Conformationally constrained serine analogues: Synthesis of new 2-amino-3-hydroxynorbornanecarboxylic acid derivatives

A synthesis of the new oxazolone 2, functionalized with the ethoxycarbonyloxy group on methylenic carbon, is presented, starting from 4-hydroxymethylenoxazolone 1 and ethyl chlorocarbonate. Oxazolone 2 was reacted with cyclopentadiene in the presence of EtAlCl 2, giving the two diastereoisomeric cycloadducts exo-3 and endo-3 in a 70:30 ratio. Selective hydrolysis of the lactone ring (THF, HCl) gave the corresponding acids 5 and 6 which were transformed into hydroxyacid derivatives 7 and 8, respectively, operating in an ethanolic solution of Me 2NH. The new 3-hydroxy-2-aminonorbornane-2-carboxylic acids 11 and 12, in which the serine skeleton is included, were obtained by reduction of acids 5 and 6 to derivatives 9 and 10 and a subsequent hydrolysis with HCl.