A Ru Catalyzed Divergence: Oxidative Cyclization vs Cycloisomerization of Bis-homopropargylic Alcohols

During the course of investigating the development of catalytic reactions involving ruthenium vinylidene intermediates, a novel divergence of reactivity was discovered. The oxidative cyclization of bis-homopropargylic alcohols with Ru(+2) complexes as catalysts and N-hydroxysuccinimide as oxidant, which requires formation of a ruthenium vinylidene intermediate, is complicated by the simple electrophilically initiated direct attack of the hydroxyl group on a π-complex of the alkyne and ruthenium. A catalytic system composed of CpRu[(p-CH3O6H4)3P]2Cl and excess (p-CH3O−C6H4)3P directs the reaction toward the oxidative cyclization to form δ-lactones in good yields. Significantly, a simple switch of catalyst to CpRu[(p-FC6H4)3P]2Cl redirects the reaction to a cycloisomerization to form dihydropyrans in good yields. The synthetic utility of the oxidative cyclization is illustrated by the synthesis of oviposition attractant pheromone of the mosquito Culex pipens. The utility of the cycloisomerization to dihydropyrans is demonstrated by an iterative process leading to the antiviral agent narbosine B. A rationale for this dramatic switch by simple ligand modification is proposed.