1.4.2- Dioxazoline

Table 2 Ring proton NMR data for 1,4,2-dioxazoline derivatives 13...

As previously mentioned, 1,4,2-dioxazoline rings may be used as a protected group for hydroxamic acids (see Section 6.04.6.3). In fact, this ring is susceptible to hydrogenolysis conditions (H2/Pd) affording carbonyl compounds <1996CHEC-II(4)525, 2002JOC4833>. 

An improved synthesis of 1,4,2-dioxazolines from hydroxamic acids 119 and 2,2-diethoxypropane in the presence of camphorsulfonic acid as an acidic catalyst via a transketalization reaction has been reported (Scheme 23)... 

Benzoyl nitrene 247 (R = Ph) generated by photolysis of benzoyl azide in the presence of carbonyl compounds affords 1,4,2-dioxazolines 248-251 (Scheme 34). Moderate yields are obtained upon irradiation at 254 nm, while better yields are obtained upon irradiation at 365 nm in the presence of a sensitizer, such as Michler s ketone, which diminishes the photo-Curtius side reaction of acylnitrene to isocyanate. With ethoxycarbonylnitrene 247 (R=—OEt) and acetone, compounds 251 and 252 are obtained the latter is possibly formed from acetone and the dipolar intermediate 252 <1995T7181>. In another report on reactions of chiral aroylnitrene 254, the obtained dioxazolines 255 do not show any diastereoselectivity, although the presence of a chiral auxiliary in the ortfe-position ensures its vicinity to the reaction center (Scheme 35) <2001S1125>. 

Adduct and dioxazoline formation were not suppressed in the decomposition of the heptanoyloxyl derivative 163c in the presence of tributyltin hydride. 

Thermolysis or photolysis of 1,2,4-dioxazolidines leads to rupture of the OO bond to give acyclic molecules (84CHEC-l(6)897>. Thermal decomposition of 3-acyl-1,2,4-dioxazolines at room temperature gives diacylamines (see Section 4.13.6.2.2) and l,2,4-dioxazolin-3-ones give isocyanates <84CHEC-I(6)897>. 

Dioxazolines (113) undergo a facile rearrangement to triacylamines (115) apparently via imino anhydrides (114) the reaction rate does not depend on solvent polarity (Scheme 26) <90JCS(P1)1011, 91JCS(P2)1085>. 

Dye-sensitized photooxygenation of oxazoles (242) in aprotic solvents with 650 W halogen lamp leads to unstable 1,2,4-dioxazolines (244) via intermediate cyclic peroxides (243) (Scheme 49) <90JCS(Pl)10n, 91JCS(P2)1085>. 

For all the above examples, the nucleophilic attack takes place at the tertiary carbon atom of the (oxa/thia)azoline ring. However in two particular cases, namely for the fluoromethyl substituted dioxazoline (158) <84JOC9i9> and the unusual oxathiazoline (159) <74ZOR39>, the nucleophiles attack at the sp ring carbon according to Scheme 25, and Equation (4)). 

JAP77148076). Some other oxathiazoline derivatives (279) have central nervous system (CNS) activity, as tranquilizers and ataraxics with low toxicity (68SAP6804299) and some dioxazoline derivatives (280) have a modest CNS depressant activity (77JPS772). 

Dioxazolines, synthesis, 30 271 Dioximes, technetium(III) complexes, 41 33-36 Dioxocyclam, 45 96 Dioxohexakis(trimethylsilylmethyl)-... 

The synthesis of 1,2,4-dioxazolines and 1,2,4-dioxazolidines is based on the use of singlet oxygen, ozone, or peroxides. Thus, photoaddition of O2 to 2-alkoxyoxazoles followed by a rearrangement leads to unstable 3-acyl-... 

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