Efficient Iminophosphorane‐Mediated Preparation of Benzofuro[3,2‐d]pyrimidin‐4(3H)‐ones and Unexpected Ring Opening Products

The carbodiimides 4 , obtained from aza‐Wittig reactions of iminophosphorane 3 with aromatic isocyanates, reacted with secondary amines, phenols or alcohols in the presence of catalytic amounts of K₂CO₃ or sodium alkoxide to give 2‐substituted benzofuro[3,2‐d]pyrimidin‐4(3H)‐ones 6 . However, when 2,2′‐iminobis[ethanol] was used, the unexpected ring opening product 7 was formed instead of 6 . Reaction of 4 with primary amines RNH₂ (R=Et, Pr, Bu, etc.) gave guanidine intermediates 8 , which were further treated with EtONa to give only one regioisomer 9 via a base catalyzed cyclization. However, another regioisomer 11 was obtained when NH₃ or ‘small’ amines RNH₂ (R=Me, NH₂) were used in the absence of EtONa via a spontaneous cyclization of 8 .