Young Ho Rhee

Gold(I)-Catalyzed Divergence in the Reactivity of 3-Silyloxy 1,6-Enynes: Pinacol-Terminated vs Claisen-Terminated Cyclization Cascades

On activation with catalytic amounts of gold(I) complexes, 3-silyloxy 1,6-enynes can react through two alternative pathways. In one, a cascade reaction consisting of carbocyclization and subsequent pinacol rearrangement takes place. In the second pathway, a heterocyclization is followed by a Claisen rearrangement. The reaction outcome differs depending on the substitution pattern of the 3-silyloxy 1,6-enynes and, more importantly, the electronic properties of the gold-bound phosphane ligand.

Mass-Balanced 1H/2H Isotope Dipeptide Tag for Simultaneous Protein Quantitation and Identification

Mass-balanced (1)H/(2)H isotope dipeptide tags (MBITs) are presented for simultaneous protein quantitation and identification. MBIT is derived from N-acetyl-Ala-Ala dipeptide and conjugated to primary amines of target peptides. (1)H/(2)H isotopes are encoded in the methyl groups of N-acetylated dipeptide: one tag deuterated on the N-acetyl group and another on the C-terminal alanine. MBIT-linked peptides comigrate in reversed-phase liquid chromatography without significant (1)H/(2)H isotope effects and provide 2-plex quantitation signals at 114 and 117 Th as well as peptide sequence information upon MS/MS analysis with MALDI TOF/TOF. MBIT shows good quantitation linearity in a concentration range of 20-250 fmol. The performance of MBIT on protein quantitation and identification is further tested with yeast heat-shock protein (Hsp82p) obtained from three different physiological states. MBIT using nanogram-scale samples produces the relative abundance ratios comparable to those obtained from optical imaging of microgram-scale samples visualized with SYPRO Ruby stain. The MBIT strategy is a simple and low-cost alternative for 2-plex quantitation of proteins and offers possibilities of tuning the 2-plex signal mass window by replacing the N-terminal alanine with other amino acid residues.

A Ru-catalyzed one-pot synthesis of homopropargylic amines from alkyl azides under photolytic conditions

A new synthetic method for homopropargylic amines from alkyl azides is presented. A salient feature of this reaction is the involvement of N-unsubstituted imines as the key intermediates, which are generated from alkyl azides by Ru catalysis under photolytic conditions. Notably, this method avoids the use of a protective group strategy in the homopropargylic amine synthesis.

De Novo Synthesis of Furanose Sugars: Catalytic Asymmetric Synthesis of Apiose and Apiose-Containing Oligosaccharides

A deu2005novo synthetic method towards apiose, a structurally unusual furanose, is reported. The key feature is sequential metal catalysis consisting of the palladium-catalyzed asymmetric intermolecular hydroalkoxylation of an alkoxyallene and subsequent ring-closing metathesis (RCM). This strategy enabled the efficient synthesis of various apiose-containing disaccharides and a unique convergent synthesis of trisaccharides.

Aliphatic dipeptide tags for multi-2-plex protein quantification.

Mass-balanced (1)H/(2)H-isotope dipeptide tag (MBIT) is diversified as aliphatic tags for multiplexed protein quantification. Aliphatic MBITs are based on the N-acetyl-Xxx-Ala dipeptide, where Xxx is an artificial amino acid with a linear alkyl side chain from C(2)H(5) to C(8)H(17) (C(2)-C(8) tags). (1)H/(2)H isotopes are encoded in the methyl groups of N-acetyl and Ala to yield a pair of isobaric tags with 2-plex quantitation signals separated by 3 Da. C(2)-C(5) tags are prepared by solid-phase synthesis, while C(6)-C(8) tags are synthesized by olefin metathesis in solution. These aliphatic tags are made reactive toward the primary amines of peptides, and the relative abundances of quantitation signals are characterized using both matrix-assisted laser desorption ionization and electrospray ionization tandem mass spectrometry. MBIT-linked peptides co-migrate in reverse-phase liquid chromatography (LC), and their tandem mass spectra exhibit 2-plex quantitation signals as well as sequence ions in similar abundances. As the length of alkyl side chain increases, C(2)-C(8) tags show a stepwise increase in both the LC retention time and the relative abundance of quantitation signals. In addition, the quantitation linearity is well-maintained in a 15-250 fmol range. The multiplexing capability of aliphatic MBITs is demonstrated by applying three different tags (C(6)-C(8) tags) to the quantification of yeast heat shock proteins expressed under four different physiological conditions.

Palladium-Catalyzed Asymmetric Nitrogen-Selective Addition Reaction of Indoles to Alkoxyallenes

A new palladium-catalyzed asymmetric addition reaction of indoles to alkoxyallenes is reported. Remarkably, the reaction showed complete regioselectivity toward the nitrogen. A new mechanism distinct from that of conventional π-allyl chemistry is proposed to explain this unique selectivity. The utility of the reaction is demonstrated by highly efficient and flexible synthesis of N-glycosylindoles.

Synthesis of Enamides by Ruthenium‐Catalyzed Reaction of Alkyl Azides with Acid Anhydrides in Ionic Liquid

Enamides were synthesized by a ruthenium‐catalyzed one‐pot, one‐step procedure from alkyl azides and acid anhydrides. The substrate scope includes not only secondary azides, but also primary aliphatic ones to give a wide range of enamides containing various functional groups. This one‐step procedure was based on the newly discovered activity of Severins diruthenium complex ([Cp^RuCl2]2: Cp^=η5‐1‐methoxy‐2,4‐di‐tert‐butyl‐3‐neopentylcyclopentadienyl) for the transformation of alkyl azides into the corresponding N−H imine intermediates in ionic liquids. The formation of ruthenium tetrazene complexes was observed in the stoichiometric reaction of Severins complex with alkyl azides, which acted as the catalyst for the formation of N−H imine intermediates.

Access to trans‐3,4‐Dihydroxy‐2‐alkylpyrrolidines and Piperidines by Use of Stereodefined Cyclic N,O‐Acetals as a Diversity‐Generating Element

A highly efficient and stereoselective synthetic pathway towards trans-3,4-dihydroxy-2-alkylpyrrolidines and piperidines is described. The nature of the protecting groups on the hydroxyl moieties played a crucial role on the trans selectivity. By using this method, a concise total synthesis of (-)-2-epilentiginosine has been achieved.

Novel Catalyst System for Hydrostannation of Alkynes

A catalyst system was developed for the highly regio- and stereoselective hydrostannation of a range of alkynes with tributylstannane under mild conditions. The active catalytic species was generated from a stable diruthenium complex by illuminating household fluorescent light (30 W) at room temperature.

Synthesis of 1H-azadienes and application to one-pot organic transformations

1H-Azadienes were synthesized from allyl azides by ruthenium catalysis under mild and neutral conditions. Applications of the 1H-azadienes were demonstrated for the one-pot synthesis of N-benzyl-1-azadienes, hydrazones, tertiary carbinamines, dieneamide, and N-cyclohexenylacetamides. Two 1H-azatrienes were also synthesized, which were transformed to pyridine derivatives.