Richard G. Rogers, 56, will retire Dec. 31 as president and chief operating officer of Syntex Corp., Palo Alto. He has held his current position and has been a member of the corporation’s board of directors since 1982. He joined Syntex in 1964 as advertising manager for Syntex’s pharmaceutical and chemical products. He was named executive vice president in 1978. Chairman and Chief Executive Albert Bowers will assume the presidency on Rogers’ departure.