Diastereoselective Synthesis of Triterpenoid 1,2,4-Trioxolanes by Griesbaum Co-ozonolysis

Diastereoselective synthesis of triterpenoid
1,2,4-trioxolanes
by Griesbaum co-ozonolysis was shown for the first time. Ozonolysis
of 2-methoxyoximes (syn–anti-isomers mixture) of allobetulin or methyl oleanoate with CF3-ketones resulted in asymmetrical spiro-1,2,4-trioxolanes
as mixtures of diastereomers in yields up to 80–85%. The configuration
of the spiro-C-2 center of individual ozonides was determined by 2D
NMR spectra and X-ray crystallographic analysis. The products of ozonolysis
of triterpenoid 3-methoxyoximes were mixtures of regioisomeric N-methoxylactams. Thus, the fundamental differences in the
oxidation of homologous triterpenoid 2- or 3-methoxyoximes with ozone
have been established. These results may afford a new stage in the
development of the Griesbaum method as applied to natural compounds
and biologically active peroxides.