Diels-Alder 2,5-endoperoxide

Characteristic reactions of singlet oxygen lead to 1,2-dioxetane (addition to olefins), hydroperoxides (reaction with aHyhc hydrogen atom), and endoperoxides (Diels-Alder "4 -H 2" cycloaddition). Many specific examples of these spectrally sensitized reactions are found iu reviews (45—48), earlier texts (15), and elsewhere iu the Engchpedia. 

In addition, particularly micro-channel processing is demanded here as the reaction is dangerous owing to the explosive nature of the endoperoxide formed as intermediate. This handling of explosive or toxic compounds for Diels-Alder reactions is also described in [21]. Owing to the use of only small volumes, the hazardous potential can be substantially minimized. This was exemplarily shown for the addition of singlet oxygen. 

The product distribution resulting from P-carotene oxidization by 02 was studied by Stratton et al. (1993) using reverse-phase HPLC, UV-vis spectrophotometry, and mass spectrometry. The oxidation products were identified as [3-ionone, P-apo-14 -carotenal, (i-apo-IO -carotenal, P-apo-8 -carotenal, and P-carotene-5,8-endoperoxide. The formation of 5,8-endoperoxide derivative by a [4+2] Diels-Alder addition mechanism was also reported in the 02-mediated oxidation of P-carotene in reverse micelles (Montenegro et al. 2002), P-ionone (Borsarelli et al. 2007), and of the A1E retinoid derivative (Jockusch et al. 2004). 

The corresponding endoperoxides of furans usually ring-open to afford 1,4-diketones. Such a sequence, followed by an intramolecular Diels Alder reaction was utilized in the synthesis of the tts-indacenyl dianion (6.7) 613>. 

C. Kinetics of photoperoxidation reaction. In the sensitized photooxygenation by Type lib mechanism the 02 (JAg) acts as dienophile and adds to give the endoperoxide in concerted addition reaction similar to Diels-Alder... 

The photochemical dimerization of unsaturated hydrocarbons such as olefins and aromatics, cycloaddition reactions including the addition of 02 ( A ) to form endoperoxides and photochemical Diels-Alders reaction can be rationalized by the Woodward-Hoffman Rule. The rule is based on the principle that the symmetry of the reactants must be conserved in the products. From the analysis of the orbital and state symmetries of the initial and final state, a state correlation diagram can be set up which immediately helps to make predictions regarding the feasibility of the reaction. If a reaction is not allowed by the rule for the conservation of symmetry, it may not occur even if thermodynamically allowed. 

With these compounds (as with other dienes), 02 undergoes a so-called Diels-Alder-reaction (March 1992), forming an endoperoxide intermediate that further reacts to yield various products. For example, the reaction of FFA with 02 is (Haag etal., 1984a) ... 

Tetradehydrodianthracene undergoes 4 + 2-cycloaddition with electron-deficient dienes such as 1,2,4,5-tetrazines.263 The Diels-Alder reactions of [3.3 ortho-anthracenophanes witli A-(p-nitro, chloro, or methoxy-substituted phenyl)malehnides yield approximately equal quantities of inside and outside adducts.264 The photooxidation of bulky water-soluble 1,4-disubstituted naphthalenes with singlet oxygen yields both the expected 1,4- and the unexpected 5,8-endoperoxides.265... 

Review. The varied uses of singlet oxygen in organic syntheses have been reviewed (70 references). The best known are the ene reaction of alkenes, which results in an allylic hydroperoxide, and the Diels-Alder reaction with 1,3-dienes to form 1,4-endoperoxides. 

In 1989, the irradiation of (E,E)-2,4-hexadiene S3 sensitized by meso-porphyrin IX dimethyl ester led to the formation of cis-3,6-dimethyl-l,2-dioxene (62), which was the major product detected at — 78 °C in Freon 11 [69]. Endoperoxide 62 was purified under vacuum at 0.75 mmHg, and collected in a trap (98% isolated yield). Dienes that can adopt a cisoid conformation, such as 53 or ( , )-l,4-di phenyl butadiene, were photooxidized by the corresponding endoperoxides in high or quantitative yield in a suprafacial Diels-Alder reaction [60, 70], Dienes that cannot readily adopt cisoid conformations, such as (fc, Z)-2,4-hexadienes and (Z, Z)-2,4-hexadienes, lose their stereochemistry in the singlet oxygen [2 + 4]-cyclo-addition [60], (E,Z)- and (Z,Z)-dienes give a complex mixture of hydroperoxides and aldehydes, which suggests the intervention of intermediate zwitterions or 1,4-diradicals [71]. 

It was suggested27-288 that all these compounds would be generated from the unstable endoperoxide (20) (originated from a Diels-Alder reaction of singlet oxygen with the dienio pyrrole system) through various paths, as indicated in Scheme I. 

Just as anthracene and its homologues behave like conjugated dienes in the Diels-Alder reaction, they react with singlet oxygen to form 9,10-endoperoxides (equation 135) [16, 24]. 

The cyclopentadienyltin(IV) compounds undergo Diels-Alder cycloadditions with reactive dienophiles such as maleic anhydride, diethyl maleate, and diethyl acetylenedi-carboxylate," and an endoperoxide has been identified from the reaction with singlet oxygen.123... 

Thiiranoradialene sulfoxide (38) was also found to participate in [4 + 2] cycloaddition with singlet 02 to afford endoperoxide (46), which underwent a second Diels-Alder reaction in furan to give the... 

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