naphthyl each alkyl substituent being c 1 c 8 and when together cyclized r 3 with r 4 may represent c 4 c 12 cycloalkyl and c 1 c 8 alkyl substituted cycloalkyl r 6 r 7 r 8 and r 9 each represent c 1 c 12 alkyl or when together cyclized r 6 with r 7 and r 8 with r 9 may represent c 4 c 12 cycloalkyl and c 1 c 8 alkyl substituted cycloalkyl r 10 is selected from the group consisting of hydrogen c 1 c 8 alkyl and str2 wherein r 13 represents hydrogen c 1 c 18 alkyl or alkenyl phenyl or naphthyl r 11 and r 12 independently represent hydrogen and c 1 c 18 alkyl and n is an integer in the range from 1 to about 8 specific examples of such 3 5 dhpzna stabilizers are identified by the following code numbers and structures in which ch 2 groups at the intersection of lines are not otherwise identified and projecting lines represent ch 3 groups the substituents on the alpha c atom of the acetamide are critical in the above identified stabilizer compound it will be appreciated that when r 10 is to be acyl it is introduced by an acylation step after formation of the 3 5 dhpzna in which there is no.