Syntheses of O-Methylated-1,2,4-dioxazolidines by Ozonolyses of O-Methylated Dioximes

Abstract

On the basis of the above results, monoozonolyses of Omethylated dioximes of dicarbonyl compounds in the absence of added carbonyl compounds would be expected to give the corresponding bicyclic dioxazolidines. Recently, K. Griesbaum has made use of this by ozonizing O-methylated dioximes of type 6 with n = 2 and n = 3 to prepared the corresponding 1,2,4-dioxazolidines. Extending these studies, we have now ozonized the acyclic O-methylated dioximes 6a-6d with n = 4 to n = 6 and the aromatic O-methylated dioximes 9a and 9b. Ozonolyses of 6a-d in dichloromethane at -78 C afforded the corresponding bicyclic 1,2,4-dioxazolidines 8a-d in yields of 67 %, 59%, 31% and 53%, respectively. These results show that intramolecular trapping of the carbonyl oxide moiety with the C=N bond of intermediate 7 can be effectively performed as outlined in Scheme 1. Ozonolyses of the aromatic O-methylated dioximes 9a and 9b in dichloromethane at -78 C afforded the corresponding aromatic 1,2,4-dioxazolidines 11a and 11b via intermediates 10 in yields of 65% and 35%, respectively (Scheme 2). All of the 1,2,4-dioxazolidines 8 and 11 have been isolated by column chromatography on silica gel and their structures were established by H and C NMR spectroscopy. Characteristic signals in the H NMR spectra of all 1,2,4-dioxazolidines of type 8 and 11 were those for the OCH3 and CH2 groups. The OCH3 groups showed singlet signals in the range of δ = 3.48-3.81 and the CH2 groups showed singlet signals in the range of δ = 5.29-6.00. Characteristic signals in the C NMR spectra of all dioxazolidines of type 8 and 11 were those for the OCH3-carbon atoms and the carbon atoms in the heterocyclic rings. The signals for the OCH3carbon atoms appeared in the range of δ = 61.05-62.56 and those for the carbon atom in the heterocyclic rings appeared in the range of δ = 94.67-103.91. Reductions of 8a-d with TPP gave the corresponding monooximes 12, whereas reductions of 11a and 11b afforded both of the two possible monooximes 13 and 14. Reduction of 11a gave 13a and 14a in a ratio of ca. 1 : 1 and 11b gave 13b and 14b in a ratio of ca. 7 : 1.