1.9.5.5 Oxidative Cyclization

DDQ has been extensively employed for the cyclodehydrogenation reactions of phenolic and carboxylic acids. These processes afford effective route for the synthesis of oxygen heterocycles

Scheme 11

Scheme 12

such as coumarins, chromones, benzofurans and lactones. For example, 8-diphenylmethyl-1-naphthoic acid undergoes oxidative cyclization to give δ-lactone (Scheme 11).  Likewise, DDQ can be employed for the cyclodehydrogenation of phenols (Scheme 12).

1.9.5.4 Isomerization

The compounds that are difficult to dehydrogenate give a mixture of isomeric products (Scheme 13).

Scheme 13

The isomeric products could be formed due to fragmentation of intermediate carbonium ion followed by ring closure and dehydrogenation.