Geometry of Some Strained Oxygen Ring Compounds: 1,3-Benzodioxole and 2,3-Dihydro-1,4-benzodioxin (Benzodioxan). Application to the Geometry of (6aR-cis)-3-Methoxy-6a,12a-dihydro-6h-[1,3]dioxolo-[5,6]benzofuro[3,2-C] [1]benzopyran (Pterocarpin)

Abstract

The geometry of benzodioxole, benzodioxan and the strained five- membered rings of pterocarpin are studied by the MNDO method. We show that in benzodioxole there is bond alternation towards a Kekule -like structure such as in indan (Mills-Nixon effect). In benzodioxole, owing to the smaller perimeter of the fused ring and enhanced strain, alternation is more pronounced than in indan . An explanation is offered for the strong distortion in ring angles in benzodioxole compared with indan. The effect of strain in benzodioxan and pterocarpin is discussed.