COUMARINS IN ORCHIDACEAE: A REVIEW CONNECTING ETHNOMEDICINE AND THERAPEUTIC EXPECTATIONS

Orchids hold a strong ethnomedicinal background with varied bioactive compounds including coumarins. The current review explores the coumarin compounds reported in the family of Orchidaceae to achieve better therapeutic expectations. This review was conducted from November 2019 to April 2021 by applying the mixed-method review approach. Ayapin, Coumarin, dihydroayapin, denthyrsin, 3,4-dihydrocoumarin, dendrocoumarin, esculetin, scoparone, scopoletin, scopolin, sinensin A and sinensin B were found to have been reported in orchids. Even though preliminary phytochemical screening of several orchids revealed the presence of coumarins, only a few of them were further investigated for their isolation, identification and pharmacological activity. This lacuna on the role of coumarins in Orchidaceae demanded a further investigation on their ethnomedicinal background. Ethnobotanical surveys conducted by various researchers have substantiated the use of orchids in a varied range of illnesses. Interestingly, the documented folklore claims correlate with the unique anti-inflammatory and anti-edema potential of the coumarin entity. Thus the review highlight the coumarin compounds reported in orchids that could provide better insight for further investigation of this secondary metabolite in the family of Orchidaceae owing to its therapeutic excellence.


Coumarins in Orchidaceae
The review determined coumarins including ayapin, coumarin, dihydroayapin, denthyrsin, 3,4dihydrocoumarin, dendrocoumarin, esculetin, scoparone, scopoletin, scopolin, sinensin A and sinensin B in Orchidaceae and was displayed in Table-1. Figure-1 represents the chemical structure of isolated coumarins from different plant parts of Orchidaceae. Coumarins in Dendrobium thyrsiflorum D. thyrsiflorum is characterized by a long cluster of yellow flowers with dark orange lip. 21,22 The investigation of coumarins was initiated by the study of Wrigley, when the researcher detected ayapin, scopoletin and scoparone (6,7-dimethoxy coumarin) from the leaf hydrolysate of D. thyrsiflorum. 23 A study by Zhang et al. has detected scoparone and denthyrsin in the stems of D. thyrsiflorum. Further, the study investigated the cytotoxic potential of these isolated compounds on MCF-7, HeLa and K-562 cancer cells by in-vitro MTT assay. Erianin was used as the positive control for the study. The results of the research revealed that denthyrsin exhibited more cytotoxic activity (IC50 between 0.45-18.1μM) than scoparone (IC50> 100μM) and this was attributed to the coumarins turning more active after coupling with benzofuran. 24 Also, a qualitative and quantitative analysis of ayapin and scoparone in D.thyrsiflorum was validated using HPLC-DAD-ESI-MS method. 25 A study by Zheng et al. on the localization and relative quantity of coumarins in the top, middle and basal parts of stem of D. thyrsiflorum using laser scanning confocal microscopy (LSCM) suggested the collection of the two-year-old stem during February would be favorable for high yield of coumarins. 26 Furthermore, the coumarin content and its distribution in the leaf and root of D. thyrsiflorum at different developmental stages were studied using LSCM. The study reported coumarins at their highest level during the flowering stage than the fruiting or vegetative period. 27 Another similar study was carried out by Zheng et al. in the top, middle and basal parts of D. thyrsiflorum stems at the flowering and initial fruiting stages of 1, 2 and 3-year-old plants. Coumarins reached their maximum content during the flowering stage, suggesting that harvesting could be carried out when the plants enter the flowering period to achieve the maximum yield of coumarins. The study further reported the distribution of coumarins in vascular bundles and the cell wall of the stems of D. thyrsiflorum. 28 Additionally, scoparone and ayapin were isolated and characterized from the ethanol extract of D. thyrsiflorum. 29 Coumarins in Dendrobium densiflorum D. densiflorum, also known as D. clavatum is characterized by cream to butter yellow-colored flowers. 21 The leaves were used to set fractures while the pseudobulbs were applied on pimples and boils. [30][31][32] According to the study by Zheng et al., the ethanol extract obtained from the fresh stems of D. densiflorum was subjected to successive soxhlet extraction for 4hours with petroleum ether (60-90°C) followed by chloroform and ethylacetate. Chromatography of the ethylacetate extract over silica gel column using ethyl acetate/petroleum ether (3:7) yielded dihydroayapin. Moreover, scopolin from the petroleum ether extract and ayapin from the chloroform extract was also reported in the study. 33 Fan et al. reported ayapin, scoparone and scopoletin in the stems of D. densiflorum. Further, among the isolated compounds, scopoletin and scoparone were found to possess potent anti-platelet aggregation activity on Sprague-Dawley rats. 34

Coumarins in Dendrobium moschatum
The leaves of D. moschatum are used to relieve earaches and the pseudobulbs in treating fractures. 35 Gas Chromatography/Mass Spectroscopy (GC/MS) analyses, the coumarins accounted for 2157.1μg/g (48.8%) and 180.1μg/g (47.8%) of the total volatiles from the inflorescence and leaves of D. moschatum respectively. Interestingly, 3, 4-dihydrocoumarin was reported in the inflorescence of D. moschatum, but the same was found absent in leaves. 37 Coumarins in Dendrobium denneanum A systematic phytochemical study carried out by Pan et al. reported coumarin in the stems of D. denneanum. According to the study, the ethanol extract of stems of D. denneanum was further partitioned with n-butanol and chloroform, and the respective fractions were subjected to column chromatography and preparative HPLC. 38

Coumarins in Spiranthes sinensis
In a study by Peng et al., two new prenylatedcoumarins, namely sinensin A and sinensin B were reported in the roots of S. sinensis. Sinensin A and B are chemically 5-γ,γ-dimethylallyl-8-[2-(2,6dihydroxyphenyl)-3-dimethyl-but-2-enyol]-umbelliferon and 4,6-di(γ,γ-dimethylallyl)-8-lavandulylumbelliferon respectively. 43 Orchids with coumarins remaining unrevealed The phytochemical screening of secondary metabolites in various parts of Eria tomentosa studied by Akter et al. revealed coumarins to be substantially higher in stems and roots compared to leaves. In contrast, the study by Sohag et al. showed higher coumarin content in the leaves and stems of Luisia zeylanica, than that in the roots. 44 46 However, isolation, identification and pharmacological screening of coumarins in these orchids remain uninvestigated. This existence of an incomplete bridge in the role of coumarins in Orchidaceae further directed the investigation upon ethnomedicine behind these orchids.

Orchidaceae and Ethnomedicine
Traditional folklore practices remain a primary source of traditional knowledge and a promising guide in natural products research. 47 Even, most of the well-established natural leads have been developed from their traditional claims. 48 The earliest documentation in orchids dates back to 28 th century B.C., when

CONCLUSION
Coumarins remain as an under-explored secondary metabolite in the family of Orchidaceae. The planar, aromatic and lipophilic nature of the 2H-chromen-2-one ring allows better interaction with several protein targets exhibiting a wide range of pharmacological activity. 15 Apart from its scientifically proven potential against cancer, burns, cardiovascular and rheumatic diseases, reports on natural coumarins also add a special note on its unique anti-edema and anti-inflammatory activity that may be attributed to the presence of coumarin ring. 59,60 Coumarins are reported to be found in abundance in the families of Apiaceae, Caprifoliaceae, Clusiaceae, Guttiferae, Nyctaginaceae, Oleaceae, and Rutaceae. 61 Moreover, the long-established medicinal practice of orchids further kindles several therapeutic expectations, in turn, reflects an apparent lacuna in their transformation into scientific research. Hence, the current review will highlight the diversified biopharmacological activity of coumarin entity with a promising traditional background of Orchidaceae, leading to further phytochemical and phytopharmacological studies in orchid research.