Naturally occurring, bioactive isomer of vitamin A, q.v. The 13-cis- and all-trans-forms come to equilibrium in solution with ≈33% as 13-cis. Isoln from shark liver oil: C. D. Robeson, J. G. Baxter, J. Am. Chem. Soc. 69, 136 (1947). Synthesis: M. Matsui et al., J. Vitaminol. 4, 178 (1958). Stereoselective synthesis: J. Otera et al., Chem. Lett. 1985, 1883. HPLC determn in food samples: E. Brinkmann et al., J. Chromatogr. A 693, 271 (1995). Review of vitamin A compounds: W. Friedrich in Vitamins (Walter de Gruyter, Berlin, 1988) pp 63-140.