Thermal and photochemical routes to compounds containing the tricyclo[3.3.1.0(^2,7)]nonane skeleton using polyfluoro-aryl and heteroaryl prop-2-enyl ethers

Abstract

This work is concerned with the sythesis of fluorine containing derivatives of the novel tricyclo[3.3.1.0(^2,7)]nonane skeleton by both thermal and photochemical routes from Claisen rearrangement products obtained from polyfluoroaryl and heteroaryl prop-2-enyl ethers. Fluorine is used as a blocking group to prevent an intermediate dienone from rearranging to a tautomeric phenol. Thermal rearrangements of 2,3,5,6-tetrafluoro-4-pyridyl prop-2-enyl ether 41 in the vapour phase gives (via an intramolecular Diels-Alder addition reaction of the intermediate dienone) 2,4,5,7-tetrafluoro-3-aza-tricyclo[3.3.1.0(^2,7)]non-3-en-6-one 42. The structure of this product was confirmed by complete hydrolysis with 50% sulphuric acid to give 4-hydroxybenzoic acid. Hydrolysis and hydration of the Diels-Alder adduct with water gave 2,5,7-trifluoro-6,6-dihydroxy-3-aza-tricyclo [3.3.1.0(^2,7)]nonan-4-one 43. This compound was readily dehydrated by sublimation under high vacuum to give 2,5,7-trifluoro-3-aza-tricyclo[3.3.1.0(^2,7)]nonan-4,6-dione 44. l,3,5,6,7,8-Heptafluoro-2-naphthyl prop-2-enyl ether 77 readily underwent Claisen rearrangement in xylene at reflux to give l,3,4,5,6,7,8-heptafluoro-l-(prop-2-enyl)-napnthalen-2-one 80. Irradition of the napnthalenone in solution at 366nm gave rise to two isomeric tricyclic compounds 2,5,7-trif luoro-3,4-(tetrafluorobenzo) -tricyclo[3.3.1.0(^2,7)]non-3-en-6-one 95 and l,2,7-trifluoro-3,4-(tetrafluorobenzo)-tricyclo[3.3.1.0(^2,7)]non-3-en-6-one 103. Compound 103 is formed by an initial [3,5] sigmatropic shift, followed by a [2+2] cycloaddition and then a [1,3] sigmatropic shift. This compound was easily hydrated to give the corresponding gem-diol l,2,7-trifluoro-3,4-(tetrafluorobenzo)-tricyclo[3.3.1.0(^2,7)]non-3-en-8,8-diol 104 The structure of the second compound 95, formed by a direct [2+2] cycloaddition reaction was shown by hydrolysis and methylation to give 1β,4β,5,6,7,8,8bβ - heptafluoro-1α-methoxycarbonyl-8b,2a,3,4-tetrahydro-8bα,2aα-ethanonaphthalene 96c which was identified by (^19)F nmr and i.r. spectroscopy. Pyrolysis of compound 95 at 450ºC gave the naphthalenone 80 whilst at higher temperatures (490ºC) a new compound 1-fluorovinyl-4,5,6,7,8-pentafluoro-1-naphthyl ketone105 was formed. Pyrolysis of 1,3,4,5,6,7,8-heptafluoro-1- (prop-2-enyl)-naphthalen-2-one 80 at 455ºC gave the the compound 55 in very low yield. At higher temperatures (490ºC) the 1-fluorovinyl ketone 105 was formed.