Enolboration of conjugated ketones and synthesis of beta-amino alcohols and boronated alpha-amino acids

Abstract

Enolization–aldolization of conjugated ketones, enantioselective synthesis of benzofuryl beta-amino alcohols, and synthesis of p-dihydroxyborylphenylalanine (BPA) and its analogs are described. Aldolization of benzaldehyde with lithium dienolates derived from unhindered conjugated cyclohexenones favored anti- selectivity, whereas syn selectivity was favored for hindered cyclohexenones. Anti-aldols were preferentially formed from dienolborinates derived from conjugated cyklohexenones, however,competing aldolization at the 2-position was observed for hindered ketones. Benzofuryl beta-amino alcohols were prepared using as a key step the enantioselective reduction of the corresponding alpha-bromoacetylbenzofurans with (–)-B- -chlorodiisopinocampheylborane. Ionic liquids were used as solvents for the synthesis of BPA by the Suzuki cross-coupling reaction. The reaction time is short, and a solution of the catalyst in the ionic liquid can be recycled.