| abstract |
A compound of Formula IIa or IIb: ** Formula ** wherein R1 and R2 are independently alkyl, alkenyl, alkynyl, arylalkyl, cycloalkylalkyl, heteroalkyl, or heterocycloalkyl, each of which is unsubstituted or substituted by halogen; Qh and Tg are each independently alkylene, R7 and R8 are independently -H, alkyl, alkenyl, alkynyl, arylalkyl, cycloalkylalkyl, heteroalkylalkyl, or heterocycloalkyl; R10 and R11 are -H; R12 is -H or alkyl; and g and h are each independently an integer from 1 to 5. |