and/or a salt thereof, wherein R1 is —OH or —OP(O)(OH)2, and X1, X2, X3, R2, R2a, Ra, Rb, and Rc are defined herein. Also disclosed are methods of using such compounds as selective agonists for G protein-coupled receptor S1P1, and pharmaceutical compositions comprising such compounds. These compounds are useful in treating, preventing, or slowing the progression of diseases or disorders in a variety of therapeutic areas, such as autoimmune diseases and vascular disease.