The instant N-oxyl compounds can be prepared in a number of ways.  One method is described in U.S. Pat.  No.  4,665,185 where a hindered amine having an N--H moiety is oxidized with a hydroperoxide in the presence of an appropriate metal carbonyl, oxide or alkoxide catalyst, such a tert-butyl hydroperoxide in the presence of molybdenum trioxide catalyst. 
The oxidation of a hindered amine with hydrogen peroxide in the presence of sodium tungstate catalyst is reported by V. Kaliska et al.,  Chem.  Pap.,  42 (2), 243-8 (1988)[Chem.  Abst.  109, 92734n (1988)].  The use of ultrasound facilitates the reaction when the hindered amine has a fatty acid group at the 4-position of the piperidine ring. 
The preparation of the instant N-oxyl compounds has also been described by the transesterification of the lower alkyl ester of a carboxylic acid with the 1-oxyl hindered amine alcohol as taught by I. Dr agutan, Free Radical Res.  Comm.  9 (3-6), 379 (1990) and by V. D. Sholle et al.,  Izv.  Akad.  Nauk.  SSSR, Ser.  Khim, 11, 2578 (1981)[Chem.  Abst.  96, 84950c (1982)] using sodium methoxide or sodium ethylate as catalyst.  It is noted that these catalysts including lithium amide cause severe decomposition and the formation of oxidation-reduction by-products in many of the reactions causing little of the desired N-oxyl ester product to form. 
U.S. Pat.  No.  5,218,116 teaches that hindered amines can be oxidized to their N-oxyl derivatives with hydrogen peroxide in the presence of a titanium catalyst which is a titanium silicalite or synthetic zeolite.  EP 574,667 A1 teaches that divalent metal catalysts are useful for the same purpose. 
Russian (SU) patent No.  1,168,556 describes the preparation of 1-oxyl-2,2,6,6-tetramethylpiperidin-yl esters of carboxylic acids by reaction of the corresponding 4-hydroxy compound with a lower alkyl ester of a mono- or di-carboxylic acid at 125.degree.-140.degree.  C. in the presence of a tetraalkyl orthotitanate transesterification catalyst in xylene.  The yields are reported to vary from 51 to 84%.  The patent further states that, when the reaction is run at a temperature below 125.degree.  C., the yields obtained are considerably lower. 
In actuality, due to the reactive nature of the N-oxyl group, using the conditions described in the Russian reference, namely in xylene at 140.degree.  C., the desired N-oxyl product is produced in low yield accompanied by a host of various by-products. 
Although the Russian reference genetically describes a method for making such N-oxyl ester products, discrete chemically significant improvements in said process have resulted in an improved process for making the instant N-oxyl ester products in higher yields and with excellent purity at lower temperature and with more benign solvent systems.