Prantares was a liberty port that was known to the Federation by the late 23rd century.
In a discussion about Federation-Klingon tactics with the USS Enterprise senior staff in 2277, Fleet Admiral Karis Tatenen referred to Prantares when he said that there was a chance that you could be "jumped", conned or sold into slavery. However, if you walk down the street like you'll "eat" the first one that intends to harm you then you'll leave them along. He used this example to support his stance that the Federation has got to be bold and hit the Klingons at the first opportunity, avoiding diplomacy. (TOS - Star Trek II Short Stories short story: "The Blaze of Glory")