1.2.4- Dioxazoline ring

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>. 

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)). 

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

In contrast to the above, the more reactive ethoxycarbonyl nitrene 4 is able to attack the carbonyl group of ketones forming three-membered rings such as compound 90 shown in Sch. 24. The ground state for this nitrene is the triplet state, meaning that the cycloaddition reaction occurs in at least two steps. Indeed is has been found that acetone can be attacked by the reactive intermediate 4 primarily at the carbonyl O-atom (see Sch. 24) the dipolar intermediate is able to add a second acetone molecule to yield the dioxazoline (compound 91) [21]. 

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