AA 1799 then UA 3376 at O'Hare
Cancún → Chicago → St Louis
Timetable, Chicago time
19:20AA 1799 from Cancún at the gate
11:00UA 3376 to St Louis leaves the gate
15 h 40 apart on the timetable, overnight
UA 3376 leaves 15 h 40 after AA 1799 arrives on the timetable: more than half a day, so not a connection we count.