The discovery of the karrikin class of plant growth regulators (PGRs) is a milestone accomplishment in plant biochemistry and physiology, with significant potential in agriculture and horticulture. These compounds have in common a fused furano-pyran ring system featuring various permutations of methyl substitution. Chief amongst these compounds is karrikinolide (KAR1), identified as the key germination stimulant present in plant-derived smoke, which together with five other closely-related structures (KAR2–KAR6) make up the karrikin class of PGRs. By contrast, the germination inhibitor 3,4,5-trimethyl-2(5H)-furanone has also been identified in plant-derived smoke. Various synthetic endeavours have been undertaken for structure–activity relationship study purposes as well as to probe the molecular mechanics of these compounds. In this study, syntheses of KAR1, KAR3 and S-KAR1 were carried out and their structures verified by X-ray crystallography. Effects on germination were measured against the inhibitor 3,4,5-trimethyl-2(5H)-furanone in Grand Rapids lettuce seeds. X-ray crystallographic data and germination promotory activity for S-KAR1 are described for the first time.