Compounds, particularly, glucopyranosyl lipid adjuvant (GLA) compounds, having the following structure (I) are provided: or a pharmaceutically acceptable salt thereof, wherein L1, L2, L3, L4, L5, L6, L7, L8, L9, L10, Y1, Y2, Y3, Y4, R1, R2, R3, R4, R5, R6, are as defined herein. Pharmaceutical compositions, vaccine compositions, and related methods for inducing or enhancing immune responses, are also provided.