The stereochemistry of the Wittig reactions of allylic phosphoranes and phosphonate esters with aldehydes

Abstract

Methyl Z- and E-4-bromo-3-methylbut-2-enoate 13 and 14 react with triethyl phosphite stereospecifically with retention of configuration to give the corresponding Z- and E-phospho-nate esters 8 and 9. The Z- and E-phosphonates react with aldehydes with extensive stereomutation of the double bond. Rapid equilibration to a mixture of one part E-phosphonate to two parts Z-phosphonate occurs when either geometric isomer is treated with base. Despite this, the E-2,E-4 ester 16 is the chief product of the reaction. Methyl E-β-ionylideneacetate 16b is the major product of the reaction of either isomer with β-cyclocitral and with cyclohexene car-boxaldehyde only the E-2,E-4 isomer 16c is formed. Benzaldehyde reacts with the phosphonates to give a mixture of the E-2,E-4 and Z-2,E-4 isomers 16a and 15a in a 4:1 ratio. Although the corresponding Z- and E-phosphonium salts 6 and 7, obtained stereospecifically from the bromo esters, react by the same general mechanism as the phosphonates the main product with benzaldehyde is the Z-2,Z-4 isomer 17a.