Abstract
1,3-Diketo-cyclohexanes are part of approximately 790.000 structures, but compound 2 is the first example of an E ring annelated triterpenoid carrying a 1,3-diketo moiety in an additional ring. This compound was obtained starting from methyl platanoate (1) by epimerization at position C-19 of the triterpenoid skeleton followed by an intramolecular Claisen condensation. The mechanistic assumptions leading to this product were supported by extensive NMR and deuteration experiments. Furthermore, reaction of 2 with formaldehyde gave a “Vorländer-adduct” 4 that exists at room temperature in two conformers 4A and 4B. The rotational barrier in 4A/4B is higher than 19 kcal/mol; this might be caused by stronger steric hindrance in 4. The synthesis of 2 is the first example of a successful synthesis of a 19-epi-configurated triterpenoic acid, and its reaction product with formaldehyde, 4, is the first example of a “Vorländer-adduct” derived from a triterpene.
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