6 alkyl c u2550 o u2014 r 1 u2014 o u2014 c 1 c 6 alkyl c u2550 o u2014 r 1 u2014 nr 1 u2014 c 1 c 6 alkyl c u2550 o u2014 r 2 u2014 c 0 c 6 alkyl nr 1 u2014 c u2550 o u2014 or 2 u2014 c 0 c 6 alkyl o u2014 c u2550 o u2014 nr 1 r 2 u2014 c 0 c 6 alkyl nr 1 u2014 c u2550 nr 2 u2014 nr 3 r 4 u2014 c 0 c 6 alkyl nr 1 u2014 c u2550 o u2014 nr 2 r 3 u2014 o u2014 c 2 c 6 alkyl nr 1 u2014 c u2550 o u2014 nr 2 r 3 u2014 nr 1 u2014 c 2 c 6 alkyl nr 2 u2014 c u2550 o u2014 nr 3 r 4 and u2014 c 0 c 6 alkyl nr 1 u2014 c u2550 s u2014 nr 2 r 3 any two radicals of a m a 1 and a 2 may be taken together to form an optionally substituted 3 to 10 membered carbocyclic or heterocyclic ring r 1 r 2 r 3 and r 4 are each independently hydrogen or an optionally substituted radical selected from the group of u2014 c 1 c 6 alkylhalo u2014 c 1 c 6 alkyl u2014 c 1 c 6 alkylcyano u2014 c 3 c 7 cycloalkyl u2014 c 4 c 10 alkylcycloalkyl heteroaryl u2014 c 1 c 6 alkylheteroaryl.