Four heteroatoms

Replacement of one of the carbon atoms in the sugar moiety by oxygen, in effect converting that ring to an acetal, leads to yet another false substrate for viral reverse transcriptase. Glycosdation of the sdylated purine (79) with chiral dioxolane (80), prepared in several steps from anhydro-mannose, in the presence of ammonium nitrate affords the coupling product as a mixture of anomers. The mixture of products is then separated on a chromatographic column. The desired diastereomer (81) is the reacted 

The antitumor chemotherapy agent clofarabine (104) sports halogen on the purine as well as on the sugar. The major portion of the synthesis involves preparation of the requisite fluorine-substituted furanose (101). The sequence begins with the conversion of the only free hydroxyl in allofuranose bis(acetonide) (95) to its tosylate (96). This group is then displaced by fluorine in its reaction with potassium fluoride to afford 

Nerve cells are known to elaborate proteins that guide early development of the nervous system and in later life play a role in cell repair and regeneration. A purine that crosses the blood-brain barrier has shown activity similar to those endogenous factors in vitro as in in vivo models of nerve damage. Conjugate addition of adenine to ethyl acrylate in the presence of base leads to the ester (106). Reaction of that product with sodium 

The search for better tolerated folate antagonist antitumor agents continues to produce yet more variants on methotrexate [see pemetrexed (46) for structure]. The analogue pralatrexate (202) retains most of the features of the prototype, but replaces the side-chain tertiary amine by a 

Alkyl groups and halogen atoms in tetrazoles are not highly activated unless the ring is quatemized. 

Development of conductive organic molecular assemblies (organic metals, superconductors, and exotic functional materials, particularly, tetrathia(selena)fulvalenes and their derivatives) 07BCJ1. 

Magnetic exchange interactions in perfluorophenyl 1,2,3,5-dithiadiazolyl radicals 05CCR(249)2631. 

Methods of functionalization of tetrazoles - strategy and prospects 06ZOR487. 

Organic azides in reactions with participation and preparation of tetrazoles 05AG(E)5188. 

Organometallic tetrazole derivatives, their preparation, and synthetic application 05ZOR1599. 

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For compounds with three or four heteroatoms in the ring the number of measurements is much fewer, and these are summarized in Table 1. 

Table 1 X-Ray Structures of Compounds with Five-membered Rings and Two, Three or Four Heteroatoms ...

Orientation in azole rings containing three or four heteroatoms Effect of azole ring structure and of substituents Proton acids on neutral azoles basicity of azoles Proton acids on azole anions acidity of azoles Metal ions... 

Orientation in azole rings contqining three or four heteroatoms... 

Vll. Five Membered Heterocyclo-Quinolines with Four Heteroatoms. 173... 

Only a few representatives of scvcn-membcred ring compounds with maximum unsaturation containing four heteroatoms that fall within the scope of this section have been reported. 

Besides cyclazines, there are a large number of examples of this angularly fused heterocycle containing up to four heteroatoms leading to three heterocyclic rings fused (5 5 5). When X = N+ in G, the nitrogen atom will carry a positive charge and such cationic cyclazines have not been described so far. 

Five-Membered Heterocycles with Four Heteroatoms... 

Interestingly, the dithiadimetallaborane [S2B5H5M2] (M = CoCp) seems to prefer the ni-9<(IV) structure with the four heteroatoms in the 4k positions 6,8,7,9 at the tetragonal aperture [174]. 

Three or Four Heteroatoms including at least One Selenium or Tellurium... 

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