2,3-Ethylene- and 2,3-trimethylene-bridged analogues of the group III metabotropic glutamate receptor ligand 2-amino-4-phosphonobutanoic acid

The racemic trans- and cis-isomers of 1-amino-2-phosphonomethyl-cyclobutanecarboxylic acid (5 and 6) and 1-amino-2-phosphonomethyl-cyclopentanecarboxylic acid (7 and 8) were synthesized as extensions of the mGluR4 agonists trans- and cis-1-amino-2-phosphonomethyl-cyclopropanecarboxylic acid (3 and 4). Although the methylene bridge in 3 and 4 allows for retention of affinity toward the mGluR4 receptor, increasing the bridging unit to the ethylene group as in 5 and 6 or to the trimethylene group as in 7 and 8 introduces sufficient steric hindrance to eliminate affinity for the mGluR4 receptor.