Synthesis of spiro[2 H ‐indole‐2,1′‐1 H ‐isoindole]‐3,3′‐diones, spiro[1 H ‐isobenzofuran‐1,2′‐2 H ‐indole]‐3,3′‐diones and spiro[benzofuran‐2,1′‐isobenzofuran]‐3,3′‐diones via transannular reactions of eight membered ring intermediates

An auto oxidation‐rearrangement product 4 was isolated from a high dilution reaction of ninhydrin with 3,4,5‐trimethoxyaniline in water. A general synthesis of this compound and its derivatives 4–6 was devised by oxidation of tetrahydroindeno[1,2‐ b ]indol‐10‐ones 1–3 with sodium periodate to give isoindolo[2,1‐ a ]‐indole‐6,11‐diones 4–6 in good yield. Compounds 4–6 can be easily transformed into spiro[1 H ‐isobenzofuran‐1,2′‐2 H ‐indole]‐3,3′‐diones 8–10 , spiro[2 H ‐indole‐2,1′‐1 H ‐isoindole]‐3,3′‐diones 11–13 and isoindole[1,2‐ a :2′,1′‐ b ]pyrimidine‐5,15‐diones 15, 16 in high yields. Analogous reactions were performed on 3‐amino‐5a, 10a‐dihydroxybenzo[ b ]indeno[2,1‐ d ]furan‐10‐one ( 17 ) to give a dibenzoxocintrione 18 , spiro‐[benzofuran‐2,1′‐isobenzofuran]‐3,3′‐dione 19 and an isoindol‐1‐one 20 .