Photochemical Beckmann rearrangement

Nitrones derived from 2-azabicyclo[5.3.0]decane give quinolizidine compounds by photochemical Beckmann rearrangement which implies simultaneous ring expansion and ring contraction reactions. Intramolecular Schmidt reactions in 2(4-azidobutyl)-cyclopentanones also give quinolizidinone derivatives by ring expansion. Examples of both types of reactions are given in Sections 12.01.11.1 and 12.01.11.3, respectively. 

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. 

There are a number of thermal and photochemical reactions for which oxaziridine intermediates have been proposed but never isolated. These include, among others, the photochemical Beckmann rearrangement of oximes, many photochemical reactions of aromatic A -oxides, and the thermal rearrangement of nitrones to amides. A brief discussion of the first two seems warranted in this review because they have been studied extensively and some strong inferential evidence for oxaziridine intermediates has been obtained. 

The first photochemical Beckmann rearrangement of aromatic aldoximes was reported in 1963. Subsequently, cyclohexanone oxime was shown to rearrange, upon photolysis, to caprolactam. Although the presence of oxaziridines in the solutions of photolyzed oximes was demonstrated, no oxaziridines have been isolated from these reaction mixtures presumably because of the general instability of oxaziridines that have no substituents on the ring nitrogen. The qualitative results are consistent with the intermediacy of oxaziridines in the photolysis of oximes to amides, yet the possibility of the reactions following other pathways cannot be ruled out. ... 

Evidence that the photochemical Beckmann rearrangement of oximes proceeds through an oxaziridine intermediate ccmes fran the observation that the relative migratory tendencies of H and aryl groups in the rearrangement of RCH NCH (R=Ph, 4-MeOCgH, 4- 1 ) are similar to those of (352)25. ... 

Acetic acid irradiation Photochemical Beckmann rearrangement Lactams from cyclic oximes... 

Toray. The photonitrosation of cyclohexane or PNC process results in the direct conversion of cyclohexane to cyclohexanone oxime hydrochloride by reaction with nitrosyl chloride in the presence of uv light (15) (see Photochemical technology). Beckmann rearrangement of the cyclohexanone oxime hydrochloride in oleum results in the evolution of HCl, which is recycled to form NOCl by reaction with nitrosylsulfuric acid. The latter is produced by conventional absorption of NO from ammonia oxidation in oleum. Neutralization of the rearrangement mass with ammonia yields 1.7 kg ammonium sulfate per kilogram of caprolactam. Purification is by vacuum distillation. The novel chemistry is as follows ... 

Solid-state photochemistry of (—)-2-chloro-2-nitrosocamphane 275 was studied145 by irradiation of the blue-crystal with red light to invert the configuration at C(2) (equation 123). This also causes a photochemically initiated Beckmann rearrangement to form chloroxime 276 to give nitroxide radical 278 (equation 124). The intermediate chloro oxime 276 is proposed to arise from the njr excitation and is believed to be the common intermediate for the photo-epimerization and Beckmann rearrangement. Extended... 

The photochemically induced oxidation of the isoxazolidines (220), (221), and (222) has been investigated.Typically, the major product from the photolysis of (220) in acetone was the nitro-compound (223), whereas in benzene it was the dimer (224). In an attempt to establish the general mechanism, photo-Beckmann rearrangements were studied for a series of oximes. In each case a pair of... 

Further study of the photo-Beckmann rearrangement of oximes suggests that the reaction occurs by way of a concerted pathway from an excited singlet oxaziridine intermediate.84 Thus, in the rearrangement of syn- or anti-5jS-cholestan-6-one oximes (107), which are photochemically interconvertible, the original configuration at C-5 is retained in both lactam products (108) and (109). 

The photochemical transformations of anomeric diazides, 1-methoxy azides, and cyano azides were studied by French and Japanese research groups. A nitrene proved to be the key intermediate. Its stability is dependent on stereochemical and structural features of the molecule, and in particular the anomeric configuration exerts an important influence. Thus for the 1-methoxy azides 323a and 323p it was shown that both migration of the methoxy group as well as the Beckmann rearrangement proceed differently for the anomers. 

Lessard, Spino, and coworkers developed the formation of iV-heterocycles from cycloalkanones using a photochemical ring contraction reaction coupled with a Beckmann rearrangement ring expansion." Their methodology is an impressive approach to transform disubstituted cycloalkanones into N-heterocycles of the same ring size tSchemp The... 

Useful ring expansion reactions occur in the photo-Beckmann rearrangements, the photolysis of aromatic N-oxides and pyridinium N-ylides, and in the photochemical rearrangement of N-alkylphthalimides. ... 

En gros, the N insertion reactions can be subdivided into a Beckmann type and a Schmidt-type rearrangement part. Furthermore, some photochemical rearrangements of chiral oxaziridines are known to generate a range of optically active lactams. 

The well-known photochemical rearrangement of citral has been shown to be temperature-dependent at elevated temperatures the major products are the formyl-migrated (65), with the c -aldehyde predominating. " Sensitized photochemical rearrangement of the corresponding geranonitrile at 132 °C yields predominantly (66), a known acid-catalysed Beckmann fragmentation product of... 

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