Keteniminium Ion-Initiated Cascade Cationic Polycyclization

Abstract

A novel and efficient keteniminium-initiated cationic polycyclization is reported. This reaction, which only requires triflic acid or bistriflimide as promoters, affords a straightforward entry to polycyclic nitrogen heterocycles possessing up to three contiguous stereocenters and seven fused cycles. These complex, polycyclic molecules can be obtained in a single operation from readily available ynamides which were shown to be remarkable building blocks for multiple, consecutive cationic transformations.

Metal‐Free Enantioselective Electrophilic Activation of Allenamides: Stereoselective Dearomatization of Indoles

“…However, it should be mentioned that no examples of metal-free electrophilic activation of allenamides in enantioselective transformations have been reported so far. [8] To verify the feasibility of this working hypothesis, we considered the challenging C3-site-selective intermolecular …”

Reactions of Trifluoroacetyl Alkynes Under Electrophilic Activation with Brønsted Acids or Acidic Zeolites

“…Subsequent aqueous workup of the reaction mixture affords triflates 3 in this case (see also formation of triflates from alkynes in the recent works [23,24] ). In relatively low acidic H 2 SO 4 , the reaction proceeds through formation of monocations A.…”

“…The same way is realized in the system TfOH-Py leading to Oprotonated forms of vinyl triflates C (X = CF 3 ). Subsequent aqueous workup of the reaction mixture affords triflates 3 in this case (see also formation of triflates from alkynes in the recent works [23,24] ). However, most probably the reaction in stronger triflic acid leads to the formation of dications B.…”

A Journey in the Chemistry of Ynamides: From Synthesis to Applications

“…The first cationic cyclization based on this strategy was developed starting from ynamides 51 embedded with an ortho-substituted aromatic ring on the alkyne and a benzyl or allyl group on the nitrogen atom, substrates which were found to be smoothly cyclized to complex tri/tetracyclic nitrogen heterocycles 56 upon treatment with excess triflic acid or catalytic amounts of bistriflimide (Scheme 16). 46 In this quite remarkable transformation, the key step which triggers the cationic transformation is the [1,5]-sigmatropic hydrogen shift from activated keteniminium ion 52, an intermediate generated by protonation of the starting ynamide 51. This hydrogen shift furnishes conjugated iminium ion 53, in resonance with 54, which then undergoes a 4π conrotatory electrocyclization followed by trapping of the benzylic carbocation in 55 by the allyl or benzyl group to form the last cycle in 56 possessing a cis ring junction.…”