Melifoliox B, a novel phloroglucin derivative isolated from <i>Melicope barbigera</i> (Rutaceae) and synthesis of new oxidation products from melifoliones A and B

In addition to new acetophenones and 2 H -chromenes, the dichlormethane extract from leaves of Melicope barbigera A. Gray (Rutaceae) afforded a mixture of the isomeric melifoliones A ( 1 ) and B ( 2 ) as well as an oxidation product of 2 , whose structure was elucidated as the para -quinol 4 . For an independent synthesis of 4 and its possible isomer 3 , the required compounds 1 and 2 were synthesized as a mixture of the isomers starting from chromene 5 , briefly heated in a closed microwave apparatus with catalytic amounts of acetic acid. Forced heating of 5 in acetic acid or use of stronger acids lead to the benzoxocin derivatives 6 and 7 . Oxidation of melifoliones 1 and 2 under a great variety of oxidants and conditions failed to give 3 and 4 . Iodine-containing oxidants yielded the products 8 , 9 , and 10 . Combined oxidation with hydrogen peroxide and ferricyanide in alkaline solution resulted in an unexpected contraction of the acetyl phenol to a furanone ring, forming the derivatives 11 and 12 , whose structures were confirmed by X-ray analysis. A hypothetical mechanism for the oxidative ring contraction is proposed. 11 and 12 are the first representatives of new heterocyclic ring systems that have not previously been described in the literature.