The present invention discloses compounds of Formula (Ia), and pharmaceutically acceptable salts, thereof: which inhibit coronavirus replication activity. The invention further relates to pharmaceutical compositions comprising a compound of Formula (Ia) or a pharmaceutically acceptable slat 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 (Ia) or a pharmaceutically acceptable salt thereof.