Structure of a thionucleoside: 5'-deoxy-5',6-epithio-5,6-dihydro-2',3'-O-isopropylidene-3-methyluridine

C<SUB>13</SUB>H<SUB>18</SUB>N<SUB>2</SUB>OS, M<SUB>r</SUB>=314.35 orthorhombic,P2<SUB>1</SUB>2<SUB>1</SUB>2<SUB>1</SUB>, with a=39.526(4), b=6.607(2),c=5.661(2)Å,Z=4, V=1478.36Å<SUP>3</SUP>, D<SUB>c</SUB>=1.412 Mgm<SUP>-3</SUP>, Cu Kα radiation. Final R = 0·073 for 1154 observed counter reflections. The sulphur atom is in a pseudo-equatorial position with respect to the dihydrouracil ring. The sugar pucker is predominantly O(l')exo unlike the C(3')-exo,C(4'}endo observed for 2'.3'·O·isopropylideneuridine (lSPU). The five­ membered dioxolane ring has C(7) displaced by 0·497 (7) Å from the best plane through atom, 0(2'), C(2'), e(3'), 0(3'), in contrast to ISPU where 0(3') shows the maximum deviation