Dioxolanones as synthetic intermediates. Part 4. Biomimetic synthesis of multicolanic acid
Regiospecific cyclisation of the intermediate formed by reaction of the dioxolanone (9) with methyl α-lithioheptanoate gave (Z)-t-butyl multicolanate (13) in 53% yield. Photochemical stereomutation of (13) gave (E)-t-butyl multicolanate (16) which was converted by trifluoroacetic acid into (E)-multicolanic acid (3), a metabolite of P. multicolor. In the same way, reaction of (9) with methyl α-lithiophenylacetate and, if desired, subsequent photochemical irradiation, provided (Z)- and (E)-4-carboxymethylene-2-phenyltetronic acids (21) and (23).