Pyridine-catalysed chromenylation of mono-chelated meta-dihydric phenols with mono-, sesqui- and di-terpene aldehydes: synthesis of rubranine and flemingins A-, B- and C-methyl ethers

Abstract

meta-Dihydric phenols in which one hydroxy-group is engaged in chelation, undergo pyridine-catalysed reaction with citral to give chromens, further cyclisation to ‘citrylidene’ types being inhibited. The reaction is deployed to synthesise the three natural flemingins, A (VIII), B (IX) and C (X) as their methyl ethers. Pinocembrin (XXIV) condenses with citral, with cleavage of the chromone accompanying the initial chromenylation. By use of the newly formed hydroxy-group, cyclisation continues and the product is the ‘citrylidene’-type (XXVI) having a cinnamoyl substituent. This has proved to be identical with the new natural product rubranine, recently isolated from rosewood. Chromenylation of 2-methyl-5,7-dihydroxychromone with citral has been studied. Farnesal and phytal condense with mono-chelated meta-dihydric phenols in the same way as citral, giving long-chain chromens.