A compound of formula (I) or salts thereof, where R.sup.0 is hydrogen or together with R.sup.7 O denotes a C.sub.1 -C.sub.15 hydrocarbylidenedioxy group, R.sup.1 is hydrogen, R.sup.1.sub.a or a group of formula (II), R.sup.1.sub.a is R.sup.1.sub.b or a protecting group Q, R.sup.1.sub.b is C.sub.1 -C.sub.20 alkyl, C.sub.2 -C.sub.20 alkenyl, C.sub.3 -C.sub.10 cycloalkyl, C.sub.6 -C.sub.15 aryl, C.sub.7 -C.sub.16 aralkyl or a 5- or 6-membered heterocyclic group attached by a carbon atom in the heterocyclic group to the indicated phosphorus atom, R.sup.2 is R.sup.2.sub.a or --OR.sup.15 or together with R.sup.5 denotes an oxy group --O--, provided that R.sup.2 is --OR.sup.15 when R.sup.1 is a group of formula (II), R.sup.2.sub.a is a C.sub.1 -C.sub.20 aliphatic group, a C.sub.3 -C.sub.10 cycloaliphatic group, a C.sub.6 -C.sub.15 aromatic group, a C.sub.7 -C.sub.16 araliphatic group, or a 5- or 6-membered heterocyclic group attached by a carbon atom in the heterocyclic group to the indicated phosphorus atom, R.sup.3 is R.sup.3.sub.a or Z, R.sup.3.sub.a is hydrogen, halogen, hydroxy, R.sup.16, --OR.sup.16, OCOR.sup.16, --OSO.sub.2 R.sup.16, or tri(C.sub.1 -C.sub.15 hydrocarbyl)silyloxy, R.sup.4 is hydrogen, halogen or R.sup.17, R.sup.5 is R.sup.5.sub.a or Z or together with R.sup.2 denotes an oxy group --O--, R.sup.5.sub.a is hydrogen, halogen, hydroxy, R.sup.18, --OR.sup.18, --OCOR.sup.18, --OSO.sub.2 R.sup.18, or tri(C.sub.1 -C.sub.15 hydrocarbyl)silyloxy, or together with R.sup.6 denotes a C.sub.1 -C.sub.15 hydrocarbylidenedioxy group, R.sup.6 is halogen, hydroxy, --OR.sup.19, --OCOR.sup.19, --OSO.sub.2 R.sup.19 or B.sup.1, or together with R.sup.5 denotes a C.sub.1 -C.sub.15 hydrocarbylidenedioxy group, provided that R.sup.6 is not B.sup.1 when R.sup.1 is a group of formula (II) in which R.sup.14 is B.sup.2, R.sup.7 is hydrogen or R.sup.7.sub.a, R.sup.7.sub.a is R.sup.20, --COR.sup.20, --SO.sub.2 R.sup.20 or tri(C.sub.1 -C.sub.15 hydrocarbyl)silyl, or together with the attached oxygen atom and R.sup.0 denotes a C.sub.1 -C.sub.15 hydrocarbylidenedioxy group, R.sup.8 is hydrogen, halogen, hydroxy, R.sup.21, --OR.sup.21, --OCOR.sup.21, --OSO.sub.2 R.sup.21 or together with R.sup.10 denotes a valence bond, R.sup.9 is hydrogen, halogen or R.sup.22, R.sup.10 is hydrogen, halogen, hydroxy, R.sup.23, --OR.sup.23, --OCOR.sup.23 or --OSO.sub.2 R.sup.23, or together with R.sup.8 denotes a valence bond, R.sup.11 is hydrogen, halogen or R.sup.24, R.sup.12 is hydrogen or R.sup.12.sub.a, or R.sup.12 O-- together with R.sup.13 denotes a C.sub.1 -C.sub.15 hydrocarbylidenedioxy group, R.sup.12.sub.a is R.sup.25, --COR.sup.25, --SO.sub.2 R.sup.25 or tri(C.sub.1 -C.sub.15 hydrocarbyl)silyl, R.sup.13 is R.sup.13.sub.a or Z, R.sup.13.sub.a is hydrogen, halogen, hydroxy, R.sup.26, --OR.sup.26, --OCOR.sup.26, --OSO.sub.2 R.sup.26 or tri(C.sub.1 -C.sub.15 hydrocarbyl)silyloxy, or together with R.sup.14 denotes a C.sub.1 -C.sub.15 hydrocarbylidenedioxy group, or together with R.sup.12 O-- denotes a C.sub.1 -C.sub.15 hydrocarbylidenedioxy group, R.sup.14 is halogen, hydroxy, --OR.sup.27, --OCOR.sup.27, --OSO.sub.2 R.sup.27, or B.sup.2 or together with R.sup.13 denoted a C.sub.1 -C.sub.15 hydrocarbylidenedioxy group, provided that R.sup.14 is not B.sup.2 when R.sup.6 is B.sup.1, R.sup.15 is hydrogen or R.sup.15.sub.a, R.sup.15.sub.a is a C.sub.1 to C.sub.10 aliphatic group, a C.sub.3 to C.sub.8 cycloaliphatic group, a C.sub.6 to C.sub.15 aromatic group or a C.sub.7 to C.sub.16 araliphatic group, R.sup.16, R.sup.17, R.sup.18, R.sup.19, R.sup.20, R.sup.21, R.sup.22, R.sup.23, R.sup.24, R.sup.25, R.sup.26 and R.sup.27 are independently a C.sub.1 to C.sub.10 aliphatic group, a C.sub.3 to C.sub.10 cycloaliphatic group, a C.sub.6 to C.sub.15 aromatic group or a C.sub.7 to C.sub.30 araliphatic group, B.sup.1 and B.sup.2 are independently each a monovalent nucleoside base radical, and Z is substituted or unsubstituted C.sub.6 to C.sub.10 aryloxythiocarbonyloxy. ##STR1##