A practical, scalable synthesis of the C.1-C.13 segment of halichondrin B has been developed starting from L-mannonic-gamma-lactone, using C-allylation/oxy-Michael cyclization and Ni(II)/Cr(II)-mediated vinyltrimethylsilane addition to set the C.6/C.3 and C.11 stereocenters, respectively. Copyright (C) 1996 Elsevier Science Ltd