Selective Anomeric Deacetylation of Per-Acetylated Carbohydrates Using (i-Pr)3Sn(OEt) and Synthesis of New Derivatives

In this study natural carbohydrates such as glucose, galactose, xylose, fructose andlactose, are acetylated by acetic anhydride and sodium acetate catalyst. Anomeric configuration is deacetylated by (i-Pr)3Sn(OEt)as a catalyst, an easy synthetic regioselective deacetylation of full acetylated carbohydrates using (i--Pr)3Sn(OEt) is described. The acetylated carbohydrates reacted with HBr (solution in AcOH, 32 wt.%) for the bromination of anomeric position. The synthesis oxazaphosphorine, and bromo hexa alkyl Methylsulfonate derivatives from anomeric position of carbohydrates was reacted. FT IR, 1H, 13C NMR, 31PNMR spectroscopy techniques were employed to examine the synthesized compounds.


Introduction
Carbohydrates are a large class of organic compounds which can be found in the structure of living organisms. They are major energy source for both plants, and animals that play an important role in feeding the living organisms. Likewise, these compounds are quintessential reactions of ATP, RNA and DNA. Carbohydrates are polyhydroxy ketone or aldehydes that can be found in nature. Carbohydrates can be divided into three major classes: monosaccharides, disaccharides and polysaccharides. Monosaccharides can be classified into classes, aldose and ketose. The presence of the heteroatom in-ring has an appreciable effect on both formation and reactivity of carbohydrates. The existence of an electron-withdrawing substituent (as halogens) onto C1 in axial position is more stable due to the anomeric effect. This tendency not only is not limited to carbohydrates but also observes in annular systems like substitutability of 2-tetrahydropyrans. This phenomenon is known as the anomeric effect. (Sorg, Hull, Kliem, Mier, & Wiessler, 2005) The hydroxyl groups in carbohydrates are esterified easily. Acetylation is the most common reaction for esterification. Selective anomericde acetylation is considered as an important stage in synthesis of glycoside. Acetylated-1-hydroxy of carbohydrates contains valuable structures for diverse reactions of glycosides.
Carbohydrates are used in medicine usages such as heparin of anticoagulant, antibiotics, vaccines, anticancer medicines, antibacterial and antifungal (Liang, Huang, & Duan, 2007). carbohydrates hasn't been extended for cellular biology improvements in drug receiving (Mazur, Opydo-Chanek, & Stojak, 2011). Thence, carbohydrates performance hasn't been studied widespread in biology because of the complicated structures of oligosacarids, non existence of synthesis methods and analysis of structure.
In this paper, a sample of carbohydrates consisting of glucose, galactose, xylose, fructose and lactose is acetylated using Ac 2 O/AcONaunder the 50-60°C and anomeric configuration of acetylated carbohydrates using ( i Pr) 3 Sn(OEt) as a new catalyst has been released. Then, it is reacted with HBr (AcOH solution), new derivatives of oxazaphosphorine, and alkyl sulfunate from anomeric position reacted. All of these compounds are identified by FT-IR, 1 H, 13 C and 31 PNMR spectroscopy techniques.

Experiment
All materials were obtained from Merck Co. 1 H, 13 C and 31 P NMR spectra of in CDCl 3 and/or DMSO-d6 were measured using Brucker 400 AC spectrometer as a solvent at room temperature (University of Tabriz, Tabriz, Iran and ShahidBeheshti University, Tehran, Iran).

Preparation of 2,3,4,6-Tetra-O-Acetyl-ß-D-Galactopyranozyl Bromide with HBr/AcOHAa
HBr solution in acetic acid (4.5ml) was mixed with galactosepentaacetate. The reaction mixture was stirred until it became a homogeneous mixture and was stirred for 30 min. The obtained precipitate became smooth and rinsed with water. Then it was dissolved in ether and separated by separatory funnel. Organic phase was transferred into another container and dried. Solvent, was extracted without heating in vacuum and during two steps at 5 minutes interval, insignificant amount of petroleum ether was added. After a milky precipitate was formed, it kept in freezer for 24h. After that time, 2.5ml of petroleum ether was mixed with 2.5ml of ether and added to it. Precipitate was smoothed and washed with mixture of ether and petroleum ether, a white precipitate yielded with 59% efficiency.
In order to prepare 6a, a compound 5a (0.01mol) and ethanol amine(0.015mol) in 30mL of dry DMF, and 0.01mol pyridine were cooled at 0°C were cooled to 0°C in N 2 atmosphere.the solvent was removed in vacuum pump. After removing solvent, 6a was obtained as a red oil.
In order to prepare 7a SOCl 2 was reacted for 1h and finally, the deprotection process of acetyl group was occurred.

Results and Discussion
This paper aimed to describe the full acetylation of some carbohydrates such as D-glucose (1a), D-galactose (1b), D-xylose (1c), D-fructose (1d) and D-lactose (1e) in the presence of Ac 2 O/AcONa as well as selective deacetylation of anomeric position using ( i Pr) 3 Sn(OEt) as a new catalyst (Scheme 1, Tables 1 and 2).Anomeric configuration is deacetylated by (iPr) 3 Sn(OEt)as a catalyst, A convenient synthetic approach to regioselectivedeacetylation of full acetylated carbohydrates using ( i Pr) 3 Sn(OEt)is described. The acetylated carbohydrates are reacted with HBr (solution in AcOH, 32 wt.%) for the bromination of anomeric position. As a representative bromination of 2a is yielded 4a (Scheme 1, Table 3).
Representatively, the 1 H NMR spectrum of glucose penta acetate (2a) is shown in Figure 1.
Its structure was characterized by 1 H, 13 C NMR and FT-IR spectra. The 1 H NMR spectrum of 2a showed doublets at δ 6.6ppm (J = 3.6Hz) for H-1, a double of doublet at δ 5.24ppm (J = 10.5Hz) for H-2, a triplet at δ 5.80ppm (J = 9.9Hz) for H-3, a triplet at δ 5.33ppm (J = 9.9Hz) for H-4, a double of doublet at δ 4.29ppm (J = 12.2Hz) for H-5, a double doublet at δ 4.0ppm (J = 12.3Hz) for H-6. Methyl groups show three singlets at δ 1.50-1.70ppm. The FT-IR spectrum showed a strong absorption at 1741cm -1 due to carbonyl stretching frequency (See supporting information).  The acetyl ester plays an important role for protection of the hydroxyl group in organic synthesis. Methodologies that are widely used have been put forth for the esterification. The deprotection of the acetyl esters is much less studied despite its practical significance in synthetic processes. A convenient methodology has been developed for the selective removal of the anomeric acyl group of carbohydrate derivatives using ( i Pr) 3 SnOEt conditions. Representatively, the proposed mechanism of the formation of 3b is shown in Scheme 2. Scheme 2. Representatively, proposed mechanism of the formation of 3b Scheme 3. Synthesis of 8a from 3a as representative In IR spectrums the existence of 1730cm -1 peaks shows a Carbonyl group and peaks 1000cm -1 are related to aliphatic group. absorption vibrate of esters are appeared in 1000-1300cm -1 acetylation of galactos while organizing Carbonyle in position 2 which shows the effect of neighbor group so the existing quitter group easily take apart from number one carbon and organize stable Carbocation. In this mode there is a possibility of attacking carbon anomeric by nucleofilic agent. Absorption band in 1379cm -1 location is related to symmetrical jmbr.ccsenet.org Vol. 10, No. 1;2020 curvature of methyl CO-CH 3 and seen absorption in 1221cm -1 is related to neighbor C-O vibration of ester Carbonyl group and absorption band in 1100cm -1 is related to ester tension of alcoholic section of ester.

Journal of Molecular Biology Research
In order to synthesize the new oxazaphosphorine derivatives, the anomeric hydroxyl group should be deprotected. For this purpose, we used ( i Pr) 3 SnOEt for the selectively deacetylation of the anomeric position. Therefore, as a representative, 2a was selectively converted to 3a in good yield (Scheme 1). The reaction of compound 3a with POCl 3 yielded 5a. The reaction of this compound with 2-eyhanolamine gave 6a, then in the presence of thionyl chloride yielded 7a. The deprotection of acethyl groups of later compound (7a) in the presence of trimethylsilane yielded 8a.
ppm ( The 31 P NMR spectrum of 7a is shown in Figure 2. A singlet peak at δ -12.82 ppm corresponded to a unique phosphorous atom connected to oxygen, nitrogen and halogen atoms.  As a representative, FTIR spectrum of 2b showed vibration bands of carbonyl (C=O), C-O groups are appeared in 1746 and 1224cm -1 , respectively, and vibration band at 1371cm -1 is related to bending vibration of methyl group. No hydroxyl group stretching frequency was observed at 3500-3600cm -1 . These observations demonstrated the full acetylation of 1b the acetylation of D-galactose within formation of acethoxy group in position 2 demonstrates neighboring group participation (NGP). In this manner, charging of nucleophilic agent to anomeric carbon is only possible from one side. The selective deacetylation of 2b was carried out in the presence of ( i Pr) 3 Sn(OEt) and yielded 3b. Carbohydrate is one of the natural materials with the biggest organic compounds classification which exist in a high level biomolecule such as lipids or glycoproteins. Carbohydrates play an important role in different kinds of biological processes. Mutual and specific performances of protein-carbohydrate in processes such as cell breakdown, cell adhesion, safety reaction, congestion and tumor cell.
Metamorphosis, glycolipid, glycoprotein and Polysaccharide creation in lektins and proteins, are the biological responsibilities of carbohydrates.
The FT -IR spectrum in combination with it, showed that there was no penetration with the OH absorption range. All compounds synthesized in Table 4 are confirmed by 31 PNMR spectroscopy.
In order to prepare (1F), compound 3b obtained from previous stage (1.4mmol) was dissolved in pyridine (0.4mL) and deprotonated by NaOH. A mixture is cooled at 0' C in an ice water bath, then bromohexaalkylsulfunat is added (2mmol, 0.2mL), and then the reaction mixture is stirred for 6 h at room temperature. After gathering sediment, it is dissolved in dichloromethane (12mL) and organic phase is extracted by salt water (2*6mL) and then dried by using sodiumsolphate, the solution is filtered and removed by rotary, and crystallized by ethanol with 53% efficiency (scheme 4).

Conclusion
The present study investigated a productive manner for deacetylation of anomeric position. This method has several advantages such as the good-natured reaction conditions, experimental simplicity, good yield .then the basic reagents are unfavorable for anomeric deacetylation this approach can be very effective. In all cases, The yields were good to excellent. However, in this paper carbohydrates such as glucoss, xylose, fructose, galactose, lactose by acetic anhydride in the presence of sodium acetate were acetylated. Then the deacetylation of anomeric position by( i Pr) 3 Sn(OEt) was reacted. In order to prepare derivatives such as oxazaphosphorine and bromoheptyl methanesulfonate, deporptonation of the hydroxyl group was reacted with carbohydrates to supply new compounds.