The Scientific World Journal

Volume 2014 (2014), Article ID 307895, 6 pages

http://dx.doi.org/10.1155/2014/307895

Research Article

Cyanuric Chloride as an Efficient Catalyst for the Synthesis of 2,3-Unsaturated O-Glycosides by Ferrier Rearrangement

Xiaojuan Yang¹ and Na Li²

¹College of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang, Henan 453003, China

²School of Pharmacy, Xinxiang Medical University, Xinxiang, Henan 453003, China

Received 19 August 2013; Accepted 20 October 2013; Published 19 January 2014

Academic Editors: H. Miyabe, H. Pellissier, and I. Shibata

Copyright © 2014 Xiaojuan Yang and Na Li. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Cyanuric chloride has been found to be an efficient catalyst for the synthesis of 2,3-unsaturated O-glycosides from the reaction of 3,4,6-tri-O-acetyl-D-glucal and a wide range of alcohols in dichloromethane at room temperature. The experimental procedure is simple, and the products are obtained in high yields.

1. Introduction

2,3-Unsaturated O-glycosides play a substantial role in the synthesis of oligosaccharides [1], uronic acids [2], complex carbohydrates [3], and various natural products [4, 5]. Moreover, the unsaturated part of the sugar ring is amenable to easy functionalization such as hydrogenation, hydroxylation, epoxidation, and aminohydroxylation, which contribute to their diversities and complexities. One of the most common procedures to achieve 2,3-unsaturated O-glycosides is the acid-catalyzed allyl rearrangement of glucals, which was discovered by Ferrier and Prasad [6]. The Ferrier rearrangement is a reliable procedure for the formation of 2,3-unsaturated-O-glycosides, which has known extensive development over decades [7]. This rearrangement typically occurs by treatment of glucals and alcohols with Lewis acids such as BF₃·Et₂O [8], FeCl₃ [9], Montmorillonite K-10 [10], I₂ [11], InCl₃ [12], SiO₂-Bi(OTf)₃ [13], ZnCl₂/Al₂O₃ [14], or protic acids such as H₂SO₄-SiO₂ [15], H₃PO₄ [16], H₂SO₄/4 Å molecular sieves [17], CF₃SO₃H-SiO₂ [18], and (S)-camphorsulfonic acid [19]; Er(OTf)₃ [20, 21] as catalyst has also been reported. Although some of these methods have been used for the synthesis of 2,3-unsaturated O-glycosides with good to high yields, the majority of them suffer at least from one of disadvantages, such as prolonged reaction times, low yields, harsh reaction conditions, difficulty in the preparation and moisture sensitivity of the catalysts, and high cost and toxicity of the reagents. Therefore, there is a scope to develop an alternative method for the synthesis of 2,3-unsaturated O-glycosides.

Cyanuric chloride (TCT) is an inexpensive, easily available reagent of low toxicity and less corrosiveness than other similar reactants and has been widely used in organic reactions [22–24], but it has not been carefully studied as a catalyst in the synthesis of 2,3-unsaturated O-glycosides until now. In this paper, we wish to report a rapid and highly efficient method for the synthesis of 2,3-unsaturated O-glycosides in the presence of TCT at room temperature (Scheme 1).

Scheme 1: Synthesis of 2,3-unsaturated O-glycosides.

2. Experimental

2.1. General

¹H NMR and ¹³C NMR spectra were determined on Bruker AV-400 spectrometer at room temperature using tetramethylsilane (TMS) as an internal standard, coupling constants (J) were measured in Hz; elemental analyses were performed by a Vario-III elemental analyzer; commercially available reagents were used throughout without further purification unless otherwise stated.

2.2. General Procedure for the Preparation of 3

To a solution of 3,4,6-tri-O-acetyl-D-glucal (272 mg, 1 mmol) in CH₂Cl₂ (5 mL) were added the corresponding alcohol (1.2 mmol) and TCT (18 mg, 0.10 mmol). The mixture was stirred at room temperature for an appropriate time (Table 2). After completion of the reaction (TLC), the reaction mixture was treated with 10 mL water and extracted with CH₂Cl₂ (3 × 10 mL). The combined organics were dried over anhydrous Na₂SO₄. The solvent was removed under vacuum. All the products were purified by silica gel column chromatography (hexane/ethyl acetate = 9/1). The structures of products were identified through comparison of these spectra data with those in the known literatures [8–19]. Spectral data for selected compounds are as follows.

2.3. Spectral Data of Some Selected Compounds

n-Butyl 4,6-di-O-acetyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranoside (3c). ¹H NMR (400 MHz, CDCl₃): δ 5.80–5.65 (m, 2H), 5.18 (dd, J = 1.2, 9.6 Hz, 1H), 4.90 (s, 1H), 4.06 (dd, J = 5.6, 12.4 Hz, 1H), 4.2 (dd, J = 2.4, 12 Hz, 1H), 3.98–3.94 (m, 1H), 3.66–3.35 (m, 2H), 1.95 (s, 3H), 1.94 (s, 3H), 1.47–1.23 (m, 4H), 0.81 (t, 3H); ¹³C NMR (100 Hz, CDCl₃): δ 170.5, 170.1, 129.0, 128.1, 93.6, 68.9, 67.0, 65.6, 62.9, 31.8, 20.8, 20.7, 18.9, 13.8. Anal. Calcd. for C₁₄H₂₂O₆: C 58.73, H, 7.74. Found: C 58.65; H, 7.69.

Benzyl 4,6-di-O-acetyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranoside (3l). ¹H NMR (400 MHz, CDCl₃): δ7.33–7.25 (m, 5H), 5.93–5.83 (m, 2H), 5.29 (dd, J = 1.6, 2.8 Hz, 1H), 5.12 (s, 1H), 4.82 (d, J = 11.6 Hz, 1H), 4.64–4.55 (m, 1H), 4.32–4.08 (m, 3H), 2.07 (s, 3H), 2.05 (s, 3H); ¹³C NMR (100 Hz, CDCl₃): δ 170.5, 170.1, 137.4, 129.5, 128.3, 128.0, 127.8, 127.7, 92.9, 70.6, 67.2, 64.9, 63.0, 20.8, 20.6. Anal. Calcd. for C₁₇H₂₀O₆: C 63.74, H, 6.29. Found: C 63.88; H, 6.20.

Cyclohex-2-enyl 4,6-di-O-acetyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranoside (3j). ¹H NMR (400 MHz, CDCl₃): δ5.87–5.75 (m, 4H), 5.30 (d, J = 8.8 Hz, 1H), 5.20 (d, J = 18.0 Hz, 1H), 4.27–4.14 (m, 4H), 2.09 (s, 3H), 2.08 (s, 3H), 2.03–1.93 (m, 3H), 1.84–1.70 (m, 2H), 1.56 (m, 1H); ¹³C NMR (100 Hz, CDCl₃): δ170.7, 170.3, 131.6, 128.8, 128.6, 128.4, 128.3, 93.8, 72.9, 67.0, 65.6, 62.9, 29.9, 24.8, 21.2, 19.8. Anal. Calcd. for C₁₅H₂₄O₆: C 59.98, H, 8.05. Found: C 60.02; H, 8.02.

Pregnenolonyl 4,6-di-O-acetyl-2,3-dideoxy-α-D-erythro-hex-2-enopyranoside (3o). ¹H NMR (400 MHz, CDCl₃): δ5.86 (d, J = 10.0 Hz, 1H), 5.75 (d, J = 10.0 Hz, 1H), 5.31 (s, 1H), 5.24 (d, J = 10.0 Hz, 1H), 5.13 (s, 1H), 4.20–3.55 (m, 4H), 2.52 (t, J = 8.0 Hz, 1H), 2.38–2.14 (m, 3H), 2.11 (s, 3H), 2.06 (s, 3H), 2.05 (s, 3H), 2.02 (d, J = 9.5 Hz, 1H), 1.99–1.42 (m, 11H), 1.23–1.11 (m, 2H), 1.06–0.96 (m, 5H), 0.62 (s, 3H); ¹³C NMR (100 Hz, CDCl₃): δ208.9, 170.8, 170.3, 140.5, 128.7, 128.2, 121.3, 93.0, 78.2, 67.0, 65.5, 63.7, 63.2, 56.4, 50.0, 44.1, 40.5, 38.3, 37.3, 36.2, 31.8, 31.7, 31.4, 27.9, 24.3, 22.8, 21.2, 21.0, 20.8, 19.5, 13.5. Anal. Calcd. for C₃₁H₄₄O₇: C 70.43, H, 8.39. Found: C 70.29; H, 8.27.

3. Results and Discussion

In a typical reaction procedure, 2,3-unsaturated O-glycosides 3 were prepared, from the reaction of 3,4,6-tri-O-acetyl-D-glucal 1 and alcohols 2 in CH₂Cl₂ at room temperature in the presence of TCT as catalyst (Scheme 1). Since in this new kind of methodology, there is a great challenge to find new catalysts able to perform the reactions with high activities, we considered TCT as a potential promoter due to its good features for such a transformation. During our investigation, we firstly chose the reaction of 3,4,6-tri-O-acetyl-D-glucal (1 mmol) and methanol (1.2 mmol) as model reactants and examined the effect of the different amounts of TCT (Table 1). When above model reactants were conducted in CH₂Cl₂ (5 mL) in the presence of 0, 0.05, 0.10, 0.15, and 0.20 mmol of TCT separately, the best result was obtained using 0.10 mmol of catalyst (yield = 90%). Using lower amounts of catalyst resulted in lower yields, while higher amounts of catalyst did not affect the reaction times and yields (Table 1, entries 2–5). In the absence of catalyst, the product was not formed (Table 1, entry 1). In order to evaluate the effect of the solvent, the reaction of 3,4,6-tri-O-acetyl-D-glucal and methanol in the presence of TCT was performed in various solvents such as CH₂Cl₂, CH₃CN, acetone, THF, and ethyl ether, in which the best result was obtained in CH₂Cl₂ (Table 1, entry 3); thus, we chose it as the solvent for all further reactions (Table 1, entries 6–9).

Table 1: Reaction conditions optimisation for the synthesis of 3a.

Entry
Catalyst/mmol
Solvent
Time/min
Yield/%

1
0
CH₂Cl₂
120
Trace

2
0.05
CH₂Cl₂
60
79

3
0.10
CH₂Cl₂
30
90

4
0.15
CH₂Cl₂
30
89

5
0.20
CH₂Cl₂
30
90

6
0.10
CH₃CN
30
82

7
0.10
Acetone
30
80

8
0.10
THF
30
69

9
0.10
Ethyl ether
60
42

Table 2: Preparation of 2,3-unsaturated O-glycosides.

Entry
Alcohols
Products
Time/min
Yield/%
α/β a

1
MeOH

30
90
7 : 1

2
EtOH

30
91
10 : 1

3

45
84
8 : 1

4

30
89
8 : 1

5

30
93
α

6

45
89
α

7

45
90
α

8

30
88
8 : 1

9

45
85
11 : 1

10

45
94
11 : 1

11

45
91
10 : 1

12

60
88
9 : 1

13

60
87
9 : 1

14

60
86
α

15

60
86
α

16

45
88
α

17

45
85
α

The α/β ratio was determined from the anomeric proton ratio in the ¹H NMR spectra.

Under the optimum conditions, a wide range of alcohols, including primary, secondary, and allylic alcohols, reacted with acetylated glucal according to Scheme 1 using 10 mol % of TCT at room temperature to give the corresponding 2,3-unsaturated O-glycosides in good to excellent yields and in very short reaction time with α-anomer as a major product (Table 2).

Encouraged by these results, we next explored the scope of this methodology for the synthesis of pseudoglycals connected to various biologically important natural products (Table 2, entries 15–17). Under our catalytic conditions, pregnenolone, L-menthol, and borneol pseudoglycosides were obtained in 86%, 88%, and 85% yields, respectively, with exclusive α-anomeric selectivity.

The formation of products can be explained as follows: initially TCT reacts with the adventitious moisture to form 3 mol of HCl and cyanuric acid (removable by water washing) as a byproduct. The in situ generated HCl assists the formation of the intermediate cyclic allylic oxocarbenium ion I from 3,4,6-tri-O-acetyl-D-glucal, which was originally proposed by Ferrier. Then, alcohol is added to the intermediate I in a quasiaxial fashion preferentially to provide the product 2,3-unsaturated O-glycosides 3 having major α-selectivity as shown in Scheme 2.

Scheme 2: Plausible mechanism for the formation of 2,3-unsaturated O-glycosides.

4. Conclusion

In conclusion, we have developed a mild, eco-friendly approach for the synthesis of 2,3-unsaturated O-glycosides using a catalytic quantity of TCT. The main advantages of this method include good α-selectivity, the use of inexpensive catalyst, and environmentally benign character. We believe that the methodology reported here is synthetically quite attractive and would spur on further interest toward the synthesis of complex glycosides.

Conflict of Interests

The authors have declared that no conflict of interest exists.

Acknowledgment

The authors are pleased to acknowledge the financial support from Xinxiang College.

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