Synthesis of Conjugated Enynones

B-l-Alkynyls-9-BBN easily and quantitatively prepared from lithium methyl al-kynyldialkylborinate [12] undergo a facile condensation with readily available 4-methoxy-3-buten-2-one in hexane at room temperature to provide, in excellent yield, conjugated enynones (Eq. 7.9) [18]. 

The simple workup procedure provides highly pure products ( 97%) without the need for further purification of the crude material. The small excess of 4-methoxy-3-buten-2-one utilized in the reaction is conveniently hydrolyzed to water-soluble and/or highly volatile products (Eq. 7.10). The B-methoxy-9-BBN byproduct is oxidized to water-soluble cis-1,5-cyclopentanediol and boric acid. The whole sequence thus provides [18] clean products after simple extraction of the reaction mixture. 

Soderquist and coworkers have reported [19] that hydroboration with 9-BBN of 1-silylacetylenes, having bulkier groups on the Si, place the boron exclusively at the internal position of the alkyne as a kinetically less preferred process. Consequently, triisopropylsilyl-substituted alkynes afford the novel P-silylated vinyl-boranes, which on oxidation with alkaline hydrogen peroxide affords the corresponding ketones (Eq. 7.11) in excellent yields, a truly noteworthy example of the stability imparted to this functionality by the triisopropyl substitution on silicon [20]. It should be noted that earlier studies have shown failures to obtain (3-ketosilanes from suitable organoprecursors due to their known sensitivity to bases and nucleophiles [21]. 

The l-metallacyclohexan-4-ones are efficiently synthesized [22] from the corresponding divinyl compounds. The intermediate borinanes are prepared in a 

The syntheses of 1-sila- and l-germacyclohexan-4-ones are summarized in Table 7.19 [22]. 

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The reaction is general for other (3-methoxy- , S-unsaturated ketones that can assume the cisoid conformation and provides a useful synthesis of conjugated enynones. 

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