Rearrangement acyliminium ions

With certain substituents, such as methoxy150 or (substituted) phenyl53 functions, in the allylie position the reaction outcome completely changes, giving rise to predominant or exclusive formation of five-membered ring products via a preceding 2-aza-Cope rearrangement of the initially formed A -acyliminium ion. These substituents clearly stabilize the intermediary carbo-cation 3. 

The domino cycloaddition-iV-acyliminium ion cyclization cascade has been extensively reviewed. Tandem reactions combining Diels-Alder reactions and sigma-tropic rearrangement reactions in organic synthesis have been extensively reviewed. The tandem Diels-Alder reaction between acetylenedicarboxaldehyde and N,N -dipyrrolylmethane has been extensively studied at the RHT/3-21G and RHF/6-31G levels.The molecular mechanism of the domino Diels-Alder reaction between hexafluorobut-2-yne and A,A -dipyrrolylmethane has been studied using density functional theory. 

However, a considerable difference should be noted between a-hydroxyalkyl nitrilium ions 24 which contain an intramolecular nucleophilic center (OH group), and N-alkylnitrilium ions 26 which are the intermediates of a Ritter reaction45. The transformation of ions 24 into the N-acyliminium ions 25 (equation 11) is evidently an intramolecular rearrangement (see Sections V.B and V.C), while the nitrilium salts 26 can be converted into ions 25 only by an oxidative dehydrogenation of amides 273 (equation 12). 

In these reactions attention should be paid to the 118 -> 121 as well as the 126 -> 127 rearrangements, which were unknown until recently for A -acyliminium ions. It is possible, however, that in this situation an alternative mechanism involving an isomeriza-the stage of the (9-protonated intermediate, i.e. a 1,2-shift in the hydroxycarbenium ion 13386 formed from 132 (equation 45), may take place. 

A-acyliminium ions (equations 41, 43, 55 and 56) Danilov rearrangement of aldehydes 347 into ketones 349149 (equation 95), the acyloin rearrangement 350 -> 35290 (equation 96) and dienone-phenol rearrangement 353-355150 (equation 97). 

The reaction differs from the Ritter reaction by the two types of electrophilic activation of the reagents and by the two types of rearrangement of nitrilium 285 and carboxonium ions 288 (equation 94). Besides, this interaction proceeds at an oxidation level of two, while the Ritter reaction occurs at an oxidation level of one17. While it may be shown that A-acyliminium ions 365 can be obtained from a carbonyl compound and a nitrile via the Ritter reaction, then it is only the second step b) in a three-step process where the first step (a) is the reduction of carbonyl compound 364 to alcohol 366 and the third step (c) is an oxidative dehydrogenation of amide 369 obtained3 (equation 105). 

Cyclopropyl-containing carbocycles can be prepared from the initial [4 + 2] cycloadducts by an Af-acyliminium ion-enamide rearrangement. The unsaturated bicyclic lactam also undergoes 1,3-dipolar cycloadditions with azomethine ylides. Reduction of the bicyclic lactam with alane followed by hydrogenation affords enantiomerically pure 2-substituted pyrrolidines (eq 11). ... 

Hart, D. J., Tsai, Y.-M. N-Acyliminium ions detection of a hidden 2-aza-Cope rearrangement. Tetrahedron Lett. 1981,22,1567-1570. 

It is known that nitrilium salts can be prepared by alkylation of nitriles , via the reaction of imidoyl chlorides with Lewis acids as well as by Beckmann rearrangement of oximes . The application of nitrilium salts in organic synthesis is summarized in a number of reviews " . In this connection it is appropriate to mention another three-component one-pot synthesis of 5,6-dihydro-4. -l,3-oxazines 40 based on the reaction of aldehydes with iV-tcr/-butylnitrilium salt 41 which, however, proceeds without the participation of Af-acyliminium ions . The reaction is carried out by mixing the aliphatic or aromatic aldehyde, tcrt-butyl chloride and Lewis acid (SbCljjSnC ) in benzonitrile solution (equations 16 and 17). 

Recently, cyclizations of the isomeric unsaturated carbamates (141) and (142) have been investigated (equation 107). These fundamental reactions are not as regio- and stereo-selective as similar cyclizations starting from cyclic A -acyliminium ions (equations 25 and 26). The loss of stereoselectivity might have to do with the occurrence of cationic aza-Cope rearrangements in combination with chair-chair interconversions (equation 108). Normal cyclization of (142) occurs via conformation (145) and should lead to (144). If sigmatropic rearrangement competes with normal cyclization, (146) arises, but this conformation still leads to (144). Only after chair-chair interconversion to (147) and subsequent cyclization, (143) is formed. Via the same mechanism (141) can produce the abnormal product (144). 

Another strategy for the formation of /V-acyliminium ions as substrates for 2-aza-Cope rearrangements consists of the intramolecular cyclization of carboxylic amides of type 17. The Cope products are cyclized to afford substituted pyrrolidines, e.g., 20 and 21964. 

Acyliminium ions are also derived from rearrangement of Bischler-Napieralski cyclization products, which are obtained from amide product of reaction of arylethylamines and 2-methoxycarbonyl benzoyl chloride. These can be captured, for example, with methoxide anion (Scheme 43) (94TL(35)2751). 

In other reactions that proceed via an acyliminium ion, O-vinyl iV,0-acetals rearrange smoothly to /3-(A-acylainino)aldehydes at 0 °C in CH2CI2 in the presence of TMSOTf with moderate to high diastereoselectivities (eq 78). However, TMSOTf failed to promote aprotic alk)fne-iminium cyclizations, which are readily enhanced by TMSCl, TMSBr, or SiCU. On the other hand, TMSOTf assists in the addition of enols to heteroaromatic imines or hydroxyaminal intermediates. ... 

N-Acyliminium ions are hot and will even add to unactivated olefins. For example, N, 0-acetal 20 reacts with formic acid to provide 23. The stereochemistry of the process is relatively clean and can be rationalized by an twft -periplanar addition of electrophilic carbon and nucleophilic oxygen across the carbon-carbon double bond with a transition state that resembles a chair N-acylpiperidine. In reality, this process is mechanistically more complex than this simple model (for example, possible carbocation intermediates and in some cases, some underlying sigmatropic rearrangements), but the model is simple, easy to remember, and has excellent predictive value. How was this chemistry used to prepare perhydrohistrionicotoxin ... 

W-Acyliminium Ion Cyclizations /. Am. Chem. Soc. 1980,102, 397—398. Hart, D. J. Tsai, Y-M. N-Acyliminium Ions Detection of a Hidden 2-Aza-Cope Rearrangement Tetrahedron Lett. 1981, 22, 1567-1570. 

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