Methods and compositions for delivery of active agents

A compound of formula IA:

• or a salt thereof, wherein
• X and Y are each, independently, alkyl or aryl;
• L1 and L2 are each, independently, alkylene linking units, which may optionally contain one or more double bonds and further may optionally be interrupted by one or more heteroatoms and/or one or more cycloalkylene groups;
• R1 and R2 are each, independently, C10-C30 aliphatic group, each of which may optionally contain one or more double bonds, and may optionally be interrupted by one or more heteroatoms and/or one or more cycloalkylene groups;
• Z is —C(O)O—, —OC(O)—, —C(O)N(R3)—, —N(R3)C(O)—, —O(CO)N(R3), —N(R3)C(O)O—, —C(O)N(R3)C(O)O—, —OC(O)N(R3)C(O)— or —N(R3)C(O)N(R3)—;
• each occurrence of R3 is, independently, H, alkyl or aryl, and
• --- represents a single bond or a double bond, where when --- represents a single bond, C* is further substituted by hydrogen or alkyl,
• wherein L2 is a linear alkylene unit and contains one or more double bonds and one or more cycloalkylene groups.