Cook pasta according to package directions, omitting salt and fat.
Heat oil in a medium saucepan over medium heat. Add garlic; sauté 2 minutes. Stir in marinara sauce, tomato paste, and red pepper; bring to a simmer. Drain clams in a sieve over a bowl, reserving liquid; set clams aside. Stir reserved clam liquid into marinara-sauce mixture. Simmer 10 minutes. Stir in clams, and simmer 3 minutes. Serve pasta with sauce.