Compounds having activity as inhibitors of G12C mutant KRAS protein are provided. The compounds have the following structure (I): or a pharmaceutically acceptable salt, stereoisomer or prodrug thereof, wherein R1, R2a, R2b, R2c, R3a, R3b, R4a, R4b, R5a, R5b, R6, A, G1, G2, L1, L2, m1, m2, n, X and E are as defined herein, and wherein at least one of R3a, R3b, R4a or R4b is not H. Methods associated with preparation and use of such compounds, pharmaceutical compositions comprising such compounds and methods to modulate the activity of G12C mutant KRAS protein for treatment of disorders, such as cancer, are also provided.