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Dichlorodiethoxysilane was treated with isoprene in the presence of magnesium in THF. It was found that new silacyclopentene derivatives i.e., 1,1-diethoxy-3-methyl-1-silacyclopentene-3(1), and 2,7-dimethyl-5-silaspiro[4.4]nonadiene-2,7(2), were produced in good yields, while sodium was used instead of magnesium in the reaction 2 was obtained in a low yield.

The in situ reaction of dialkoxydichlorosilanes, such as dimethoxy-, diethoxy-, dipropoxy-, dibutoxy-, and diisobutoxydichlorosilane, with isoprene and magnesium in tetrahydrofuran (THF) was investigated. It was found that the double annelation product, i.e., 2,7-dimethyl-5-silaspiro[4.4]nona-2,7-diene (1), was obtained from diethoxy-, dipropoxy-, and dibutoxydichlorosilane in 87.0, 65.2, and 66.5% yields respectively. On the other hand, the reaction of dimethoxy- and diethoxydichlorosilane with butadiene and magnesium in THF gave 5-silaspiro[4.4]nona-2,7-diene (7) in 20.7 and 35.4% yields respectively. Several silaspirononane derivatives were synthesized from 1.