Asymmetric synthesis and biological activities of natural product (+)-balasubramide and its derivatives

The natural product (+)-balasubramide (3j) and its derivatives (3a–3i) were synthesized using a two-step asymmetric synthesis, and the biological activities of 3a–3j were determined in vitro. Methyl (2S,3R)-(+)-3-phenyloxirane-2-carboxylate (1h), the asymmetric synthesis of which was described in a previous paper, was selected as the starting material. Compounds 3a–3j were evaluated for their neuroprotective, antioxidative, and anti-neuroinflammatory effects. (+)-Balasubramide and its derivatives with different electronegative groups in the 6-phenyl ring produced little neuroprotection and antioxidation, but induced potent anti-neuroinflammatory effects in BV-2 microglial cells (with the exception of 3g). Compound 3c, with a trifluoromethyl group in its 6-phenyl ring, was a particularly potent anti-neuroinflammatory agent. These results demonstrated that the electronegativity of the 6-phenyl ring of (+)-balasubramide is an important determinant of its inhibitory effect on neuroinflammation. More electronegative substituents result in more potent anti-neuroinflammatory effects. Moreover, cytotoxicity assays indicated no significant effects of the tested compounds.