Oxygen sensitivity, substituent effects

Substituent effects in the dye-sensitized (Rose Bengal or methylene blue) singlet oxygen photooxidation of a wide variety of furans have been investigated.287 Solvents effects on the value of p, which became less negative as polarity of the medium was increased was indicative of... 

Tables 3 and 4 contain results from a study of a series of coumarins and fiirocoumarins in, 1,2-dibromoethane, which found that the lactone carbonyl signal consistently appeared near 350 ppm more variability was seen for the single bonded oxygen, especially for the furocoumarins, typically appearing at 220 ppm [85]. Synthetic 3-aryl coumarins also exhibit 6(C=0) and 5(-0-) values near those mentioned above [86]. Recently, a study of a series of 7-substituted-4-methylcoumarins, with a wide range in electronic character of the substituents, demonstrated that the carbonyl signal is quite sensitive to substituent effects and that the NMR chemical shift is reasonably well correlated with the carbonyl oxygen AMI estimated electron density [87].

This paper wiii present the effects of sensitizer and solvent on singlet oxygenation of (1) and the substituent effect on the chemical behavior of (2). 

The synthesis of porphyrins from bilanones is free of all symmetry restraints.77 The oxo function is necessary to stabilize the bilane system by its electron-withdrawing effect. The synthesis of porphyrins from the parent bilane without the oxo function and with /3-substituents is possible,54 but the method gives rise to preparative problems due to the sensitivity of these compounds to oxidation, to electrophiles and to acids. Nature circumvents these problems in the cellular environment by exclusion of oxygen, when porphyrinogens, the precursors of porphyrins, are produced from bilanes55 in the course of their biosynthesis. 

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