Asymmetric synthesis of four diastereomers of 3-hydroxy-2,4,6-trimethylheptanoic acid: Proof of configurational assignment

Four unique diastereomers of 3-hydroxy-2,4,6-trimethylheptanoic acid-(2R,3R,4R), (2S,3R,4R), (2S,3R,4S), and (2R,3R,4S)-the fatty acid component of callipeltin A and D, have been synthesized from commercially available (+)- and (-)-pseudoephedrine propionamide in 6 steps and 59% average overall yield. Comparison of the H-1 and C-13 NMR and optical rotation data of the resulting isomers with the natural fragment unambiguously verifies the configurational assignment of the natural isomer as (2R,3R,4R).