Total syntheses of Cep-212 and Cep-210, predicted biosynthetic intermediates of tetrodotoxin isolated from the Japanese toxic newt, have been accomplished from geraniol by an intramolecular hetero Diels-Alder reaction as a key step in a highly stereoselective manner. The success of these syntheses enabled us to determine their absolute configurations by using a chiral normal-phase HPLC/MS analysis of the bis-dinitrobenzene derivative of natural Cep-212 and reference derivatives prepared from chemically synthesized enantiomers.