Lipase-mediated asymmetrization of the meso-diol (4) afforded the optically pure monoacetate (5) in good yield. To prepare potential chiral 2,5-cyclohexadienone synthons, 5 was first oxidized to the enone (8) in an excellent yield. On the other hand, 5 was transformed into the mono-pivalate (7) which in turn was oxidized to the enone (9) in good overall yield. Both 8 and 9 are virtually enantiomeric and serve as potential synthons of chiral 4-oxygenated 2,5-cyclohexadienones. Fortuitously, we found that the enone [(-)-2], a potential chiral synthon of 2,5-cyclohexadienone (1), was generated in one step in an excellent yield when the acetate (5) was refluxed with ammonium formate in the presence of a catalytic amount of PdCl_2(PPh_3)_2. The same reaction also occurred with the pivalate (7) to furnish the enantiomeric enone [(+)-2]. The reaction was found to take place by unprecedented suprafacial 1,4-hydrogen transfer mechanism which was proven by deuterium labeling experiment. To demonstrate potential of the optically pure enones thus obtained as potential chiral 2,5-cyclohexadienone synthons, synthesis of some natural products and the related compounds was investigated. As we have anticipated the reactions occurred in completely diastereoselective manners to both enone double bond and the carbonyl group owing to the biased frameworks. Furthermore, the regeneration of the masked double bond could also be carried out without difficulty by thermal retro-Diels-Alder cleavage. Reflecting these stereochemical characteristics of the substrates, the following four products, 2-aminoheptanedioic acid (16), conduritol C (27), eutipoxide B (39), and carvone (46) as well as 5-substituted-2-cyclohexenones (17) including 5-trialkylsilyl- and 5-trialkylstannyl-2-cyclohexenones have been synthesized in both enantiomeric forms.
