Acyliminium cations

A substrate containing an amine carboxylate moiety is converted in an electrolyte solution in the presence of a strong acid to a cationic intermediate, an N-acyliminium cation, by electrooxidative reaction. This species is immediately reacted with an allylsilane [66, 67]. By nucleophilic reachon, C-C bond formahon is achieved. 

Tryptophan (and also tryptophanol) undergoes a stereoselective cyclocondensation with racemic compound 249, in a very interesting process involving a kinetic resolution with epimerization of the tryptophan stereocenter and simultaneous desymmetrization of the two diastereotopic acetate chains <2005CC1327>, affording the enantiomeri-cally pure lactam 250. A subsequent treatment of the latter compound with trifluoroacetic acid led to the indolo[2,3- ]quinolizidine 251 through an intermediate acyliminium cation (Scheme 50) <20050L2817>. 

Derivatives 30 (R OH, R2 = iV-acetyl-indol-3-yl-(CH2)2 and (R = OEt, R2 = Ph-(CH2) (n — 2, 3)) were transformed into intramolecular Friedel-Crafts type reactions involving A-acyliminium cation intermediates into the cis-and trans-isomeric mixtures of the corresponding fused derivatives on treatment with strong acid <2000TL2745, 2001JOC5731>. 

However, a better known version of the 2-aza-Cope rearrangement is that carried out by using 2-aza-l,5-hexadienes 619 (equation 269) and particularly their iminium ion counterparts, usually N-acyliminium cations 620 (equation 270)365,366 (for reviews, see also Reference 367). Aza-Cope rearrangement of the norbomene ester 621 leads to tetrahydropyridine ester 622 when allowed to stand in solution at room temperature for... 

As mentioned above, the polar 1,4-cycloaddition of N-acyliminium ions 14 and 18 to olefins gives 5,6-dihydro-4H-l,3-oxazinium salts 16 (Section fI,A,l), or forms the 1,3-oxazinium salts with acetylenes (Section III, A). The same ions 89 and 91 add nitriles to furnish 1,3,5-oxadiazinium salts 90 and 92, little investigated until now (65CB334 88ZOR230). Salts 92 are unstable and can dissociate at elevated temperatures with nitrile liberation. Therefore, they may serve as original reservoirs for active jV-acyliminium cations. 

The mechanism of the reaction is not clear, but it is possible that aldehydes react with benzyl carbamate first, to give a (l-hydroxyalkyl)carbamate and then eliminate HzO providing a route to the TV-acyliminium cation that, in the next step, is trapped by triphenyl phosphite. 

The structures of the 2-oxazolinium, 1,3,5-oxadiazinium and 3-azapyrylium salts obtained by means of electrophilic catalysis by acylium ions102,113 point to the fact that their formation proceeds not via Af,7V-bis-acyliminium ions 332 and 333, but via the TV-acyliminium ions 322 (i.e. those protonated at the nitrogen atom). In our opinion142, the acetoxy azaallenium ions 335 can be transformed to the Af-acyliminium cations 337 by a deacylation to the N-acylimines 336, followed by protonation of the latter (equation 91). Such a process is quite possible under the conditions used for... 

As indicated by the experimental and theoretical results described above, both pathways a and b of reaction of carbonyl compounds with nitriles (equation 77) include two variants of the rearrangement of nitrilium salts having an oxygen function at a-position (285) and a-iminoalkyloxycarbenium ions 288. Both rearrangements lead to A-acyliminium cations 289 see equation 94. 

Jochims and coworkers48 138 139 cf equations 79, 80 and 84) as well as Lukyanov and colleagues49 cf equation 15) suggested that oxazetium salts like 344 and 346 are intermediates (or transition states) in these rearrangements. However, at the same time the opinion was advanced137 that the conversions of both nitrilium ions 285 and carboxonium ones 288 into A-acyliminium cations 289 are a second type of acid-catalyzed rearrangement of carbonyl compounds. 

Scheme 5.2.32 Intra-molecular allylations for the synthesis of polyether natural products 5.2.8 Reactions of Af-Acyliminium Cations...

Scheme 5.15 Generation and reaction of N-acyliminium cation pool from pyrrolidine carbamate...

A zirconium-catalyzed oxidative coupling reaction between N-methylindole 192 and N-methylpyrrolidinone furnished 5-substituted pyrrolidinone 193 <04AG(E)4231>. The regioselective preference for 3-substitution of the indole ring suggests that an N-acyliminium cation intermediate might be involved. 

The mnide nitrogen attacks the aldehyde, generating an aminal which losses a hydroxide after oxygen protonation, generating an acyliminium cation. 

The position 3 of the indole reacts with the acyliminium cation. 

The amide nitrogen reacts intramolecularly with the aldehyde, forming an aminal, which looses water under acidic catalysis, producing an acyliminium cation. 

Masson and coworkers reported a-alkylation of imines through a three-component radical/cation domino process using bromomalonate 11, an enamide 13, and an alcohol as starting materials. First, an iridium species forms the alkyl radical 12 from 11 by single electron transfer (SET). Then enamide 13 reacts with 12 to produce the amidoalkyl radical 14. An oxidative radical-polar crossover reaction then furnishes an N-acyliminium cation 15, which finally reacts with nucleophilic alcohol to produce a-alkylated a-amido ether 16 (Scheme 5.5) [11]. 

The first diastereoselective total synthesis of 5-substituted morpholine-3-phosphonic acids (416) has been reported by Ordonez. Nucleophilic addition of trimethyl phosphite in the presence of BFsxOEtj to 3-methoxylated (5S)-5-phenyl- and (5S)-5-benzylmorpholine (413) via the intermediacy of N-acyliminium cation (414) allowed the construction of cyclic a-aminophosphonates (415) from L-amino acids (412). Finally, the N-Boc cleavage and hydrolysis of the dimethyl phosphonate (415) produced desired (31 ,55 )-5-phenyl- and (31 ,55 )-5-benzylmorpholine-3-phosphonic acids (416) in almost quantitative yields (Scheme 125). ... 

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