Synthesis of functionalised β-lactones via silylcarbocyclisation/desilylation reactions of propargyl alcohols

Functionalised β-lactones were prepared starting from propargyl alcohols by means of an efficient rhodium-catalysed silylcarbocyclisation reaction. This process took place in high yields and complete stereoselectivity using DBU as the base, a sterically hindered alcohol and/or hydrosilane. Otherwise, a silylformylation competitive reaction occurred affording 3-hydroxy-2-[(aryldimethylsilyl)methylene]alkanals with complete regioselectivity. Indeed, while α-(aryldimethylsilylmethylene)-β-lactones were generated exclusively starting from an alkynol characterised by two alkyl groups on the propargyl carbon atom, significant amounts of the aldehydes were observed for secondary alcohols. In these cases, the use of ortho-substituted arylsilanes improved the chemoselectivity towards the α-methylene-β-lactones. Such molecules were then successfully converted into α-methylaryl-β-lactones by a fluoride-induced rearrangement of the aryl group that migrates from silicon to carbon with retention of configuration.