Uncarialines A-E, new alkaloids from Uncaria rhynchophylla and their anticoagulant activity

Uncarialines A-E (1–5), five undescribed monoterpene indole alkaloids, together with five known analogues were obtained from the stems of Uncaria rhynchophylla. Alkaloids 1–3 were unique 3,4-seco-tricyclic alkaloids with a 6/5/10 ring system, while 4 and 5 possessed a rare rearranged scaffold originated from corynantheine-type alkaloids with C-2/C-7 oxidation. Their structures were characterized by a comprehensive analysis of MS, NMR, and ECD. Their effects on blood clotting times of human plasma were evaluated and alkaloid 5 had a slight prolongation effect on both thrombin time and activated partial thromboplastin time (p < 0.001). Graphical Abstract Supplementary Information The online version contains supplementary material available at 10.1007/s13659-023-00377-0.

The possible biogenetic routes for 1-5 is presented in Scheme 1. Biogenetically, the H-3 of uncarialin A or dihydrocorynantheine (10) was initially oxidized to hydroxyl group, and then the hydroxyl derivatives undergo hydrolysis of tertiary amine under acidic conditions to yield intermediates i and ii, which eventually undergoes reduction, oxidation, and methylation to form compounds 1-3, respectively. Likewise, dihydrocorynantheine (10) was oxidized to 2,7-dihydroxydihydrocorynantheine N-oxide, followed by hydrolysis of quaternary ammonium under acidic conditions to form the key intermediate iii, which finally undergoes rearrangement reaction to form compounds 4 and 5.

Anticoagulant activity
The anticoagulant activity of the new isolates is represented by the following parameters: thrombin time (TT), prothrombin time (PT) and activated partial thromboplastin time (APTT) [26]. Compounds 1-4 were inactive on TT, PT and APTT (p > 0.05), while compound 5 had a slightly prolongation effect on both TT and APTT (p < 0.001) ( Table 3).

General experimental procedures
The experimental apparatus is as previously reported [2, 3]

Plant material
The stems of U. rhynchophylla were obtained in Jianhe, Guizhou Province, China, on May 2020 and identified by Prof. Hongping He, one of our co-authors. The sample specimen (No. Z20200520) was deposited at Kunming Institute of Botany.

Extraction and isolation
The crushed stems of U. rhynchophylla were cold soaked 3 times in methanol (MeOH) to obtain the extract. The crude alkaloids (2030 g) were obtained by the previously procedures [2,3], which were divided to six fractions (A-F) using silica gel column chromatography (DCM/ MeOH