Convergent First Total Synthesis of Melovinone: A Densely Substituted 3-Methoxy-4-quinolone Isolated from Melochia tomentosa L.

The first total synthesis of melovinone, a nonrutaceous 3-methoxy-4-quinolone alkaloid isolated from Melochia tomentosa L., is reported. The target was acquired in a convergent fashion through the Suzuki–Miyaura cross-coupling reaction between an ortho-nitrobenzoic acid acetonyl ester derivative prepared from vanillin and potassium 5-phenyl-1-pentyltrifluoroborate, obtained from β-phenethyl bromide. The coupling was followed by a chemoselective reduction of the nitro group and a microwave-assisted and AcOH-promoted cyclization with rearrangement of the resulting acetonyl anthranilate. This afforded a pseudane intermediate, which was selectively methylated on the 3-OH. The synthetic pathway enabled to reach the objective in 11 steps and 18% overall yield. The 1H NMR spectra of the synthetic and natural product were in full agreement.