In studies directed towards the synthesis of 25-hydroxybrassinolide 1 we fo
und, that upon reaction of intermediate 7 with trimethylaluminium-n-butylli
thium an alkylating fission of the epoxide ring and rearrangement of the te
trahydropyranyl unit takes place to afford after deprotection the new dioxo
nane-annelated brassinosteroid 9.