The enantiomers of the valproic acid analogue 2-n-propyl-4-pentynoic acid (4-yn-VPA): asymmetric synthesis and highly stereoselective teratogenicity in mice

The teratogenic activities of R(+)- and S(-)-2-n-propyl-4-pentynoic acid (R and S-4-yn-VPA), the enantiomers of the highly teratogenic valproic acid (VPA) analogues (+/-)-4-yn-VPA, were investigated in mice. The enantiomers were prepared via asymmetric synthesis, each in three steps employing the chiral auxiliaries (4R,5S)-4-methyl-5-phenyl-2-oxazolidinone and S-4-benzyl-2-oxazolidinone. The determination of the absolute configurations and the optical purities is described. R(+)-4-yn-VPA contained 7%, and S(-)-4-yn-VPA 8%, of the respective antipodes. The aqueous solutions of the sodium salts of R- and S-4-yn-VPA were administered as single i.p. injections during early organogenesis in the mouse (day 8 of gestation) using the induction of exencephaly as the teratological end point. Dose/exencephaly curves indicated that S-4-yn-VPA is 7.5 times more teratogenic than its antipode, 1.9 times more teratogenic than (+/-)-4-yn-VPA and 3.9 times more teratogenic than the parent drug VPA. In contrast, the neurotoxicity (maternal toxicity) of the 4-yn-VPA enantiomers was found to be independent of the stereo-chemical configuration and lower than achieved after VPA administration. Due to its low neurotoxicity and highly stereoselective neural tube-inducing activity, S-4-yn-VPA should prove an important tool for the investigation of molecular mechanism of the teratogenic action in this class of compounds; R-4-yn-VPA could act as the negative control in these studies.