The invention relates to compounds of structural formulas (I), (VII) and (XI): or a pharmaceutically acceptable salt, solvate, clathrate, or prodrug thereof, wherein X.sub.1, X.sub.2, X.sub.3, Y, Z, L, R.sub.1, R.sub.2, R.sub.3, R.sub.18 and n are defined herein. These compounds are useful as immunosuppressive agents and for treating and preventing inflammatory conditions, allergic disorders, and immune disorders.