Linear oligomerization

Under similar conditions bisnitrofurazans afforded oligomeric linear or macro-cyclic ethers (96JOC1510, 99ZOR1555). An attempt to extend this reaction to nitrofuroxans failed. 

While the inorganic chemistry of divalent tin and lead is well established, there are few well-characterized divalent organotin and organo-lead compounds. The R2Sn formulation seen in the older literature has proved to be an empirical formula, the formally divalent stannylene actually being an oligomeric linear or cyclic organostannane. Neumann has commented on this point several times (1-3). 

Regardless of the t)q)e of soluble metal silicates used, they are subject to the same molecular speciation in aqueous solution resulting in a mixture of monomeric tetrahedral ions, oligomeric linear or cyclic silicate ions, and polysilicate ions. Sodium metasilicate, an example of a soluble metal silicate, can be prepared in anhydrous form or in the presence of water of crystallization as the penta- or nona-hydrate. It is readily soluble in water [8]. 

The successful syntheses of the above host-guest assemblies pointed the way to the formation of molecular threads - namely, compounds formed by threading a cyclic species, such as the cyclobis(paraquat-/ -phenylene) tetracation, on to a separate extended linear component the product is a pseudorotaxane. Pseudorotaxanes are a derivative category of the rotaxanes in which one of the assembled components is a longer (often oligomeric) linear fragment. They are thus similar to rotaxanes, with the exception that the linear component is not terminated by bulky stoppers . 

Polymer supports also can enable high yields in the synthesis of cyclic peptides. Cyclo-(Gly-His)3 was produced in 42% overall yield from attachment, five deprotection-coupling sequences, and cleavage from the resin.It was isolated by chromatography with no detectable amounts of oligomeric linear or cyclic peptides. 

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