Stereoelectronic Control of Additions of Vinylmagnesium Bromide to 1-Methyland 3-Methylpentacyclo[5.4.0.02,6.03,10.05.9]undecane-8,11-diones and to 1-Methylhexacyclo[10.2.1.0.02,11.04,9.04,14.09,13]pentadeca-5,7-diene-3,10-dione