Six new C-20 and one new C-19 quassinoids, named perforalactones F-L (1–7), were isolated from twigs of Harrisonia perforata. Spectroscopic and X-ray crystallographic experiments were conducted to identify their structures. Through oxidative degradation of perforalactone B to perforaqussin A, the biogenetic process from C-25 quassinoid to C-20 via Baeyer–Villiger oxidation was proposed. Furthermore, the study evaluated the anti-Parkinson’s disease potential of these C-20 quassinoids for the first time on 6-OHDA-induced PC12 cells and a Drosophila Parkinson’s disease model of PINK1^B9. Perforalactones G and I (2 and 4) showed a 10–15% increase in cell viability of the model cells at 50 μM, while compounds 2 and 4 (100 μM) significantly improved the climbing ability of PINK1^B9 flies and increased the dopamine level in the brains and ATP content in the thoraces of the flies.