A simple and convenient method for the synthesis of pyranoid glycals

Carbohydrate Research 345 (2010) 168–171

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A simple and convenient method for the synthesis of pyranoid glycals Jinzhong Zhao a,b,c, Shanqiao Wei a,c, Xiaofeng Ma a,c, Huawu Shao a,* a b c

Natural Products Research Center, Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu 610041, China Shanxi Agriculture University, Taigu Shanxi 030801, China Graduate School of Chinese Academy of Sciences, Shijingshan, Yuquan Road, 19(A), Beijing 100049, China

a r t i c l e

i n f o

Article history: Received 5 May 2009 Received in revised form 30 September 2009 Accepted 12 October 2009 Available online 15 October 2009

a b s t r a c t A simple, mild, and environmentally benign synthesis procedure of pyranoid glycals is described. In a novel fashion, protected glycopyranosyl bromides undergo the reductive elimination in the presence of zinc in phosphate buffer at room temperature. The pyranoid glycals were obtained in good-to-excellent yields (18 examples). Ó 2009 Elsevier Ltd. All rights reserved.

Keywords: Glycals Phosphate buffer Glycopyranosyl bromides Zinc Reductive elimination

Glycals are used extensively in the synthesis of O-glycosides,1–3 C-glycosides,4–11 S-glycosides,12,13 N-glycosides,14–16 cyclopropanated carbohydrates,17,18 and natural products.19–23 Many glycoconjugates can be synthesized via the glycal method, and glycals have also been used in glycosylation reactions as glycosyl donors or glycosyl acceptors.24–27 Most 2-C-branched sugars can be synthesized through 1,2-cyclopropanation followed by selective ring opening via solvolysis,28–32 and a majority of the 1,2-cyclopropane derivatives have been prepared from glycals. Hence it is necessary to develop a simple, convenient, and environmentally benign method to produce a variety of glycals of different configurations. The traditional synthesis of glycals involves treating a peracetylated glycopyranosyl bromide with zinc in acetic acid.33 The Fischer–Zach method has been one of the most popular methods for synthesizing glycals, because the products of this method are suitable for the synthesis of many other natural products. Over the years, numerous synthetic methods for glycals have been developed, including the reduction of protected glycosyl halides by Na,34 lithium naphthalenide, Li–NH3,35 Zn–Ag, (Cp2TiCl)2,36–38 Cr(II),39,40 Al–Hg, K-graphite,41 Zn–AcOH–THF,42 Zn–N-base,43 Zn–MeOH–vitamin-B12,44 Zn–Ag, Na naphthalenide, and Al–Hg,45 or by using thiophenyl glycoside,46 glycosyl sulfones,47 glycosyl sulfoxides,48 and an electrochemical approach.49 However, there are several drawbacks in the above-mentioned methods, including, for example, the use of very expensive and toxic reagents, low temperatures, and intricate laboratory operations. In view of the * Corresponding author. Fax: +86 28 85222753. E-mail address: [email protected] (H. Shao). 0008-6215/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.carres.2009.10.003

importance of glycals and their derivates in the synthesis of oligosaccharides and natural products, a strong impetus has been given to develop a mild, simple, less toxic, economically convenient, and user-friendly reaction protocol for the synthesis of glycals. However, there have been no reports on the use of phosphate buffer for the synthesis of pyranoid glycals. As part of our continuing interest in the development of new synthetic methodologies, we disclose our results on the synthesis of pyranoid glycals from protected glycopyranosyl bromides in the presence of zinc in phosphate buffer at room temperature. Initially, treatment of the tetra-O-acetyl-a-D-glucopyranosyl bromide (1) with a Zn–CuSO4 in acetic acid following the Fischer–Zach method afforded the corresponding D-glucal (2, Scheme 1), but the method requires a lower temperature and intricate laboratory operations (Table 1, entry 1). When 1 was treated with Zn–CuSO4 in phosphate buffer at room temperature, a good result was obtained (Table 1, entry 2), and the yield was also the same as that obtained by using Zn alone in phosphate buffer at room temperature (Table 1, entry 3). But the reaction of 1 with Fe in phosphate buffer gave only a modest yield of 2 (Table 1, entry

AcO AcO

OAc

OAc O

1

O

AcO AcO

OAc Br

2

Scheme 1. Synthesis of glucal 2 from the protected tetra-O-acetyl-a-D-glucopyranosyl bromide (1).

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J. Zhao et al. / Carbohydrate Research 345 (2010) 168–171 Table 1 Optimization for the synthesis of pyranoid glycal 2 from the protected tetra-O-acetyla-D-glucopyranosyl bromide (1) Entry

Conditions

Yield (%)

1 2 3 4 5 6 7 8 9 10

Zn, CuSO4, NaOAc, HOAc, H2O, 15 to 0 °C, 6 h Zn, CuSO4, phosphate buffer (pH 4.5), rt, 6 h Zn, phosphate buffer (pH 4.5), rt, 6 h Fe, saturated NaH2PO4 solution, rt, 4 h Zn, phosphate buffer (pH 6.0), rt, 6 h Zn, phosphate buffer (pH 7.0), rt, 6 h Zn, phosphate buffer (pH 8.0), rt, 8 h Zn, saturated NaH2PO4 solution, rt, 5 h Zn, saturated NaH2PO4 solution, acetone, rt, 2 h Zn, saturated NaH2PO4 solution, EtOAc, rt, 2 h

67 80 80 45 80 75 40 85 90 90

Table 2 Synthesis of substituted pyranoid glycals from protected pyranosyl bromides Entry

Starting material

Glycal

OAc

OAc 1

O

AcO AcO

1

OAc

90

Br

2 OAc

2

O

AcO AcO

O

AcO AcO

83

2

Br

3

OAc OAc

OAc

O

O 3

81

AcO

AcO OAc

4

20

Br

AcO

O

AcO

O

AcO 4

90

OAc

OAc

21

Br

5 5

O

AcO AcO

6

6

OAc Br

Me AcO

O OAc

7

22 O Me AcO

OAc

8

23

OAc

Br

AcO AcO

9

9

AcO AcO

OAc Br

24

10

AcO AcO

11

12

25

Br

O

AcO AcO

Br

OAc OTs O AcO OAc Br

71

25 OAc OMs O 92

AcO

26 OAc OTs O 80

AcO

27 N3

N3 OAc O

14

65

OTs

OAc OMs O AcO OAc Br 12

AcO AcO

O

24

13

13

75

AcO AcO

OAc O

11

O

AcO AcO

OMs

OAc O

10 OTs

89

OTs O

OMs

O

AcO AcO

OTs 8

87

OAc OMs

O

AcO AcO

76

AcO

OMs 7

O

AcO

Br

1. Experimental The reactions were monitored by thin-layer chromatography (TLC) using silica gel HSGF254 plates. Flash chromatography was performed using silica gel HG/T2354-92. 1H NMR and 13C NMR (600 and 150 MHz, respectively) spectra were recorded in CDCl3. 1 H NMR chemical shifts are reported in ppm (d) relative to tetramethylsilane (TMS) with the solvent resonance employed as the internal standard (CDCl3, d 7.26 ppm). Data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet,

O

AcO AcO

OAc OAc

OAc

4). Encouraged by the results obtained with the tetra-O-acetyl-a-Dglucopyranosyl bromide 1 and Zn in phosphate buffer, we turned our attention to Zn and a different phosphate buffer. Interestingly, treating tetra-O-acetyl-a-D-glucopyranosyl bromide 1 with Zn in different phosphate buffers (pH 6.0, 7.0, and 8.0), all afforded Dglucal 2 (Table 1, entries 5–7). Saturated NaH2PO4 solution proved to be a very efficient system, giving a good yield of 2 (Table 1, entry 8). When 1 was treated with Zn-saturated NaH2PO4 solution in the presence of a little acetone or EtOAc, the yield of 2 was increased to 90%, and the reaction time was shortened to only 2 h (Table 1, entries 9 and 10). Meanwhile, the glucal can also be synthesized under neutral conditions (Table 1, entry 6). Hence, this method is superior in comparison with the classical Fischer–Zach method. To explore the synthetic potential of the method, reactions involving 18 examples (Table 2, 1 and 3–19) were performed on a preparative scale in Zn-saturated NaH2PO4 solution (Scheme 2). To our delight, treatment of the acetylated glycopyranosyl bromides 3–7 with Zn gave pyranoid glycals 2 and 20–23 in goodto-excellent yields. Benzylated pyranoid glycals 29–31 and 6-Oacetyl benzylated pyranoid glycals 32 could be afforded in modest yield. In all cases the pyranoid glycals (2 and 20–23) were obtained in 64–92% isolated yields (Table 2, entries 1–18). Importantly, 6-Omesyl, 6-O-tosyl, and 6-azido pyranoid glycals 24–28 were accessible in good yield using this method. A variety of protecting groups, including acetyl, benzyl, methanesulfonyl, and p-tolylsulfonyl groups, were stable under the reaction conditions. An advantage of the new methodology is that the groups of the starting material are typically either acid or base stable, and a wide variety of reactions can be utilized in modifying the protecting groups on the glycal products. In summary, we have developed a mild and efficient protocol for the synthesis of variously protected pyranoid glycals using Zn-saturated NaH2PO4 as an inexpensive and environmentally friendly system. In the preparation of pyranoid glycals, various methods have been used for enhancing the activity of zinc, but ordinary zinc dust has high activity and does not need to be activated by using other reagents in phosphate buffer. The method offers significant advantages, such as low toxicity, simple operation, mild reaction conditions, high yields, and low cost, which makes it an attractive process for the synthesis of pyranoid glycals.

Yield (%)

O

AcO AcO

Br

85

28 (continued on next page)

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J. Zhao et al. / Carbohydrate Research 345 (2010) 168–171

Table 2 (continued) Entry

Starting material

Glycal

Me BnO

O

BnO

14

Yield (%)

O

Br

OBn OAc

67

OBn

29

15

BnO BnO

15

OBn

O

BnO BnO

OAc

17 OBn OBn O BnO OAc

17

18

O

BnO BnO

Br

64

30 OBnOBn O 70

BnO Br

OAc O

BnO BnO

OAc

19

O

BnO BnO Br

65

32

R" R'O R'O

R" Zn-saturated NaH2PO4

O

rt

OAc

The glycopyranosyl bromide (1 mmol) was dissolved in acetone (6.0 mL), and then sodium dihydrogen phosphate dihydrate (3.4 g, 21.7 mmol) and zinc dust (1.0 g, 15.3 mmol) were added. The mixture was stirred for 10 min, then H2O (0.6 mL) was added. The reaction mixture was stirred for 3 h at room temperature, and TLC (1:1 petroleum ether–EtOAc) indicated that the reaction was complete. The solution was extracted with EtOAc. The organic phase was washed with water, satd NaHCO3, and brine, and then dried over MgSO4 and concentrated. The residue was purified by silica gel column chromatography (2:1 petroleum ether–EtOAc).

31

OAc 18

75

30

OBn 16

O

BnO BnO

Br

16

1.2. General procedure for the synthesis of pyranoid glycals 21 and 22

OBn

OBn OAc O

1.1.2. 3,4,6-Tri-O-acetyl-D-galactal (20) 50 ½a23 Colorless syrup: ½a25 D 9 (c 1.0, EtOAc); lit. D 17 (c 1.1, CHCl3); Rf 0.38 (1:2 petroleum ether–EtOAc). 1H NMR (CDCl3): dH 2.03 (s, 3H), 2.09 (s, 3H), 2.13 (s, 3H), 4.22 (dd, 1H, J = 11.6, 5.2), 4.28 (dd, 1H, J = 11.6, 7.2), 4.33 (m, 1H), 4.73 (m, 1H), 5.43 (dd, 1H, J = 3.8, 1.1), 5.56 (d, 1H, J = 1.0), 6.46 (d, 1H, J = 5.2).

R'O R'O

O

Br

1.2.1. 3,4-Di-O-acetyl-D-arabinal (21) 40 ½a25 Colorless syrup: ½a25 D +200 (c 0.9, CH2Cl2), lit. D +262 (c 0.9, 1 CHCl3); Rf 0.55 (2:1 petroleum ether–EtOAc). H NMR (CDCl3): dH 2.07 (s, 3H), 2.08 (s, 3H), 3.98 (dd, 1H, J = 10.6, 9.6), 4.03 (m, 1H), 4.85 (dd, 1H, J = 5.8, 5.2), 5.19 (m, 1H), 5.44 (dd, 1H, J = 4.8, 4.2), 6.50 (d, 1H, J = 6.0). HRESIMS: calcd for C9H12NaO5 [M+Na] m/z 223.0577; found m/z 223.0564. 1.2.2. 3,4-Di-O-acetyl-D-xylal (22) 40 ½a25 Colorless syrup: ½a25 D 301 (c 1.9, CHCl3), lit. D 303 (c 2.3, 1 CHCl3); Rf 0.57 (2:1 petroleum ether–EtOAc). H NMR (CDCl3): dH 2.07 (s, 3H), 2.10 (s, 3H), 3.98 (dd, 1H, J = 12.3, 1.4), 4.19 (m, 1H), 4.95–4.97 (m, 2H), 5.00 (m, 1H), 6.60 (d, 1H, J = 6.3).

Scheme 2. Synthesis of pyranoid glycals from protected pyranosyl bromides.

1.3. General procedure for the synthesis of pyranoid glycals 23–32 m = multiplet), integration, and coupling constants (Hz). 13C NMR chemical shifts are reported in ppm from tetramethylsilane (TMS) with the solvent resonance as the internal standard (CDCl3, d 77.0 ppm). ESIHRMS spectra were recorded on a BioTOF Q instrument. Optical rotations were acquired on a Perkin–Elmer 341 Digital Polarimeter. 1.1. General procedure for the synthesis of pyranoid glycals 2 and 20 The glycopyranosyl bromide (1 mmol) was dissolved in acetone (2.0 mL), and then satd sodium dihydrogen phosphate solution (4.0 mL) and zinc dust (0.82 g, 12.5 mmol) were added. The reaction mixture was stirred for 5 h at room temperature, at the end of which time TLC (1:1 petroleum ether–EtOAc) indicated that the reaction was complete. The solution was extracted with EtOAc. The organic phase was washed with water, satd NaHCO3, and brine, and then dried over MgSO4 and concentrated. The residue was purified by silica gel column chromatography (2:1 petroleum ether–EtOAc). 1.1.1. 3,4,6-Tri-O-acetyl-D-glucal (2) White powder: mp 50–51 °C, lit.40 mp 50–52 °C; ½a25 D 22 (c 4.1, CHCl3), lit.40 ½a20 D 22 (c 2.1, CHCl3); Rf 0.38 (2:1 petroleum ether–EtOAc); 1H NMR (CDCl3): dH 2.05 (s, 3H), 2.08 (s, 3H), 2.10 (s, 3H), 4.20 (dd, 1H, J = 12.4, 3.1), 4.26 (m, 1H), 4.40 (dd, 1H, J = 12.2, 5.8), 4.85 (dd, 1H, J = 9.5, 3.3), 5.22 (dd, 1H, J = 7.5, 6.0), 5.35 (dd, 1H, J = 4.2, 3.7), 6.47 (d, 1H, J = 6.2).

The glycopyranosyl bromide (1 mmol) was dissolved in EtOAc (2.0 mL), and then saturated sodium dihydrogen phosphate solution (4.0 mL) and zinc dust (0.82 g, 12.5 mmol) were added. The reaction mixture was stirred for 3 h at room temperature, at the end of which time TLC (2:1 petroleum ether–EtOAc) indicated that the reaction was complete. The solution was extracted with EtOAc. The organic phase was washed with water, satd NaHCO3, and brine, then dried over MgSO4 and concentrated. The residue was purified by silica gel column chromatography (4:1?2:1 petroleum ether–EtOAc). 1.3.1. 3,4-Di-O-acetyl-L-rhamnal (23) 51 ½a25 Colorless syrup: ½a25 D +65 (c 1.1, CH2Cl2), lit. D +55 (c 1.0, CHCl3); Rf 0.63 (2:1 petroleum ether–EtOAc). 1H NMR (CDCl3): dH 1.31 (d, 3H, J = 6.6), 2.04 (s, 3H), 2.08 (s, 3H), 4.11 (m, 1H), 4.78 (dd, 1H, J = 6.2, 3.1), 5.03 (dd, 1H, J = 8.2, 6.2), 5.34 (m, 1H), 6.43 (d, 1H, J = 6.0). 1.3.2. 3,4-Di-O-acetyl-6-O-methanesulfonyl-D-glucal (24) 52 ½a17 Colorless syrup: ½a25 D 4 (c 0.3, CHCl3), lit. D +15 (c 1.6, 1 EtOH); Rf 0.43 (2:1 petroleum ether–EtOAc); H NMR (CDCl3) dH 2.06 (s, 3H), 2.10 (s, 3H), 3.07 (s, 3H), 4.35 (m, 2H), 4.47 (dd, 1H, J = 11.6, 6.2), 4.89 (dd, 1H, J = 6.2, 3.5), 5.21 (dd, 1H, J = 7.4, 5.6), 5.35 (m, 1H), 6.48 (d, 1H, J = 6.2). 1.3.3. 3,4-Di-O-acetyl-6-O-p-toluenesulfonyl-D-glucal (25) White powder: mp 106–107 °C, lit.53 mp 106–107 °C; ½a25 D +16 1 +14 (c 1, CHCl ); H NMR (CDCl ): d (c 1.5, CHCl3), lit.53 ½a22 3 3 H 2.03 D

J. Zhao et al. / Carbohydrate Research 345 (2010) 168–171

(s, 3H), 2.04 (s, 3H), 2.46 (s, 3H), 4.23 (m, 3H), 4.82 (dd, 1H, J = 6.2, 3.5), 5.13 (dd, 1H, J = 3.7, 3.7), 5.27 (dd, 1H, J = 6.2, 5.5), 6.35 (d, 1H, J = 6.0), 7.35 (d, 2H, J = 7.9), 7.80 (d, 2H, J = 8.4). 1.3.4. 3,4-Di-O-acetyl-6-O-methanesulfonyl-D-galactal (26) Colorless syrup: ½a25 D 5 (c 0.4, CH2Cl2); Rf 0.25 (2:1 petroleum ether–EtOAc). 1H NMR (CDCl3): dH 2.06 (s, 3H), 2.10 (s, 3H), 3.07 (s, 3H), 4.33–4.37 (m, 2H), 4.48 (dd, 1H, J = 6.0, 2.9), 4.89 (dd, 1H, J = 6.0, 3.3), 5.21 (dd, 1H, J = 7.2, 5.5), 5.35 (dd, 1H, J = 4.3, 3.5), 6.48 (d, 1H, J = 6.2). 13C NMR (CDCl3): dC 20.8, 20.9, 37.8, 65.6, 66.9, 67.0, 73.6, 99.4, 145.3, 169.8, 170.3. HRESIMS: calcd for C11H16Na1O8S1 [M+Na] m/z 331.0464, found m/z 331.0474. 1.3.5. 3,4-Di-O-acetyl-6-O-p-toluenesulfonyl-D-galactal (27) Colorless syrup: ½a25 D +3 (c 0.2, CH2Cl2); Rf 0.45 (2:1 petroleum ether–EtOAc). 1H NMR (CDCl3): dH 2.01 (s, 3H), 2.05 (s, 3H), 2.46 (s, 3H), 4.14 (dd, 1H, J = 10.5, 4.4), 4.28 (dd, 1H, J = 10.6, 7.7), 4.32 (m, 1H), 4.72 (dd, 1H, J = 6.1, 3.1), 5.37 (s, 1H), 5.48 (s, 1H), 6.35 (d, 1H, J = 6.1), 7.36 (d, 2H, J = 8.2), 7.79 (d, 2H, J = 8.2). 13C NMR (CDCl3): dC 20.5, 20.7, 21.7, 63.5, 63.8, 66.7, 72.4, 98.9, 128.0, 129.9, 132.6, 145.2, 169.8, 170.1. HR-ESIMS: calcd for C17H20Na1O8S1 [M+Na] m/ z 407.0777, found m/z 407.0779. 1.3.6. 3,4-Di-O-acetyl-6-azido-6-deoxy-D-glucal (28) 54 [a] 6 (c 1.9, Colorless syrup: ½a25 D 46 (c 1.2, CH2Cl2), lit. CHCl3); Rf 0.61 (2:1 petroleum ether–EtOAc). 1H NMR (CDCl3) dH 2.06 (s, 3H), 2.10 (s, 3H), 3.56 (dd, 1H, J = 11.0, 6.5), 3.61 (dd, 1H, J = 11.2, 5.0), 4.29 (dd, 1H, J = 11.8, 6.0), 4.87 (dd, 1H, J = 6.0, 3.5), 5.28–5.31 (m, 2H), 6.50 (d, 1H, J = 6.1). 1.3.7. 3,4-Di-O-benzyl-L-rhamnal (29) 55 ½a22 Colorless syrup: ½a25 D +37 (c 0.9, CH2Cl2), lit. D +64 (c 1.0, 1 CHCl3); Rf 0.72 (5:1 petroleum ether–EtOAc). H NMR (CDCl3): dH 1.40 (d, 3H, J = 6.5), 3.51 (dd, 1H, J = 9.0, 7.3), 3.97 (m, 1H), 4.24 (m, 1H), 4.59 (d, 1H, J = 11.7), 4.68 (d, 1H, J = 11.8), 4.73 (d, 1H, J = 11.3), 4.87–4.91 (m, 2H), 6.38 (d, 1H, J = 6.1), 7.29–7.38 (m, 10H). 1.3.8. 3,4,6-Tri-O-benzyl-D-glucal (30) White powder: mp 56–57 °C, lit.56 mp 55 °C; ½a25 D 6 (c 1.9, CH2Cl2), lit.56 ½a22 D 3 (c 16.5, CHCl3); Rf 0.53 (4:1 petroleum ether–EtOAc). 1H NMR (CDCl3): dH 3.77–3.85 (m, 2H), 3.88 (m, 1H), 4.08 (m, 1H), 4.23 (s, 1H), 4.56–4.62 (m, 3H), 4.64–4.68 (m, 2H), 4.85 (dd, 1H, J = 11.3, 4.1), 4.88 (m, 1H), 6.44 (m, 1H), 7.26– 7.34 (m, 15H). 1.3.9. 6-O-Acetyl-3,4-di-O-benzyl-D-glucal (32) 57 ½a25 Colorless syrup, ½a25 D 7 (c 1.2, CH2Cl2), lit. D +2 (c 1.1, 1 CHCl3); Rf 0.44 (4:1 petroleum ether–EtOAc). H NMR (CDCl3): dH 2.06 (s, 3H), 3.79 (dd, 1H, J = 13.6, 7.0), 4.11 (m, 1H), 4.24 (m, 1H), 4.37 (dd, 1H, J = 12, 5.4), 4.42 (d, 1H, J = 12.1), 4.57 (d, 1H, J = 11.6), 4.67 (d, 1H, J = 12.1), 4.68 (d, 1H, J = 11.4), 4.87 (d, 1H, J = 11.2), 4.92 (dd, 1H, J = 3.8, 2.2), 6.41 (d, 1H, J = 6.0), 7.28–7.37 (m, 10H). Acknowledgments We are grateful for the financial support from the Chinese Academy of Sciences (Hundreds of Talents Program). We also thank the Analytical and Testing Center of Chengdu Institute of Biology for NMR and MS analyses.

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Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.carres.2009.10.003. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19.

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