Beckmann rearrangement systems

Some benzannulated azocine systems can be obtained via Beckmann rearrangement of the corresponding benzocycloheptatrienone oxime derivatives.27... 

Beckmann rearrangement of anthraquinone dioximes leads to 1.5-diazocine systems, a reaction which indirectly proves the C2-symmetry of the starting material.51,52 The reaction is induced preferentially by heating in polyphosphoric acid. 

Lasubines I and II are alkaloids containing a 4-arylquinolizidine substructure that have been isolated from plants of the Lythraceae family and have attracted the attention of synthetic chemists for some time. While numerous racemic syntheses of these and related compounds have been reported, only a few enantioselective syntheses are known. Some examples of these syntheses are given below, and the strategies involved in these examples are summarized in Scheme 92. Three of these syntheses involve the creation of the quinolizidine system by formation of one bond at the a- or 7-positions, while the fourth approach is based on a ring transformation associated with a photochemical Beckmann rearrangement. 

Conceptually interesting is the synthesis of the oxazole system 94 through a Beckmann rearrangement of a-formyl ketoxime dimethyl acetals 93 which demonstrated the possibility of a non-amino acid pathway in the biosynthesis of marine derived oxazoles <06CC1742>. 

Several explosions or violent decompositions dining distillation of aldoximes may be attributable to presence of peroxides arising from autoxidation. The peroxides may form on the -C=NOH system (both aldehydes and hydroxylamines perox-idise [1]) or perhaps arise from unreacted aldehyde. Attention has been drawn to an explosion hazard inherent to ketoximes and many of their derivatives (and not limited to them). The hazard is attributed to inadvertent occurence of acidic conditions leading to highly exothermic Beckmann rearrangement reactions accompanied by potentially catastrophic gas evolution. Presence of acidic salts (iron(III)... 

The previous referred inconveniences have prompted an increasing interest in the development of alternative, essentially neutral and more environmental-friendly catalysts to promote the rearrangement of O-unsubstituted oximes. The development of highly efficient and selective transformations and also of processes for catalyst recovery and its reuse are the aim of some of the more recent studies. Much of this work is being done in industry to improve current production processes and is the subject of new patent applications. During the last two decades environment concerns have led to the development of green, simple and cost-effective catalytic systems for the Beckmann rearrangement. 

The intermediate may be trapped by other nucleophiles (different from water) and diverse products may be obtained. The interception of the intermediate may occur inter- or intra-moleculary, the latter providing a helpful tool to produce a new ring system (Scheme 9, pathway 2). These reactions are sometimes referred to, respectively, as Beckmann rearrangement-addition and Beckmann rearrangement-cyclization reactions. 

In fused ring systems, the Beckmann rearrangement of oximes at the carbon adjacent to a common carbon of the two rings usually proceeds by preferential migration of the ring system (the common carbon atom undergoes migration preferentially). This behaviour can be mainly attributed to the increased stability of the oxime in which the Af-hydroxyl (or derivative) is directed towards the outside of the molecule. 

Expansions of oximes of B-ring steroid systems are much less common. Recently, during the synthesis of a 6-aza-pregnane derivative 387, the introduction of the nitrogen was made using the Beckmann rearrangement reaction (equation 156). 

The reactor systems were also improved, concerning the Beckmann rearrangement of cyclohexanone oxime and the production of the unwanted ammonium salts ", by-products and contaminants" ". 

These derivatives may be obtained by reduction of the appropriate nitro derivative catalytically or with a metal-acid system, or by Beckmann or Hofmann rearrangements of suitable acyl or carboxamido derivatives. 4-Aminobenzo[6]thiophene has also been prepared by means of a Bucherer reaction with 4-hydroxybenzo[6 Jthiophene. Several 5-aminobenzo[6]thiophenes have been prepared by cyclization reactions of p-acetamino-phenylthio derivatives. 6-Acetaminobenzo[6 Jthiophenes may be obtained from the corresponding 6-acetyl derivative by Schmidt or Beckmann rearrangements. 7-Aminobenzo[6 ]thiophene can also be prepared from 7-hydroxybenzo[6 ]thiophene by a Bucherer reaction (70AHC(ll)l77). 

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