expression
Fixed phrase used to express regret or disappointment about a situation. Often placed at the beginning of a sentence.
See also: 残念
残念なことに、彼は来られなかった。
Unfortunately, he couldn't come.
残念なことに、その店は閉まっていた。
Regrettably, the shop was closed.
残念 is an adjective meaning 'disappointing' or 'regrettable', while 残念なことに is a set phrase meaning 'unfortunately'.
From 残念 (regrettable) + な (attributive) + こと (thing) + に (adverbial particle), literally 'as a regrettable thing'.