Stereocontrolled Synthesis and Cycloaddition of 1,2,4-Trioxygenated 1,3-Dienes

A new stereocontrolled synthetic pathway to 1,2,4-trioxygenated 1,3-dienes from pyruvic aldehyde dimethyl
acetal (14a) is described. Reacting the cyclohexylamine-derived imine of this starting material with
chloroalkyl ethers under basic conditions affords ketoacetals 18−20, which were then transformed into
eight different enoxysilanes 12. A δ-elimination triggered by tert-butyllithium yields 1,2,4-trioxygenated
dienes 13. Increasing the bulkiness of the silyloxy group or that of the acetal moiety leads stereoselectively
to the (1E,3E) or (1Z,3E) isomers of 13, respectively. Hyperbaric [4 + 2] cycloadditions between 13
(13c, 13d, 13g) and N-methylmaleimide or methyl- and phenylacrylates give access to the expected
cycloadducts with fine stereo- and regiocontrol.