Compounds of the formula (I) wherein: R1 is independently H1—OR3, —NHR4; C1-C4 alkyl; or, when n is 2, adjacent R1 together define an olefinic bond; R2 is H; or when the gem R1 is C1-C4 alkyl, that R2 may also be C1-C4 alkyl; or when the gem R1 is —OR3, that R2 may also be —C(═O)OH or a pharmaceutically acceptable ester thereof; R3 is independently H, or a pharmaceutically acceptable ester thereof; R4 is independently H or a pharmaceutically acceptable amide thereof; R5 and R6 are H or an amine prodrug moiety n is 1, 2 or 3; and pharmaceutically acceptable salts thereof; have utility in the treatment or prophylaxis of HIV, especially reverse transcriptase mutants which allow an obligate chain terminating nucleoside- or nucleotide phosphate to be excised from the nascent DNA strand by ATP- or pyrophosphate-mediated excision.