Synthesis of 1,3‐Oxazines and Furo[2,3‐b]pyrans by reaction of 2‐amino‐4,5‐dihydro‐3‐furancarbonitriles with dibenzoyldiazomethanes

2-Amino-4,5-dihydro-3-furancarbonitriles (1) react with a slight excess of dibenzoyldiazomethane in the presence of rhodium(II) acetate to give 1,3-oxazin-4-ones (2). With three equivalents of dibenzoyldiazomethane compounds 1 react to afford furo[2,3-b]pyran-3a-carbonitriles (3). Compound 3a was also obtained by treatment of 2a with two equivalents of dibenzoyldiazomethane.