Synthesis by Photoaddition

This section is concerned with the synthesis of heterocyclic systems by intermolecular addition. The topic can be conveniently divided into a discussion of 1,2- and 1,4-cycloaddition processes. 

The synthesis of oxetanes by the photochemical 1,2-cycloaddition of the carbonyl function in aldehydes and ketones to alkenes [Eq. (69)] was first reported by Paterno274 in 1909, and later reinvestigated by Biichi 275 in 1954. This reaction has recently been extensively reviewed.276, 277 The formation of the oxetane is apparently the result of addition of excited n, n triplet carbonyl to an alkene, although for certain aromatic aldehydes and ketones the mechanism is less clear.278 

When the reaction is applied to an unsymmetrically substituted alkene, the major photoproduct or products are those arising from the most stable diradical intermediate. This is illustrated in the irradiation of benzaldehyde in 2-methylbut-2-ene in which the principal products are the stereoisomeric oxetanes arising from the same diradical intermediate [Eq. (70)].278 The oxetanes resulting from addition in the alternative sense [Eq. (71)] are minor products, as are other products arising by allylic hydrogen abstraction. 

The addition of aliphatic aldehydes and ketones to alkenes is less successful as a preparative procedure for oxetanes. An essential requirement for addition is that the triplet energy of the alkene must be considerably greater than that of the carbonyl. If this condition is not fulfilled, energy transfer to the alkene can occur,279 sensitizing, for example, dimerization of the alkene. This is clearly illustrated 280, 281 for norbornene (264) which on irradiation in the presence of benzophenone (ET 68.5 kcal/mole) forms the adduct 265 photolysis in acetone (ET 75 kcal/mole) affords only norbornene dimers (266 and 267), whereas acetophenone, which has intermediate triplet energy (Et 73.6 kcal/mole) forms both oxetanes and norbornene dimers. 

The photocycloaddition of aliphatic ketones to the electron-deficient double bonds of frans-l,2-dicyanoethylene282,283 and maleic anhydride 282 [Eq. (72)] is believed to proceed by an alternative mechanism involving nucleophilic attack on the double bond by the n, ir singlet state of the carbonyl, 

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