Greti Rihs

Synthesis and reactivity of compounds with cyclobutane rings. Part III. Cyclobut-1-ene-1,2-dicarboxylic acid. X-Ray crystal structure and exceptional stereoselectivity in its Diels–Alder reaction with cyclopentadiene

Cyclobut-1-enes with 1,2-dicyano, -bisalkoxycarbonyl, or -bischloroformyl groups react with cyclopentadiene to give predominantly [4 + 2] adducts with the electron-withdrawing groups in exo-positions regardless of their size. In contrast, the addition of cyclobut-1-ene-1,2-dicarboxylic acid (Ie) to cyclopentadiene yields predominantly (88 : 12) the endo-stereoisomer. This reversal of the stereochemical course is rationalised by the existence of an intramolecular hydrogen bond (2·625 ± 0·004 A) in (Ie) and its influence on non-bonded interactions in the cycloaddition transition states. Compound (Ie) crystallises in the monoclinic space group Cc with lattice parameters a= 5·551 ± 0·002, b= 13·541 ± 0·005, c= 8·714 ± 0·003 A; β= 98·91 ± 0·03°; Z= 4. The structure was solved by packing considerations and refined by least-squares calculations to R 0·041 for 975 reflections.