Functionalisation of Alkylalkoxysilanes. Studies Towards Annulations of Diterpenoids

Ortho-(2-triethoxysilylethyl) derivatives of aryl ketones undergo oxidative desilylation with H2O2. Tetralone derivatives have served as model substrates for 14-2-(triethoxysilylethyl)-7-oxopodocarpanes, which have been converted into 2-arylethanols in a four-step sequence (BH3·BMS, H2O2, Et3SiH/CF3COOH, TBAF) without isolation of intermediate products (oxasilepin, diol, trifluoroacetate). Use of the 14-(2-diethoxysilylmethyl) analogue improved the overall yield significantly (75 versus 40%). Re-oxidation of ring B with CAN gave the Δ5-7-oxo derivative, which was converted into an enone–aldehyde by treatment with Collinsʹ reagent. The conjugated alkene was reduced stereoselectively with SmI2, which, however, did not promote pinacol coupling of the resulting 1,5-keto aldehyde.