Acyl halides 3,7-unsaturated

Cyclo acylations leadUy take place in intermoleculai acylations involving bifunctional acylating agents. Both functional groups may be acyl (as in the case of a,CO-diacyl halides) or one may be an alkylating group (as in unsaturated acyl halides or certain haloacyl halides) (18). 

Using a,P-unsaturated acyl halides, alkenes are acylated to give a,P,a, P -unsaturated ketones, which undergo spontaneous intramolecular Na2arov cyclizations to cyclopentenones, important precursors of natural products (173). 

The reaction is of wide scope (R = alkyl, aryl) however the substrate molecule should not contain other functional groups that can react with diazomethane. With unsaturated acyl halides the yield can be poor, but may be improved by modified reaction conditions. ... 

It was noted in Section 3 that unsaturated acyl halides often formed anomalous products. This tendency is even more pronounced in the case of aldehydes. The decarbonylation of 2-allylbenzaldehyde gives a high yield of indan, and very... 

Ketones containing a double bond have also been prepared by the reaction of unsaturated acyl halides with aromatic hydrocarbons in the usual Friedel-Crafts manner. Acylation of benzene and its homologs with /S,/S-dimethylacroyl chloride leads to dimethylvinyl aryl ketones, (CHj)jC = CHCOAr (75-90%). °° The latter compounds are stable and do not undergo intramolecular condensation. 

Acylations by a,3-unsaturated acyl halides provide routes to a,3,a, 3 -unsaturated ketones. Care must be taken in choice of reaction conditions, since Lewis acids are excellent catalysts for Nazarov cy-clizations to cyclopentenones (Scheme 2). Indeed, this can be exploited as a synthesis of the five-mem-bered ketones without isolation of the intermediate divinylic ketones. Cyclizations are also observed after acylations of cyclohexenes with vinylacetyl chloride derivatives (equation The acylation-cycloalkylation sequence provides a complement for the Robinson annelation, since the carbonyl function is located adjacent to the bridgehead position. This potential has been realized in natural product syntheses. ... 

A mechanism for the formation of jS-unsaturated acyl halides from (7i-C3HsNiX)2 (X = Br or I) and CO has been suggested, equation (7-17). 

---