Alessadrini made up for the missed chance on 14 minutes, though, when he struck from 25 yards to make it 1-0.
Bastia could not get a grip on the game and were lucky not to be 2-0 down after Jean-Armel Kana-Biyik hit the bar with a header from a corner.
Rennes doubled their lead on 29 minutes through Toivonen, who put a precise and powerful volley past the goalkeeper to make it 2-0.
Bastia struggled to get hold of the ball late into the second half, with Rennes coming forward with attack after attack.
Jean Makoun had an effort cleared off the line from a corner before the goalkeeper eventually gathered the ball.
Bastia had more intensity in the second half, but they continued to struggle with the pace of Rennes's counter-attacks.
Makoun made it 3-0 for the home side on 59 minutes after a perfect one-two with Toivonen allowed him to beat the keeper from inside the six-yard box.
Bastia had a couple of sights on goal as Rennes seemingly began to relax, but they didn't really trouble the shot-stopper.