Leucascine, a β-alkylpyridine from the Okinawan sponge <i>Leucascus protogenes</i>

A new β-alkyl pyridine alkaloid, leucascine (<b>1</b>), was isolated from the Okinawan marine sponge <i>Leucascus protogenes</i>. Its planar structure was elucidated using a combination of 1D and 2D NMR spectroscopy, mass spectrometry, and comparison with reference compounds. The configuration at the stereogenic centre C-2 was determined through Mosher's method using MPA derivatives (compounds <b>2</b> and <b>3</b>), supported by the analysis of ¹H NMR chemical shifts. To validate this assignment, model compounds <b>7</b> and <b>8</b> were synthesised and subjected to density functional theory (DFT) calculations. Conformational analysis revealed that both enantiomers favour the <i>anticlinal-E</i> (<i>ac-E</i>) conformation, and the computed chemical shifts closely matched the experimental data, further confirming the possibility of the 2<i>S</i> configuration of leucascine (<b>1</b>). Notably, compound <b>1</b> demonstrated significant cytotoxicity, exhibiting an IC₅₀ value of 9.2 µM against NBT-T2 tumour cells.