Nucleophilic Organosilicon Intermediates Turned Electrophilic: (Trimethylsilyl)methyl, Trimethylsilyloxy and also 2-Tetrahydropyranyloxy as Terminators of Cycloadditions of Allyl Cations. A Short Route to Dehydrozizaenes (6-Methylenetricyclo[6.2.1.01,5]-undec-9,10-enes) and Related Tricycles and [3.2.1]-Bicycles.