Ring cleavage selective

Earlier we indicated that an alternative strategy to prevent ring cleavage is to use selective inhibition of the enzymes which catabolise the ring structure. 

Reductive decoupling, 181, 187 Reductive elimination, 171-174, 190-197 refined catalytic cycle, 206-211 and selectivity, 212-214 steric and electronic factors, 202-205, 217 Retro-Wittig decomposition, 37, 78, 79-83 thiobetaines, 57-58, 60, 65 Ring cleavage, 105, 276, 281 Ring closure in oligomerization, 172, 174, 190-197... 

The most strained benzocycloalkene studied, cyclobutabenzene, undergoes extensive ipso attack upon sulphonation. In 95% sulfuric acid, cyclobutabenzene forms polyxylylene and ring cleavage products exclusively. The extent of ipso attack can be decreased in sulfur trioxide media where moderate -selectivity is observed. ... 

Regioselective ring-cleavage of cyclic derivatives of carbohydrates is a useful method for achieving their selective deprotection. 3, 5 -0-(Tetraisopropyldisiloxane-l,3-diyl) derivatives of nucleosides (51, B = uracil-1-yl, cytosin-l-yl, N4-benzoylcytosin-l-yl, guanin-9-yl, or adenin-9-yl) have been prepared,317 and selectively cleaved in 95% yield at the silicon-0-3 bond on treatment with 0.2 M sodium hy-... 

Since dehydrocyclization and ring cleavage are reversible processes, the same mechanisms may be operative in both transformations. Two basic cleavage patterns—a selective and a nonselective ring opening—are known. In the selective process only secondary-secondary carbon-carbon bonds are broken. Conversely, only the formation of carbon-carbon bonds between methyl groups is allowed in ring formation. In contrast, all bonds are cleaved equally in the nonselective reaction. Bonds adjacent to quaternary carbon atoms are exceptions, since they are not cleaved at all. 

---