Three-membered heterocycles photochemical

The first chapter is devoted to the formation and subsequent modification of three-membered heterocycles—aziridines. Synthesis and properties of aziridinyl ketones, bi- and tricyclic aziridine derivatives, cycloaddition and photochemical reactions are described. The second chapter deals with... 

Photochemical preparations of three-membered heterocycles, such as epoxides and aziridines, are fewer in number than those of cyclopropanes. Some recent examples of synthetic interest are described below. 

Thermal and photochemical electrocyclic reactions in the chemistry of three-membered heterocycles 89KGS1443. 

Concerted cycloadditions are observed with heterocyclics of all ring sizes. The heterocycles can react directly, or via a valence tautomer, and they can utilize all or just a part of unsaturated moieties in their rings. With three-membered rings, ylides are common reactive valence tautomers. Open chain 47T-systems are observed as intermediates with four-membered rings, and bicyclic valence tautomers are commonly reactive species in additions by large rings. Very often these reactive valence tautomers are formed under orbital symmetry control, both by thermal and by photochemical routes. 

Both target compounds discussed in this review, kelsoene (1) and preussin (2), provide a fascinating playground for synthetic organic chemists. The construction of the cyclobutane in kelsoene limits the number of methods and invites the application of photochemical reactions as key steps. Indeed, three out of five completed syntheses are based on an intermolecular enone [2+2]-photocycloaddition and one—our own—is based on an intramolecular Cu-catalyzed [2+2]-photocycloaddition. A unique approach is based on a homo-Favorskii rearrangement as the key step. Contrary to that, the pyrrolidine core of preussin offers a plentitude of synthetic alternatives which is reflected by the large number of syntheses completed to date. The photochemical pathway to preussin has remained unique as it is the only route which does not retrosynthetically disconnect the five-membered heterocycle. The photochemical key step is employed for a stereo- and regioselective carbo-hydroxylation of a dihydropyrrole precursor. 

Photochemical or thermal extrusion of molecular nitrogen from ot-diazocarbonyl compounds generates a-carbonylcarbenes. These transient species possess a resonance contribution from a 1,3-dipolar (303, Scheme 8.74) or 1,3-diradical form, depending on their spin state. The three-atom moiety has been trapped in a [3 + 2] cycloaddition fashion, but this reaction is rare because of the predominance of a fast rearrangement of the ketocarbene into a ketene intermediate. There are a steadily increasing number of transition metal catalyzed reactions of diazocarbonyl compounds with carbon-carbon and carbon-heteroatom double bonds, that, instead of affording three-membered rings, furnish hve-membered heterocycles which... 

The non-planar polyene nature of azepines renders them susceptible to a variety of intra-and inter-molecular pericyclic processes. The azepine-benzeneimine valence isomerization has been discussed in Section 5.16.2.4, and the ring contractions of azepines to benzenoid compounds in the presence of electrophiles is covered in Section 5.16.3.3. In this section the thermal and photochemical ring contractions of azepines to bicyclic systems, their dimerizations and their isomerizations via sigmatropic hydrogen shifts are discussed. Noteworthy is a recent comprehensive review which compares and contrasts the many and varied valence isomerizations, dimerizations and cycloadditions of heteroepins (conjugated seven-membered heterocycles) containing one, two and three heteroatoms (81H(15)1569). 

Photochemistry of Three-membered Ring Heterocycles N. R. Bertonniere and G. W. Griffin, Org. Photochem. 105 (1973). 

The 1,3-dipole 96 has been generated exclusively by thermal and photochemical ring-opening reactions of other heterocycles, both with and without elimination of another molecule. Precursor heterocyclic rings have been five-,82-89 four-,90 or three-membered.91-102 Generally, the dipolarophile... 

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