Stereoselective synthesis of (-)-alpha-kainic acid and (+)-alpha-allokainic acid via trimethylstannyl-mediated radical carbocyclization and oxidative destannylation

(-)-alpha-Kainic acid (1) and its C4 epimer (+)-alpha-allokainic acid (2) have been prepared from L-serine. The requisite stereochemical array in (-)-alpha-kainic acid (1) was introduced using a trimethylstannyl radical carbocyclization of a diene, which gave the 2,3-trans/3,4-cis and 2,3-trans/3,4-trans compounds in a 2.8:1 ratio and in high yield. The destannylation of the trisubstituted pyrrolidine nucleus was achieved via an oxidative cleavage of the C-Sn bond with eerie ammonium nitrate. This provided a dimethyl acetal. that was further transformed into the intended alpha-kainic acid. When the same radical carbocyclization was attempted on a triene, the 2,3-trans/3,4-trans and the 2,3-trans/3,4-cis adducts were obtained in a 2.5:1 ratio, respectively. This approach was used to synthesize (+)-alpha-allokainic acid.