THERMAL ISOMERIZATION OF 2-ACYLOXY-4,6-DIMETHOXY-1,3,5-TRIAZINES TO 1-ACYL-3,5-DIMETHYL-1,3,5-TRIAZIN-2,4,6(1H,3H,5H)-TRIONES. CRYSTAL STRUCTURE OF 1- (2,2-DIMETHYLPROPANOYLOXY)-3,5-DIMETHYL-1,3,5-TRIAZIN-2,4,6(1H,3H,5H)-TRION E