Tether cleavage

Information on the three-dimensional structure of RNA can be obtained by tethering cleavage agents to known positions and then by studying the fragments produced by the cleavage reactions. This has been used to study Escherichia coli ribosomal... 

Scheme 10-5 A short tether ensures complete regio- and stereocontrol with vinyl silyl ethers Subsequent tether cleavage is also readily achieved.

In most cases the radical generated after cyclization is quenched by H-abstraction. However, another possibility is to utilize the cyclized radical in another C-C bond-forming event. Fraser-Reid and co-workers utilized a silyl-tethered radical cyclization of the (L)-rhamnal-derived silyl ether 142 to generate the anomerically. stabilized radical 143, which could be trapped in the presence of an excess of acrylonitrile to generate acetate 144 after tether cleavage and peracetylation (Scheme 10-48) [55a]. This reaction sequence occurred with complete regio- and stereoselectivity. The same group has also used an acetal tether (vide infra) to effect similar transformations [55 b, 56]. 

Tamao et al. have investigated (dichloromethyl)dimethylsilyl ethers as radical cyclization precursors [69]. Silyl ether 174 was readily prepared from the commercially available silyl chloride and isophorol. 5- xo-trig cyclization could be effected under high-dilution conditions, affording the bicycle 175 as a 6 4 mixture of stereoisomers. Subsequent oxidative tether cleavage afforded 2-formyl alcohols 176 and 177 where unfortunately, the presence of base caused partial epimerization of the center a to the formyl group (Scheme 10-56). 

A variety of oxidative tether cleavage protocols have also been evaluated which allow regioselective monoprotection of the resulting 1,3-diol products, facilitating differentia-... 

FIGURE 16.11 Comparison of the relative configurations of the product of a double intermolecular cycloaddition (325) with the products of the fused mode of inteimolecular-[4 -I- 2]/intramolecular-[3 -I- 2] cycloaddition before (326, 329) and after (327, 328, 330, 331) tether cleavage. 

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