This invention relates to novel heterocyclic compounds of formula I wherein R1nullR4, X1 and X2 are as defined in the summary and pharmaceutically acceptable salts and solvates thereof, methods to inhibit or modulate Human Immunodeficiency Virus (HIV) reverse transcriptase with compounds of formula I, pharmaceutical compositions containing of formula I admixed with at least one solvent, carrier or excipient and processes to prepare compounds of formula I. The compounds are useful for treating disorders in which HIV and genetically related viruses are implicated 1