Electron Transfer-Initiated Diels-Alder Cycloadditions of 2'-Hydroxychalcones

摘要

A number of biologically active prenylflavonoid natural products have been isolated from the mulberry tree and related plants. For example, kuwanon G (1) and multicaulisin (2) are Diels-Alder cycloadducts between prenylflavonoid dienes and 2'-hydroxychalcones (Figure 1). Related Diels-Alder cycloadducts include (-)-panduratin A (3) and nicolaioidesin C (4). To access these natural products, we wished to develop methodology to construct the cyclohexenyl chalcone nucleus employing electron-rich 2'-hydroxychalcone dienophiles. In this Communication, we report examples of such [4 + 2] cycloadditions in a process likely involving electron transfer.rnOur studies began with model reactions of frans-2'-hydroxy chalcone 5 and 2,3-dimethylbutadiene 6 (Table 1). Owing to our inability to effect cycloaddition using Lewis acid-promoted ("LUMO" lowering) conditions,9 we considered alternative modes of catalysis. On the basis of a recent report involving Diels-Alder dimerization of piperine, we evaluated Co(I) catalysis for cycloaddition.