On the reaction of prop-2-enylidenetriphenylphosphorane derivatives. Novel synthesis of the azulene ring system

Abstract

Reaction of prop-2-enylidenetriphenylphosphorane derivatives with several tropones has been studied in an attempt to provide a short new route to the azulene ring system. (2-Ethoxyprop-2-enylidene)triphenylphosphorane 4 reacts with 2-chloro-, 2-methoxy- and 2,3,5,7-tetrachlorotropones 7a–c to give azulene derivatives 8 and 9 in moderate yields. On the other hand, reaction of [2-ethoxy-3-(ethoxycarbonyl)prop-2-enylidene]triphenylphosphorane 6 with tropones 7a,c results in the formation of azulene esters 10 and 11 in low yields, whilst that with tropone 7b gives no azulene and substrate 7b is recovered. In order to gain insight into the mechanistic pathways, reaction of phosphoranes 4 and 6 with deuteriated tropones 14a,b which are the corresponding trideuteriated derivatives of compounds 7a,b, have also been studied. Furthermore, compound 4 reacts also with 5-(dimethylaminomethylene)cyclopenta-1,3-dienecarbaldehyde 33 to give 6-ethoxyazulene 37 in moderate yield.