Building the Housane: Diastereoselective Synthesis and Characterization of Bicyclo[2.1.0]pentane Carboxylic Acids

An approach to 1,3-disubstitued bicyclo[2.1.0]­pentane (housane) derivatives was developed. The method relied on lithium bis­(trimethylsilyl)­amide-mediated intramolecular cyclization of trisubstitued cyclopentane carboxylates bearing a leaving group (at the C-4 position) and an additional substituent (at the C-3 atom), in turn synthesized from cyclopent-3-ene carboxylate. The synthetic sequence allowed for the preparation of both <i>cis</i>- and <i>trans</i>-1,3-disubstituted housane-1-carboxylic acids in diastereoselective manner on up to 80 g scale. In particular, bicyclic γ-amino acidsγ-aminobutyric acid analogueswere synthesized. It was shown that the bicyclo[2.1.0]­pentane did not significantly affect p<i>K</i>_a of the corresponding derivatives and slightly increased their hydrophilicity (by 0.07–0.25 Log <i>P</i> units) as compared to cyclopentane. X-ray diffraction studies showed that <i>cis</i>- and <i>trans</i>-1,3-disubstituted housanes can be considered as flattened analogues of the corresponding cyclopentane derivatives with fixed envelope conformation of the five-membered ring.