Stereoselective synthesis of 4-amino-3-hydroxy-4,5,6,6a-tetrahydro-3aH-cyclopenta[d]isoxazole-4-carboxylic acid, a conformationally constrained analogue of aspartic acid

An alternative synthesis of 4-amino-3-hydroxy-4,5,6,6a-tetrahydro-3aH- cyclopenta[d]isoxazole-4-carboxylic acid, a conformationally constrained analogue of aspartic acid, is described. The synthetic strategy is based on a regioselective 1,3-dipolar cycloaddition to give the cyclopenta[d]isoxazoline framework; subsequent condensation of this 4-oxocyclopenta[d]isoxazoline with 4-methoxybenzylamine gives a 4-imino derivative, which undergoes a highly stereoselective nucleophilic attack by the cyanide ion. This gives a 4-(methoxybenzylamino)-4-cyano derivative, which is oxidized to the corresponding 4-amino-4-cyano derivative, which itself is transformed into the target 4-carboxylic acid. This amino acid is obtained in 27% overall yield in five steps, whereas the previously described synthetic strategy gave the target derivative in only 0.5% overall yield over 15 steps.