Ocotea alkaloids: variabiline, a novel aminoaporphine

Synthesis, Stereochemical Resolution, and Analogue Synthesis of Variabiline, an Aporphine Alkaloid That Sensitizes Acinetobacter baumannii and Klebsiella pneumoniae to Colistin

Increasing antimicrobial resistance, coupled with the absence of new antibiotics, has led physicians to rely on colistin, a polymyxin with known nephrotoxicity, as the antibiotic of last resort for the treatment of infections caused by Gram-negative bacteria. One approach to increasing antibiotic efficacy and thereby reducing dosage is the use of small-molecule potentiators that augment antibiotic activity. We recently identified the aporphine alkaloid (±)-variabiline, which lowers the minimum inhibitory concentration of colistin in Acinetobacter baumannii and Klebsiella pneumoniae. Herein, we report the first total synthesis of (±)-variabiline to confirm structure and activity, the resolution, and evaluation of both enantiomers as colistin potentiators, and a structure-activity relationship study that identifies more potent variabiline derivatives. Preliminary mechanistic studies indicate that (±)-variabiline and its derivatives potentiate colistin by targeting the Gram-negative outer membrane.

Rapid structural characterisation of benzylisoquinoline and aporphine alkaloids from Ocotea spixiana acaricide extract by HPTLC-DESI-MSn

INTRODUCTION

Lauraceae alkaloids are a structurally diverse class of plant specialised secondary metabolites that play an important role in modern pharmacotherapy, being useful as well as model compounds for the development of synthetic analogues. However, alkaloids characterisation is challenging due to low concentrations, the complexity of plant extracts, and long processes for accurate structural determinations.

OBJECTIVE

The use of high-performance thin layer chromatography coupled with desorption electrospray ionisation multistage mass spectrometry (HPTLC DESI-MSⁿ ) as a fast tool to identify alkaloids present in Ocotea spixiana extract and evaluate the extract's acaricide activity.

METHODS

Ocotea spixiana twigs were extracted by conventional liquid-liquid partitioning. HPTLC analysis of the ethyl acetate extract was performed to separate isobaric alkaloids prior to DESI-MSⁿ analysis, performed from MS³ up to MS⁷ . The extract's acaricide activity against Rhipicephalus microplus was evaluated by in vitro (larval immersion test) and in silico tests.

RESULTS

HPTLC-DESI-MSⁿ analysis was performed to identify a total of 13 aporphine and four benzylisoquinoline-type alkaloids reported for the first time in O. spixiana. In vitro evaluation of the extract and the alkaloid boldine showed significant activity against R. microplus larvae. It was established in silico that boldine had important intermolecular interactions with R. microplus acetylcholinesterase enzyme.

CONCLUSION

The present study demonstrated that HPTLC-DESI-MSⁿ is a useful analytical tool to identify isoquinoline alkaloids in plant extracts. The acaricide activity of the O. spixiana ethyl acetate extract can be correlated to the presence of alkaloids.

Lauraceae alkaloids

Lauraceae is one of the most representative botanical families, presenting 67 genera, with over 2500 species and more than 300 different alkaloids reported, mainly isoquinolines.