A series of cyclic dimethoxysilanes has been prepared using a one-step strategy which consists in reacting magnesium dibromoalkanes with tetramethoxysilane. The dimethoxysilacycloalkanes obtained were evaluated as the external electron donor in Ziegler-Natta catalysts for propylene polymerisation. Comparison of the experimental results with theoretical predictions using a molecular model confirmed the essential structural characteristics that are required to achieve stereospecific polymerisation of propylene. Very good activity and stereospecificity were obtained with 2,6-diethyl-1,1-dimethoxysilacyclohexane which led to results close to that of actual industrial external electron donors, cyclohexylmethyldimethoxysilane and dicyclopentyl dimethoxysilane.