Dominique Vichard

ENANTIOSELECTIVE SYNTHESES OF ORGANOSULFUR COMPOUNDS VIA 2,3 SIGMATROPIC REARRANGEMENTS OF YLIDES DERIVED FROM DI(ALLYL), DI(PROPARGYL), AND DI(BENZYL ) SULFIDE COMPLEXES ; CONTROL OF CARBON CONFIGURATION BY AN EASILY RESOLVED AND RECYCLED RHENIUM AUXILIARY

The (2,3) sigmatropic rearrangement of sulfur ylides with S-ally1 or relat- ed substituents finds widespread use in organic synthesis, but enantioselective ver- sions of this carbon-carbon bond forming reaction are relatively unexplored. We re- port that the chiral rhenium Lewis acid ((q5-C5H5)Re(NO)(PPh3))+ is a highly effect- ive auxiliary for desymmetrizations of di(allyl), di@ropargyl), and di(benzy1) sulf- ides. Additions of t-BuOK to the air-stable cationic sulfide adducts give neutral thio- late complexes of high diastereomeric purities. The thiolate ligands can be S-alkylated and detached as sulfides of high enantiomeric purities. The rhenium auxiliary is eas- ily recovered and recycled without loss of configuration. Crystal structures establish product configurations, and mechanisms of diastereoselection are analyzed in detail. Sulfur ylides that have ally1 substituents undergo rapid (2,3) sigmatropic rearrangements to give sul- fides or thioethers (1). This carbon-carbon bond forming reaction commonly generates a new carbon stere- ocenter, and sees extensive use in organic synthesis. The ylides are most often accessed by deprotonations of sulfonium salts. Surprisingly, there are only two cases in which sulfides have been generated in an enantioselective manner from sulfonium salts that lack resolved carbon stereocenters (2). These are depicted in Scheme 1. One involves an adamantyl sulfonium salt with a resolved sulfur stereocenter, and gives a sul- fide of high enantiomeric purity. The other involves an achiral sulfonium salt and a chiral solvent and base, and gives a sulfide of low enantiomeric purity. Both were reported over twenty years ago by Trost.

A new feature in the chemistry of nitrobenzofuroxans: ambident reactivity in Diels–Alder condensations

The 4,6-dinitrobenzofuroxan (DNBF) structure is shown to act both as a dienophile and a heterodiene upon treatment with 1-trimethylsilyloxybuta-1,3-diene and vinyl ethyl ether, providing in two steps a highly functionalised stereoselective compound.