Synthesis of Naturally Occurring Curcuminoids and Related Compounds

Six known naturally occurring curcuminoids1) like curcumin, [5‐hydroxy‐1,7‐bis(4‐hydroxy‐3‐methoxyphenyl)‐1,4,6‐heptatrien‐3‐one (4a)] and 31 other analogs (Scheme 1) have been synthesized. Among them, 25 curcuminoids were prepared by condensing different substituted 2,4‐pentanediones and benzaldehydes. The boron complex 12 has been used to avoid Knoevenagel condensation at C‐3 of 2,4‐pentanedione. Some curcuminoids containing hydroxy groups in the aromatic moiety have been acylated. Alkylation of the tetrabutylammonium salt of 5‐hydroxy‐1,7‐diphenyl‐1,4,6‐heptatrien‐3‐one (1a) by benzyl bromide gave only C‐alkylated 4‐benzyl‐1,7‐diphenyl‐1,6‐heptadiene‐3,5‐dione (1d*). The 13C NMR data, the UV absorptions, and the tautomerism of the curcuminoids are discussed.