The application is directed to compounds of Formula (I) and pharmaceutically acceptable salts and solvates thereof, wherein R1, R2, R3, R4, Y, and Z are defined as set forth in the specification. The invention is also directed to use of compounds of any one of Formulae I-VIII, and the pharmaceutically acceptable salts and solvates thereof, to treat disorders responsive to the modulation of one or more opioid receptors, or as synthetic intermediates. Certain compounds of the invention are especially useful for treating pain.