Process for preparing purine derivatives and intermediates thereof

A process for the preparation of a compound of formula (A):

• ##STR37## wherein:
• X is hydrogen, hydroxy, chloro, C.sub.1-6 alkoxy or phenyl C.sub.1-6 alkoxy;
• and R.sub.a and R.sub.b are hydrogen, C.sub.2-5 alkanoyl or phosphate derivatives thereof, which process comprises:
• (i) the preparation of a compound of formula (I):
• ##STR38## wherein R.sub.1 is C.sub.1-6 alkyl, or phenyl C.sub.1-6 alkyl in which the phenyl group is optionally substituted with one or two substituents selected from the group consisting of hydroxy, C.sub.1-6 alkyl, C.sub.1-6 alkoxy or halogen;
• R.sub.2 is hydrogen, hydroxy, chlorine, C.sub.1-6 alkoxy, phenyl C.sub.1-6 alkoxy or amino;
• and R.sub.3 is halogen, C.sub.1-6 alkylthio, C.sub.1-6 alkylsulphonyl, azido, an amino group C.sub.2-5 alkanoylamino, benzoylamino or benzylamino, which preparation comprises the reaction of a compound of formula (II):
• ##STR39## wherein R.sub.2 and R.sub.3 are as defined for formula (I) with:
• (a), a compound of formula (III):
• ##STR40## wherein R.sub.4 and R.sub.5 are independently hydrogen, C.sub.1-6 alkyl, or phenyl, or R.sub.4 and R.sub.5 together are C.sub.5-7 cycloalkyl, to give a compound of formula (IV):
• ##STR41## and thereafter converting the intermediate compound of formula (IV) to a compound of formula (I) via transesterification, and, as necessary or desired, interconverting variables R.sub.1, R.sub.2 and R.sub.3 to further values of R.sub.1, R.sub.2 and R.sub.3 ;
• and thereafter
• (ii) the conversion of the resulting compound of formula (I) to a compound of formula (A) by
• a. converting variable R.sub.3, when other than amino, to amino,
• b. reducing the ester groups CO.sub.2 R.sub.1 to CH.sub.2 OH and optionally forming C.sub.2-5 alkanoyl or phosphate derivatives thereof, and
• c. as necessary or desired converting variable R.sub.2 in the compound of formula (I) to variable X in the compound of formula (A).