Conjugate addition of radical species has been recognized as a versatile tool for introducing an alkyl group into the b position of a,b-unsaturated carbonyl compounds, [1] and the subsequent trapping of the intermediate radical species with allyltin compounds has been widely studied. [2,3] Tandem reactions are among the most efficient synthetic methods. However, those that proceed sequentially through ionic species sometimes exhibit problematic drawbacks such as the need for exacting reaction conditions, whereas undesired polymerization may result from tandem radical reactions. A combination of radical and ionic processes may alleviate these problems and could thus be a promising approach. Oshima and co-workers first demonstrated that a tandem radical addition-aldol condensation of enones or enals could be performed via the formation of an intermediate boryl enolate. [4] Other groups have recently reported tandem reactions involving radical and ionic processes for the convenient synthesis of highly complex molecules. [5] However, there are only limited examples which employ enolate intermediates formed by a radical addition reaction. The goal of our work is to develop a highly efficient carbon-carbon bond-construction method by taking advantage of a novel hybrid radical-ionic reaction involving the radical addition to a,b-unsaturated oxime ethers and subsequent ionic trapping of the resulting N-boryl enamine by aldehydes.
Building upon our syntheses of a-and b-amino acids by radical addition to oxime ethers, [6] we extended our use of a,bunsaturated oxime ether 1 bearing Oppolzers camphorsultam to the synthesis of g-amino acids (Figure 1). Recent studies on the radical addition to imines showed that aminoboranes were effectively formed by trapping of intermediate aminyl radicals with triethylborane. [7][8][9][10] Thus, we expected [