The synthesis of fragment B started from homoallylic TIB ester 10 (Scheme 2B), which is also the precursor of recursive reagent sulfoxide 8; this maximized the convergency of our approach by reducing the number of distinct building blocks for the construction of bastimolide B. Enantioselective Pt-catalyzed diboration with B2Pin2 gave enantioenriched 1,2-bis(boronic ester) 23 with 96:4 er on multi-gram scale. 28 Homologation of 23 with sulfoxide 8, followed by oxidation of the 1,3-bis(boronic ester), yielded diol 24 as a single diastereoisomer after chromatographic purification (>20:1 dr). The anti configuration of the diol was confirmed by 13 C NMR analysis of acetonide 25. Oxidation and protection of the C19-C21 diol at this stage not only ensured diastereopurity of this intermediate, but also provided a suitable protecting group to aid in the late-stage lithiation-borylation at C23 for the coupling of fragments A and B (vide infra). Iridium-catalyzed hydroboration of the terminal olefin with pinacolborane gave primary boronic ester 26 in 85% yield. Homologation of 26 with sulfoxide 9 proceeded smoothly to afford secondary boronic ester 27 as a single diastereoisomer in 77% yield. Once the remote stereocenter at C15 was set via reagent-controlled homologation, catalyst-controlled diastereoselective diboration of the terminal olefin was conducted with (S,S)-L1 to afford the 1,2-bis(boronic ester) 28. One further homologation with the recursive reagent 9 allowed the installation of the desired 1,5,7-tris(boronic ester) with the correct stereochemistry at C9, C11 and C15. Simultaneous oxidation of the three boronic esters, followed by O-TES protection, led to fragment B (5). Overall, our iterative approach utilising a combination of diboration and hydroboration coupled with boron homologations with recursive reagents enabled fragment B to be constructed in 10 steps from 10 with excellent stereocontrol.