Carbonylative coupling of organozinc reagents in the presence and absence of aryl iodides: synthesis of unsymmetrical and symmetrical ketones

Abstract

The utility of the palladium(0) catalysed reaction of the iodoalanine-derived organozinc reagent 6a with functionalised aryl iodides, under a carbon monoxide atmosphere, to give protected 4-aryl-4-oxo α-amino acids 8, is illustrated by a short synthesis of L-kynurenine 4. Treatment of functionalised organozinc reagents with catalytic tetrakis(triphenylphosphine)palladium(0) under an atmosphere of carbon monoxide in the absence of any electrophile leads to the formation of symmetrical functionalised ketones 9 in good yields. This reaction is illustrated by a one-step synthesis of protected (2S,6S)-4-oxo-2,6-diaminopimelic acid 9a from commercially available compounds. It has been established that adventitious molecular oxygen plays a key role in the formation of the symmetrical ketones 9, and that rigorous exclusion of oxygen can result in substantially higher yields of ketones 8 in the cross-coupling with some aromatic iodides.