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

In addition to new acetophenones and 2H-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.