These conditions for the lithiation-borylation of model TIB ester 25 were successfully implemented in the coupling of the advanced fragments A and B (Scheme 4), and subsequent oxidation gave alcohol 2 in 68% yield with excellent stereocontrol at C23 (>20:1 dr). Although a significant excess of TIB ester 5 was required for optimum yield, unreacted 5 could easily be recovered by chromatographic purification. The lithiation-borylation between 5 and 4 not only provided the open-chain backbone of bastimolide B with all the correct functionality and stereochemistry in place, boronic ester oxidation also revealed the alcohol required for closure of the 24-membered macrolactone without having to manipulate protecting groups. The final stages of the synthesis involved esterification of the alcohol at C23 under modified Yamaguchi conditions with acid (Z)-3. In the following one pot protocol, the terminal olefin underwent hydroboration with 9-BBN followed by stereoretentive Suzuki crosscoupling with the (Z)-iodo alkene, giving the 24-membered macrolactone 35 in 42% yield. This macrocyclization approach demonstrated the power of intramolecular Suzuki cross-coupling both for macrolactone formation and for the installation of a Z-configured alkene in a particularly challenging setting. Finally, deprotection of the O-TES and acetonide groups in macrolide 35 occurred smoothly in methanol with DOWEX acidic resin, affording a pure sample of bastimolide B (1, 94% yield). The synthetic material matched the natural sample in all aspects ( 1 H NMR, 13 C NMR, HRMS, IR, optical rotation, ECD), confirming the proposed structure, and its absolute stereochemistry. Using very high 13 C-resolution (<0.01 ppm) data from pure-shift HSQC and HSQC-TOCSY NMR methods, we were also able to establish the two-and three-bond connectivity of the molecule and fully assign every 1 H and 13 C NMR signal. It is worth noting that, although the 1 H and 13 C NMR of our synthetic sample matched the data for the natural product, many of the assignments of the NMR signals did not (see SI for details). Fortunately, these errors in the 1 H/ 13 C assignments did not detract from the 2D and 3D structure determination in the original report because their structure proposal was informed by comparison to X-ray analysis of the related natural product bastimolide A, rather than relying exclusively on NMR. In conclusion, we have shown that homologation of boronic esters using a magnesiated butenyl TIB ester followed by hydroboration of the terminal alkene provides an iterative method for the generation of 1,5-polyols with full stereocontrol. This method has been applied to a 16-step (LLS) synthesis of bastimolide B (26 steps in total) with full stereocontrol. Reagent-controlled boronic ester homologations were used to create all the key C-C bonds, including a late-stage fragment coupling, as well as installing eight of the ten stereogenic centers, with the remaining two installed by asymmetric diboration. Four of the six 1,5-related stereocenters were generated using the magnesiated-butenyl building block in homologations of poly(boronic esters) which showed essentially complete selectivity in favor of reaction at the primary boronic ester, even in the presence of multiple secondary boronic esters. This versatile methodology will undoubtedly find further applications in synthesis.