An alternative fate of taraxasteryl cation in young shoots of Rosa multiflora: Deprotonation of H-19 produces taraxast-19-en-3β-ol and related triterpenoids

Taraxast-19-en-3β-ol (1) and taraxast-19-en-3-one (2) were isolated from the surface lipids of young shoots of Rosa multiflora Thunb. The results provide the first evidence that deprotonation of H-19 can occur to produce the tetra-substituted triterpenes, in addition to the known formation taraxasterol and ϕ-taraxasterol by deprotonation of H-30 and H-21, respectively. Furthermore, taraxast-19(29)-ene-3β,20α-diol (3), which is presumed to be formed from compound 1 was isolated. The structures of the new compounds 1-3 with the α-orientation of H-18 were determined by spectroscopic methods including NOE studies. In addition, 19,20-seco-3β-hydroxyursane-19,20-dione (4), a 3β-hydroxy analog of the known 19,20-seco-ursane-3,19,20-trione was isolated. It is of note that compound 4 has the β-orientation of H-18, which is opposite to the α-orientation of H-18 of compounds 1-3.