Aldolization of meso dialdehyde 3 with a borylenolate obtained from chiral N-propionylsultam 4 yields efficiently lactols 5 with simultaneous generation of five stereogenic centers.Dithioketalization/O-desilylation of 5 affords acyclic diol 9 which is converted into the marine polypropionates 1a and 1b via the asymmetric alkylation of N-propionyltoluenesultam 11 and the aldol coupling 10 + 15 → 16.