Chiral beta-aminoalkylboronates generate growing interest as versatile synthetic building blocks to access beta-aminoalcohols and other useful compounds, and also as bioisosteres of beta-amino acids in drug discovery. In this study, the lack of methodology to access both syn and anti diastereomers of optically enriched, acyclic alpha,beta-disubstituted beta-aminoalkylboronates is addressed with the development of a divergent, diastereoselective strategy for the monoprotodeboration of beta-amino gem-bis-(boronate) precursors. To this end, new reaction conditions were successfully optimized to provide the elusive anti diastereomer by inverting a sequence of desulfinylation and proto-deboronation. The desired syn or anti isomers are isolated independently in good yields and excellent diastereoselectivity (up to >20:1 dr) for a wide scope of substituents. The diastereotopic group selectivity of the new conditions yielding the anti isomer is rationalized by invoking a reactive rotamer featuring two ammonium-boronate hydrogen bonds, which enables phosphate coordination to boron with a concomitant, stereoretentive protonation of the least sterically hindered C-B bond. The accessibility and utility of both diastereomers of these alpha,beta-disubstituted beta-aminoalkylboronates is exemplified with the functionalization of the amino group, stereospecific oxidation to beta-amino alcohols and C-C bond transformations of the secondary alkylboronate, and the preparation of free boronic acids and hemiboronic heterocycles.