The present invention relates to methods and novel intermediates useful in the preparation of a compound of formula (A):, or a pharmaceutically acceptable salt thereof, wherein R1is H or C1-C6alkyl.本發明係關於有用於製備式(A)化合物:, 或其醫藥上可接受之鹽的方法及新穎中間產物,其中R1為H或C1-C6烷基。