Compounds, particularly, glucopyranosyl lipid adjuvant (GLA) compounds, having the following structure (I) are provided: or a pharmaceutically acceptable salt thereof, wherein L 1 , L 2 , L 3 , L 4 , L 5 , L 6 , L 7 , L 8 , L 9 , L 10 , Y 1 , Y 2 , Y 3 , Y 4 , R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , are as defined herein. Pharmaceutical compositions, vaccine compositions, and related methods for inducing or enhancing immune responses, are also provided.