Efficient Iminophosphorane‐Mediated Preparation of Benzofuro[3,2‐ d ]pyrimidin‐4(3 H )‐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 K 2 CO 3 or sodium alkoxide to give 2‐substituted benzofuro[3,2‐ d ]pyrimidin‐4(3 H )‐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 RNH 2 (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 NH 3 or ‘small’ amines RNH 2 (R=Me, NH 2 ) were used in the absence of EtONa via a spontaneous cyclization of 8 .