First transformations of pyrano[3,4-b]indol-3-ones to salvadoricine and 2,3-diacylindoles

The readily available methylated pyrano[3,4-b]indol-3-ones 1a and 1b were hydrolyzed to furnish the 2-acetylindol-3-alkanoic acids 2 and 4 . Compound 2 was easily transformed selectively to 2-acetyl-3-methylindole ( 3 , salvadoricine). Substrate 1b reacts with molecular oxygen from the air only in the presence of a catalyst to give 2,3-diacetylindole ( 5 ) while 1a reacts with nitrosobenzene via a proposed Diels-Alder step to yield 2-acetylindole-3-carbaldehyde ( 6 ). The latter product can also be obtained in low yield from the reaction of 1a with molecular oxygen from the air.