Cascade Assembling of Isatins and Barbituric Acids: Facile and Efficient Way to 2′′ H ‐Dispiro[indole‐3,5′‐furo[2,3‐ d ]pyrimidine‐6′,5′′‐pyrimidine]‐2,2′,2′′,4′,4′′,6′′‐(1 H ,1′ H ,1′′ H ,3′ H ,3′′ H )‐hexone Scaffold