| dc.description.abstract | Carbohydrates are a family of polyfunctional natural products and can be chemically modified in numerous ways. The primary significance of carbohydrates rests in their importance in biological functions. A particular class of sugars, namely, 2-deoxy or C-2 modified sugars has received a special attention, due to their importance in biological functions. These sugars are defined as carbohydrates carrying a hetero-atom, other than the hydroxyl group, and their derivatives. There is an ever-leading requirement to synthesize various carbohydrates-containing natural and un-natural products, such as, oligonucleotides, glycopeptides, antitumor drugs and cardiac glycosides, having C-2 modified sugars. Chapter 1 describes various synthetic modifications, particularly at the C-2 of a monosaccharide, as relevant to the work presented in this Thesis.
1, 2-Unsaturated glycopyranosides, namely, glycals, are versatile synthetic intermediates for the elaboration to a number of functionalized glycosyl derivatives. A major utility of the glycals is their conversion to the 2-deoxy glycosyl derivatives. In a programme, it was desired to identify a synthetic method to prepare 2-deoxy sugar derivatives that are endowed with an anomeric activation. In particular, a thioglycoside activation was desired. In the event, a methodology was identified, which allowed the synthesis of activated 2-deoxy-1-thioglycosides.The method involved reaction of a glycal with EtSH, in the presence of ceric ammonium nitrate (CAN) as the catalyst. The reaction was applicable to different epimeric glycals. Apart from the 2-deoxy-1-thioglycosides, formation of the 2, 3-unsaturated enoses, corresponding to the Ferrier product, also observed. Optimal conditions for the formation of the 2-deoxy-1-thioglycosides were identified (Scheme 1) and the reaction was proposed to proceed through a radical oxocarbenium ion and a thiolate intermediate.
(Fig)
Scheme1
Upon synthesis of 2-deoxy-1-thioglycosides, few glycosylation reactions with both aglycosyl and glycosyl acceptors were performed and the α-anomeric 2-deoxy glycosides were obtained exclusively.
Chapter 2 summarizes synthesis, characterization of 2-deoxy-1-thioglycosides and their glycosyl donor properties towards several glycosyl acceptors.
Many naturally-occurring antibiotic and antitumor drugs contain 2-deoxy glycosides as important structural components. For example, 2,6-dideoxy-hexopyranoses are common structural units of chromomycin A3, olivomycin A and mithramycin. The most common structural features of these molecules are: (i) the presence of 2-deoxy sugar residues and (ii) the sugar residues are connected to the aromatic moiety, through a β-glycosidic linkage. The synthesis of these biologically important 2-deoxy glycosides encounters difficulties, due to the absence of stereoelectronic influences at C-2 of the 2-deoxy glycosyl derivatives.
Direct glycosylation of phenols and naphthols with activated 2-deoxy-1-thio-glycosides, in the presence of the thiophilic activator N-iodosuccinimide/triflic acid (NIS/TfOH), lead to the formation of the α-anomer, as the major glycosylated product (Scheme 2).
(Fig)
An effort was under taken to identify methods to prepare the 2-deoxy aryl glycosides, in the β-anomeric configuration. A nucleophilic substitution reaction was anticipated to lead to the formation of β-anomeric glycosides. A halide substitution at C-1 for an effective nucleophilic substitution was adopted. Thus, conversion of the activated 2-deoxy-1-thioglycosides with Br2 in the first step, followed by reaction of the resulting bromide with aryloxy anions, led to the facile conversion to 2-deoxy glycosides in a nearly quantitative f-anomeric configuration at C-1(Scheme 3).
Scheme 3 (Fig)
Chapter 3 presents details of the methodologies that allow a facile preparation of each of the anomers of aryl 2-deoxy-D-glycosides from a common precursor, namely, 2-deoxy-1-thio-glycosides.
An easy access to activated 2-deoxy-1-thioglycosides from the 1, 2-unsaturated sugar and their synthetic utility towards various glycosyl and aglycosyl acceptors led towards synthesis of 2-deoxy disaccharides. Synthesis of six new 2-deoxy-arabino-hexopyranosyl and 2-deoxy-lyxo-hexopyranosyl sugar containing disaccharides were accomplished. These are: (i) 2-deoxy-α-D-arabino-hexopyranosyl-(1→4)-D-glucopyranose (2'-deoxy maltose); (ii) 2-deoxy-α-D-lyxo-hexopyranosyl-(1→4)-D-glucopyranose; (iii) 2-deoxy-α-D-arabino-hexopyranosyl-(1→4)-2-deoxy-D-arabino-hexopyranose (2,2'-dideoxy maltose); (iv) 2-deoxy-α-D-lyxo- hexopyranosyl-(1→4)-2-deoxy-D-arabino-hexopyranose; (v) α-D-glucopyranosyl-(1→4)-2 deoxy-D-arabino-hexopyranose (2-deoxy maltose) and (vi) β-D-galactopyranosyl-(1→4)- deoxy-D-arabino-hexopyranoside (2-deoxy lactose).
The 2'-deoxy and 2, 2'-dideoxydisaccharides were synthesized using a 2-deoxy glycosyl donor and a normal glycosyl acceptor (in case of 2'-deoxy disaccharides) and a 2-deoxy glycosyl acceptor (in case of 2, 2'-dideoxy disaccharides) with a free OH group at C-4, while the remaining hydroxyl groups protected suitably (Scheme 4).
Scheme 4 (Fig)
On the other hand, the syntheses of 2-deoxy disaccharides were initiated from a D-maltose and D-lactose, respectively. The conversion of these disaccharides to a disaccharide glycals was targeted first and conversion of these glycals to a 2-deoxy-1-thioglycosides or a 2-deoxy-1-acetates, followed by a hydrolysis of the thiol moiety or the acetate group, afforded the 2-deoxy disaccharides (Scheme 5). (Fig)
Chapter 4 describes synthesis, characterization of 2-deoxy, 2,2'-dideoxy and 2'-deoxy disaccharides.
Continuing the efforts to establish the utility of 2-deoxy-1-thioglycosides as potential glycosyl donor, synthesis of 2-deoxy cyclic and linear oligosaccharides was undertaken. Prominent among cyclic oligosaccharides are the cyclodextrins. Due to their unique structural and physical properties, cyclodextrins find manifold applications. Known methods to synthesize cyclic oligosaccharides are (i) the cyclization of linear oligosaccharides to produce the cyclic oligosaccharides and (ii) the synthesis of designed monomers and subjecting them to cyclooligomerization protocols. The cyclooligomerization was adopted to synthesize new types of 2-deoxy cyclic-and linear oligosaccharides. After a series of trials, a disaccharide monomer, namely, ethyl 4-O-(6-O-benzoyl-2,3-di-O-methyl-α-D-glucopyranosyl)-2-deoxy-3,6-di-O-methyl-arabino-hexopyranoside (1), was identified as a suitable monomer for thecyclooligomerization protocol. For an effective oligomerization, the concentration of the monomer and the choice of the reagents are important.
The reaction was conducted at three different monomer concentrations, 2 mM, 10 mM and 25 mM, using two thiophilic activators, namely, (i) NIS/TfOH and (ii) NIS/AgOTf. Better yields of the cyclic oligosaccharides, namely, the cyclic tetrasaccharide (2) (40 %) and cyclic hexasaccharide (3) (25 %), were isolated when the monomer (1) concentration was 25 mM and NIS/TfOH acid was used as the promoter (Scheme 6). The formation of linear disaccharide (4) (10 %) and tetrasaccharide (5) (18 %) was also observed at this concentration.
On the other hand, when the reaction of the monomer was performed in the presence of NIS/AgOTf, the oligomerization reaction led to the formation of linear oligosaccharides, consisting of di-to eicosa-saccharides. Synthesis of different monomers, their characterization and oligomerization reaction using these monomers through a polycondensation protocol are
discussed in Chapter 5.
Scheme 6(fig)
In summary, the Thesis establishes the chemistry of 2-deoxy sugars, formation of activated 2-deoxy sugars, formation of alkyl and aryl glycosides, 2-deoxy disaccharides, 2-deoxy cyclic and linear oligosaccharides. Routine physical methods were used to characterize the newly formed 2-deoxy sugars and the oligosaccharides. Single crystal X-ray structural determination was performed for an aryl 2-deoxyglycosides, which provided the solid state configurational features of the 2-deoxy pyranose.
(For structural formula pl see the pdf file) | en_US |
