S‐((Phenylsulfonyl)difluoromethyl)thiophenium Salts: Carbon‐Selective Electrophilic Difluoromethylation of β‐Ketoesters, β‐Diketones,and Dicyanoalkylidenes

S‐((Phenylsulfonyl)difluoromethyl)thiophenium salts were designed and prepared by a triflic acid catalyzed intramolecular cyclization of ortho‐ethynyl aryldifluoromethyl sulfanes. The thiophenium salts were found to be efficient as electrophilic difluoromehtylating reagents for introduction of a CF₂H group to sp³‐hybridized carbon nucleophiles such as of β‐ketoesters and dicyanoalkylidenes. The (phenylsulfonyl)difluoromethyl group can be readily transformed into CF₂H under mild reaction conditions. Enantioselective electrophilic difluoromethylation was also achieved in the presence of bis(cinchona) alkaloids.     

Individual (f,tA)‐ and (f,tC)‐Fullerene‐Based Nickel(II) Glycinates: Protected Chiral Amino Acids Directly Linked to a Chiral π‐Electron System

Stereoselective electrosynthesis of the first individual (^f,tA)‐ and (^f,tC)‐1,4‐fullerene derivatives with a non‐inherently chiral functionalization pattern is described, as well as the first example of an optically pure protected primary amino acid directly linked to the fullerene through only the chiral α‐amino‐acid carbon atom. An application of an auxiliary chiral nickel‐Schiff base moiety as derivatizing agent allowed separation of (^f,tA)‐ and (^f,tC)‐1,4‐fullerene derivatives using an achiral stationary phase, a separation which has never been done before.     

Stereospecific α‐Sialylation by Site‐Selective Fluorination

Sialic acids are ubiquitous components of mammalian cell membranes and key regulators of cellular recognition events. Located at the non‐reducing termini of bioactive gangliosides, these essential building blocks are fused to the polysaccharide core via a characteristic α‐linkage, and rarely occur in the monomeric form. Effective chemical strategies to enable α‐sialylation are urgently required to construct well‐defined tools for glycomics. To complement existing chemoenzymatic strategies, an α‐selective process has been devised based on the site‐selective introduction of fluorine at C3 (more than 20 examples, up to 90 % yield). Predicated on localized particle charge inversion (C?H^δ+→C?F^δ?), fluorine insertion simultaneously augments the anomeric effect, enhances electrophilicity at C2 and mitigates elimination. A stereochemical induction model is postulated that spans the S_N continuum and validates the role of the C?F bond in orchestrating α‐selectivity.     

Total Syntheses of (+)‐Sarcophytin, (+)‐Chatancin, (−)‐3‐Oxochatancin,and (−)‐Pavidolide B: A Divergent Approach

A concise and divergent approach for the total syntheses of four cembrane diterpenoids, namely (+)‐sarcophytin, (+)‐chatancin, (?)‐3‐oxochatancin, and (?)‐pavidolide B, has been developed, and it also led to the structural revision of (?)‐isosarcophytin. The key steps of the strategy feature a double Mukaiyama Michael addition/elimination, a Helquist annulation, two substrate‐controlled facial‐selective hydrations, and a pinacol rearrangement. The described syntheses not only achieved these natural products in an efficient manner, but also provided insight into the biosynthetic relationship between the two different skeletons.     

Diastereo‐ and Enantioselective Synthesis of β‐Aminoboronate Esters by Copper(I)‐Catalyzed 1,2‐Addition of 1,1‐Bis[(pinacolato)boryl]alkanes to Imines

Reported herein is an efficient copper(I)‐catalytic system for the diastereo‐ and enantioselective 1,2‐addition of 1,1‐bis[(pinacolato)boryl]alkanes to protected imines to afford synthetically valuable enantioenriched β‐aminoboron compounds bearing contiguous stereogenic centers. The reaction exhibits a broad scope with respect to protected imines and 1,1‐bis[(pinacolato)boryl]alkanes, thus providing β‐aminoboronate esters with excellent diastereo‐ and enantioselectivity. The synthetic utility of the obtained β‐aminoboronate ester was also demonstrated.