Synthetic studies in aromatic hemiterpenes of natural origin, Part 5: Synthesis of 6-acetyl-2,2-dimethyl-8-methoxychromene via benzylic oxidation route

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The synthesis of 6-acetyl-2,2-dimethyl-8-methoxychromene (1c), a naturally occurring isomer of encecalin (1a) has been described starting from 2,2,6-trimethyl-8-methoxychromene (2e) which was obtained from creosol (4) in two steps involving condensation of the phenol with malic acid to the coumarin (3), followed by Grignard reaction with CH2MgI. The transformation of (2e) to the natural product (1c) was effected by oxidative dehydrogenation by DDQ of the 6-methyl function to the formyl group (2f), Grignard reaction to the carbinol (2g) and finally its oxidation to the acetyl moiety (1c), the sequence of the essential steps schematically summarised as: Ar-CH3 → Ar-CHO → Ar-CH (OH) CH3 → Ar-COCH3.