A straightforward sequential synthetic strategy has been developed for the synthesis of a pentasaccharide repeating unit corresponding to the cell wall O-antigen of the Escherichia albertii O4 strain in very good yield with the desired configuration at the glycosidic linkages using thioglycosides and trichloroacetimidate derivatives as glycosyl donors and perchloric acid supported over silica (HClO4/SiO2) as a solid supported protic acid glycosyl activator. The expected configuration at the glycosidic linkages was achieved using a reasonable selection of protecting groups in the manosaccharide intermediates.
Keywords: Escherichia albertii O4; glycosylation; HClO4/SiO2; O-antigen; pentasaccharide
Diarrheal outbreaks are serious concerns all over the world particularly in the developing countries due to inadequate sanitation systems . In most of the cases, the enteric infections originated due to the intake of less cooked food and contaminated water . Several strains of Shigella , Salmonella  and enteropathogenic Escherichia coli  are commonly known for causing diarrheal infections. Besides the mainstream enteropathogenic bacterium, Escherichia albertii (E. albertii) is an emerging human pathogen causing gastroenteric infections in different countries . Although, this species was identified earlier as Hafnia alvei, later it was redesignated as E. albertii . E. albertii acted as a causative agent for diarrheal diseases in children with vomiting, fever and abdominal distension . Several strains of E. albertii have been identified till date, which significantly contributed to the spreading of devastating diarrheal infections in different countries . The role of cell wall O-polysaccharides in regulating the virulence properties of bacteria is well established . Recently, Naumenko et al.  reported the structure of the repeating unit of the cell wall O-polysaccharide of the E. albertii O4 strain , which is a pentasaccharide comprising of α-linked ᴅ-galactosamine, β-linked ᴅ-glucosamine, β-linked ᴅ-galactose, α-linked ʟ-fucose and α-linked ʟ-rhamnose moieties. In the recent past, several vaccine candidates have been developed to control bacterial infections by conjugating cell wall polysaccharides with suitable proteins, which include vaccines against Haemophilia influenza type b (Hib) [12,13], meningitis , pneumococcal infections [15,16] and enteric diseases such as cholera , diarrhea  and urinary tract infections . Despite the possibility of isolating the polysaccharides by fermentation techniques, it is difficult to get a significant quantity of polysaccharide fragments from natural sources with adequate purity. Therefore, the development of chemical synthetic strategies is quite pertinent to obtain a requisite quantity of oligosaccharide fragments with adequate purity. In this direction, the total synthesis of the pentasaccharide repeating unit corresponding to the cell wall O-antigenic polysaccharide of the E. albertii O4 strain using a sequential glycosylation strategy is presented herein (Figure 1).
The synthesis of pentasaccharide 1 was achieved using a convergent as well as a block synthetic strategy. For this purpose, a series of suitably functionalized monosaccharide intermediates 2 , 3 , 4 , 5 , 6  and 7  were prepared from the commercially available reducing sugars utilizing the reaction conditions reported in the literature (Figure 1). Although the monosaccharide intermediates used for the construction of the pentasaccharide derivative 15 are known in the literature, preparation of these intermediates required multiple step reaction sequences. Having obtained the monosaccharide intermediates, it was decided to proceed through a step-economic block synthetic strategy to achieve the target pentasaccharide derivative. Accordingly, stereoselective glycosylation of a ᴅ-galactosamine derivative 2 with a ᴅ-galactose thioglycoside derivative 3 in the presence of a combination [26,27] of N-iodosuccinimide (NIS) and perchloric acid supported over silica (HClO4/SiO2) [28,29] furnished disaccharide derivative 8 in 79% yield, which on de-O-acetylation using sodium methoxide  gave the disaccharide acceptor 9 in 95% yield. NMR spectral analysis of compound 9 confirmed its formation with appropriate configuration at the glycosidic linkages [Signals at δ 5.54 (d, J = 2.5 Hz, H-1A), 5.44 (s, PhCH), 4.54 (d, J = 7.5 Hz, H-1B) in 1H NMR and at δ 105.2 (C-1B), 100.6 (PhCH), 98.2 (C-1A) in 13C NMR spectra] (Scheme 1).
In another experiment, ʟ-rhamnosyl trichloroacetimidate donor 5 was coupled with ʟ-fucosyl thioglycoside acceptor 4 in the presence of HClO4/SiO2  as activator using an orthogonal glycosylation approach to furnish disaccharide thioglycoside derivative 10 in 76% yield, which was directly used in the next level of glycosylation. NMR spectral analysis of compound 10 unambiguously confirmed its formation [signals at δ 5.26 (d, J = 1.5 Hz, H-1D), 4.23 (d, J = 9.5 Hz, H-1C) in 1H NMR and at δ 98.4 (C-1D), 84.8 (C-1C) in 13C NMR spectra] (Scheme 2). It is worth noting that sulfide linkage at the anomeric position of compound 4 remained unaffected under the reaction conditions.
Having achieved the disaccharide acceptor 9 and the disaccharide thioglycoside donor 10, a stereoselective glycosylation between them was attempted in the presence of a combination [26,27] of NIS and HClO4/SiO2 as thiophilic activator. Unfortunately, the required tetrasaccharide derivative 11 was obtained in a poor yield (22%, Scheme 3). It was decided to follow a sequential glycosylation strategy to achieve a significant quantity of compound 11. Accordingly, a stereoselective glycosylation was carried out using compound 9 with ʟ-fucose thioglycoside derivative 6 in the presence of a combination [26,27] of NIS and HClO4/SiO2 as thiophilic activator. Gratifyingly, the trisaccharide derivative 12 was obtained in 74% yield with a newly formed 1,2-cis glycosyl linkage in it. The structural confirmation of compound 12 was established by its NMR spectral analysis [signals at δ 5.67 (d, J = 3.0 Hz, H-1A), 5.60 (d, J = 3.5 Hz, H-1C), 5.50 (s, PhCH), 4.79 (d, J = 7.5 Hz, H-1B) in 1H NMR and at δ 103.3 (C-1B), 100.8 (PhCH), 99.0 (C-1A), 97.1 (C-1C) in 13C NMR spectra]. Compound 12 was subjected to a set of reactions consisting of a one-pot  de-O-acetylation and benzylation using benzyl bromide and sodium hydroxide in the presence of tetrabutylammonium bromide (TBAB) followed by oxidative removal  of the PMB group using 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to give trisaccharide acceptor 13 in 72% yield. Trisaccharide acceptor 13 was then allowed to couple with ʟ-rhamnosyl trichloroacetimidate donor 5 in the presence of HClO4/SiO2 as a solid acid activator  to provide tetrasaccharide derivative 11 in 76% yield, which was de-O-acetylated to furnish tetrasaccharide acceptor 14 in 94% yield. The formation of compound 11 with appropriate configuration at the glycosidic linkages was supported by its NMR spectral analysis [signals at δ 5.72 (d, J = 3.5 Hz, H-1A), 5.58 (d, J = 3.5 Hz, H-1C), 5.51 (s, PhCH), 4.80 (d, J = 7.5 Hz, H-1B), 4.71 (br s, H-1D) in 1H NMR and at δ 103.3 (C-1B), 100.6 (PhCH), 99.0 (C-1C), 95.0 (C-1A), 94.1 (C-1D) in 13C NMR spectra]. Finally, NIS and HClO4/SiO2-promoted stereoselective glycosylation of compound 14 with ᴅ-glucosamine thioglycoside donor 7 furnished the desired pentasaccharide derivative 15 in 70% yield. The formation of compound 15 with appropriate configuration at the glycosidic linkages was supported by its NMR spectral analysis [signals at δ 5.64 (d, J = 3.5 Hz, H-1A), 5.54 (d, J = 3.0 Hz, H-1C), 5.50 (d, J = 8.5 Hz, H-1E), 5.48 (s, PhCH), 5.41 (s, PhCH), 4.96 (br s, H-1D), 4.69 (d, J = 8.0 Hz, H-1B) in 1H NMR and at δ 103.4 (C-1B), 101.6, 100.4 (2 C, 2 PhCH), 100.1 (C-1E), 99.0 (C-1C), 94.9 (C-1D), 94.7 (C-1A) in 13C NMR spectra]. Compound 15 was subjected to a sequence of reactions consisting of (i) reductive transformation of the azido group into an acetamido group by the treatment with thioacetic acid ; (ii) transformation of the N-phthalimido group into acetamido group using hydrazine hydrate followed by selective N-acetylation ; (iii) hydrogenolysis of benzyl ethers and benzylidene acetals over Pearlman’s catalyst  to furnish the desired pentasaccharide 1 in 49% overall yield (Scheme 4). The structure of compound 1 was unambiguously characterized by its NMR spectral analysis [signals at δ 5.37 (d, J = 2.0 Hz, H-1A), 5.29 (d, J = 3.5 Hz, H-1C), 5.12 (br s, H-1D), 4.73 (d, J = 7.5 Hz, H-1E), 4.61 (d, J = 8.0 Hz, H-1B) in 1H NMR and at δ 102.2 (C-1E), 102.1 (C-1B), 96.8 (C-1A), 96.5 (C-1C), 96.0 (C-1D) in 13C NMR spectra].
In summary, a convenient stepwise synthetic strategy has been developed for the synthesis of the pentasaccharide repeating unit of the cell wall O-antigen of Escherichia albertii O4 in very good yield. Although the target compound can be achieved by block synthetic approach but a better yield of the product was obtained by a sequential approach. HClO4/SiO2 was used as a solid acid activator in the glycosylation reactions using trichloroacetimidate as well as thioglycoside donors. All intermediate steps were high yielding with excellent stereo outcome in the glycosidic linkages.
|Supporting Information File 1: Experimental and analytical data and copies of NMR spectra.|
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