Photocydoaddition reactions

One of the first reported photocydoaddition reactions of benzene was with the highly electron-deficient alkene, maleic anhydride. 

De Keukeleire, D., He, S.-L., Blakemore, D., and Gilbert, A. (1994) Intramolecular photocydoaddition reactions of 4-phenoxybut-l-enes. Journal of Photo-... 

New York, pp. 33-100 (d) Bach, T. (1998) Stereoselective intermolecular [2 + 2] photocydoaddition reactions and their application in synthesis. Synthesis, 683-703 (e) Mattay, J., Conrads, R., and Hoffmann, R. (1996) [2 + 2] Photocydoaddtions of a.S-unsaturated carbonyl compounds, in Houben-Weyl, Methods of Organic Chemistry, Vol. E 21 Stereoselective Synthesis (eds G. Helmchen, R.W. Hoffmann, J. Mulzer and E. Schaumann), Thieme Verlag, Stuttgart, pp. 3085-3178. 

The regioselectivity of intramolecular [2 + 2]-photocydoaddition reactions is predictable if five-membered ring formation is possible in the formation of biradicals of type C or C (rule of five, vide supra). If five-membered ring formation is not feasible, then six-membered rings are most readily formed. The facial diastereo-selectivity is efficiently controlled by a stereogenic center in the cyclopentenone if the intramolecular alkene is attached via a tether to this stereogenic center. The key step 16 —> 17 in the stereoselective synthesis of (—)-incarvilline (18) illustrates the point (Scheme 6.7) [28]. The side chain attached to C-4 in the cyclopentenone 16 carries the terminal alkene, which reacts intramolecularly with perfect regio- and diastereoselectivity to cyclobutane 17. 

Chiral cyclohexenones have been frequently employed in intermolecular [2 + 2]-photocydoaddition reactions directed towards natural product synthesis. A further case in point is the reaction of cydohexenone 32 with trans-1,2-dichloroethylene... 

The 3(2H)-furanones are - as the other 4-hetero-2-cyclopentenones - normally 2,2-disubstituted to avoid enolization to the respective 3-hydroxyfuran. If one of the substituents is an alkenyl side chain, then intramolecular [2 + 2]-photocydoaddition reactions are possible with the regioselectivity being dependent on the chain length (Scheme 6.20). The allyl-substituted substrate SO (n— 1) gave predominantly the formal straight product SI [62], while the butenyl-substituted substrate 50 (n = 2) resulted in formation of the crossed product 52 [63]. 

Lactams of type A4 have been successfully employed in intramolecular [2 + 2]-photocydoaddition reactions. Direct excitation has been found most useful to achieve sufficient conversion in these reactions. The reaction of [i-substituted a,P-unsatu-rated lactams 95 in CH2C12 gave the expected products 96 of five-membered ring closure in decent yields (Scheme 6.34) [92]. 

Pyridones can react photochemically along several reaction channels [94]. Besides [4 + 4]-photodimerization and 4jt]-ring closure, [2 + 2]-photocydoaddition reactions are possible in an a,P- or in a y,8-mode relative to the carbonyl carbon atom. With regard to the former reaction pathway, the [2 + 2]-photocydoaddition of olefins to 4-alkoxypyridones appears to be synthetically most useful (vide infra). 

Enantioselectivity control in a [2 + 2]-photocydoaddition reaction was achieved in a chiral, self-assembled host. Fluoranthenes and N-cyclohexylmaleimide underwent an intramolecular reaction in a cage made of M6L4, with the metal M being palladium (II) coordinated to a chiral diamine, and the ligand Lbeing2,4,6-tris(4 -pyridyl)-l,3,5-triazine. Up to 50% ee was observed [117]. 

Selig, P. and Bach, T. (2006) Photochemistry of 4-(2 -aminoethyl) quinolones enantioselective synthesis of tetracyclic tetrahydro-laH-pyrido [4, 3 2,3]-cydobuta[l,2-c] quinoline-2,11 (3H, 8H)-diones by intra- and inter-molecular [2 + 2] photocydoaddition reactions in solution. Journal of Organic Chemistry, 71, 5662—5673. 

Giilten, S., Sharpe, A., Baker, J.R., and Booker-Milburn, K.I. (2007) Use of temporary tethers in the intramolecular [2 + 2] photocydoaddition reactions of tetrahydrophthalimide derivatives a new approach to complex tricydic lactones. Tetrahedron, 63, 3659-3671. 

Haywood, D.J. and Reid, S.T. (1979) Intramolecular photocydoaddition reactions of 4-alkenyloxycoumarins. Tetrahedron Letters, 20, 2637—2638. 

Miyauchi, H., Ikematsu, C., Shimazaki, T., Kato, S., Shinmyozu,T., Shimo,T., and Somekawa, K. (2008) One-pot synthesis of macrocyclic compounds possessing two cydobutane rings by sequential inter-and intramolecular [2 + 2] photocydoaddition reactions. Tetrahedron,... 

Next, the homochiral crystals were utilized for subsequent diastereoselective reactions. Diastereoselective 2+2 photocydoaddition reaction of amide 20 with methacrylonitrUe was examined. The reaction with methacrylonitrile proceeded effectively, stereo-, and regiospecifically. The amide 20 before crystallization was irradiated in the presence of methacrylonitrile with a high-pressure mercury lamp at 20°C until most of the starting amide was consumed (2 hrs). The photochemical reaction occurred effectively, and 2 + 2 cycloadducts were obtained in 100% chemical yields both diastereomers were endo isomers, minor (lS,2aR,8bR)-23, major (lR,2aS,8bS)-23, and the de value was -25%. Since epamerization was not observed at 20°C, it seems that the de value of the photoproducts should be attributed to the ratio of the diastereomers of the amide 20 before crystallization (-20% de). 

Furthermore, we examined a photocydoaddition reaction using the homochiral molecular conformation converged by CIDT. The crystals were expected to be compwsed of a single diastereomer of (S,aR)-conformation, and the epimerization in the solution caused by the bond rotation between the quinolone and the carbonyl group was restricted at room temperature. In other words, conformation in the crystals may be retained as frozen after dissolving them in the solvent at room temperature, and molecular homochirality can be... 

Bach, T. and Spiegel, A. (2002) Stereoselective total synthesis of the tricyclic ( )-kelsoene by an intramolecular Cu(l)-catalyzed [2+2]-photocydoaddition reaction. Synlett, 1305-1307. 

Abe, M. and Nojima, M., The Paterno-Biichi photocydoaddition reactions of silyl ketene acetals controlling factors on the regio- and stereoselectivity in the oxetane formation,/. Synth. Org. Chem. Jpn., 59, 855, 2001. 

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