Asymmetric Propargylboration of Aldimines and Ketimines with the Borabicyclo[3.3.2]decanes: Novel Entries to Allenyl Carbinamines, Amino Acids, and Their α-Methyl Counterparts

Gold(I)-Catalyzed Nucleophilic Propargylation of Azinium Ions via Hydroxydihydroazine Intermediates

The nucleophilic propargylation of azinium ions with a propargylboronate proceeds efficiently under gold(I) catalysis. A range of N-alkylated pyridinium, quinolinium, and pyrazinium ions undergo propargylation with good yields and high regioselectivities to give various functionalized 1,4-dihydropyridines, 1,2-dihydropyridines, 1,4-dihydroquinolines, 1,2-dihydroquinolines, and 4,5-dihydropyrazines. No allenylation side-products are observed. Density functional theory (DFT) calculations provided insight into the mechanisms of these reactions. Hydroxydihydroazine intermediates formed by the addition of LiOH to the azinium ions were found to be the reactive electrophiles in these reactions.

Copper-catalyzed synthesis of allenylboronic acids. Access to sterically encumbered homopropargylic alcohols and amines by propargylboration

Tri- and tetrasubstituted allenylboronic acids were prepared via a new versatile copper-catalyzed methodology. The densely functionalized allenylboronic acids readily undergo propargylboration reactions with ketones and imines without any additives. Catalytic asymmetric propargylborylation of ketones is demonstrated with high stereoselectivity allowing for the synthesis of highly enantioenriched tertiary homopropargyl alcohols. The reaction is suitable for kinetic resolution of racemic allenylboronic acids affording alkynes with adjacent quaternary stereocenters.

Regio- and Stereoselective Hydroamination of Alkynes Using an Ammonia Surrogate: Synthesis of N-Silylenamines as Reactive Synthons

An anti-Markovnikov selective hydroamination of alkynes with N-silylamines to afford N-silylenamines is reported. The reaction is catalyzed by a bis(amidate)bis(amido)Ti(IV) catalyst and is compatible with a variety of terminal and internal alkynes. Stoichiometric mechanistic studies were also performed. This method easily affords interesting N-silylenamine synthons in good to excellent yields and the easily removable silyl protecting group enables the catalytic synthesis of primary amines.

Quaternized α,α'-Amino Acids via Curtius Rearrangement of Substituted Malonate-Imidazolidinones

An efficient synthesis protocol is presented for accessing quaternized α-amino acids in chiral, nonracemic form via diastereoselective malonate alkylation followed by C- to N-transposition. The key stereodifferentiating step involves a diastereoselective alkylation of an α-monosubstituted malonate-imidazolidinone, which is followed first by a chemoselective malonate PMB ester removal and then a Curtius rearrangement to provide the transposition. The method demonstrates a high product ee (89-99% for eight cases) for quaternizing a range of proteinogenic α-amino acids. The stereogenicity in targets 5a-i supports previous conclusions that the diastereoselective alkylation step proceeds via an α-substituted malonate-imidazolidinone enolate in its Z-configuration, with the auxiliary in an s-trans_C-N conformation.