Stereospecific photochemical ring expansion of lithiated benzamides.
Treatment of N-benzyl benzamides with a strong base (LDA or t-BuLi) followed by irradiation with a 500 W tungsten lamp provides, according to the substitution pattern of the starting amides, either norcaradienes or cycloheptadienones by overall insertion of the N-benzyl group into the benzamide's aromatic ring system. Chiral benzamides undergo the ring expansion with high (sometimes complete) stereospecificity. The reaction appears to occur via a series of pericyclic reactions (photochemical or thermal sigmatropic rearrangements and thermal electrocyclic reactions) following an initial dearomatizing cyclization.
Clayden
Jonathan
J
Department of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, UK. j.p.clayden@man.ac.uk
Knowles
Faye E
FE
Menet
Christel J
CJ