The present invention discloses compounds of Formula (la), and pharmaceutically acceptable salts, thereof: Formula (Ia) which inhibit coronavirus replication activity. The invention further relates to pharmaceutical compositions comprising a compound of Formula (la) or a pharmaceutically acceptable salt thereof, and methods of treating or preventing a coronavirus infection in a subject in need thereof, comprising administering to the subject a therapeutically effective amount of a compound of Formula (la) or a pharmaceutically acceptable salt thereof.