When Sanofi froze its defined benefit pension plan last year, the top brass wanted to make sure its 401(k) was seen as a worthy replacement by the company’s 24,000 U.S. employees and retirees. Sanofi has succeeded, judging by Plan Sponsor magazine’s designation of the U.S. division of the French pharmaceutical giant as 2013 “Plan Sponsor of the Year.” In corporate America, 401(k) plans are now the norm: in 2012, only 11 of Fortune magazine’s 100 largest companies still offered a traditional defined benefit pension, according to the consulting firm Towers Watson. But Sanofi U.S. had strong motivation for designing a 401(k) that is generous compared with typical 401(k)s. The company has “highly technical, highly specialized, highly skilled [employees] that w…