作者:Reuben D. Rieke、Heping Xiong
DOI:10.1021/jo00050a036
日期:1992.11
A one-step method for the synthesis of a wide variety of spirocyclic systems has been developed based on the reactions of bis-electrophiles with a series of new 1,3-diene-magnesium reagents, the magnesium complexes of 1,2-bis(methylene)cycloalkanes. The direct metalation of 1,2-bis(methylene)cycloalkanes with highly reactive magnesium in THF at ambient temperature generates the corresponding diene-magnesium reagents in high yields. Reactions of the diene-magnesium reagents with 1,n-dibromoalkanes produce a large number of spirocarbocycles containing an exocyclic double bond. The ring sizes of the accessible spiro compounds can be any combinations of four- to seven-membered rings. In most cases, the initially alkylated intermediates can be trapped by protonation, giving the corresponding bromo olefins. Significantly, treatment of the diene-magnesium reagents with bromoalkyl nitriles leads to a one-step synthesis of keto-functionalized spirocycles. The initial adduct is believed to be a Grignard reagent containing a cyano group. When a bromo nitrile containing a cyclic moiety is used as the bis-electrophile, the approach provides a direct access to dispiroenones.