Photocyclization reactions, limitations

The photocyclization reaction of 2-allylanilines, depending on the degree of alkylation at the N atom, has long been a subject of interest. When iV,iV-dialkylanilines 60a-62a were irradiated in methanol, the corresponding 2-cyclopropylanilines 60b-62b were formed in 73, 60 and 47% yield, respectively (equation 17)163. This reaction proceeds in the triplet state via a clean aromatic di-jr -methane rearrangement. However, no reaction occurs when 2-allyl-l-(Ai,Ai-dimethylamino)naphthalene is irradiated in methanol. Thus, this photochemical cyclopropanization of allylated anilines appears to have only a limited scope as preparative method, because it is successful only for a few substrates. 

The formation of benzene and pyridine presumably occurs via the bicyclo[4.2.0] valence isomers (175) and (176), although adducts of these structures with dienophiles were not obtained on heating or during irradiation. The difference in products in thermal and photochemical reactions could be explained either by interconversion of (173) and (174) and rate-limiting isomerizations of (173) to (176) and (174) to (175), or alternatively, rate-limiting isomerization of (173) to (174) and symmetry-allowed 4ir photocyclization of (173) to (175) (79JOC1264). 

Griesbeck et al. have developed the decarboxylative photocyclization (PDC) of phthalimido oo-alkylcarboxylates (15) as a versatile route to macrocyclic ring systems (16). The carboxylate serves as electron donor and C02 is eliminated during the course of the reaction. Applying this concept, the syntheses of medium- and macrocyclic amines, polyethers, lactams, lactones, as well as cycloalkynes were accessible but the limitations... 

Accumulation of the product may increase the rigidity of the medium, diminish the rate of the reaction, and ultimately limit the chemical yield of a solid-state reaction. In a recent study on the photocyclization of three chloro-substituted a-oxoamides (57-59, Scheme 36) by Hashizume et al. it was speculated that limited chemical yields may be attributed to increased internal strain caused by the... 

Photocyclization is not limited to two alkene partners, and one important reaction involves cycloaddition of a carbonyl and an alkene. This reaction was discovered by Paterno and Chieffi in 19093U and expanded by Biichi et al. 3l3 thus it is called the Paterno-Biichi reaction.319 The product is an oxetane (378) and the reaction is believed to proceed via a diradical intermediate (377). The regioselectivity of the reaction can be explained by molecular orbital considerations, however. Electron-rich carbonyls of both aldehydes or ketones can be used. The alkene moiety is usually electron rich but this is not a requirement. 

The cyclization of chloroamines in concentrated sulphuric acid (Hofmann-Loffler-Freytag method) has been known for a number of years. Photocyclization in the presence of triethylamine of a chloroamine prepared in situ from an amine and N-chlorosuccinimide has now been found to produce a similar result and is exemplified in Scheme 121. The reaction is probably limited to the preparation of... 

Enantioselective photoreactions induced by CPL have received considerable interest, particularly in regard to answering the question of how enantiomerically enriched material in the universe has been generated. From a synthetic point of view, however, this method has seen limited success. In the few asymmetric fixation reactions examined (in most cases, photocyclizations), only low enantiomeric excesses were achieved. A single example of an asymmetric photocycloaddition has been reported. " " By preferential excitation of the (-)-antipode of racemic cyclopentanone ( )-51 by/-CPL (351- to 363-nm laser) in the presence of acetylene, enantiomerically enriched (1% ee) (-)-52 was obtained (Scheme 17). The unreacted starting material 51 was optically enriched in the (-l-)-enantiomer. This method provided product of sufficient optical purity to examine the chiroptical data. 

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