THE FIRST PREPARATION OF A HETEROTRICYCLE, [1,3]OXAZINO[2,3-ALPHA]ISOINDOLE-2,6-DIONE, BY A RETRO-DIELS-ALDER METHOD

Hexahydro- 3a,c and hydroisoindolo[2,1-a][3,1]-benzoxazine-6,12-diones 3b,d were prepared by the reaction of 2-carboxybenzaldehyde (I) with diendo- and diexo-3-aminabicyclo[2.2.1]hexane/ene-2-carboxylic acids 2 a-d. When heated, 3b and 3d (containing a norbornene structural moiety ) undergo retrodiene decomposition by splitting off cyclopentadiene to give the 1,3-oxazino[2,3-a]isoindole-2,6-dione (4). This is the first example of the preparation of a condensed heterotricyclic compound in a retro Diels-Alder reaction. The results reveal that this reaction c an be extended to the formation of new condensed-skeleton polycyclic h etero compounds which have an electron-rich moiety in the terminal rin g of the fused heterocycle formed by the splitting off of cyclopentadi ene. The structures of the pentacyclic hetero compounds 3a-d were dete rmined by NMR spectroscopy.