S-Alpine-Borane, also known as (S)-Alpine-Borane or (S)-B-3-pinanyl-9-borabicyclo[3.3.1]nonane, is a chiral organoborane reagent derived from (+)-α-pinene. It is a commercially available, air- and moisture-sensitive compound typically supplied as a solution in tetrahydrofuran (THF). Its defining feature is the presence of a chiral borane moiety, which imparts high stereoselectivity in specific chemical transformations. Its primary and most significant application is as a highly enantioselective reducing agent for prochiral ketones. Specifically, S-Alpine-Borane exhibits excellent selectivity in the asymmetric reduction of α,β-acetylenic ketones to yield the corresponding chiral propargylic alcohols with high enantiomeric excess (ee). This reaction is a cornerstone in the synthesis of chiral building blocks for pharmaceuticals, natural products, and other fine chemicals. The reagent's selectivity arises from its well-defined chiral environment provided by the pinane skeleton. Due to its specialized nature and role in facilitating stereocontrolled synthesis, it is categorized as a chiral reagent within synthetic chemistry. It is not a general-purpose reducing agent but a tool for specific, high-value enantioselective reductions. Proper handling requires an inert atmosphere (e.g., nitrogen or argon) to prevent decomposition.