Photo Diels Alder reaction

Cycloadditions - Photo-Diels-Alder Reactions Using Oxygen... 

Aromatic compounds can participate in both [2+2] and [4+2] photocycloaddition reactions with uracil derivatives to give either benzocyclobutane or ethenoquinazoline (barrelene) derivatives, which can then undergo a number of subsequent photochemical reactions. The products obtained are dependent upon the reaction conditions, and thus the photocycloaddition reaction between naphthalenes 470 and l,3-dimethyl-5-fluorouracil 471 in cyclohexane gave 4a-fluoro-5,10-ethenobenzo[/]quinazolines 472 as products as a result of a [4+2] photocycloaddition (photo-Diels-Alder) reaction <2002TL3113, 2003H(61)377>. 

Irradiation of 4-(but-3-enylidene)-2-phenyl-5(477)-oxazolone 733 leads to the [4 + 2] adduct 734 derived from 733 acting as both a diene and a dienophile in a solid-state photo Diels-Alder reaction (Scheme 7.226). °... 

Such additions are very rare, apparently because the allowed ir2s + n2s process is much more accessible (see Chapter 11.3). Most photo Diels-Alder reactions that have been studied involve attack of a triplet diene on a singlet olefin and thus proceed by an intermediate biradical. Both intramolecular75 and intermolec-ular76 concerted Diels-Alder reactions have, however, recently been observed. For example, photolysis of 41, through a w4s + Tt2a transition state, stereo-specifically forms 42. Similarly, 43 is converted to 44. Nevertheless, not all intramolecular Diels-Alder reactions brought about by direct irradiation are stereospecific.77... 

However, 2-nitrothiophene (176), but also 2-acetylthiophene and thio-phene-2-carbaldehyde, irradiated in the presence of phenylacetylene, gave the corresponding diphenyl derivative ortho substituted 177. The reaction is probably a photo Diels-Alder reaction with extrusion of sulfur from the adduct to give the observed product (95TL6567). 

Three types of cycloaddition products are generally obtained (Sch. 1). While [2+2] (ortho) and [2+3] (meta) cycloaddition are frequently described, the [2+4] (para or photo-Diels-Alder reaction) pathway is rarely observed in benzene ring systems. With naphthalene systems however, the para cycloaddition occurs more frequently [6,8]. The photo-Diels-Alder reaction and other photocyclization reactions are also observed with anthracene derivatives and higher condensed aromatic compounds. However, these reaction are not treated in this chapter since they are caused by the particular photophysical and photochemical properties of these compounds [6,9]. 

The photo Diels-Alder reaction of a-acetylnaphthalene 82 with the chiral oc-enaminonitril 83 yielded the cycloadduct 84 with excellent diastereoselec-tivity (Sch. 16) [57]. The intermediary formed biradical Y is particular stable due to delocalization of the radical on the aromatic moiety and to a captodative effect on the enamine part. The chiral induction occurred in two steps [58]. In the first step, a stereogenic center is created in the a-position of the acyl group. In the second step of the diastereoselection, one of the two diasteromeric intermediates undergoes preferentially cyclization to yield the final product 84, while the other one is more readily decomposed to form the starting material. For a more detailed discussion of the mechanism see Ref. [59]. 

Enantiodifferentiating photo-Diels-Alder reactions of cyclohexadiene 96 with p-substituted p-methylstyrenes 100-103 sensitized by axially chiral tetracy-anobinaphthyl 105 and tetracyanobianthryl 106 gave the endo-[4 + 2] cycloadduct 104 in moderate ees (Scheme 18) [69],... 

This review article deals primarily with addition reactions of nucleophiles, electrophiles, and neutral radicals to photochemically generated radical ions of organic compounds and some organometallic compounds. Photocyclodimerizations of electron-rich alkenes, photo-Diels-Alder reactions between alkenes and alkadienes via dimer or heterodimer radical cations, and photocycloadditions via triplexes are also included. 

Schuster proposed a triplex as a key intermediate for the inter- and intramolecular photo-Diels Alder reaction in nonpolar solvents in the presence of an electron accepting sensitizer [105-107]. This photoreaction can be applied to an enantioselective photo-Diels Alder reaction between 1,3-cyclohexadiene and propenylbenzene by the use of a chiral sensitizer (Scheme 32) [108]. 

SMP is a useful chiral auxiliary in various cycloaddition reactions. Chiral 2-amino-1,3-dienes have been used in the Diels-Alder reaction with 2-aryl-1 -nitroethylenes, and 5-ary 1-2-methyl-substituted 4-nitrocyclohexanones were obtained in excellent enantiomeric purities (ee=95-99%) and diastereoselectiv-ities (ds = 75-95% eq 6). The photo-Diels-Alder reaction of SMP acrylonitrile with 1-acetylnaphthalene has been carried out. After hydrolysis of the adduct, the 1,4-diketone was obtained in excellent enantiomeric purity (ee > 97% eq 7). 

Electron-transfer photosensitization (2,4,6-triphenylpyrylium tetrafluorobor-ate) is reported to induce a photo-Diels Alder reaction between A-arylimines (55) and styrene or a-methylstyrene. The reaction is considered to proceed by attack of the styrene radical cation onto the arylimine and affords both dia-stereoisomers (56) and (57) in reasonable yields, although amounts of the quinoline (58) and the amine (59) are formed in some cases. 

FIGURE 20.22 Absorption of a photon results in the formation of a new HOMO, 4>3.The HOMO(dje e)-LUMO(dienDphiie) interaction involves one antibond.The photo-Diels—Alder reaction is forbidden by orbital s)mnmetry. 

Ohkura, K., Noguchi, Y, and Seki, K., Acid-catalyzed photoreaction of 6-chloro-1,3-dimethyluracil in frozen benzene formation of photocycloadducts and their isomerization through photo-Diels-Alder reaction. Heterocycles, 47, 429,1998. 

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