Unsaturated photochemical cycloaddition with

Participation in Pericyclic Reactions. 2-MP is an active participant in pericyclic reactions. A few examples include a substitution-dependent ene reaction with an alkynyl Fischer car-bene complex (eq 8), a [2-p3] reaction with 2-amino-1,4-naphthoquinone (eq 9), a solvent-dependent photochemical cycloaddition with c>-naphthoquinones, j and reaction with 2-nitro-1,3-dienes to provide a,p-unsaturated 1,4-diones (eq 10). ... 

There are a few reports in the literature in which the carbonyl group has been formally replaced by another unsaturated function. Thio-benzophenone (268) undergoes photochemical 1,2-cycloaddition to a-phellandrene (269) to give the thietane (270), together with the sulfur heterocycles (271 and 272) formed by 1,4-cycloaddition.298 Isoprene, cyclopentadiene, and 1,4-diphenylbutadiene also undergo 1,4-cycloaddition with thiobenzophenone, but 1,3-cyclooctadiene... 

Knoevenagel products of aliphatic aldehydes do not undergo photochemically induced [2 + 2] cycloaddition with alkenes, but are isomerized instead to the 3,7-unsaturated isomers. On the other hand, benzylidene-Meldrum s acids give two isomeric cycloadducts upon irradiation in the presence of cyclohexene. Unsymmetrical Knoevenagel products such as the benzylidenepyrazolones undergo cisitrans isomerization on irradiation. The photostationary state of (231) is = 2 1, whereas the thermo-... 

The most useful intermolecular [2tt+2tt] cycloadditions from a synthetic point of view involve alkenes and cyclic a,6-unsaturated carbonyl compounds. These reactions are discussed in more detail in Section 6.3.2 of Part B. Scheme 12.1 lists some examples of photochemical cycloaddition and electrocyclic reactions of the type that are consistent with the predictions of orbital symmetry considerations. 

Carbonyl ylides (155), generated photochemically from the corresponding a,p-unsaturated, y,5-epoxy dinitriles, undergo regioselective 1,3-dipolar cycloaddition with ethyl vinyl ether, leading predominantly to the exo-adduct. The very low reactivity of seven-membered ring ylides (155) is rationalised by AMI calculations which show the ylide reactivity to depend on the LUMO energy and on the Cy.Cs separation. 

For the examples reported so lar, usually in the presence of die alkene the azomethine ylide is generated in situ by photochemical disrotatory ring-opening of the aziridine (254 nm) in dioxane yielding pyrrolidines. (Obviously, thermal conrotatoiy ring-opening of aziridines demands conditions, which oppose an efficient stereocontrol of the subsequent cycloaddition with a,P-unsaturated carboxylic ester derivatives.)... 

Another class of molecules that are quite prone to undergo photochemical cycloadditions are a,/3-unsaturated carbonyl compounds. The reactive state in photochemical cycloadditions of or,-unsaturated ketones is believed to be the n-Tt triplet." Conservation of spin then implies that the initial intermediate is a triplet diradical, and the reaction need not be stereospecific with respect to the alkene component." The reaction has been most thoroughly studied in the case of cyclopen-... 

Bicyclo[3,2,0]heptane Derivatives.— These are principally prepared by the [2 -I- 2] cycloaddition of a cyclopentene with another unsaturated unit. Typical combinations which have been routinely applied in syntheses are photochemical addition of a cyclopentene to an olefin or acetylene and thermal addition of a cyclopentadiene with an olefin, a keten, or an acetylene. In the last case, cycloaddition with an alkyne is complicated both by competition between [2 -h 2] and [2 + 4] modes of addition and by the possibility of a second addition of cyclopentadiene to the initially formed bicyclo[3,2,0]heptadiene. The results for diphenylacetylene and cyclopentadiene are shown in Scheme 31. 

Photolysis of epoxy ketone 54, obtained upon oxidation of unsaturated ketone 53 with m-chloroperbenzoic acid (MCPBA) in dry benzene, afforded the aldehydoketene 55. The ketene cis-55, firstly undergoes photochemical isomerization to the trans-isomer on prolonged irradiation, and then undergoes an intramolecular [4 - - 2] cycloaddition on heating to afford the bicyclic lactone 56 (Scheme 19) (1978JOC3314). 

It is also worthwhile comparing the intramolecular photochemical cycloaddition reactions of ethylenic aldehydes and ketones with free radical intramolecular additions. For instance, irradiation of 5-hexen-2-one (470) (Scheme 161) in the gas phase gives the oxetane 471 as only cyclized product, as expected from the known photochemical intermolecular reaction between olefins and ketones. If the irradiation is conducted in solution 470 gives 471 (26%) and 472 (18%). With other y,< -unsaturated ketones, the bicyclic compound analogous to 472 may become the major product. With 2-allylcyclanones such as 473 (Scheme 161) bicyclic compounds are obtained (80% yield) as a mixture of 474 and 475, with 475 being the major product, but such compounds are difficult to isolate. " In the same manner, selective irradiation of the carbonyl group of 2-acyl-2,3-dihydro-4/f-pyrans (476) leads exclusively (23% yield) to exo-brevicomin (477) (a sex attractant), neither oxetane formation nor Norrish type II reaction being observed. The formation of the compounds 472, 475, and 477 which was considered as unexpected... 

Kosugi, H., S. Sekiguchi, R. Sekita, and H. Uda Photochemical cycloaddition reactions of a,P-unsaturated lactones with olefins, and application to synthesis of natural products. Bull. Chem. Soc. Japan 49, 520—528 (1976). 

Concerted cycloadditions are observed with heterocyclics of all ring sizes. The heterocycles can react directly, or via a valence tautomer, and they can utilize all or just a part of unsaturated moieties in their rings. With three-membered rings, ylides are common reactive valence tautomers. Open chain 47T-systems are observed as intermediates with four-membered rings, and bicyclic valence tautomers are commonly reactive species in additions by large rings. Very often these reactive valence tautomers are formed under orbital symmetry control, both by thermal and by photochemical routes. 

Recent synthetic applications of the photochemical [2 + 2] cycloaddition of unsaturated sulfones have been noted. Musser and Fuchs84 have effected an intramolecular [2 + 2] addition of a 6-membered ring vinyl sulfone and a five-membered ring vinylogous ester in excellent yield, as part of a synthetic approach to the synthesis of the mould metabolite, cytochalasin C. The stereospecificity of the addition was only moderate, however, and later problems with this synthetic approach led to its abandonment. Williams and coworkers85 have used the facile [2 + 2] photoaddition of 73 and... 

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