Exocyclic allenes constitute useful building blocks in organic synthesis and have recently been identified as key intermediates in the synthesis of natural products. Here the first general method for the most straightforward synthesis of exocyclic allenes reported to date is presented. This method is based on the Barbier-type cyclization of propargyl halides catalyzed by titanium; a safe, abundant, and ecofriendly metal. The reaction proceeds under mild conditions compatible with different functional groups and provides good yields of five-, six-, and seven-membered carbocycles and nitrogen-containing heterocycles bearing an exocyclic allene group. Experimental evidence supporting the proposed reaction mechanism is also provided. Moreover, this procedure can be carried out in an enantioselective manner by using chiral titanocene(III) catalysts. The utility of this method has been proved in the synthesis of the natural alkaloid stemoamide.
A general method for the straightforward synthesis of exocyclic allenes on five‐, six‐, seven‐, and eight‐membered oxygen heterocycles is described. A Barbier‐type titanocene(III)‐catalyzed cyclization of propargyl halides with a pendant carbonyl group is the key step of the process. This reaction is compatible with many functional groups, and can be smoothly carried out under neutral and mild conditions at room temperature.
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