Iminium ring-forming reactions

The most common form of the type e ring-closure reaction to afford 1,4-benzodiazepines is a l-endo-trig--ptoce,ss in which a substituted iminium derivative, typically generated in situ, is captured by the aryl ring. Cyclization of the A,iV -(bis-l,2,3-benzotriazol-l-ylmethyl)amine 104 by a Friedel-Crafts-type ring closure afforded the tetrahydro-1,4-... 

The mercury(II) acetate oxidation of pyrrolidino alcohols can be controlled to give fair yields of bicyclic oxazolidines in which the masked carbonyl carbon atom is at the ring junction (60JA5148). The course of the reaction is regarded as an internal nucleophilic attack by the alcohol group on the ternary iminium salt formed in the initial oxidation step (equation 60). 

As for the corresponding iminium and oxocarbenium ions, thiocarbenium salts can undergo endo- or exo-cyclic ring closure reactions. Again, carbon-carbon bond formation by exocyclic-type reactions lead to heterocyclic systems only by chance , namely when an additional heteroatom is located in the chain that will form the ring, e.g. the thiol-assisted annulation of (119) to give (120 Scheme 58). ... 

Dimethylthioformamide (DMTF) (234) reacts with oxiranes (235) in the presence of a catalytic amount of trifluoroacetic acid under mild conditions to form the corresponding thiiranes in good yield as shown in Scheme 79 and Table 37 <86S779>. A possible reaction mechanism is postulated based on the mechanism observed in the preparation of oxiranes with thiocyanate or thiourea <66CRV297>. Intermediate (236) is generated by the nucleophilic attack of the sulfur atom of (234) on the carbon atom of (235) followed by ring closure reaction of the hydroxy oxygen atom with the iminium carbon atom. Subsequent elimination of dimethylformamide affords thiirane (237). 

Viehes salt and various acyclic, cyclic, and rigid bicyclic e(-diols give iminium salts which on thermolysis yield /tf-chloro-deoxy-X-N,N-dimethylcarbamates formed in diaxial ring opening reactions (e.g., Scheme 6). 

It is observed that 1,5-hydride transfer can be accelerated by iminium activation. Therefore, it is speculated that cinnamaldehyde derivatives 194 represent ideal acceptors that are susceptible to activation by secondary amine catalysts capable of forming an iminium ion (Scheme 1.85) [132], The resulting iminium ion activation is expected to increase hydride transfer to alkene. The subsequent ring-closure reaction mediated by enamine catalysis furnishes ring-fused tetrahydroquinoline derivatives in moderate yields and high levels of enantioselectivity. 

The dye is initially linked to a ballasted thiazoUdine, which reacts with silver to form a silver iminium complex. The alkaline hydrolysis of that complex yields an alkali-mobile dye. Concomitantiy the silver ion is immobilized by reaction with the ballasted aminoethane thiol formed by cleavage of the thiazolidine ring. 

The electrophile 4 adds to the aromatic ring to give a cationic intermediate 5. Loss of a proton from 5 and concomitant rearomatization completes the substitution step. Subsequent hydrolysis of the iminium species 2 yields the formylated aromatic product 3. Instead of the highly toxic hydrogen cyanide, zinc cyanide can be used. The hydrogen cyanide is then generated in situ upon reaction with the hydrogen chloride. The zinc chloride, which is thereby formed, then acts as Lewis acid catalyst. 

The 2-azonia analog of the Cope rearrangement is estimated to be accelerated by 106, relative to the unsubstituted system.270 The product of the rearrangement is an isomeric iminium ion, which is a mild electrophile. In synthetic applications, the reaction is often designed to generate this electrophilic site in a position that can lead to a cyclization by reaction with a nucleophilic site. For example, the presence of a 4-hydroxy substituent generates an enol that can react with the iminiun ion intermediate to form a five-membered ring.271... 

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