3260, 3077, 2978, 2930, 2310, 1800, 1764, 1726, 1692, 1677, 1661, 1654, 1567, 1550, 1515, 1498, 1481, 1449, 1219, 772, 686; 1H NMR (500 MHz, CDCl3): d 5.90 (d, 1H, J = 3.3 Hz, H1), 5.70-5.63 (br s, 1H, NH), 4.82 (dd, 1H, J = 4.2, 3.3 Hz, H2), 4.46 (d, 1H, J = 11.3 Hz, OCH), 4.26 (d, 1H, J = 11.3 Hz, OCH), 3.34 (d, 1H, J = 11.3 Hz, CHN), 3.27 (d, 1H, J = 11.3 Hz, CHN), 2.32 (d, 1H, J = 14.1 Hz, H3), 1.94 (dd, 1H, J = 14.1, 5.7 Hz, H3), 1.57 (s, 3H, Me), 1.32 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 153.0, 112.4, 106.5, 80.4, 73.2, 68.1, 49.6, 38.2, 26.8, 25.5; ESIMS: 230 (M+H)+ 4.1.28. N3-(S)-b2,2-Caa-(S)-b2,2-Caa-OMe (36) To a stirred solution of ester 33 (0.50 g, 1.95 mmol) in MeOH (4 mL), aq 4M NaOH solution (4 mL) was added at 0 °C to room temperature. After 1 h, MeOH was evaporated and added aq 1M HCl solution at 0 °C to adjust pH to 2–3. The reaction mixture was extracted with EtOAc (2  10 mL), dried (Na2SO4) and evaporated to give 33b, which was used for the next reaction without purification. To a solution of 33 (0.48 g, 1.87 mmol) in MeOH (2 mL), catalytic amount of Pd-C (10%) was added and stirred at room temperature under hydrogen atmosphere for 4 h. Work up as described for 12 furnished the amine 33a, which was used for the next reaction. A cooled (0 °C) solution of 33b (0.46 g, 1.89 mmol), HOBt (0.31 g, 2.27 mmol) and EDCI (0.44 g, 2.27 mmol) in CH2Cl2 (6 mL) was stirred for 15 min and treated with the above salt 33a and DIPEA (0.49 mL, 2.84 mmol) under nitrogen atmosphere for 6 h. The reaction mixture was quenched with satd aq NH4Cl (10 mL) at 0 °C and diluted with CHCl3 (20 mL). It was sequentially washed with 1M HCl (10 mL), satd NaHCO3 (10 mL), water (10 mL) and brine (10 mL). The organic layer was dried (Na2SO4), evaporated and purified the residue by column chromatography (60– 120 mesh Silica gel, 50% EtOAc in pet. ether) to afford 36 (0.70 g, 79%) as a white solid; mp 148–150 °C; ½a25 D = 175.0 (c 0.57, CHCl3); IR (KBr): m 3436, 2992, 2947, 2113, 1758, 1677, 1535, 1436, 1381, 1321, 1256, 1223, 1164, 1117, 1063, 1015, 922, 852, 777 cm1; 1H NMR (CDCl3, 500 MHz): d 7.31 (dd, 1H, J = 1.1, 9.3 Hz, CONH), 5.91 (d, 1H, J = 3.5 Hz, H1), 5.86 (d, 1H, J = 3.1 Hz, H1), 4.77–4.70 (m, 2H, H2), 4.16 (dd, 1H, J = 9.7, 13.8 Hz, CHNCO), 3.73 (s, 3H, COOMe), 3.60 (d, 1H, J = 13.1 Hz, CHN3), 3.15 (d, 1H, J = 13.1 Hz, CHN3), 2.99 (dd, 1H, J = 13.8, 3.1 Hz, CHNCO), 2.86 (d, 1H, J = 14.2 Hz, H3), 2.64 (d, 1H, J = 14.5 Hz, H3), 2.26 (dd, 1H, J = 14.5, 5.5 Hz, H3), 1.95 (dd, 1H, J = 14.2, 4.5 Hz, H3), 1.44 (s, 3H, Me), 1.39 (s, 3H, Me), 1.28 (s, 3H, Me), 1.26 (s, 3H, Me); 13C NMR (CDCl3, 75 MHz): d 171.7, 171.4, 112.3, 112.1, 106.6, 106.3,

17

89.1, 86.1, 80.4, 79.7, 55.5, 52.2, 46.2, 38.5, 35.9, 25.7, 25.4, 25.2, 25.1; HRMS (ESI+): m/z calcd for C19H28N4O9 (M+Na)+ 479.17485, found 479.17371. 4.1.29. N3-(S)-b2,2-Caa-(S)-b2,2-Caa-(S)-b2,2-Caa-(S)-b2,2-Caa-OMe (37) A solution of 36 (0.20 g, 0.44 mmol) in MeOH (2 mL) was treated with 4M NaOH (1.5 mL) at 0 °C and stirred at room temperature for 1 h. Work up as described for 33b gave 36a (0.18 g, 92%) as a white solid, which was used for further reaction. A solution of 36 (0.19 g, 0.42 mmol) in MeOH (1 mL) was treated with cat. Pd-C (10%) and stirred at room temperature under hydrogen atmosphere for 4 h. Work up as described for 33a afforded the amine 36b, which was used as such for further reaction. A solution of 36a (0.18 g, 0.40 mmol), HOBt (0.06 g, 0.48 mmol) and EDCI (0.09 g, 0.48 mmol) in dry CH2Cl2 (4 mL) was stirred at 0 °C for 15 min and treated with the amine 36b and DIPEA (0.10 mL, 0.60 mmol) under nitrogen atmosphere for 5 h. Work up as described for 36 and purification of the residue by column chromatography (60–120 mesh Silica gel, 1.7% MeOH in CHCl3) afforded 37 (0.26 g, 70%) as a white solid; mp 193–195 °C; ½a25 D = 127.5 (c 0.17, CHCl3); IR (KBr): m 3419, 2989, 2945, 2108, 1748, 1678, 1523, 1444, 1381, 1314, 1255, 1214, 1164, 1116, 1017, 850 cm1; 1H NMR (300 MHz, CDCl3): d 7.41 (dd, 1H, J = 8.9, 2.6 Hz, NH), 7.32 (dd, 1H, J = 9.1, 3.8 Hz, NH), 7.27 (dd, 1H, J = 7.7, 4.5 Hz, NH), 5.94 (d, 1H, J = 3.6 Hz, H1), 5.86-5.81 (m, 3H, 3 H1), 4.77 (q, 1H, J = 4.0 Hz, H2), 4.74–4.66 (m, 3H, 3 H2), 4.11 (dd, 1H, J = 13.8, 9.1 Hz, –CH2), 4.01 (dd, 1H, J = 13.8, 9.2 Hz, –CH2), 3.78 (s, 3H, COOMe), 3.70 (dd, 1H, J = 13.4, 8.1 Hz, CH2), 3.62 (d, 1H, J = 13.3 Hz, CH2), 3.36 (dd, 1H, J = 13.0. 4.9 Hz, CH2), 3.27 (d, 1H, J = 13.3 Hz, CH2), 3.12 (dd, 1H, J = 13.6, 3.4 Hz, H3), 3.02 (dd, 1H, J = 13.6, 3.8 Hz, H3), 2.76 (d, 1H, J = 14.6 Hz, H3), 2.23 (dd, 1H, J = 14.3, 5.7 Hz, H3), 2.09 (m, 2H, –CH2N3), 1.92 (dd, 2H, J = 14.2, 4.5 Hz, 2 H3), 1.45 (s, 1H, Me), 1.42 (s, 3H, Me), 1.41 (s, 3H, Me), 1.36 (s, 3H, Me), 1.29 (s, 6H, 2 Me), 1.26 (s, 3H, Me), 1.25 (s, 3H, Me); 13C NMR (75 MHz, CDCl3): d 172.8, 172.7, 172.0, 112.6, 112.5, 112.4, 112.2, 107.0, 106.7, 106.6, 106.3, 89.5, 87.7, 87.4, 86.1, 80.5, 80.4, 80.3, 80.0, 56.4, 52.8, 46.2, 45.7, 38.6, 37.9, 37.5, 36.7, 26.2, 26.1, 25.8, 25.7, 25.5, 25.4, 25.2; HRMS (ESI+): m/z calcd for C37H54N6O17 (M+H)+ 855.36237, found 855.36102. Acknowledgments All the authors thank the UGC and CSIR, New Delhi, India, for financial support. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.carres.2014. 01.026. References 1. (a) Hill, D. J.; Mio, M. J.; Prince, R. B.; Huges, T. S.; Moore, J. S. Chem. Rev. 2001, 101, 401–3893; (b) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173–180; (c)Foldamers: Structure, Properties and Applications; Hecht, S., Huc, I., Eds.; Wiley-VCH: Weinheim, Germany, 2007; (d) Sharma, G. V. M.; Kunwar, A. C. Recent Research Developments in Foldamer Chemistry In Nageswar, Y. V. D., Ed.; Nova Science Publishers: New York, 2012. 2. (a) Seebach, D.; Overhand, M.; Kuhnle, F. N. M.; Martinoni, B.; Oberer, L.; Hommel, U.; Widmer, H. Helv. Chim. Acta 1996, 79, 913–941; (b) Appella, D. H.; Christianson, L. A.; Karle, I. L.; Powell, D. R.; Gellman, S. H. J. Am. Chem. Soc. 1996, 118, 13071–13072. 3. (a) Frackenpohl, J.; Arvidsson, P. I.; Schreiber, J. V.; Seebach, D. Chem. Biochem. 2001, 2, 445–455; (b) Arvidsson, P. I.; Ryder, N. S.; Weiss, H. M.; Hook, D. F.; Escalante, J.; Seebach, D. Chem. Biodiver. 2005, 2, 401–420.

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