Silylation-desilylation of propargyl amides: rapid synthesis of functionalised aldehydes and β-lactams

Propargyl functionalised β-silylalkenals were easily prepared starting from suitable propargyl compounds by a silylformylation process. In particular the use of propargyl tosyl amides allowed the synthesis of α,β-unsaturated aldehydes through a two-step sequence of silylformylation-desilylation reactions. TBAF was employed to induce the desilylation process that was performed under very mild experimental conditions and occurred along with an 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) α-silylmethylene-β-lactams were synthesised through a silylcarbocyclisation process. A high chemoselectivity towards the β-lactam was observed when dialkyl propargyl amides were employed. The obtained β-lactams were easily transformed into the corresponding methylaryl-β-lactams by fluoride induced aryl migration-desilylation with total retention of configuration of the migrating group and complete stereoselectivity towards the more stable β-lactam (E)-isomer.