The "Non-Oxidative" pummerer reaction: conclusive evidence for S(N)2-type stereoselectivity, mechanistic insight, and synthesis of enantiopure L-alpha-trifluoromethylthreoninate and D-alpha-trifluoromethyl-allo-threoninate(1)

Enantiopure methyl D-alpha-trifluoromethyl-allo-threoninate 18 and L-alpha-trifluoromethylthreoninate 19 were synthesized using (R)-ethyl p-tolylsulfoxide as chiral alpha-hydroxyethyl anion equivalent. The key step was the S(N)2-type replacement of the sulfinyl auxiliary with a hydroxy group, via trifluoroacetic anhydride promoted "non-oxidative" Pummerer reaction (NOPR) of the diastereomeric intermediate beta-sulfinyl amines 14 and 15, obtained by condensation of (R)-ethyl p-tolylsulfoxide 13 with the N-Cbz imine of methyl trifluoropyruvate 12. The conclusive evidence for S(N)2-type stereoselectivity of the NOPR was achieved by X-ray diffraction of both the starting diastereomer 14 and the p-bromobenzoate 25, obtained from the threoninate 19. NMR monitoring of the NOPR performed on 15 allowed the detection of a transient intermediate, which was identified as the four membered cyclic sigma-sulfurane 27. This intermediate spontaneously rearranged (40 min, rt) into the corresponding sulfenamide 17, probably via an intramolecular displacement of the sulfinyl by a trifluoroacetoxy group, with inversion of configuration at the carbon stereocenter. The same process occurred for the diastereomeric beta-sulfinyl amine 14, but the sulfenamide 16 was formed at a very fast rate, thus precluding NMR detection of the corresponding sigma-sulfurane intermediate 26. One-pot treatment of the diastereomeric sulfenamides 16 and 17 with NaBH(4) afforded very good yields of the corresponding threoninates 18 and 19.

Resolution of Racemic 1,2-Dibromohexafluoropropane through Halogen-Bonded Supramolecular Helices.

Halogen bonds, attractive intermolecular interactions between perfluoroalkyl bromides and bromide ions, are present in cocrystals of (-)-sparteinium hydrobromide (1) and (S)-1,2-dibromohexafluoropropane (2; shown schematically), and result in enantiopure and infinite supramolecular helices. The perfluorocarbon-hydrocarbon self-assembly allows the resolution of racemic 2.

Tsugicoline E, a new polyoxygenated protoilludane sesquiterpene from the fungus laurilia tsugicola1

Tsugicoline E (3) has been isolated from cultures of the Basidiomycetous fungus Laurilia tsugicola and its structure deduced from 1H and 13C NMR and single-crystal X-ray diffraction studies. The suggested absolute configuration is consistent with biogenetic considerations.