A highly diastereo- and enantioselective boron-mediated pentenylation reaction is presented. The chiral pentenylborane reagents, which are derived from pinene, undergo addition to various aldehydes to afford ethyl-substituted homoallylic alcohols in good yield and high stereoselectivity. The latter are then easily converted into the corresponding α,β-unsaturated δ-lactones using an acylation/ring-closing metathesis sequence. The relative and absolute stereochemistry is exclusively controlled by the reagent.