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Statement of the Problem: Lycopodium alkaloids are quinolizine, pyridine and pyridone alkaloids isolated from club mosses (Lycopodiaceae). The most notable alkaloid from this group is huperzine A, which is a potent reversible Acetylcholinesterase (AChE) inhibitor. Studies on Lycopodium alkaloids from club mosses in Southeast Asia are deficient. This work aimed to phytochemically investigate club mosses native to Thailand and the Philippines. Methodology: Whole plants of H. squarrosa collected from Thailand and the Philippines were extracted with methanol. The methanolic extracts were subjected to acid-base extraction. The obtained alkaloidal fractions were further purified through column chromatography. Findings: H. squarrosa from Thailand yielded four alkaloids. Two known Lycopodium alkaloids were identified to be huperzine A (1) and 12-epilycodoline N-oxide (4). Squarrosine A (2) was a new fawcettimine-type Lycopodium alkaloid which possessed intramolecular hydrogen bonding. (R)-2-piperidineacetic acid (5) has never been reportedly isolated. This alkaloid was speculated to derive from precursors of Lycopodium alkaloids. From Philippine H. squarrosa, huperzine A (1) and pyrrolhuperzine A (3), a new lycodine-related Lycopodium alkaloid bearing a rare pyrrole moiety, were isolated. Semi-synthetic approaches to pyrrolhuperzine A (3) were achieved to confirm its structure elucidation, and two plausible biogenetic pathways from huperzine A (1) to pyrrolhuperzine A (3) were proposed. Furthermore, huperzine A (1) was chemically transformed into three amide derivatives (6-8). The newly isolated and semi-synthetic alkaloids were assayed for their anti-AChE activities. Huperzine A derivatives 6 and 7 exhibited strong AChE inhibition.