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Abstract

Anhydrides of 3β,28-diacetoxy-21,22-secolup-18-ene-21,22-dioic acid (1) and (19R)-3β,28-diacetoxy-18β,19-epoxy-21,22-secolupane-21,22-dioic acid (11) undergo alkaline hydrolysis yielding the corresponding dicarboxylic acids. Depending on reaction conditions, these acids are further transformed yielding various lactones, liberating C-28 hydroxymethyl group, or undergoing decarboxylation leading to nor derivatives. This method has been used to prepare a diverse series of E-secolupane derivatives including lactonoacids (e.g. 2 and 15), lactones (4, 16 and 17), 28-nor derivatives (3 and 6) and 21,28-dinor derivatives (12 and 13). Derivatives of (19R)-3β,28-dihydroxy-18β,19-epoxy-21,22-secolupane-21,22-dioic acid 21,28-lactone (15c), bearing a free carboxylic group, are labile and can only be isolated as the corresponding dilactones 17. The C-22 carboxylic group forms a β-lactone by a nucleophilic α-directed attack on the C-18 epoxide ring carbon atom resulting in (19R)-3β,19-dihydroxy-21,22-secolupane-21,28:22,18α-dilactone (17b) and related derivatives. The structure and stereochemistry of the compounds discussed in this contribution were derived from IR, MS, ¹H and ¹³C NMR spectra (1D and 2D COSY, TOCSY, NOESY, HSQC, HMBC). Using these NMR techniques and measuring the solvent influence on the IR carbonyl stretching frequencies of the dilactones 17, an equilibrium between the two E-ring conformations was shown to exist.

Keywords: Triterpenes; Triterpenoids; Lupane; Secolupane; NMR spectroscopy, ¹H and ¹³C.

References: 16 live references.