Photochemical 2 + 2 cydoadditions

Crimmins, M.T. and Reinhold, T.L. (1993) Enone-olefin [2 + 2] photochemical cydoadditions, in Organic Reactions,... 

Cyclobutene (3) was used on page 269 in a photochemical cydoaddition. It is made by ionic reactions from readily available adipic add (27) (see Chapter 27), This is something of an exception and cydisation by carbonyl condensations is not normally a recommended route to four-membered rings (cf. Chapter 29). 

Addition reactions such as addition of enolates to carbonyl compounds, photochemical cydoaddition, radical addition and elimination reactions such as dehydration can be carried out in microflow reactors. Addition-elimination reactions such as Wittig reaction, Homer-Wadsworth-Emmons reaction, Baylis-Hillman reaction. 

ST. Fukuyama, Y. Hino, N. Kamata, I. Ryu, Quick Execution of [2 + 2] type photochemical cydoaddition reaction by continuous flow system using a glass-made microreactor. Chem. Lett. 2004, 33, 1430-1431. 

The first photochemical [3 + 2] cycloadditions of arenes with aikenes were reported by two groups in 1966 [76, 77]. This is one of three types of cycloaddition reaction ortho, meta, and para) of arenes with aikenes, and is referred to as the meta-cydoaddition reaction. There are many combinations of arenes and aikenes and, indeed, a large number of reports and reviews have been made on the regiochemistry and stereochemistry [78]. Because this reaction can be used easily to construct a tricyclo[3.3.0.02,8]oct-3-ene framework, the organic syntheses of many natural compounds using this method have been attempted [79], in which the photochemical [3 + 2] cydoaddition plays a key role. 

The photochemical intramolecular [3 + 2] cydoaddition of arenes with a tethered alkene leads to polycydic compounds. This reaction is better suited to the synthesis of natural products than to intermolecular [3 + 2] cydoaddition. A carbonyl group in the tether fails to produce any good results, because this chromophore can quench the... 

The use of photochemical [3 + 2] cycloaddition in natural product synthesis (up to July 2006) has recently been reviewed by Chappell and Russell [79]. Subsequently, one report has been made concerning natural product syntheses up to October 2008. Wang and Chen have used a photochemical [3 + 2] cydoaddition to build the 5,6,7-tricydic skeleton of landfodilactone F and buxapentalactone (Scheme 4.62), which were isolated from the Chinese herb Schisandra [92],... 

Singh, V., Praveena, G.D., Karki, K., and Mobin, S.M. (2007) Cydoaddition of cydohexa-2,4-dienones, ring-closing metathesis, and photochemical reactions a common stereoselective approach to duprezianane, polyquinane and... 

The most characteristic photochemical reaction of aromatic compounds is their cydoaddition with alkenes. The intramolecular reaction is suitable for the synthesis of complex structures, such as those depicted in Scheme 9.49, where [3+2]-photocycloaddition leads to structures which resemble natural products (aphidico-line and stemoclinone). An interaction of the arene singlet excited state with the alkene ground state gives rise to the meta adduct [83, 84]. 

Thiophene, 3-pentadeuterophenyl-diemical shifts, 4, 730 Thiophene, 2-phenyl-oxidation, 4, 800 phototranspositions, 4, 743 rearrangement, 4, 42 reduction, 4, 775 synthesis, 4, 865, 914 UV spectrum, 4, 735 Thiophene, 3-phenyl-photochemical rearrangements, 4, 735 phototranspositions, 4, 743 Ismeier formylation, 4, 759 Thiophene, 2-piv oyl-Birdi reduction, 4, 775 Thiophene, polybromo-reactivity, 4, 829 Thiophene, polylithio-synthesis, 4, 831 Thiophene, (propaigylthio)-rearrangement, 4, 746 Thiophene, 2-(3-pyridinyl)-synthesis, 4, 781 Thiophene, 2-(5-pyrimidinyl)-synthesis, 4, 781 Thiophene, 3-pyrrolidinyl-cydoaddition reactions, 4, 68 with dimethyl acetylenedicarboxylate, 4, 788-789... 

Scheme 6.40 Photochemical [2 + 2] cydoaddition of cyclohex-2-enone (156) with vinyl acetate (147).

Besides [2+2]-cydoadditions, also the photochemical alkylation of fuUerenes is of interest. It includes a photo-induced electron transfer, which due to the fuUer-enes acceptor properties normally means the formation of a radical anion by reductive transfer. There are, however, examples of oxidative processes as well. 

In our analysis of the synthesis of rings of various sizes, we concluded (Chapter 29) that four-membered rings are uniquely difficult. For this reason, a special method, the photochemical 2 + 2 cycloaddition is often used to make four-membered rings. Some 2 + 2 thermal cydoadditions, particularly of ketenes (Chapter 33), and some ionic reactions (page 272) are also useful. 

Intramolecular [2+2]-photocycloadditions of cycUc a,P-unsaturated enones with remote double bonds have been extensively used to synthesize a variety of interesting compounds, including natural products. An analysis of the mechanism of the additions has also been carried out. 2-Pyrones having pendant enes and dienes undergo synthetically useful photocycloaddition processes to give tricyclic lactones and lactone-bridged cyclooctadienes by [2+2]-and [4+4]-cydoadditions, respectively. The photochemical reactions... 

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