Compounds of formula (I), wherein A' and A" are independently the same or different group of formula (II) wherein: R' is H, CH.sub.3, C(CH.sub.3).sub.2, --OR.sup.a, --N(R.sup.a).sub.2, --N(R.sup.a)OR.sup.a or --DP; R'" is H or CH.sub.3 ; R.sup.a is H, C.sub.1 -C.sub.3 alkyl; D is a bond, alkylene, --C(.dbd.O)--, --S(O)-- or S(O).sub.2 --; P is an optionally substituted, mono or bicyclic carbo- or hetereocycle; R" is H, any of the sidechains found in the natural amino acids, carboxacetamide, or a group (CH.sub.2).sub.n DP; M is a bond or --C(.dbd.O)N(R'")--; Q is absent, a bond, --CH(OH)-- or CH.sub.2 --; or R" together with Q, M and R' define an optionally substituted 5 or 6 membered carbo- or heterocyclic ring which is optionally fused with a further 5 or 6 membered carbo- or heterocyclic ring; with the proviso that R' is --OR.sup.a, --, N(R.sup.a).sub.2, --N(R.sup.a)OR.sup.a or -DP, if M is a bond and Q is absent; X is H, OH, OCH.sub.3, Y is H, OH, OCH.sub.3, but X and Y are not both H; Z' and Z" are independently --(CH.sub.2).sub.m P where P is as defined above; n and m are independently 0, 1 or 2; and pharmaceutically acceptable salts and prodrugs thereof have utility as aspartyl protease inhibitors of HIV. They can be prepared in a facile two step synthesis from novel 2,5-di-O-benzyl-L-mannaro-1,4:6,3-dilactone intermediates.