Synthesis of symmetrical 2,2′,4,4′‐tetrasubstituted[4,4′‐bioxazole]‐5,5′(4H,4′H)‐diones and their reactions with some nucleophiles

Several symmetrical 2,2′,4,4′-tetrasubstituted[4,4′-bioxazole]-5,5′(4H,4′H)-diones 1a-f were obtained by dehydrodimerization of 5(4H)-oxazolones 2a-f. The configurations of four were established; one by X-ray crystallography rac-1c, and three rac-1a, meso-1a and rac-1b by 1H nmr spectroscopy of their derivatives. Upon being heated, the bioxazolones isomerized, presumably by breakage of the 4,4′-carboncarbon bond to form free radicals followed by their recombination. The results of a crossover experiment were consistent with a radical nature for this isomerization reaction. Treatment of three of the bioxazolones rac-1a, meso-1a and rac-1c with methanol and amine nucleophiles led to ester and amide derivatives 7–11 of α,α'-dehydrodimeric amino acids.

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