Total Synthesis of the Didemnins; III. - Synthesis of Protected (2R,3S)-Alloisoleucine and (3S,4R,5S)-Isostatine Derivatives - Amino Acids from Hydroxy Acids

Article abstract of DOI:10.1055/s-1989-27404

(2S,3S)-2-Acetoxy-3-methylvaleric acid (3) is prepared from L-isoleucine (2) with 96percent retention of configuration.Compound 3 is converted to optically pure methyl D-alloisoleucinate (7) as its hydrochloride salt, via the methanesulfonate 5 and the azide 6 with 76percent yield and 99.9percent inversion.Subsequent protection-saponification-activation of 7, followed by reaction with the lithium enolate of methyl trimethylsilyl malonate and reduction, yields (3S,4R,5S)-N-(9-fluorenylmethoxycarbonyl)isostatine (12). (3S,4R,5S)-Isostatine is a characteristic unit of the didemnines 1.