ortho-Alkynylphenylureas and -acetamides 1, with an alkoxy and aryl group on the nitrogen atom, were subjected to platinum-catalyzed cyclization reactions to afford the corresponding tetracyclic compounds 2, via N-O bond cleavage, in good to excellent yields. For example, N-methoxy-N'-methyt-N'-(2-(pent-1-ynyl)phenyl)-N-phenylurea (1a) was reacted for 12 h in the presence of Ptl(4) (10 mol%) in 1,4-dioxane at 100 degrees C to afford 5-methyl-12-propytindolo[1,2-c]quinazolin-6(5H)-one (2a) in 93% yield, via an iminium-bound platinum carbenoid intermediate, followed by an aromatic C-H insertion.