Sugars nitrogen heterocycles from

Oligonucleotides are nucleic acid polymers. Each nucleic acid is assembled from a sugar, a nitrogenous heterocyclic base, and a phosphate. The specific identity of the sugar and base determines the type of nucleic acid, which in turn determines the structure and function of the oligonucleotide. 

Nucleotides are derived from three different types of molecules—phosphoric acid, pentose sugar, and heterocyclic nitrogen base (Fig. 21-1). Two different pentoses are used, o-ribose for RNA and 2-deoxy-D-ribose for DNA. Henceforth, the sugars will be referred to simply as ribose and deoxyri-bose. A total of five different heterocyclic bases are used, three pyrimidine and two purine bases. Each base is symbolized by the first letter of its name, C, T, U, A, and G for cytosine, thymine, uracil, adenine, and guanine, respectively. A, G, and C are used in synthesizing both DNA and RNA nucleotides. T is used only for DNA while U is used only for RNA. 

We construct a nucleotide from three components. First, we replace the hydroxy group at Cl in the sugar with one of the base nitrogens. This combination is called a nucleoside. Second, a phosphate substituent is introduced at C5. In this way, we obtain the four nucleotides of both DNA and RNA. The positions on the sugars in nucleosides and nucleotides are designated r, 2, and so forth, to distinguish than from the carbon atoms in the nitrogen heterocycles. 

Aqueous aza-Diels-Alder reactions of chiral aldehydes, prepared from carbohydrates and with benzylamine hydrochloride and cyclopentadiene, were promoted by lanthanide triflates (Eq. 12.65).137 The nitrogen-containing heterocyclic products were further transformed into aza sugars, which are potential inhibitors against glycoprocessing enzymes. 

Many nitrogen- and sulfur-containing heterocycles have been identified in the aroma fractions of foods [214]. In roasted products (e.g., coffee) and heat-treated foods (e.g., baked bread or fried meat), these heterocycles are formed from reducing sugars and simple or sulfur-containing amino acids by means of Maillard reactions [215, 216]. Their odor threshold values are often extremely low and even minute amounts may significantly contribute to the aroma quality of many products [217, 218]. Therefore, N- and N,S-heterocyclic fragrance and flavor substances are produced in far smaller quantities than most of the products previously described. 

As with other nitrogen-containing heterocycles, TV-alkylation is a reaction of some interest. This is particularly true when the products are nucleosides. One interesting example of nucleoside formation arises from reaction of a suitable furo[3,4-J]pyrimidine (41) and l-O-acetyl-2,3,5-tribe nzoyl-/ -D-ribose the product formed seems to be solvent dependent. The N-l substituted nucleoside (42 R = tribenzoylribose) is formed in 81 % yield when the heterocycle and the protected sugar are treated with tin(IV) chloride in acetonitrile. However, use of 1,2-dichloroethane as solvent affords only 17% of this product together with 81% of the N-l,N-3 disubstituted product (Equation (10)) <83CPB3074>. 

An interesting methodology to highly oxygenated p)razolidines and indolizidines from 2,3-unsaturated sugar lactones was presented by Chmielewski [84]. The synthesis was initiated by a 1,4-addition of nitrogen nucleophiles to unsaturated lactones 93, which resulted in formation of appropriate heterocyclic derivative 94 (O Scheme 38). 

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