Cycloalkenes intramolecular McMurry reaction

Intramolecular free radical cyclization of dicarboxylic esters leads to a-hydroxy ketones (acyloins). Reductive coupling of dicarbonyl compounds provides 1,2-diols ipinacols) and further reaction of these yields cycloalkenes (McMurry reaction). These cyclization reactions are especially valuable for the preparation of medium and large rings that are not readily accessible by other methods. 

In 1977, McMurry and Kees [152] developed a titanium-induced intramolecular coupling procedure to form cycloalkenes from dicarbonyl compounds. Mechanistically, as shown in Scheme 85, the coupling reaction proceeds by an initial pinacol dimerization of the dicarbonyl 253 to 254, followed by titanium-induced deoxygenation to afford alkene 255. 

Cycloalkenes. McMurry and Kees have described an intramolecular coupling of dicarbonyl compounds to cycloalkenes. In the intermolecular version of this reaction titanium(O) powder obtained by reduction with potassium or lithium is used (7, 368). In the intramolecular reaction a superior coupling reagent is prepared by reduction of TiClg with a zinc/copper couple in DME. 

---