N-heterocyclic carbenes catalyze the rearrangement of 1,1-bis(arylsulfonyl)ethylene to the corresponding trans-1,2-bis(phenylsulfonyl) under mild conditions. Tandem rearrangement/cycloadditions have been developed to capitalize on this new process and generate highly substituted isoxazolines and additional heterocyclic compounds. Preliminary mechanistic studies support a new conjugate addition/Umpolung process involving the ejection and subsequent unusual re-addition of a sulfinate ion.

N-heterocyclic carbenes (NHCs) promote the addition of 1,1-bis(phenylsulfonyl)ethylene to the α-position of α,β-unsaturated aldehydes. The proposed reaction pathway for this Rauhut-Currier-type reaction includes an unusual conjugate addition of an NHC to a bis-vinyl sulfone.

The rhodium(II)-catalyzed oxidative cyclization of glycal 3-carbamates with in situ incorporation of an alcohol nucleophile at the anomeric position provides access to a range of 2-amino sugars having 1,2-trans-2,3-cis stereochemistry, a structural motif present in compounds of medicinal and biological significance such as the streptothricin group of antibiotics and the Chitinase inhibitor allosamidin. All of the diastereomeric d-glycal 3-carbamates have been investigated, revealing significant differences in anomeric stereoselectivity depending on substrate stereochemistry and protecting groups. In addition, some substrates were prone to forming C3-oxidized dihydropyranone byproducts under the reaction conditions. Allal- and gulal 3-carbamates provided uniformly high stereo- and chemoselectivity, while for glucal substrates, acyclic, electron-withdrawing protecting groups at the 4 O and 6 O positions were required. Galactal 3-carbamates have been the most challenging substrates; formation of their amidoglycosylation products is most effective with an electron-withdrawing 6 O-Ts substituent and a sterically demanding 4 O-TBS group. These results suggest a mechanism whereby conformational and electronic factors determine the partitioning of an intermediate acyl nitrenoid between alkene addition, leading to amidoglycosylation, and C3-H insertion, providing the dihydropyranone byproduct. Along the amidoglycosylation pathway, high anomeric selectivity results when a glycosyl aziridine intermediate is favored over an aziridine-opened oxocarbenium donor.

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N-Heterocyclic Carbene-Catalyzed Rearrangements of Vinyl Sulfones. -The carbene generated from a triazolium salt and NaOtBu catalyzes the rearrangement of 1,1-bis(arylsulfonyl)ethenes to the corresponding (E)-1,2-bis(arylsulfonyl)ethenes. The combination of this rearrangement with a subsequent [3 + 2] cycloaddition reaction to a nitrone in a single flask provides highly substituted isoxazolines as single diastereomers. The method can be also applied to the [3 + 2] cycloaddition reaction of azomethine imine (X). -(ATIENZA, R. L.; ROTH, H. S.; SCHEIDT*, K. A.; Chem. Sci. 2 (2011) 9, 1772-1776, http://dx.doi.org/10.1039/c1sc00194a ; Dep. Chem., Northwest. Univ., Evanston, IL 60208, USA; Eng.) -Klein 52-125