B-Alkenyl-9-borabicyclo [3.3. 1] nonanes (B-alkenyl-9-BBN)(l), readily prepared by the hydroboration of acetylenes with 9-BBN,'undergo a smooth 1, Caddition to methyl vinyl ketone (MVK) and related ketones in refluxing tetrahydrofuran. Hydrolysis of the initially formed enol borinate intermediate (2) provides the corresponding 4-alkenyl-2-butanones (3)(eq 1).