A STEREOSPECIFIC SYNTHESIS OF (+/-)-ABSCISIC ACID

A convenient separation of (E)- and (Z)-3-methylpent-2-enedioic acids was devised, and it was shown that with acetyl chloride or thionyl chloride the (Z)-acid yields the cyclic anhydride while the (E)-acid forms 6-chloro-4-methylpyran-2-one. The chloropyranone by conventional chemistry gave 4-methyl-6-(2'-oxopropyl)pyran-2-one which condensed with 4-methylpent-3-en-2-one in the presence of pyrrolidine, yielding 4-methyl-6-(2',6',6'-trimethyl-4'-oxocyclohex-2'-enyl)pyran-2-one. Oxidation with selenium dioxide or t-butyl chromate then gave 6-(1'-hydroxy-2',6'6'-trimethyl-4'-oxocyclohex-2'-enyl)-4-methylpyran-2-one, which on reduction by lithium aluminium hydride and reoxidation afforded (+/-)-abscisic acid stereospecifically.