Chromanones Friedel-Crafts reaction

The usual range of solvents associated with Friedel-Crafts reactions is available carbon disulfide and nitrobenzene have found frequent use. The choice of solvent may influence the extent of cyclization. Thus, in ether, 1,2,3,5-tetramethoxybenzene yielded the uncyclized product with 3,3-dimethylacryloyl chloride, whereas in a mixture of ether and tetrachloroethane the major product was the chromanone. This example is also of interest since ring closure could involve either the 2- or 6-methoxy group. In fact, the formation of the 5,6,7-trimethoxy isomer was not observed. The identity of the product was established by conventional 14C labelling (60BSB593). 

A closely related reaction involves that between a saturated acyl halide and a phenol or phenolic ether. A necessary feature of the acid chloride is that it contains a bromine atom at C-2 which allows formation of a double bond during the reaction by loss of bromide. Normal Friedel-Crafts conditions are employed in the first step which leads to an o-hydroxyphenyl 2-bromoalkyl ketone (589). In boiling diethylaniline, hydrogen bromide is lost and the resulting acrylophenone spontaneously cyclizes to the chromanone <24LA(439)132). 

This year has seen the publication of the first convenient synthesis of crotonophenones, which are open chain forms of benzopyranones (chromanones). Previous attempts at their synthesis via Friedel-Crafts or simple aldol reactions gave poor yields. However, it has now been shown that the dianion of n-hydroxyacetophenones will react with aldehydes, and dehydration of the products constitutes a simple synthesis of crotonophenones and also of 2-methyl chromanones. 

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