Salvileucalin B

While identifying new medicinal clues from the plant Salvia leucantha (Figure 1), Aoyagi et al. successfully isolated Salvileucalin B in 2008.1 Plants of the Salvia type have a very rich history in medicine. The name Salvia actually originates from the Latin word salvare, which means ‘to heal’. This isolated molecule exerts cytoxic activity against A594 (human lung adenocarcinoma) and HT-29 (human colon adenocarcinoma) cells with IC50 values of 5.23 and 1.88 µg/mL, respectively.1 Salvileucalin B is a unique member of the clerodane family because it contains an attractive carbon framework. The structure of this molecule is shown in Figure 2 below:

Figure 1: Salvia Leucantha Figure 2: Salvileucalin B

The challenging synthetic feature of Salvileucalin B is that is possesses a stable norcaradiene core fixed within a polycylic skeleton.2 The heart of the synthetic appeal is the norcaradiene core. This six membered diene ring fused to a fully substituted cyclopropane is a puzzle for synthetic chemists. Structures of this type often rearrange to seven-membered cycloheptatriene rings, but in this case, the polycyclic framework around it stabilizes it. Studies have shown that due to the geometrical constrains, the norcaradiene is stable at room temperature,11 which means that the electrocyclic ring-opening of the norcaradiene core would result in two bridgehead olefins within a strained [4.3.1] bicycle, which is highly unfavorable.12 This norcaradiene core of Salvileucalin B is also decorated with two γ-lactones, a furan moiety and two tertiary carbon stereocenters. The formation of the stable norcaradiene demands the construction of a fully substituted cyclopr...

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