Syntheses of highly substituted furan and pyrrole derivatives via lithiated 3-aryl-1-methoxyallenes: Application to the synthesis of codonopsinine

Lithiated 1-methoxy-3-phenylallenes were generated in situ from a phenyl-substituted propargylic ether. They smoothly combined with aldehydes, ketones, or imines to give allenyl adducts, which cyclized to highly substituted heterocycles either under basic conditions or with silver nitrate assistance. Analogously, a dihydropyrrole derivative was obtained by the addition of anisyl-substituted lithiated methoxyallene derivative to an N-tosyl imine and subsequent cyclization of the intermediate. The two diastereomers obtained were subsequently transformed into the alkaloid (+/-)-codonopsinine and one of its epimers. The key steps of these sequences are highly diastereoselective hydroborations of silyl enol ethers quantitatively leading to hydroxylated intermediates.