Development of New Methods for Constructing Medium Sized Carbocycles by Using Acetylene Dicobalt Complexes

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(1)New aspects of [5+2] cycloaddition reactionsIn order to explore the scope and limitations of the [5+2] cycloaddition reaction using the five-carbon unit having an acetylene dicobalt complex moiety and an enol silyl ether moiety, the reactions with various electron rich olefins as well as aromatic compounds were examined. Although aromatic compounds including anisols and aniline derivatives gave no good results, the reaction with a siloxyallene derivative was found to give the desired seven-membered cycloadduct in good yield.(2)Development of the higher-order cycloaddition reactionsA new six-carbon unit, a homologue of the conventional five-carbon unit, underwent a smooth [6+2] cycloaddition reaction with an enol triisopropylsilyl ether giving rise to an eight-membered acetylene dicobalt complex with high stereoselectivity. Use of an allyl silyl ether instead of an enol silyl ether resulted in formation of a nine-membered ring via a [6+3] cycloaddition reaction. The resulting cyclic acetylene dicobalt complex was converted into the corresponding cyclononyne derivative by treating with CAN. On the other hand, the reaction of the six-carbon unit and furan afforded a ten-membered carbocycle having an ether bridge through a [6+4] cycloaddition reaction.(3)Application to the synthesis of natural productsTotal synthesis of furanether B was accomplished on the basis of a new method for constructing a hydroazulene skeleton through the [5+2] cycloaddition reaction of a siloxy allene derivative.