N + C – S H 3 C R1 R2 N + S H 3 C R1 R2 H H 3 C O O OH N + S H 3 C R1 R2 OH H 3 C O – O N + S H 3 C R1 R2 C O O O – H 3 C N + C – S H 3 C R1 R2 O H 3 C H N N H + H + + - + +