Alkynes to (E)-enolates using tandem catalysis: stereoselective anti-aldol and syn-[3,3]-rearrangement reactions

Bi(OTf)3-, TfOH-, and TMSOTf-Mediated, One-Pot Epoxide Rearrangement, Addition, and Intramolecular Silyl-Modified Sakurai (ISMS) Cascade toward Dihydropyrans: Comparison of Catalysts and Role of Bi(OTf)3

“…Initial studies focused on the production of HBr and TfOH in situ . 33 Consistent with the hypothesis that the purported Lewis acid acts as a source of HX which in turn functions as the catalyst or initiator, addition of 15 mol% of the hindered, non-nucleophilic base, 2,6-di- tert -butyl-4-methylpyridine (DTBMP) completely suppressed the reaction (entries 2, 3, 7, 14 and 15). 30a Given Ollevier and co-workers' report that DTBMP-TfOH efficiently catalyzed the three-component Mannich reaction, 34 we added 5 and 10 mol% of this salt to a solution of the epoxide and ( Z )-δ-hydroxyalkenylsilane (entries 10-11).…”

Harnessing the Electrophilicity of Keteniminium Ions: A Simple and Straightforward Entry to Tetrahydropyridines and Piperidines from Ynamides

“…Finally,aiming at introducing an additional substituent on the piperidine ring, we envisioned trapping the intermediate iminium ion 8 (Scheme 2) by anucleophile.Logically,and in accordance with previously reported results, [6] when this nucleophile is added at the beginning of the reaction, it directly reacts with the keteniminium ion before the hydride shift can occur.Knowing that the tetrahydropyridine was fully protonated (to 8)u nder the reaction conditions, [14] we surmised that the nucleophile would be best added after the cyclization. To our delight, this strategy turned out to be quite efficient and indeed allowed for an efficient synthesis of highly substituted piperidines 16 ( Figure 2).…”

“…Indeed, such intermediates-which are conveniently obtained from readily available ynamides 3 [5] and electrophiles-have been shown to be incredibly electrophilic species. [6] Provided that this exceptional level of reactivity can be finely tuned and controlled, it can be used to promote reactions in which simple keteniminium ions 2 fail and, notably,t oactivate poorly reactive C À Hb onds through hydride abstraction or hydrogen shift. [6k, 7] In this context, we recently reported that activated keteniminium ions were indeed reactive enough to initiate a [ 1,5]-sigmatropic hydrogen shift from an unactivated benzylic position, which triggered ac ationic polycyclization reaction.…”

Alternative Approaches to Enolate Chemistry

“…Very recently, a method for the catalytic addition of silanols to acetylinic oxazolidinones has been reported (Scheme 10). 14 In this case, a Lewis acidic metal salt such as Sc(OTf) 3 or Zn(OTf) 2 was used to mediate the addition of the silanol across the alkyne. The Lewis acid also serves to activate The reaction presumably proceeds via co-ordination of the silanol to the Lewis acid, and protonation of the electron-rich alkyne (Scheme 11).…”