Advances in the stereoselective synthesis of antifungal agents and ...

Chapter 5 117Complete degradation of the substrates occurred with compounds (±)-59e-g carrying nitro, cyano or methoxy groups in the para position(entries 5-7). The reaction did not work at room temperature, at close to50°C the reaction took place for compounds (±)-59a-d while for (±)-59e-g the reaction mixtures became dark and it was impossible to detectany products.Also milder conditions than KF in DMF at 60°C were tried for thedesilylation, and TBAF in dry THF was used as an alternative (scheme5.4).RSiMe 3TBAF dry THFRHOH(±)-59a,c,e-15/RT °COH(±)-58a,c,eScheme 5.4: Desilylation of (±)-59a,c,e by TBAF.This procedure was applied only to three substrates (±)-59a,c,e.Again (±)-59a and (±)-59c gave the best results, while (±)-59e wasdegraded during the reaction. The results for this reaction are reported intable 5.3.Entry R T C° Product Yield1 4H -15/RT (±)-58a 94.82 4F -15/RT (±)-58c 96.93 4CN -15/RT (±)-58e 0Table 5.3: Reaction conditions and chemical yields for the desilylation of (±)-59a,c,eusing TBAF in THF.A last attempt of desilylation was carried out under basic conditions.A saturated solution of K2CO3 in absolute MeOH was used and thereaction was tried on (±)-59g (one of the arylpropynoles whichdecomposed in presence of the KF). The reaction was very slow and gavea complex mixture of products. In fact, in order to achieve a completeconsumption of the starting material, five days were required and only43% of the desired product (±)-58g was recovered after purification.(Scheme 5.5).