Sintesi e trasformazione di organosilani funzionalizzati

Propargyl functionalised b-silylalkenals were easily prepared starting from suitable propargyl compounds by a silylformylation process. In particular the use of propargyl amides allowed the synthesis of a,b-unsaturated aldehydes through a two steps sequence of silylformylation-desilylation reactions. TBAF was employed to induce the desilylation process that was performed under very mild experimental conditions and occurred with a contemporary elimination step of the tosylamido moiety affording 2-methylaryl-2-alkenals with good yields and stereoselectivity. When the tosyl amides were reacted with a hydrosilane in the presence of catalytic amounts of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) a-silylmethylene-b-lactams were synthesized through a silylcarbocyclisation process. A high chemo selectivity towards the b-lactam was observed when dialkyl propargyl amides were employed. The obtained b-lactams were easily transformed into the corresponding aryl methyl-b-lactams by fluoride induced aryl migration-desilylation transformation with total retention of configuration of the migrating group and complete stereoselectivity towards the more stable b-lactam E isomer. Also Polyfunctionalised aldehydes and dihydropyrans are prepared from easily available functionalised 1-alkynes through a silylformylation-aryl migration two-steps sequence. The silylformylation process is performed under mild experimental conditions and affords the corresponding b-silylalkenals with high yields. The fluoride-promoted migration step occurs instantaneously with quantitative conversion. The chemo-, regio- and stereoselectivity can be modulated according to the nature and the position of the functional group on the acetylene precursors. When a good leaving group is present in the w position of the aliphatic chain of the alkyne a cyclisation product is obtained, while a,b-unsaturated aldehydes are generated from propargylic tosylamides.