The synthesis of the 14C and 2H‐isotopomers of (R)‐N‐[2‐(2′‐ethoxyphenoxy)‐ethyl]‐N‐2‐[3‐(4′‐methoxy‐3′‐sulfonamido)‐phenyl]‐propylamine hydrochloride, an α1‐adrenoreceptor antagonist