Singlet oxygen natural products

Moan, J. and Wold, E. 1979. Detection of singlet oxygen production by ESR. Nature 279 450 151. 

Several natural products (Scheme 60) have been synthesized in vitro via singlet oxygen ene reactions. For example, clavukerin C was prepared in racemic form, in a biomimetic synthesis by Kim and Pak, whereas a-farnesene hydroperoxide was obtained starting from geraniol. 

SCHEME 60. In vitro synthesized natural products via singlet oxygen ene reactions... 

Bald and coworkers synthesized four out of the ten possible diastereomers of querci-tols (Scheme 62), some of which are natural products, by using the ene reaction of singlet oxygen as the key step and the commercially available 1,4-cyclohexadiene as starting material. In this fashion, they managed to avoid the use of natural products, or compounds which require many steps to synthesize, employed as starting materials in all previously reported syntheses. 

The total synthesis of litseaverticillols D, F and G (172 and 173), which are natural products with anti-HIV activity, was achieved recently via a singlet oxygen initiated cascade proposed to be biomimetic (Scheme 64). Finally, allylic alcohols 174a and 174b (Scheme 64) were isolated in a 4/1 ratio via a stereoselective singlet oxygen ene reaction. Stereoisomer 174a is an intermediate in the synthetic route of staurosporine, which is a bioactive alkaloid with hypotensive, antimicrobial and cell cytotoxic properties. 

As has been pointed out earlier, the kinetic results (ft = k6/k9 independent of the nature of the sensitizer) as well as the observed product distributions obtained in photosensitized oxygenation reactions and in (chemically prepared) singlet oxygen oxygenation reactions (see p. 45, for example) leave but little doubt that 102(1Ag or 1S9+) is the common oxygenating species. ... 

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%),... 

---