Seven new polyaromatic bis-spiroketal-containing butenolides, the prunolides D-I (4-9) and cis-prunolide C (10), a new dibrominated β-carboline sulfamate named pityriacitrin C (11), alongside the known prunolides A-C (1-3) were isolated from the Australian colonial ascidian Synoicum prunum. The prunolides D-G (4-7) represent the first asymmetrically brominated prunolides, while cis-prunolide C (10) is the first reported with a cis-configuration about the prunolide's bis-spiroketal core. The prunolides displayed binding activities with the Parkinson's disease-implicated amyloid protein α-synuclein in a mass spectrometry binding assay, while the prunolides (1-5 and 10) were found to significantly inhibit the aggregation (>89.0%) of α-synuclein in a ThT amyloid dye assay. The prunolides A-C (1-3) were also tested for inhibition of pSyn aggregate formation in a primary embryonic mouse midbrain dopamine neuron model with prunolide B (2) displaying statistically significant inhibitory activity at 0.5 μM. The antiplasmodial and antibacterial activities of the isolates were also examined with prunolide C (3) displaying only weak activity against the 3D7 parasite strain of Plasmodium falciparum. Our findings reported herein suggest that the prunolides could provide a novel scaffold for the exploration of future therapeutics aimed at inhibiting amyloid protein aggregation and the treatment of numerous neurodegenerative diseases.