Ribose ribofuranose forms

The two anomeric forms are called a- and P-ribofuranose. Again, in solution there exists an equilibrium between the open chain carbonyl form and the two anomeric hemiacetal forms. Ribose also forms six-membered pyranose anomers, and... 

Efforts to condense 5-benzoyl-D-ribofuranose with a variety of pyrimidine diamines failed,162 although 5-benzoyl-2,3,4-triacetyl-D-ribose was found to form a Schiff base with 4,6-diamino-2-methyl-thiopyrimidine.163... 

With D-ribose, the OH group used to form the five-membered furanose ring is located on C4. Cyclization yields two anomers at the new stereogenic center, which are called a-D-ribofuranose and p-D-ribofuranose. 

FIGURE 8-3 Conformations of ribose. (a) In solution, the straight-chain (aldehyde) and ring (/3-furanose) forms of free ribose are in equilibrium. RNA contains only the ring form, /3-D-ribofuranose. Deoxy-ribose undergoes a similar interconversion in solution, but in DNA exists solely as /3-2 -deoxy-o-ribofuranose. (b) Ribofuranose rings in nucleotides can exist in four different puckered conformations. In all cases, four of the five atoms are in a single plane. The fifth atom (C-2 or C-3 j is on either the same (endo) or the opposite (exo) side of the plane relative to the C-5 atom. 

With benzaldehyde, and zinc chloride as catalyst, n-ribose yields 2,3-0-benzylidene- 8-D-ribofuranose and 1,5 2,3-di-0-benzylidene-D-ribofur-anose. The 1,5-cyclic acetal system has not been found in other sugars. With zinc chloride and acetic acid as catalyst, Vis and Fletcher reported the formation, also, of l,5-anhydro-2,3-0-benzylidene-/3-D-ribofuranose. At higher temperatures, the dianhydride (21) was formed. 

Ribose is an aldopentose. It composes the carbohydrate portion of the ribonucleotides that form a cell s RNA. Ribose, like the aldohexoses, can form a ring. However, ribose forms a five-membered ring, called a fura-nose because of its similarity to the compound furan. Once again, there are two possible forms this ring can have a-D-ribofiiranose and /3-D-ribofura-nose. RNA contains /3-D-ribofuranose. 

Anomers also appear in cyclic form as five-membered rings, for instance in the monosaccharides fructose and ribose. These cyclic molecules are named furanoses because of their similarities to the cyclic ether furan. Starting from D-fructose and D-ribose, the cyclization yields anomers a- and p-D-fructofuranose, as well as a-and P-D-ribofuranose, respectively. 

Aldopentoses also form cyclic hemiacetals. The most prevalent forms of D-ribose and other pentoses in the biological world are furanoses. Following are Haworth projections for a-D-ribofuranose (a-D-ribose) and /3-2-deoxy-D-ribofuranose (j8-2-deoxy-D-ribose) ... 

Five particularly important nucleosides are derived from d-ribofuranose two purine derivatives—adenosine (113) with an adenine unit on the ribose and guanosine (119) with a guanine unit on the ribose—and three pyrimidine derivatives—uridine (120) with a uracil unit, cytidine (115) with a cytosine unit, and thymidine (108) with a thymine unit. Two important deoxyribofura-nose derivatives are formed by using a ribose unit without an OH at C2. Purine... 

The pentose sugars obtained by complete hydrolysis of the nucleic acids are of two types. The pentose derived from RNA is o-ribose and that from DNA is o-2-deoxyribose. When present in nucleotide combination they are found in the furanose form, i.e. as o-ribofuranose and D-2-deoxyribofuranose respectively. 

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