Small-sized ring

Five-membered rings are preferentially formed in such cyclizations. Though the exo-attack is favored in most cases, //< (9-cyclization takes place instead, particularly, when it would lead to small size rings (Equation (28)). 

Addition. Two molecules combine to yield a single molecule. Addition frequently occurs at a double or triple bond and sometimes at small-size rings. 

Reaction of 4-bromo-1,2-epoxybutane and 4-bromo-2-methyl-l,2-epoxybutane with the activated copper gave only the 3-membered ring alcohols cyclopropylmethanol and l-methyl-l-(hydroxymethyl)cyclopropane in 89% and 96% yields, respectively. For the small-sized ring closure, ring strain imposed by the connecting chain becomes crucial. The geometry of... 

Ring-closing metathesis (RCM) reaction, which is one of the most powerful methods for the formation of small-sized rings or cyclopentenol ring [100, 101], was successfully applied in the synthesis of chiral cyclopentenol, as the key intermediate of carbocyclic nucleosides. Interestingly, the 1,2,3-triazole NPA analog... 

---