Photo-Beckmann

Photohydration of 3-oxo-A -compounds in water-methanol (4 1) gave the corresponding 5a-hydroxy-3-oxo-compounds. ° Photolysis of 3-oxo-4-tosyl-oxycholest-4-ene (204) gave the hydroxy-compound (205) whereas the simple cyclohexenone derivatives (206) afforded the 3-aryl-2-hydroxycyclohexenones (207). It was established that photo-Beckmann rearrangement of A-nor-5a-cholestan-3-one oxime proceeded with retention of configuration at C-5. 

The photo-Beckmann rearrangements of the oximes of 5a- and 5/ -cholestan-6-ones preserve the stereochemical integrity at C(5), giving in each case an isomeric pair of lactams the rearrangement must occur without dissociation of the C(5)-C(6) bond. "... 

Mechanisms of the photo-Beckmann rearrangement have been advanced to involve the transformation of an excited singlet state of the oxime into an oxaziridine followed by the reorganization of the singlet excited intermediate to a lactam or amide. ... 

Oxaziridines have been proposed as intermediates in the photo-Beckmann rearrangement of oximes. A reinvestigation of the photochemistry of (+)-camphor oxime (60) has revealed that the... 

A novel variant of the photo-Beckmann rearrangement was utilized bv I a u ... 

Photochemical transpositions of steroids into the naphtho[2,1-f]quinoline system include a low-yielding photo-Beckmann (H.Suginome and T. Uchida, Bull, chem Soc. Japan, 1974, f , 687) and the conversion of 17-nitrite esters to the 17-a hydroxamic acids, reduction of which affords the lactam (S.H. Imam and B.A. Marples, Tetrahedron Letters, 1977, 2613). 

Suginome, H., Takahashi, H., Stereochemical Aspects of Photo Beckmann Rearrangement Stereochemical Integrity of Terminus of Migrating Carbon in Photo Beckmann Rearrangements of 5 a Cholestan 6 one and 5 3 Cholestan 6 one Oximes, Bull. Chem. Soc. Jpn. 1975, 48, 576 582. 

Suginome, H., Kaji, M., and Yamada, S., Photo Induced Molecular Transformations. 87. Regiospecihc Photo Beckmann Rearrangement of Steroidal a,P Unsaturated Ketone Oximes Synthesis of Some Steroidal Enamino Lactams, J. Chem. Soc., Perkin Trans. 1, 1988, 321 326. 

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). 

Photolysis of the oxime (684) of 3a,5-cyclo-5a-cholestan-6-one gave a very poor yield of ring b lactams,unlike the photo-Beckmann reactions of 5a- and 5j8-cholestan-6-one oximes, reported last year. ° The major products of photolysis of the 3a,5-cyclo-oxime were derived via initial conversion into the parent 6-ketone. ... 

Scheme S.11 Photo-Beckmann nxa/irirline rearrangement mechanisuL Scheme S.12 Total synthesis nf vitamin B 2-...

The photo-Beckmann reaction provides the regioisomeric amide compared to the amide t5 ically obtained from the traditional acid-catalyzed Beckmann rearrangement in which the carbon bond anti to the hydroxyl group of the oxime migrates. Instead, the photo-Beckmann... 

Scheme 5.11 Photo-Beckmann oxaziridine rearrangement mechanism...

Natural products are derived from plant, marine, and microbial sources and often possess biological activity. They are a source of new drug products each year and continue to play a significant role in the drug discovery and development process. Herein, we will discuss examples of recent natural product synthesis that utilize the previously discussed Beckmann reactions rearrangement, fragmentation, and photo-Beckmann. 

The total synthesis of (+)-sparteine (85) was achieved from 2,5-norbomadione 84 in 15 steps. The key steps in the synthesis were the two ring expansion reactions, one involving an intramolecular Schmidt reaction and the other a variant of the photo-Beckmann rearrangement ring-... 

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. ... 

Subsequent to this discovery, the formation of amides and lactams was found to be the major competing photoreaction of oximes Just and colleagues found that the irradiation of a methanolic solution of cyclohexanone oxime gave caprolactam.- We found that the chirality of the migrating carbon center a to the hydroxyimino group in the formation of lactams in the photo-Beckmann rearrangement is... 

Subsequent to some early studies concerning the photoreactions of a, 3-unsaturated ketone oximes, Bonet and collaborators reported that the photolysis of 17 3-hydroxyandrost-l,5-dien-3-one oxime in methanol gave the corresponding lactam arising from a photo-Beckmann rearrangement as outlined in Scheme 9. °... 

While the factor that controlled the regioselectivity of the photo-Beckmann rearrangement of steroidal a,P-unsaturated oximes remains obscure, the different regioselectivity in the lactam formation may originate from a stereoelectronic effect — the relative geometry between a lone pair on the nitrogen and the migrating CC bond in the oxaziridine intermediate. 

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