Stereoselective Synthesis of (−)-α-Kainic Acid and (+)-α-Allokainic Acid via Trimethylstannyl-Mediated Radical Carbocyclization and Oxidative Destannylation

(−)-α-Kainic acid (1) and its C4 epimer (+)-α-allokainic acid (2) have been prepared from l-serine. The requisite stereochemical array in (−)-α-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 ceric ammonium nitrate. This provided a dimethyl acetal that was further transformed into the intended α-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 (+)-α-allokainic acid.