Three-Atom 2-5-Linkages

Non-phosphate three-atom 2 -5 -linked oligonucleotides constitute a significantly different modification, although at the first glance they resemble 2 -5 -phosphate-linked ODNs and ORNs Like natural nucleic acids, they exhibit six connecting atoms in their SRU. Molec- 

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Dimers with a novel three-atom 2 —>5 -linkage, as in 233, have been synthesized, and their binding affinity to poly-U was reported. ... 

Bichromophoric Compounds With Single Flexible Three Atom Linkages 2.43.5.1 Survey of Interunit Linkages... 

Pofychromopkorie Compounds Without Three-Atom Linkages... 

In the effort to prepare l,5-methano[10]annulene, elimination of HI from iodotetraene 83 was found to give the rearranged pentaene 84 (126). A reasonable explanation for its formation is a walk rearrangement of the intermediate 1,7-bridged bicyclo[5.1.0]octadienyl cation. In this case the walk may be favored by the steric effect of the three-atom linkage, which probably constrains the homotropylium cation to the closest bicyclo[5.1.0] octadienyl form. 

Figure 29.2, 5 -three-atom linkages compared with 2, 5 -phosphates or 3, 5 -esters. From [238]. 

Some of polymers 17 and 19 formed excimers in aqueous solution. Exci-mers are observed in bichromophoric molecules, where the aromatic chromo-phores are separated by a three-atom linkage. Excimer formation in these systems requires rotational motion around the bond of the linkage to allow two chromophores to have, within the lifetime of the excited state, a conformation suitable for complex formation in which two aromatic rings overlap in a sandwich-like arrangement [35, 36]. When these geometrical requirements are satisfied for the pendent chromophores on the polymer chains, the polymer shows an excimer fluorescence as observed in polyfvinyl aromatic)s [37] and polymers containing chromophoric pendant groups [38]. 

If a tether is attached to the C(3) atom of nitronates (214), the reaction gives rise to spiro systems (215). For these systems, generating a linkage containing three atoms between the six-membered ring and the C,C double bond is a case of choice. It necessarily leads to cis -fusion of two five-membered rings. 

If a tether is considered as a linkage between the reacting fragments (nitronate and the C=C bond), it contains three atoms in all cases. 

If a cyclophosphazene precursor molecule has more than two reactive sites (e.g. chlorine atoms), linkage reactions with diamines of dialkoxides will generate a three-dimensional structure rather than a linear chain. Such materials are known as cyclomatrix polymers (3.18). These species form a half-way stage between linear polymers and ceramics, and occupy a critical position in materials science. The term ultrastructure covers a broad range of materials of this type. 

Tetraene 165, with the dienes separated by two two-atom linkages, undergoes photocycloaddition to give the strained 166 (Sch. 37) [112]. Double extrusion of carbon monoxide from 167 yields tetraene 168 with the dienes separated by two three-carbon chains. This tetraene yields pentacyclic 169 in 50% overall yield [113]. 

Norlignans lack one carbon atom in comparison to lignans. Three different linkage types are found 8-8, 7-8 and 9-8. They typically occur in mono-cotyledonous plants and in the heartwood of conifers, putatively as defence against rotting, but they have also been found in certain dicots (Suzuki and Umezawa, 2007). 

Figure 7. Diagram of linkage patterns between three atomic states showing pump (P) and Stokes (5) transitions and the various detunings for (a) A, (b) ladder, and (c) V systems.

The ambident nature of the cyanide ion leads to a four-atomic sequence N—C—M , which may be connected in different ways to form a crystal lattice. The coordination behavior of the two different metal ions decides whether there will be a one-, two-, or three-dimensional linkage. Examples with typical chains are represented by the crystals of AgCN 2 AgNOa (19) and Ni(en)2Pd(CN)4 (20). A two-dimensional linkage is seen in the sheets of the cyanide-ammonia clathrates (21, 22). The vast majority of the pol3muclear cyanides, however, contain an octahedral hexacyanometalate, M (CN) e , and have a three-dimensional framework structure (19, 23). 

Further, it should be noted that the same macrocycle as in Figure 1 could be obtained by using condensation dimers of acetone, such as diacetone alcohol or mesityl oxide (8). It was also found that several kinds of a,0-unsaturated ketones and 0-hydroxyketones may be used to prepare macrocyclic diamino-diimines (8 ). This observation is very important, because by using suitable ketones it becomes possible to introduce desired substituent(s) into the three carbon atom linkage. In fact, a variety of diamino-diimine macrocycles has been prepared and used as ligands in recent years (9 - 14 ). However, almost all of the macrocycles were synthesized under reaction conditions not requiring metal ions such as nickel(II). 

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