isobutyl especially preferred is a compound wherein r 1 is isobutyl and r 6 is cyclohexylmethyl another set of more preferred compounds within this third sub group are those wherein q is s r 1 is selected from h methyl and isobutyl and r 2 is isobutyl especially preferred is a compound wherein r 1 is isobutyl and r 6 is cyclohexylmethyl radicals which exemplify the x substituent of formula v are as follows str12 within any of these radicals exemplifying formulae iii iv and v the substituent r represents a linear or branched alkyl group of one to about ten carbon atoms or preferably one to about five carbon atoms a fourth sub group of preferred compounds consists of those compounds of formula ii wherein x is str13 wherein each of r 7 and r 8 is independently selected from the groups h lower alkyl cycloalkyl phenyl benzyl naphthyl and naphthylmethyl any one of which groups having a substitutable position may be optionally substituted with or more of lower alkyl alkoxy alkenyl alkynyl halo haloalkyl cyano and phenyl with the proviso that at least one of r 7 and r 8 is an aryl group of this fourth subgroup more preferred compounds are those wherein r 1 is isobutyl r 2 is isobutyl r 3 is benzyl r 4 is h or methyl r 5 is h or methyl r 6 is cyclohexymethyl and each of r 7 and r 8 is independently selected from h lower alkyl and phenyl with at least one of r 7 and r 8 being phenyl an especially preferred compound is wherein r 7 is h and r 8 is phenyl unless otherwise described the chemical groups recited herein shall have meanings as follows 34 lower alkyl 34 means alkyl radicals containing one.