Synthesis of naturally occurring phototoxic thiophenes and their derivatives.

Abstract

Analogues and derivatives of naturally occurring $\alpha$-terthienyl 3 have been synthesized by using the nickel-catalyzed Grignard-Wurtz cross-coupling reaction. This method has proven useful for the introduction of non-polar substituents on 2-bromothiophene which upon subsequent cross-coupling with 5-iodo-$\alpha$-dithienyl 185 led to a successful variety of 5-substituted $\alpha$-terthienyls. Derivatives of $\alpha$-terthienyl bearing polar functional groups were synthesized either by: (1) introduction of electron-withdrawing and electron-donating at C-5 and C-5$\sp{\prime\prime}$; or (2) chain extension from an appropriate substituted derivatives. In order to establish structure/activity relationships, various substituent at C-5 on the aromatic ring were made. Among the derivatives of $\alpha$-terthienyl, compound 165 was synthesized by the palladium-catalyzed coupling reaction with CuI. By adapting this methodology on $\alpha$-dithienyls, the reaction was found to be useful for coupling aryl halides such as 5-iodo-$\alpha$-dithienyl 185 with a variety of substituted 1$\sp\prime$-alkynes derivatives. A series of functional $\alpha$-dithienyl acetylene derivatives were made. The biological activity of $\alpha$-terthienyl and $\alpha$-dithienyl derivatives, as determined by feeding trials with the mosquito larvae (Aedes atropalpus) and brine shrimp, is discussed in light of the relative toxicities of these compounds.