Synthesis of Small Libraries of Natural Products—Part III: Identification of New Esters from Pelargonium graveolens L’Her. (Geraniaceae) Essential Oil

<i>Pelargonium graveolens</i> (rose geranium) essential oil contains numerous aroma-active esters that are challenging to identify at low abundance. We obtained by preparative chromatography an ester-rich fraction of the essential oil and constructed a synthetic reference library of 159 structurally related esters (spectral/GC data provided; 102 newly synthesized). This enabled dereplication and detection of constituents not apparent in direct GC-MS of the unfractionated oil. Nine esters (5-methylhexyl formate, (<i>Z</i>)-hex-3-en-1-yl 3-methylpentanoate, 3-methylbutyl 3-methylpentanoate, 3-methylpentyl 4-methylpentanoate, 5-methylhexyl hexanoate, 3-methylbutyl 6-methylheptanoate, 2-phenylethyl 6-methylheptanoate, 5-methylhexyl tiglate, and 6-methylheptyl tiglate) were confirmed as new natural products (eight of them new compounds overall), by combined evidence from retention indices, EI mass spectra, co-injections with synthesized references, and, in selected cases, by 1D/2D NMR. Systematic RI trends across acid and alcohol isomers were delineated, aiding rapid differentiation of regio-isomeric esters that share similar EI patterns. This library-guided workflow offers a robust path to differentiate structurally close volatiles in complex matrices and provides transferable RI/spectral benchmarks for future natural product identification.