New gas-phase cyclisation reactions leading to benzofurans and flavones

Abstract

The main focus of the project was the synthesis of oxo-stabilised phosphonium ylides with an o-methoxy functionalised benzene ring, and flash vacuum pyrolysis (FVP) of the ylides leading to cyclisation forming a flavone or ring-fused benzopyranone. The first class examined were β,γ-dioxo ylides with two carbonyls on the same side. Two compounds of this type were prepared and in one case the structure was determined by X-ray diffraction. Upon FVP, the ylides underwent the desired extrusion of Ph₃PO and a flavone was formed, however a large amount of a benzofuran byproduct was produced regardless of the temperature. To elucidate the mechanism of the apparently novel heterocyclic reaction by which the flavone was decarbonylated to give the benzofuran, both flavone and 3-iodoflavone were prepared. While flavone was stable to FVP, the 3-iodoflavone readily underwent decarbonylation to give 2-phenylbenzofuran as the main product, thus confirming that decarbonylation occurred via a spiro radical intermediate. In an attempt to overcome this problem, ylides in which one carbonyl was protected as a 1,3-dioxolane were designed but all routes to these failed and the target structures were not accessible, which was mainly attributed to the neopentyl effect. Despite the possibility of Ph₃PO extrusion occurring on either side, leading to mixtures of cyclised products, a range of β,β'-dioxo ylides were prepared and their FVP was examined in one case. As expected, the product mixture was quite complex but three different fused tetracyclic products were tentatively identified. Attempts were made to use ylides with both carbonyl and thiocarbonyl groups present to achieve regioselective extrusion, but surprisingly there was almost equal extrusion of Ph₃PO and Ph₃PS suggesting that no useful selectivity could be achieved in this way. Finally one example of an ylide with stabilising imidoyl and carbonyl groups was prepared and its FVP did result in exclusive elimination of Ph₃PO to give the desired iminobenzopyran, although a higher temperature will apparently be required to bring about a cascade reaction leading to a tetracyclic product.