Structure Determination of New Isomeric Naphtho[2,3‐b]furan‐4,9‐diones from Tabebuia avellanedae by the selective‐INEPT technique

Beside the known naphthoquinones, dehydro‐α‐lapachone ( 17 ) and lapachol ( 20 ), four new naphtho[2,3‐ b ]‐furan‐4,9‐diones, i.e. the 2‐acetyl‐5‐hydroxy. 2‐acetyl‐8‐hydroxy. (−)‐5‐hydroxy‐2‐(1′‐hydroxyethyl), and (±)‐8‐hydroxy‐2‐(1′‐hydroxyethyl) derivatives 16, 15, 12 , and 13 , respectively, and the new compound benzo[ b ]furan‐6‐carboxaldehyde ( 8 ) have been isolated from a CHCl 3 extract of the inner stem bark of Tabebuia avellanedae L ORENTZ ex G RISEB ., together with four known naphthofurandiones, a dihydroisocoumarin derivative, (−)–6‐hydroxymellein, and five benzoic‐acid and three benzaldehyde derivatives which have not been reported previously from this plant. Structure determination of the isomeric 5‐ and 8‐hydroxynaphtho[2,3− b ]furan‐4,9‐diones was carried out unambiguously by a combination of selective‐I NEPT experiments and X‐ray crystallographic analysis.