Cycloaddition of singlet oxygen

Recently the solvent effect on the [4+2] cycloaddition of singlet oxygen to cyclic dienes has been subjected to a multiparameter analysis. A pre-equilibrium with charge-transfer character is involved, which is affected by the solvent through dipolarity-polarisability (n ) and solvophobic interactions ( Sjf and Another multiparameter analysis has been published by Gajewski, demonstrating the... 

Cas/liquid reaction 25 [CL 25) [4 + 2] Cycloaddition of singlet oxygen to a-terpinene... 

The reaction mechanism for the aerobic oxidation of the pz to seco-pz can be attributed to a formal 2 + 2 cycloaddition of singlet oxygen to one of the pyrrole rings, followed by cleavage (retro 2 + 2) of the dioxetane intermediate to produce the corresponding seco-pz (160). This mechanism is shown in Scheme 29 for an unsymmetrical bis(dimethylamino)pz. Further photophysical studies show that the full reaction mechanism of the photoperoxidation involves attack on the reactant by singlet oxygen that has been sensitized by the triplet state of the product, 159. As a consequence, the kinetics of the process is shown to be autocatalytic where the reactant is removed at a rate that increases with the amount of product formed. 

As already hinted at above, chiral dioxetanes, obtained through the highly stereoselective [2 + 2] cycloaddition of singlet oxygen to the chiral enecarbamate, provide a convenient preparation of optically active 1,2 diols as building blocks for asymmetric synthesis (Scheme 5) . Reduction of the dioxetane 2c by L-methionine, followed by release of the oxazolidinone auxiliary by NaBH4/DBU reduction, affords the enantiomerically pure like-5 diol (for additional cases, see Table 4 in Reference 19e). 

The synthesis of fenozan BO-7 4 involves two key steps, the first of which employs a 4-1-2 cycloaddition of singlet oxygen to the diene This provides the endoper-... 

Cycloadditions of singlet oxygen to particular fused 1,3-cyclohexadienes can be highly diastereoselective, and therefore useful in the total syntheses of some complex natural products. For example, chemically generated 02 effectively and stereoselectively adds to the termini of the 1,3-diene system in 515 to give a single endoperoxide 516 (81%),... 

The synthesis of fenozan BO-7 4 involves two key steps, the first of which employs a 4 + 2 cycloaddition of singlet oxygen to the diene 83a122-20 123. This provides the endoper-oxide 83b that can be transformed into the target cis-fused 1,2,4-trioxane by treatment with the Lewis acid, TMSOTf, in the presence of a carbonyl compound. The reaction proceeds by Lewis acid promoted heterolysis of the C—O bond to give an intermediate peroxy allyl cation 83c that is captured by the carbonyl compound (in this case, cyclopen-tanone) to give the product (Scheme 30). A number of different carbonyls have been used in this reaction along with a number of different endoperoxide templates and detailed SAR have been developed (Scheme 30). 

Wasserman et al. [286] reported a closely related reaction of an oxazole as shown in Scheme 63. The major product was formed via [4 + 2] cycloaddition of singlet oxygen, followed by rearrangement to 5-cyanopentanoic benzoic anhydride 161 and methanolysis to the cyano ester 162. In the case of the oxazole-linked [60]fullerene, however, the treatment with methanol for a long time to ensure methanolysis might result in further conversion of the once formed cyano... 

Pierlot, C., Poprawski, J., Marko, J. and Aubry, J.-M. (2000) Effects of oxygenated substituents on the [4 + 2] cycloaddition of singlet oxygen in the photooxygenation of water-soluble naphthyl ethers. Tetrahedron Letters, 41 (25), 5063-5067. 

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