Total syntheses of gerberinol I and the pterophyllins 2 and 4 using the Casnati–Skattebøl reaction under different conditions
The concise and efficient total syntheses of the naturally-occurring coumarin derivatives gerberinol I, and the pterophyllins 2 and 4, from 5-methyl-4-hydroxycoumarin as a common precursor employing different Casnati–Skattebøl reaction conditions, are reported. The synthesis of the key intermediate coumarin was achieved by the organocatalytic condensation of acetylacetone and crotonaldehyde followed by a LiCl-assisted cyclization, CuCl2-promoted aromatization and a final Et2CO3-mediated cyclization. A Casnati–Skattebøl formylation under high-temperature conditions afforded gerberinol I, whereas milder conditions resulted in an unstable 3-formyl-4-hydroxycoumarin derivative, which was subjected to a basic alumina-mediated one pot O-alkylation with chloroacetone and intramolecular aldolization to furnish pterophyllin 4. Wittig methylenation of the latter conveniently afforded pterophyllin 2.