EVALUATION OF AMINO SUBSTITUENTS AS NUCLEOFUGAL CONTROLLERS OF REGIOSELECTIVITY AND AS CHELATE MODULATORS OF STEREOSELECTIVITY IN SQUARATE ESTER CASCADES

The tandem addition of an alkenyllithium reagent and a 2-lithioallylam ine to squarate esters has been systematically examined. The effect of the sequencing of this twofold addition has been investigated. The ex tent to which the amino group is eliminated was found to be dependent on the structural features of the companion nucleophile. Also assessed was the comparative ease with which the amino and methoxy groups expe rience competitive beta-elimination from the highly reactive, medium-r ing dianionic intermediates. Attempts were made to curtail the level o f competing 1,4-addition, and success was achieved by increasing the e ffective size of the O-alkyl groups in the squarate ester. The highly stereocontrolled transformations described represent a notably direct means for producing highly fused polycyclic compounds, Mechanistic con siderations sm rounding these reactions, which are characterized by an impressive enhancement of molecular scaffolding, are discussed.