Elsevier

Tetrahedron

Volume 56, Issue 24, 9 June 2000, Pages 3817-3856
Tetrahedron

Tetrahedron Report Number 529
New Developments in the Chemistry of N-Acyliminium Ions and Related Intermediates

https://doi.org/10.1016/S0040-4020(00)00159-9Get rights and content

Introduction

Reactions between N-acyliminium ions and nucleophiles—also described as amidoalkylation or Mannich type condensations—have been frequently utilized to introduce substituents at the α-carbon of an amine. Details of these reactions, including methods of generation,1 preparation of suitable precursors,2 stereochemical aspects3 and application in natural product syntheses,4 have been extensively reviewed while the development of chiral variations5 has constituted part of a major reference work. Since the appearance of the most recent surveys, however, a substantial number of valuable and pertinent contributions have appeared in the literature covering significant improvements in the accessibility of precursors and generation of the reactive species while remarkable developments have also taken place in the utilization of this technique, especially in the control of diastereo- and enantio-selectivity. It is therefore considered appropriate to summarise these novel and relevant findings, which is the subject of this review. Emphasis will be placed upon the studies pertaining to the core chemistry of the N-acyliminium synthetic method which have not been fully covered by other authors in reviews on the synthesis of particular classes of compounds. Discussion of the latter results will be included only insofar as the findings are of relevance to the contents of this survey. These reviews cover synthesis of chiral pyrrolidines,6 chiral piperidines,7 chiral bicyclic lactams,8 pyroglutamic acid derivatives,9 1,3-dipolar additions of mesoionic compounds,10 peptide mimetics,11 α-cation equivalents of amino acids,12 benzotriazoles in amidoalkylation,13 peptide chemistry,14 silicon compounds in natural product synthesis,15 Mannich reactions16 and N-sulfonyl imines17 and all contain essential parts on N-acyliminium chemistry. Results of standard procedures which lack new or unexpected findings as compared to previous reviews will be mentioned as references in the appropriate section.

Section snippets

Structure and reactivity

Throughout the last two decades it has been well-established that substitution with electron-attracting groups at nitrogen renders the Mannich-intermediate 1 considerably more reactive by enhancing its cationic character. Of these modified cations the N-acyl derivative 2 and the carbamate 3 have been most widely exploited although the use of other electronegative substituents such as the amide 4 and N-tosyl 5 cations have also been examined. Depending on the type of R1, R2 and R3 various cyclic

Cyclic precursors

The controlled preparation of a suitable cyclic precursor for generating an endocyclic iminium is discussed in this section. A great variety of structures have been described including various types of substituted and heterocyclic variants of the imide system. These substrates will not be reviewed separately but all of the results will be discussed in the appropriate section. Of the nearly 400 references in the period 1992–1999, only those adding relevant material to the existing documentation

Intramolecular C–C bond formation

The synthesis of fused ring systems starting from a cyclic N-acyliminium precursor is discussed in this section. Both endocyclic 115 as well as exocyclic 116 variants will be described, the ring size of the starting precursor being the sole criterion for classification of the sections. The type of nucleophile, size of the ring to be formed and positional attachment of the tether are also discussed here.

Carbon–Carbon Bond Formation of Linear N-Acyliminium Intermediates

The availability of stable linear precursors for the acyclic N-acyliminium intermediate—as reviewed in Section 3.2—has greatly expanded its synthetic potential. In this section a survey is presented of intra- and intermolecular applications both on achiral and chiral forms.

Applications

In the previous sections, novel and extended pathways were discussed regarding the preparation and reactions of N-acyliminium precursors. In this section, work on the synthesis of natural products of relatively simple and more complex structures and also on the construction of non-natural molecules of possible biological interest will be summarized. The discussion will not be focused on separate compound classes, rather the elements of structure, type of ring to be formed and stereochemical

Outlook and Update

A century of developments in cationic carbon–carbon bond coupling by the use of N-acyl substituted carbocations has provided the organic chemist with a plethora of methods to achieve a desired transformation. The question may therefore be posed whether this technology has now reached a state of maturity and the reader of this and other reviews will have no doubts about answering in an affirmative way. Yet both in the further extensions of this method as well as in the area of current

Acknowledgements

The contributions of Professor Henk Hiemstra to various parts of this report are highly valued.

W. Nico Speckamp obtained his Ph.D on the Total Synthesis of 6-Aza-steroids at the University of Amsterdam. As a professor at the Institute of Molecular Chemistry his interests were mainly focused on the development and applications of new synthetic intermediates among which the N-acyliminium method figured prominently. Currently he is associated as guest-professor at the same Institute.

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      Iminiums such as 1 (Fig. 1) are one of the most important electrophiles in synthetic organic transformations for the creation of carbon–carbon and carbon–heteroatom bonds. Covalent attachment of electron withdrawing groups at the nitrogen atom enhances its cationic character making the species a more reactive intermediate (Fig. 1) [1,2]. Amongst these modified cations, much interest has centered around N-acyliminium ions 2 and 3, although ureas 4, N-tosyl derivatives 5, and hydrazonium ions 6 have also been studied [1].

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    W. Nico Speckamp obtained his Ph.D on the Total Synthesis of 6-Aza-steroids at the University of Amsterdam. As a professor at the Institute of Molecular Chemistry his interests were mainly focused on the development and applications of new synthetic intermediates among which the N-acyliminium method figured prominently. Currently he is associated as guest-professor at the same Institute.

    Marinus J. Moolenaar started at the University of Amsterdam under the supervision of Prof. H. O. Huisman (polyenes and terpenes) and joined the group of Prof. Nico Speckamp in 1983, where he was involved in the development of N-acyliminium ion chemistry. His recent publications deal with the enantiopure synthesis of natural products, such as biotin and epibatidine. Since 1997, he has been working at the Institute of Molecular Chemistry under the supervision of Prof. Henk Hiemstra.

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