Rearrangement of O-cinnamoyltaxicin I to a novel C-13 spiro-taxane

During the large-scale synthesis of an O-cinnamoyltaxicin I acetonide, an intermediate for the semisynthesis of 7-deoxypaclitaxel derivatives, side-product 3 was formed via a vinylogous retro-aldol reaction and a long-range hydride shift from O-cinnamoyltaxicin I (1) under alkaline reaction conditions. Compound 3 has two hemi-acetal bridges at C-1,C-9 and C-10,C-13. Compound 4 was formed from side-product 3 under acidic reaction conditions and is the first C-13 spiro-taxane described in the literature. This spiro-taxane has two acetal bridges between C-1, C-13 and C-10,C-13.