Abstract The boranes I and II, obtained by hydroboration of 1, 1-tetra-and 1, 1- pentamethyleneallenes with 9-BBN, were used as key reactants in the synthesis of a series of unsaturated gem-substituted carbinols (VIIa-VIIg, Xa-Xc, and XII), as well as 1, 1- divinylcyclopentane and 1, 1-divinylcyclohexane (XV and XVIII). The addition reactions of the boranes I and II to the carbonyl compounds and ethoxyacetylene studied, and the ...