Dienones, photocycloaddition

In the category of more elaborate calculations are an SCF—PMO treatment of the reaction of cyclopentadiene with cyclopropene 33>, and calculations including overlap on the photodimerization of linear steroidal dienones. 31> A very fine, detailed theoretical study of thymine photodimerization has been recently presented. 47> Other PMO calculations on photocycloadditions of biochemical interest have also been published. 48>49>... 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

Cyclobutane formation via light-induced [2 + 2] cycloaddition is probably one of the best studied photochemical reactions and has been reviewed thoroughly up to 1972 (Houben-Weyl, Vols. 4/5 a and 4/5 b). The most important types of C —C double-bond chromophores undergoing such reactions arc alkenes, 1,3-dienes, styrenes, stilbenes, arenes, hetarenes, cycloalk-2-enones, cyclohexa-2,4(and 2,5)-dienones, 1,4-benzoquinones, and heteroanalogs of these cyclic unsaturated carbonyl compounds. For p notocyciodimerizations see Houben-Weyl, Vol. 4/5 a, p 278 and for mixed [2 + 2] photocycloadditions of these same chromophores to alkenes see Section 1.3.2.3. 

Orfanopoulos et al. studied the photochemical reaction of alkenes, aryalkenes, dienes dienones, and acyclic enones with [60]fullerene to obtain various substituted cyclobutylfullerenes [240,241,243,247], For example, the photocycloaddition of cis- and Irans-1 -(p-mcthoxyphenyl)-1 -propenc 68 to C6o gives only the trans [2 + 2] adducts (Scheme 27), thus the reaction is stereospecific for the most thermodynamically stable cycloadduct. A possible mechanism includes the formation of a common dipolar or biradical intermediate between 3C o and the arylalkene. Subsequent fast rotation of the aryl moiety around the former double bond leads exclusively to the trans-69 [2 + 2] adduct. Irradiation of this product, yielded 90% trans-68,10% cis-68 and cycloreversion products. Thus, a concerted mechanism can be excluded because the photocycloreversion is expected to give the trans-68 as the only product. These results can be explained by the formation of a common dipolar or diradical intermediate. Similarly, cycloreversion products from C6o and tetraalkoxyethylene... 

A number of model esters related to lac resin have been prepared. These esters have the basic cedrane skeleton (149) or the C-2 epimeric structure. In an approach to the synthesis of cedrene (152), the intramolecular photocycloaddition of the dienone (150) was carried out at low temperature. A major tricyclic compound (151) was obtained and its structure was verified by X-ray analysis of its anisylidene derivative. Further work is required to bring about the rearrangement of a derivative of (151) to give cedrene. Further details on the hydroxylation (with O3) and chlorination (with PhICl2) of cedrol, patchoulol, and their derivatives have been presented. The results of this study are in keeping with those obtained previously. 

Matlin, A. R., Photocycloaddition/Trapping Reactions of Cross conjugated Cyclic Dienones Capture of Oxyallyl Intermediates. In Horspool, W. M., Lenci, F. (eds), CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn, CRC Press LLC, Boca Raton, FL, 2004, Chapter 81, PP- 1 12. 

An example of product-controlled [2 -l- 2] is in Schultz et al. s work with [2 -I- 2] photocycloaddition of dienones [113]. In this work, cyclohexadienone 105 was irradiated at -78 C in pentane to yield a single product 106, which was found to have the isopropyl group in the more stable exo orientation as shown in Scheme 34. An extension of this study showed that the isopropyl group governed the regioselectivity as well as the stereoselectivity of the reac-... 

Diastereomer-differentiating intramolecular [2+2] photocycloaddition has been reported for the diastereomeric 2,6-dienones (105) and (106). Both direct and sensitised irradiation of (105) yields (107) whereas (108) is formed from (106). Reinvestigation, at -78 °C, of the photocycloadditions of 1-cyanonaphthalene (1-CN) and 2-cyanonaphthalene (2-CN) with 1,3-cyclohexadiene has suggested that the primary major singlet-derived products are the corresponding exo-[4+4]adducts, (109) and (110) respectively, and the s>>/i-[2+2]adducts, (111) and (112) respectively. Consideration of primary and secondary orbital interactions has been used to interpret these observations. ... 

The following additional examples offer a hint of the utility of organic photochemistry in s)mthesis. A [2 + 2] photochemical cycloaddition was a key step in the synthesis of cubane. S)mthesis of 2-bromocyclopenta-2,4-dienone resulted in a spontaneous Diels-Alder reaction, which produced 132. The monoketal of the dimer (133) underwent intramolecular photocycloaddition to produce 134 (equation 12.85). The Favorskii reaction of 134 gave 135, which was decarboxylated to 136 (equation 12.86). Hydrolysis of the ketal gave 137. Subsequent Favorskii reaction to 138 and then decarboxylation produced cubane, 139. Similarly, de Meijere and co-workers reported that the intramolecular [2 + 2] photochemical cycloaddition of 140 produced octacyclopropylcubane (141, equation 12.87) Meder and coworkers utilized the reaction sequence in equation 12.88 to convert the diazo compound 142 into tetra-f-butylcyclobutadiene (143) and tetra-f-butyltetra-hedrane (144). " ... 

Very few compounds containing the [2,2,0,0 ] ring system have been prepared. The ketone (838) has been made by photocycloaddition of a mixture of chlorohexa-dienones and cyclization with KOBu of the appropriate [2,1,1] epimer. Above 50 °C, (838) isomerizes to cyclobutenylketen (839) while cleavage to the known acid (840) occurs with KOBu in wet DMSO. The ketone shows two carbonyl bands in the i.r., at 1775 and 1759 cm and H-1 appears at 63.58 and all other protons between 51.9 and 2.9 in the n.m.r. spectrum. 

An extensive review of 2 + 2-photocycloadditions of dienones and quinones has been published. The photocycloaddition of heterocyclic 2,3-diones (18) with electron-rich alkenes in the presence of the photosensitizer benzophenone yields 2-1-2-cycloadducts (19) and (20) with high regio- and stereo-selectivity (Scheme 1) Time-resolved spectroscopy has shown that in the photochemical cycloaddition between stilbenes and quinones, the (singlet) ion-radical pair [S ", Q+ ] is the primary reaction intermediate and therefore establishes the electron-transfer pathway for this typical Paterno-Biichi transformation. A kinetic study of the Paterno-Biichi cycloaddition of stilbene to chloranil shows that solvent polarity and donicity control the formation as well as the reaction path of the ion-radical. The photoirradiation of chloranil with... 

Photocycloaddition/ Trapping Reactions of Cross-Conjugated Cyclic Dienones Capture of Oxyallyl Intermediates... 

Suishu, X., Obata, X, Shimo, X. and Somekawa, K., Species specificity and regio-selectivities in the photocycloadditions of cyclic conjugated enones and heterocycHc conjugated dienones and then-frontier MO analysis, Nippon Kagaku Kaishi, 167, 2000 Chem. Abstr., 132, 293339m, 2000. 

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