Sigmatropic rearrangements 2 -oxazolones

The reaction of aliphatic and aromatic ketone oximes 97 with a dialkyl carbonate 98 in the presence of K2CO3 at 180-190 °C yields 3-alkyl-4,5-disubstituted-2(3//)-oxazolones 104 in 22-48% yields. Mechanistically, it is proposed that N-alkylation of the initially formed oxime O-carbonate 99 yields 100, which affords the enamine 101 in the presence of base. A [3,3] sigmatropic rearrangement ensues to produce 102, which then cyclizes to 104. In cases where 97 contains two methylene groups in proximity to the C=N bond, one of which is benzylic, the above reaction sequence is regioselective for the benzylic methylene group (Fig. 5.25 Table 5.6, Fig. 5.26). ... 

It is known that 5-acyloxyoxazoles 132 rearrange to 4-acyl-5(4/l/)-oxazolones 133 in the presence of 4-(dimethylamino)pyridme or 4-(pyrrohdino)pyridine. Recently, an asymmetric variant of this nucleophUe-catalyzed rearrangement that employs a chiral derivative of 4-(pyrrolidino)pyridine has been described. This procedure allows the construction of quaternary stereocenters with high levels of enantioselectivity (Scheme 7.38). Representative examples of saturated 5(4//)-oxazolones prepared via sigmatropic rearrangements are shown in Table 7.16 (Fig. 7.18). 

TABLE 7.16. SYNTHESIS OF SATURATED 5(4H)-OXAZOLONES VIA SIGMATROPIC REARRANGEMENT OF 5-ALKOXYOXAZOLES... 

Allyl-5(4//)-oxazolones undergo a thermal 3,3 sigmatropic rearrangement (equation... 

Allyloxy-substituted 4,5-diphenyloxazoles undergo [3,3]-sigmatropic aza-Claisen rearrangements to 3-allyl-2(3//)oxazolones <84JOC399>. The 2-propynyloxy-oxazole (61) affords an allene... 

---