Synthesis and Antimicrobial Activity of Calycanthaceous Alkaloid Analogues

A series of 24 novel derivatives of the calycanthaceous alkaloids with a tetrahydropyrroloindol-based core structure was synthesized from tryptophan in good yields. Their structures were characterized by IR, ¹H NMR, and ¹³C NMR spectroscopy and ESI-MS. The synthesized compounds were evaluated against a wide variety of plant pathogenic fungi. Compound a9 exhibited a high degree of activity against Curvularia lunata, with 91.0% activity at a concentration of 100 μg mL⁻¹ and with an EC50 of 44.6 μLg mL⁻¹. a7, a8, a13, and a17 exhibited high degrees of activity against Sclerotinia sclerotiorum, with a8 being the most effective with an EC₅₀ of 38.4 μg mL⁻¹. Compound a9 illustrated activity against Botrytis cinerea, with an EC₅₀ of 79.5 μg mL⁻¹. Considering the compounds evaluated, the alkyl substituents of the chain may contribute to the significant variations in fungicidal potency. The structure antifungal activity relationships are also discussed. These results will pave the way for further design, structural modification, and development of calycanthaceous alkaloids as antimicrobial agents.