A Stereospecific Synthesis of (Z)-δ-Halo-γ,δ-unsaturated Ketones via Haloboration Reaction of Terminal Alkynes

Article abstract of DOI:10.1021/ja00304a032

Michael-type reactions of (Z)-β-bromo- and iodoalkenyl-9-borabicyclo<3.3.1>nonanes (5), readily available by haloboration of 1-alkynes, with acyclic α,β-unsaturated ketones (2) in a nonpolar solvent under Lewis acidic conditions are presented.The products, (Z)-δ-halo-γ,δ-unsaturated ketones (6), are obtained in stereochemically pure form (>98 percent).Since the haloalkenylboranes (5) are prepared in situ from haloboranes (4) and 1-alkynes, the present reaction provides a stereospecific, one-pot, and general synthesis of the title compounds (6).When methyl vinyl ketone (MVK) is used as the Michael acceptor, aldol condensation of the intermediate boron enolate with an excess of MVK occurs.However, the aldol (7) is transformed into the parent haloenone (6') without difficulty upon subsequent, in situ treatment with a base.The same product (6') is prepared directly by the reaction with 3-(trimethylsilyl)-3-buten-2-one.Synthetic utility of the present method is demonstrated by selective syntheses of several natural products.Sulcatol (11) is obtained in an overall yield of 63 percent starting from propyne.In a similar manner, trans-geranyl acetone (14) and trans-nerolidol (15) are prepared stereospecifically (>98 percent) in 62 and 72 percent yields, respectively, from 6-methyl-5-hepten-1-yne (12).