1.2- Dioxetanes 1,3-diene cycloaddition

In a thorough study on photooxidation of 2,5-dimethyl-2,4-hexadiene (455) it was found that 1,2-dioxene 456, 1,2-dioxetane 457, hydroperoxy dienes 458 and 459 and, when methanol was used as solvent, also hydroperoxy(methoxy)octene 460 are formed (Scheme 124) . Product distribution was found to be highly solvent dependent. These results led investigators to postulate a mechanism involving the intermediacy of perepoxide 461 and zwitterion 462 (Scheme 124). Accordingly, the product of [4-1-21-cycloaddition 456, the product of [2 + 2]-cycloaddition 457, as well as the products 458 and 459 deriving from ene-addition would originate from polar intermediates 461 and... 

Singlet oxygen ( Ag) generated by triplet sensitization (Section 6.7.1) can readily oxygenate alkenes, dienes and aromatic compounds the reactions are formally interpreted as [2jt + 2jt] ([2 + 2]) or [47t + 27t] ([4 + 2]) cycloaddition reactions, producing 1,2- or 1,4-endoperoxides, respectively. Scheme 6.250 shows examples of the formation of 1,2-dioxetane (513), 3,6-dihydro-l,2-dioxine (514) and 2,3-dioxabicyclo[2.2.2]octa-5,7-diene (515). 

As a model substrate, 2,5-dimethyl-2,4-hexadiene 51 has been intensively investigated by several research groups. Depending on the solvent polarity, hve peroxidic products have been isolated in varying relative yields the endoperoxide 52 (from [4 + 2]-cycloaddition), the aUylic hydroperoxide 53 (from ene reaction), the 1,2-dioxetane 54 (from [2 + 2]-cycloaddition), the diene hydroperoxide 55 (either from a vinylogous ene reaction or a radical-induced rearrangement of 53), and the methoxy-substituted hydroperoxide 56 (a methanol trapping product) were observed. A maximum of 23% of the [4 -I- 2]-cycloaddition product 52 was detected in tetrachloromethane, whereas the dioxetane 54 dominated in polar acetonitrile. 

The terpenes a- and 6-myrcene are classical examples of substrates controlled by factor (ii). In these polyenes the relative reactivity of the nonconjugated C=C double bond (di- versus trisubstituted) controls the first reaction event. 6-Myrcene, a monoterpene triene with a trisubstituted nonconjugated double bond, undergoes preferentially the ene reaction with subsequent [4 + 2]-cycloaddition,whereas a-myrcene 57 solely reacts in a [4 + 2]-mode to give the endoperoxide 58. Alkoxy-substituted 1,3-dienes, which cannot undergo ene reaction, have been shown to be excellent substrates for [2 + 2]-cycloaddi-tion. The l,4-di-t-butoxy-l,3-butadienes were the first substrates that showed nonstereoselective [2 + 2]-cycloaddition reactions. Formation of the endoperoxide 60 was observed only from the ,Z-isomer 59, which indicates a pronounced activation effect by the alkoxy groups for dioxetane formation. 

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