We began our synthesis by targeting fragment A, which required initial synthesis of the sterically hindered neopentyl boronic ester 15 (Scheme 2A). In principle, there are two possible homologation reactions to construct 15: (1) reaction of metalated butenyl TIB ester A with t BuBPin or (2) reaction of a metalated neopentyl TIB ester with allylBpin. Unfortunately, the first was unsuccessful due to the failure of A (M = Li) to form a boronate complex with t BuBPin so we explored the second method. Carbenoid formation by deprotonation of neopentyl TIB ester 12 with s BuLi in the presence of (-)-sparteine (t1/2 = ~90 min, -78 °C) followed by addition of allylBPin 14 gave 15 in only 17% 1 H NMR yield but 98:2 er. We reasoned that the high steric demand of the neopentyl carbenoid resulted in low nucleophilicity towards the boronic ester. To reduce steric hindrance, we elected to use diaminefree conditions in the homologation, which could be achieved by generating the carbenoid via tin-lithium exchange of stannane 13. Thus, treatment of 13 with n BuLi followed by trapping with allylBPin 14 and heating delivered homoallylic boronic ester 15 in high yield (94% NMR yield) and enantioselectivity (98:2 er). With 15 in hand, we explored our proposed iterative hydroboration/boronic ester homologation sequence for the construction of 1,5related stereogenic centers. Without purification, crude 15 was directly engaged in an iridium-catalyzed hydroboration with HBPin to give exclusively the anti-Markovnikov 1,4-bis(boronic ester) 16 in 72% yield over 2 steps on multi-gram scale. Homologation of 16 with the magnesiated carbenoid generated in situ from benchstable sulfoxide 8 revealed 1,5-bis(boronic ester) 17 as a single diastereoisomer. The magnesium-sulfoxide exchange and subsequent borylation and 1,2-migration proceeded under mild reaction conditions using i PrMgCl•LiCl, and no competing addition to the secondary boronic ester was observed. Pleasingly, two further iterations of this hydroboration/homologation sequence provided 1,5-tetra(boronic ester) 6 in 46% yield over 4 steps on gram scale. Notably, homologation of boronic ester 20, bearing 3 secondary boronic esters and one primary boronic ester resulted in exclusive chemoselective reaction of the primary boronic ester in 89% yield. Oxidation of the tetra(boronic ester) yielded stereochemically pure tetraol 21 with the desired stereochemistry at C27, C31, C35 and C39. Protection of 21 as triethylsilyl ether 22, followed by catalytic hydroboration of the terminal olefin provided primary boronic ester 4 (fragment A) in 11 steps from 12 (12 steps from commercially available neopentanol).