Singlet oxygen regioselectivity

Singlet oxygen reacts with olefins presumably by the "ene" reaction to form allyflc hydroperoxides (45,57), eg, l-methyl-2-propenyl hydroperoxide [20733-08-8] is produced from 2-butene (eq. 19). The regioselectivity of this reaction has been investigated (58). 

The reaction is stereospecific for at least some aliphatic ketones but not for aromatic carbonyls.197 This result suggests that the reactive excited state is a singlet for aliphatics and a triplets for aromatics. With aromatic ketones, the regioselectivity of addition can usually be predicted on the basis of formation of the more stable of the two possible diradical intermediates obtained by bond formation between oxygen and the alkene.198... 

The oxidation of 2,5-disubstuted furans by singlet oxygen was exploited for the synthesis of [5,5,5] and [6,5,6] bis-spiroketals <06OL1945>. An unusual regioselective photooxidation of 3-bromofuran to 2- and 3-bromo- hydroxybutenolides, as depicted below, was reported. The mechanism for the observed base-dependent regioselective deprotonation of the endoperoxide intermediate was not determined <06OL4831>. 

Clennan, E.L. and Sram, J.P. (2000). Photochemical reactions in the interior of a zeolite. Part 5. The origin of the zeolite induced regioselectivity in the singlet oxygen ene reaction. Tetrahedron 56, 6945-6950... 

Houk, K.N., Williams Jr., J.C., Mitchell, P.A. and Yamaguchi, K. (1981). Conformational control of reactivity and regioselectivity in singlet oxygen ene reactions Relationship to the rotational barriers of acyclic alkylethylenes. J. Am. Chem. Soc. 103, 949-951... 

Selective fomation of allylic hydroperoxides via singlet oxygen ene reaction 837 TABLE 2. Syn/anti regioselectivity in the photooxygenation of aUyUc alcohols 10- and 10-Z... 

The reaction of singlet oxygen with cis and trans alkenes shows an unexpected regioselectivity for hydrogen abstraction on the large alkyl group of the double bond . 

Trans alkenes show similar regioselectivity on their photooxygenation reactions. However, their reactivity towards singlet oxygen is much less than that of the corresponding cis aUtenes. ... 

Highly regioselective ene reactions of singlet oxygen with alkyl-substituted alkenes can also be used for synthetic purposes. For example, in the photooxygenation ofl-t-butylcycloheptene only one allylic hydroperoxide, by the exclusive geminal hydrogen... 

Regioselective singlet oxygen ene reactions have been also used, as key steps, towards the synthesis of (+)-asteriscanolide (167) , a-D-sorbofuranose (2S,3R,5R) (168) and a-D-fmctofuranose (2S,3S,5R) (169) ° (Scheme 62). 

The versatility of these [4+2] heterocyclization reactions is a consequence of the wide range of ene and diene components which can be used. In addition to alkenes and alkynes functioning as ene components, a variety of heterodienophiles is available such as electron-deficient imines (e.g. equation 89), nitriles e.g. equation 90), electrophilic carbonyl compounds (e.g. equation 91), thiocarbonyl compounds (e.g. equation 92), singlet oxygen (e.g. equation 93), nitroso compounds (e.g. equation 94), sulfenylsulfonamides (e.g. equation 95) and azo compounds (e.g. equation 96). Many of these reactions proceed with excellent regioselectivity and stereoselectivity, probably because in many instances they involve... 

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