Unsaturated carboxylic acid dienolates. Reaction with substituted cyclohexanones and unsubstituted cycloalkanones. Regio- and stereo-selectivity

Abstract

Regio- and stereo-selectivities for the reaction of lithium dienolates derived from crotonic and dimethylacrylic acids with substituted cyclohexanones (4)–(6) are found to depend on reaction time and temperature. α-Adducts resulting from equatorial attack predominate after a short time at –70 °C, but axial approach and γ-addition are favoured by longer reaction time and higher temperature. Inversion of axial/equatorial stereoselectivity is observed for some cyclohexanones. For unsubstituted cycloalkanones, regioselectivity is dependent on ring size. This effect can be explained on the basis of Brown's ‘I-strain’ rationalization. For C₅—C₁₂ketones, γ-adducts are found when reactions are carried out above room temperature, but at 0 °C, C₅, C₆ and C₁₂ cycloalkanones yield α-isomers whereas mesocyclic ketones afford γ- isomers.