Highly Enantioenriched Propargylic Alcohols by Oxazaborolidine-Mediated Reduction of Acetylenic Ketones

Straightforward access to oxazaborines, diazaborinones and triazaborines by reactions of β-enaminoamides with 4-methylbenzenediazonium tetraphenylborate

“…The most common methods for preparing heterocycles with O-B-N grouping are reactions of 1,2-and 1,3-aminoalcohols with alkyl-and arylboronic acids [10e,13] or dialkyl-and diarylborinic acids [14], boronate [15] or borane [16] and borane-dimethylsulfide [17]. Another method [18] consists in aminoboration of ketene with dialkyl(dimethylamino)boranes or bromodimethylborane and amine, or reactions of substituted vinyloxyboranes with nitriles [19,20].…”

Stereospecific Synthesis of β3‐Amino Acid Derivatives from Propargylic Alcohols: Efficient Solution‐Phase Synthesis of Oligopeptides without Coupling Agents

“…Propargylic alcohols 1 can be easily obtained by the stereoselective addition of alkynes to aldehydes [20] or by the asymmetric reduction of a,b-acetylenic ketones. [21] In our hands, the oxazaborolidine-borane reduction of prochiral 1-trialkylsylyl-1-alkyn-3-ones [22] in particular gave a high level of enantioselectivity in almost every case. The enantioenriched propargylic alcohols 1 a-l were transformed into the corresponding N-phthaloyl propargylic amines 2 a-l ( Table 1).…”

(R)-B-Methyl-4,5,5-triphenyl-1,3,2-oxazaborolidine

“…Accordingly, the experience gained with oxazaborolidine 1 suggests an order of 'empirical' size of R groups. 8 Obviously, better enantioselectivities in the reduction of the ketone carbonyl group are achieved when the substituents R L and R S are dissimilar. Reduction of Acetylenic Ketones.…”