Diastereoselective Conjugate Radical Additions to β-Pyranones

Abstract

We have developed a convenient protocol for functionalization of b-pyranones. Conjugate radical addition and tandem addition-trapping protocols allow for accessing highly substituted pyrones in a single operation with high selectivity.Radical reactions are well suited for the formation of multiple bonds in a single operation. 1,2 We have recently demonstrated that g-pyranones such as pyromeconic and kojic acid derivatives undergo highly efficient conjugate radical additions. 3 We have been interested in extending this chemistry to the functionalization of b-pyranones ( Figure 1). In an account in 2000, Feringa and co-workers had shown that ethyl radical derived from diethylzinc/oxygen adds to 1 (R = Ph) in good yield and selectivity. 4 In this work we demonstrate that a wide range of nucleophilic radicals participate in conjugate addition to readily available b-pyranones 1 in good yield and high diastereoselectivity. Furthermore, tandem protocols, i.e. radical addition-allyl stannane trapping, also proceed in good yield and selectivity to provide 2. The present methodology allows for the preparation of highly functionalized pyranones 4 in enantioenriched form. 5

Selective Functionalization of Achmatowicz Rearrangement Products by Reactions with Potassium Organotrifluoroborates under Transition-Metal-Free Conditions

“…With respect to literature precedents for conjugate addition reactions to AR products 2 , the addition of alkyl groups using organometallic nucleophiles or radicals has only been carried out on O -protected derivatives of the AR products 2 , and is amply precedented. However, the addition of vinyl or aryl groups is underdeveloped, and has only been carried out on O -protected AR products 2 . , To the best of our knowledge, the conjugate addition of C­(sp)-nucleophiles has not been previously reported.…”