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

During the large-scale synthesis of an O-cinnamoyltaxicin I acetonide, an i ntermediate for the semisynthesis of 7-deoxypaclitaxel derivatives, side-pr oduct 3 was formed via a vinylogous retro-aldol reaction and a long-range h ydride shift from O-cinnamoyltaxicin I (1) under alkaline reaction conditio ns. Compound 3 has two hemi-acetal bridges at C-1,C-9 and C-10,C-13. Compou nd 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.