Bond, ribosidic

Since it seemed likely that a xanthate function beta to a carbon radical would eliminate to furnish a double bond, we studied the tin hydride reduction of 1,2-dixanthates.1112 This provided an efficient new synthesis of olefins which was independent of the stereochemistry of the glycol. It was particularly suitable for the conversion of protected ribosides 23 via the dixanthates 24 into the corresponding olefins 25 from which biologically important dideoxynucleosides 26 were readily obtained by hydrogenation and deprotection.13... 

Table 9.1. Some important intramolecular hydrogen bonds between bases and riboside observed in nucleoside and nucleotide crystal structures...

The heterocyclic ring in l,2,4-triazine-3,5-dione (12) and its 6-methyl derivative (13) is planar. The data clearly show that in the crystal only the dione structure is present no indication of any other tautomeric structure was found. The methyl group in the 6-position of (13) has an influence on the bond lengths of the two carbonyl groups. While in (12) the two carbonyl groups are equal (1.224 A), in (13) two different distances were found C(3)=0 = 1.213 A and C(5)=0 = 1.242 A. In the ribosides (14) and (15) both carbonyl groups are nearly the same length. 

Furan adds to vinylene carbonate to produce an exolendo mixture of Diels-Alder adducts [165]. Double hydroxylation of the 7-oxanorbornene double bond is highly exo face selective (Scheme 13.88). The diol thus obtained is protected as an acetonide. Saponification of the carbonate liberates a mixture of diols that is oxidized into m 5 6>-l,5-anhydroallaric acid derivative 316. Treatment of 316 with AC2O generates the anhydride 317. Subsequent reaction of 317 with methanol gives racemic 318 that can be resolved by fractional crystallization with brucine or by chromatographic separation of the (7 )-l-((3-naphthyl)ethylamides. The individual isomers of 318 each react with ClCOOEt and Me3SiN3 in situ to provide enantiomerically pure d- and L-riboside derivatives [166]. 

The thiamin biosynthetic pathway is outlined in Fig. 22 [88]. Overall the pathway involves the separate synthesis of the thiazole 111 and the pyrimidine 114 which are then coupled. l-Deoxy-D-xylulose-5-phosphate (see sections 11 and 12 of this review) is the precursor to the five carbon unit of the thiazole [89], cysteine is the sulfur source [90, 91], and the C2-N3 atoms of the thiazole are derived from the a-carbon and the amino group of tyrosine [92-94]. The pyrimidine is derived from 5-aminoimidazole riboside (AIRs), an intermediate on the purine biosynthetic pathway. This reaction involves a complex rearrangement in which the C4 carbon of AIRs is inserted into the C4-C5 imidazole double bond, converting the imidazole to a pyrimidine, and the C2 carbon of AIRs is used to methylate the C2 position of the imidazole [95-98]. 

Figure 6.58 Representative arsenic-containing ribosides and the proposed biosynthetic C-As bond-making step.

In addition, we have studied39 charge fluctuations in the ribose and purine groups of the enzyme-bound ribonucleoside. If these fluctuations are resonant with the oxygen-stack promoting motion, it would further validate the hypothesis that the promoting motion pushes electrons across the ribosidic bond. 

In addition, we probed fluctuations across the N9-C1 ribosidic bond, since this is the bond that is broken in this reaction. The spectra are shown in the right panel of Fig. 20. They are resonant with each other and with the spectra for ribose and purine... 

The purpose of these experiments was to determine the binding mode of 6-mercaptopurineriboside to platinum using EXAFS spectroscopy. The Pt-EXAFS data presented here establish the presence of two nitrogen and two sulfur atoms at normal bonding distances from the platinum in Pt(6—mercaptopurineriboside) 2 and Pt(2-amino—6—mercaptopurine—riboside)2. For Pt(6—MPR>2 and... 

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