Singlet oxygen synthetic applications

Oxazoles, prepared from carboxylic acids (benzoin, DCC NH4OAC, AcOH, BOSS % yield), have been used as carboxylic acid protective groups in a variety of synthetic applications. They are readily cleaved by singlet oxygen followed by hydrolysis (ROH, TsOH, benzene or K2CO3, MeOH ). 

Like the porphyrins, the phthalocyanines can undergo photooxidation and act as photosensitizers for the production of singlet oxygen" One of the few chemical synthetic applications was the acceleration of the autoxidation of cumene and photooxidation of pinenes". ... 

The reaction serves as an illustrative example of the synthetic utilization of the Te(II)—>Te(IV) two-electron shuttle. Another promising application is seen in photodynamic therapy in which tellurapyrylium dyes 119 can function as photosensitizers to produce either singlet oxygen or superoxide radical-anions (via electron transfer), thereby serving as cytotoxic agents. An important useful property of tellurapyrylium dyes is their absorbance in the near-infrared region where biological tissues are most important. 

Singlet-oxygen studies have being reported to such diverse areas as chemiluminescence [14], photocarcinogenity [15], ozonolysis [16], photodynamic action [17], peroxide decomposition [7], photosynthesis [14], air pollution [18], metallocatalyzed oxygenation reactions [19,20], synthetic applications [21], and polymer degradation [10]. 

Most reactions reported in this chapter are of Type II involving singlet oxygen in the first electronically excited state ( Ag) and, in particular, photooxygenation of [4+2] and [2+2] type will be illustrated. Excellent books [5,6,8] and reviews [9] cover the extraordinarialy wide literature on the mechanistic and synthetic aspects of these reactions, and in the recent years attention has been given to the stereochemical aspects, especially in view of the application of these reactions in asymmetric synthesis [10]. 

Singlet molecular oxygen (lC>2) is a very small electrophilic reagent that can produced by photosensitization and react easily with dienes and alkenes in [4 + or [2 + 2] cycloadditions or ene-reactions. In order to develop synthetic applic tions of photooxygenations, numerous studies have recently been developed wi chiral substrates and especially in the presence of chiral auxiliaries. 

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