Catania head coach Luigi De Canio used the game to give several fringe players a runout, while opposite number Mario Beretta named a strong lineup to keep his players sharp during the Serie B winter break.
The Elefantini may be bottom of Serie A, but they got off to a perfect start after just four minutes. Francesco Lodi's free kick ricocheted off the wall and Leto rifled in the rebound from the edge of the area, giving Simone Farelli no chance. It was the Argentine's second goal in less than a week.
The goal woke Siena up, with the visitors taking control of the game. Valiani flashed wide from 25 yards and Niccolo Giannetti shot over after the Catania defence backed off.
Siena deservedly drew level on 24 minutes. Mariano Andujar could only push Angelo's low cross into the path of Paolucci, who smashed the ball in off the underside of the bar from less than six yards.
Maxi Lopez was denied a penalty after going down under Paolo Dellafiore's challenge, while Leto was off target with an ambitious bicycle kick two minutes before the break.
Siena re-took the lead around the hour mark. Valiani peeled off his marker to meet Alessandro Rosina's cross and send a glancing header past the outstretched hand of Andujar.
Catania poured forward in the hope of finding an equaliser. Farelli flew to deny Leto's acrobatic volley, before Keko expertly turned his marker to send a close range effort off the post. Farelli used his fingertips to push a Nicola Legrottaglie shot wide and Leto wasted another chance after collecting Lopez's chested pass.
Five minutes from time, Lopez spun in the box and sent a shot into the stands. Lopez then put the ball on a plate for Kingsley Boateng, but the substitute somehow missed making contact sliding in from six yards. That miss proved to be very costly, as Siena broke away and netted a third goal in stoppage time.
Pulzetti took advantage of no-one closing him down by running into the area and firing past a static Andujar. Catania capitulated and instantly gave away a fourth goal. Rossetti collected a through ball to send a low angled drive into the bottom corner.
Catania: Andujar, Peruzzi, Legrottaglie, Gyomber, Capuano, Lodi, Rinaudo, (Garufi 69) Guarente, (Boateng 60) Leto, M.Lopez, Monzon (Keko 46)
Siena: Farelli, Dellafiore, Giacomazzi, Matheu, Angelo, Feddal, Valiani, Pulzetti, Giannetti, (Rossetti 55) Paolucci, (Scapuzzi 90) Rosina (Vergassola 77)
Goals: Leto 4 (G) Paolucci 24 (S) Valiani 59 (S) Pulzetti 91 (S) Rossetti 93 (S)