Conformation and hydrogen bonding for the bicyclic compound 3-thiabicyclo[3.2.0]heptane-6,7-dicarboxylic acid...

The initial goal of this work was to verify the geometry of the product of a photochemical reaction, viz. the title compound, C(8)H(10)O(6)S, (II). Our crystallographic study firmly establishes the cis-anti-cis nature of the substituents on the cyclobutane ring. The geometry is also designated as exo, where exo signifies that the five-membered ring is on the opposite side of the central cyclobutane ring from the carboxylic acid substituents. The structure determination reveals two molecules, A and B, in the asymmetric unit that display substantially different conformations of the bicyclic core: the cyclobutane ring puckering angles are 22 and 3 degrees , and the sulfolane ring conformations are twist (S-exo) and envelope (S-endo). Intrigued by this variation, we then compared the conformations of other molecules in the Cambridge Structural Database that have sulfolane rings fused to cyclobutane rings. In this class of compound, there are five examples of saturated cyclobutane rings, with ring puckering angles ranging from 3 to 35 degrees . The sulfolane rings were more similar: four of the six molecules exhibit envelope conformations with S-endo, as in molecule B of (II). Despite the conformational differences, the hydrogen-bonding scheme for both molecules is similar: carboxyl -OH groups form hydrogen bonds with carboxyl and sulfone O atoms. Alternating A and B molecules joined by hydrogen bonds between sulfone O atoms and carboxyl -OH groups form parallel chains that extend in the ac plane. Other hydrogen bonds between the carboxyl groups link the chains along the b axis.