Naturally Occurring Chromone Glycosides: Sources, Bioactivities, and Spectroscopic Features

Chromone glycosides comprise an important group of secondary metabolites. They are widely distributed in plants and, to a lesser extent, in fungi and bacteria. Significant biological activities, including antiviral, anti-inflammatory, antitumor, antimicrobial, etc., have been discovered for chromone glycosides, suggesting their potential as drug leads. This review compiles 192 naturally occurring chromone glycosides along with their sources, classification, biological activities, and spectroscopic features. Detailed biosynthetic pathways and chemotaxonomic studies are also described. Extensive spectroscopic features for this class of compounds have been thoroughly discussed, and detailed 13C-NMR data of compounds 1–192, have been added, except for those that have no reported 13C-NMR data.


Introduction
Chromone glycosides are a class of secondary metabolites with various medicinal properties. They are widely distributed in many plant genera and, to a lesser extent, in some fungal species and other sources [1]. Several biological activities have been reported for various chromone glycosides. For example, aloesin and its analogues, from Aloe, are used in cosmetic preparations to treat hyperpigmentation induced by UV radiation, owing to their role in inhibition of tyrosinase enzyme [2,3]. Additionally, 8-[C-β-D-[2-O-(E)cinnamoyl]glucopyranosyl]-2-[(R)-2-hydroxypropyl]-7-methoxy-5-methylchromone), isolated from certain Aloe species, was reported to have potent topical anti-inflammatory activity comparable to the effect of hydrocortisone without affecting thymus weight [3]. Macrolobin, from Macrolobium latifolium, has a remarkable acetylcholinesterase inhibitory activity with an IC 50 value of 0.8 µM. Uncinosides A and B, isolated from the Chinese herbal medicine Selaginella uncinata, showed potent anti-RSV (respiratory syncytial virus) activity with IC 50 values of 6.9 and 1.3 µg/mL. Taking into consideration the broad biological activities of chromone glycosides, this review summarizes the naturally occurring chromone glycosides and categorizes these compounds on their structural basis, in addition to their sources, bioactivities and spectroscopic features. Importantly, this review will shed more light toward the NMR features of chromone glycosides to help natural product researchers in the identification of various chemical structures. Scientific databases as SciFinder, PubMed, and Google Scholar were used to collect the relevant literature data.

Biosynthesis
Chromones are biosynthesized through the acetic acid pathway by the condensa tion of five acetate molecules. These compounds, generally, have a methyl group at C-2 and are oxygenated at C-5 and C-7 [4]. Pentaketide Chromone Synthase (PCS) is a key enzyme in the biosynthesis process that catalyzes the formation of a pentaketide chro mone (5,7-dihydroxy-2-methylchrome) from five-step decarboxylative condensations o malonyl-CoA, followed by the Claisen cyclization reaction to form an aromatic ring However, it is unclear whether the heterocyclic ring closure of the pentaketide chro mone is enzymatic or not, because the ring closure can take place due to spontaneous Michael-like ring closure, as in the case of flavanone formation from chalcone in vitro PCS also accepts acetyl-CoA, resulting from decarboxylation of malonyl-CoA, as a start er substrate, but it is a poor substrate for PCS [5].The pentaketide chromone has been isolated from several plants and is known to be the biosynthetic precursor of the chro mone derivatives with additional heterocyclic rings (e.g., furano-, pyrano-and oxepino chromone glycosides). Scheme 1 ( [6] with modifications) shows the sequence of steps utilized in the biosynthesis of these compounds, fully consistent with the biosynthetic rationale developed above. The key intermediate is 5,7-dihydroxy-2-methylchromone [5,6]. For many years, the cyclization had been postulated to involve an intermediate epoxide, such that nucleophilic attack of the phenol onto the epoxide group might lead to formation of either five-membered furan, six-membered pyran or the seven membered oxepin heterocycles, as commonly encountered in natural products [6]. Scheme 1. Proposed mechanisms for the enzymatic formation of 5,7-dihydroxy-2-methylchromone and its derivatives.

Chromone Glycosides
Chromone glycosides belong to a group of oxygen-containing heterocyclic compounds with a benzo-γ-pyrone skeleton. Naturally occurring chromone glycosides can be either O-glycosides or C-glycosides. For O-glycosides, the most frequently encountered group is the 7-O-glycosides; however, 2-, 3-, 5-, 8-, 11-and 13-O-glycosides also exist but to a lower extent. For an example, only one 6-O-glycoside 11 has been reported from nature, and from fungi, not higher plants [1]. Glycosylation can also be detected at side chains for chromones, at C-11 and C-12 as in compounds 56-59, at the hydroxyprenyl and hydroxyisoprenyl side chains as in 123 and 128, respectively, or at the phenyl ethyl moiety as 139.
The most abundant among chromone glycosides is the glucoside from. However, other sugar moieties such as xylose, arabinose and rhamnose were also detected in 3-, 7-and 11-O-glycosides.

3-O-Glycosides
This category includes compounds 2-5. They share the same aglycone nucleus but with different sugar moieties at C-3. Eucryphin 4 was reported as a new compound in 1979 [8]; however, it was reported again in 1996 as a new compound under the name smiglanin [9]. In addition, 3,5,7-trihydroxychromone 3-O-β-D-xylopyranoside 2 was first reported in 2005 from Rhodadendron ovatum [10], but it was reported again as a new compound in 2013 [11]. Compounds 2-5 are shown in Figure 2. The sources and the reported biological activities are summarized in Table 2.

3-O-Glycosides
This category includes compounds 2-5. They share the same aglycone nucleus but with different sugar moieties at C-3. Eucryphin 4 was reported as a new compound in 1979 [8]; however, it was reported again in 1996 as a new compound under the name smiglanin [9]. In addition, 3,5,7-trihydroxychromone 3-O-β-D-xylopyranoside 2 was first reported in 2005 from Rhodadendron ovatum [10], but it was reported again as a new compound in 2013 [11]. Compounds 2-5 are shown in Figure 2. The sources and the reported biological activities are summarized in Table 2.

3-O-Glycosides
This category includes compounds 2-5. They share the same aglycone nucleus but with different sugar moieties at C-3. Eucryphin 4 was reported as a new compound in 1979 [8]; however, it was reported again in 1996 as a new compound under the name smiglanin [9]. In addition, 3,5,7-trihydroxychromone 3-O-β-D-xylopyranoside 2 was first reported in 2005 from Rhodadendron ovatum [10], but it was reported again as a new compound in 2013 [11]. Compounds 2-5 are shown in Figure 2. The sources and the reported biological activities are summarized in Table 2.

7-O-Glycosides
This subclass is characterized by the presence of sugar at C-7. Hyperimone A is the same as Urachromone A (22), reported at nearly the same time from different co-authors from the genus Hypericum. Takanechromone A (38) is the same as Hyperimone B, isolated from the same genus by different co-authors. They were reported each time as new compounds. We preferred to add only 13 C-NMR data of one set of these compounds (Table 23). Several biological activities have been reported to some members of this subclass. Compounds 12-53 are shown in Figure 5. The sources and the reported biological activities (if any) are summarized in Table 4.
Drynachromosides C (30) and D (33) exhibited inhibitory activity on triglyceride accumulation [22]. The effects of these compounds on mRNA expression of the three adipogenesis-related marker genes, PPARγ, C/EBPα and Ap2, in 3T3-L1 were investigated. The mRNA expression levels of PPARγ, C/EBPα and Ap2 were found to be dramatically downregulated. Compounds 40 and 43, having a unique sugar unit of 4-O-methyl-β-D-

7-O-Glycosides
This subclass is characterized by the presence of sugar at C-7. Hyperimone A is the same as Urachromone A (22), reported at nearly the same time from different coauthors from the genus Hypericum. Takanechromone A (38) is the same as Hyperimone B, isolated from the same genus by different co-authors. They were reported each time as new compounds. We preferred to add only 13 C-NMR data of one set of these compounds (Table 23). Several biological activities have been reported to some members of this subclass. Compounds 12-53 are shown in Figure 5. The sources and the reported biological activities (if any) are summarized in Table 4.

44
Lobodirin Lobodirina cerebriformis lichen [51] No reported biological activity  Chinese rhubarb (Rhei Rhizoma) [57] No reported biological activity Chinese rhubarb (Rhei Rhizoma) [57] No reported biological activity Drynachromosides C (30) and D (33) exhibited inhibitory activity on triglyceride accumulation [22]. The effects of these compounds on mRNA expression of the three adipogenesis-related marker genes, PPARγ, C/EBPα and Ap2, in 3T3-L1 were investigated. The mRNA expression levels of PPARγ, C/EBPα and Ap2 were found to be dramatically downregulated. Compounds 40 and 43, having a unique sugar unit of 4-O-methyl-β-Dglucopyranose, were isolated from the scale-insect pathogenic fungus Orbiocrella sp. BCC 33248 [23]. Uncinosides A (46) and B (48) [24], isolated from the Chinese herbal medicine Selaginella uncinata, showed potent anti-RSV (respiratory syncytial virus) activity with IC 50 values of 6.9 and 1.3 µg/mL, respectively. Uncinoside B (48) was found to have a TI value of 64.0, a large therapeutic index comparable to that of ribavirin with a TI value of 24.0, which is an approved drug for the treatment of RSV infection in humans. They also showed

No. Compound Source
Biological Activity Swertia punicea whole herb [58] No reported biological activity Rhododendron collettianum aerial parts [59] Inhibitory activity against tyrosinase enzyme with an IC50 value of 256.97 µM [59] 4.1.7. 11-and 13-O-Glycosides Compound 57 was reported in 2012 as Monnieriside A [60] and was then reported as Drynachromoside B [22,31,47]. Compounds 56-59 are shown in Figure 7. The sources and the reported biological activities (if any) are summarized in Table 6.

No. Compound Source
Biological Activity Swertia punicea whole herb [58] No reported biological activity Rhododendron collettianum aerial parts [59] Inhibitory activity against tyrosinase enzyme with an IC50 value of 256.97 µM [59] 4.1.7. 11-and 13-O-Glycosides Compound 57 was reported in 2012 as Monnieriside A [60] and was then reported as Drynachromoside B [22,31,47]. Compounds 56-59 are shown in Figure 7. The sources and the reported biological activities (if any) are summarized in Table 6.

58
Saikochromoside A Bupleurum chinense [61] Cnidium monnieri fruits [60] No reported biological activity Their structures are shown in Figure 8. The sources and biological activities (if any) of these compounds are summarized in Table 7.  Figure 8. The sources and biological activities (if any) of these compounds are summarized in Table 7.

Chromone C-Glycosides
In contrast to chromone O-glycosides, which are widely distributed and of common occurrence, C-glycoside derivatives are rarely found out.

3-C-Glycosides
This subclass includes the unusual 5,7-dihydroxychromone-3α-D-C-glucoside, named macrolobin, isolated from the aerial parts of Macrolobium latifolium [65]. Its structure is shown in Figure 9. Its source and biological activities are summarized in Table 8.   Takanechromanone B

Chromone C-Glycosides
In contrast to chromone O-glycosides, which are widely distributed and of common occurrence, C-glycoside derivatives are rarely found out.

3-C-Glycosides
This subclass includes the unusual 5,7-dihydroxychromone-3α-D-C-glucoside, named macrolobin, isolated from the aerial parts of Macrolobium latifolium [65]. Its structure is shown in Figure 9. Its source and biological activities are summarized in Table 8. Their structures are shown in Figure 8. The sources and biological activities (if any) of these compounds are summarized in Table 7.

Chromone C-Glycosides
In contrast to chromone O-glycosides, which are widely distributed and of common occurrence, C-glycoside derivatives are rarely found out.

3-C-Glycosides
This subclass includes the unusual 5,7-dihydroxychromone-3α-D-C-glucoside, named macrolobin, isolated from the aerial parts of Macrolobium latifolium [65]. Its structure is shown in Figure 9. Its source and biological activities are summarized in Table 8.

6-C-Glycosides
Compounds 65-79 are shown in Figure 10. The sources and the reported biological activities (if any) are summarized in Table 9.
Molecules 2021, 26, x FOR PEER REVIEW 15 of 58 Table 8. 3-C-Chromone glycoside with its source and biological activities.

No. Compound Source
Biological Activity

64
Macrolobin Macrolobium latifolium aerial parts [65] Inhibition of acetylcholinesterase enzyme with an IC50 value of 0.8 µM Antimicrobial activity against P. aeruginosa and Salmonella at 0.73 and 0.44 µM, respectively [65] 4.2.2. 6-C-Glycosides Compounds 65-79 are shown in Figure 10. The sources and the reported biological activities (if any) are summarized in Table 9.    Table 9. 6-C-Chromone glycosides with their sources and biological activities.

Phenyl and Isoprenyl Chromone Glycosides
This category is characterized by a hydroxyl prenyl moiety at C-6 or C-8, or a hydroxyl isoprenyl moiety at C-6 only. The sugar moiety can be either situated at C-7 hydroxyl of the chromone nucleus or C-4' of the hydroxyl prenyl or C-2' of the hydroxyl isoprenyl moiety. Most of the compounds in this category were reported from the genus Cnidium, belonging to family Apiaceae. The reported biological activity associated with several compounds in this category is their significant inhibition of fat accumulation in differentiated adipocytes employing 3T3-L1 preadipocyte cells as an assay system [60]. The compounds 123-134 are shown in Figure 14. The sources and the reported biological activities (if any) are summarized in Table 11. Aloe vera [76] No reported biological activity -D-glucosyl-isoaloeresin DII Aloe vera [76] No reported biological activity

Phenyl and Isoprenyl Chromone Glycosides
This category is characterized by a hydroxyl prenyl moiety at C-6 or C-8, or a hydroxyl isoprenyl moiety at C-6 only. The sugar moiety can be either situated at C-7 hydroxyl of the chromone nucleus or C-4' of the hydroxyl prenyl or C-2' of the hydroxyl isoprenyl moiety. Most of the compounds in this category were reported from the genus Cnidium, belonging to family Apiaceae. The reported biological activity associated with several compounds in this category is their significant inhibition of fat accumulation in differentiated adipocytes employing 3T3-L1 preadipocyte cells as an assay system [60]. The compounds 123-134 are shown in Figure 14. The sources and the reported biological activities (if any) are summarized in Table 11.   Table 11. Prenyl and isoprenyl chromone glycosides with their sources and biological activities.

Phenyl Ethyl Chromone Glycosides
Reviewing the literature, we encountered five phenyl ethyl chromone glycosides. The phenyl ethyl moiety is usually located at C-2 of the chromone nucleus. The sugar moiety is attached to C-7 of the chromone skeleton in compounds 135-137, while in compound 138, the sugar is attached to C-8. In compound 139, the sugar is not attached directly to the basic chromone skeleton. Compounds 135-139 are shown in Figure 15. Their sources are summarized in Table 12. There are no reported biological activities of these compounds.

Furano-Chromone Glycosides
This subclass of compounds is characterized by presence of an additional furan, or a tetrahydrofuran ring fused with the benzo-δ-pyrone. Khellol glucoside (140), isolated from Ammi visnaga, is one of the important members in this subclass. It possess potent coronary vasodilator and bronchodilator activities [115]. It was reported to have a significant hypocholesterolemic effect. It lowered low-density lipoprotein cholesterol (LDL-C) by 73%, high-density lipoprotein cholesterol (HDL-C) by 23%, and total-C by 44%, after a single oral dose of 20 mg/kg per day after two weeks [116]. Compounds 140-148 are shown in Figure 16. The sources and the reported biological activities (if any) are summarized in Table 13.

Oxepino-Chromone Glycosides
This subclass of compounds is characterized by the presence of an additional oxepin fused with the benzo-δ-pyrone. Only four compounds were reported from nature until now, and all of them were reported from Eranthis species. The compounds 152-155 are shown in Figure 18. The sources are summarized in Table 15. There are no reported biological activities for these compounds.

.3. Oxepino-Chromone Glycosides
This subclass of compounds is characterized by the presence of an additional oxepin fused with the benzo-δ-pyrone. Only four compounds were reported from nature until now, and all of them were reported from Eranthis species. The compounds 152-155 are shown in Figure 18. The sources are summarized in Table 15. There are no reported biological activities for these compounds.

Pyrido-Chromone Glycosides
This subclass includes only the chromone alkaloidal glycoside; Schumanniofoside. This compound was found to reduce the lethal effect of black cobra (Naja melanoleuca) venom in mice [135]. The authors proved that this effect is greatest when the venom is mixed and incubated with the extract or schumanniofoside. They concluded that the mode of action is by oxidative inactivation of the venom. Schumanniophyton magnificum is used extensively in African ethno-medicine for the treatment of various diseases and, most commonly, the treatment of snake bites [135]. Its structure is shown in Figure 19. Its source and biological activity are summarized in Table 16.

Pyrido-Chromone Glycosides
This subclass includes only the chromone alkaloidal glycoside; Schumanniofoside. This compound was found to reduce the lethal effect of black cobra (Naja melanoleuca) venom in mice [135]. The authors proved that this effect is greatest when the venom is mixed and incubated with the extract or schumanniofoside. They concluded that the mode of action is by oxidative inactivation of the venom. Schumanniophyton magnificum is used extensively in African ethno-medicine for the treatment of various diseases and, most commonly, the treatment of snake bites [135]. Its structure is shown in Figure 19. Its source and biological activity are summarized in Table 16.

.3. Oxepino-Chromone Glycosides
This subclass of compounds is characterized by the presence of an additional oxepin fused with the benzo-δ-pyrone. Only four compounds were reported from nature until now, and all of them were reported from Eranthis species. The compounds 152-155 are shown in Figure 18. The sources are summarized in Table 15. There are no reported biological activities for these compounds.

Pyrido-Chromone Glycosides
This subclass includes only the chromone alkaloidal glycoside; Schumanniofoside. This compound was found to reduce the lethal effect of black cobra (Naja melanoleuca) venom in mice [135]. The authors proved that this effect is greatest when the venom is mixed and incubated with the extract or schumanniofoside. They concluded that the mode of action is by oxidative inactivation of the venom. Schumanniophyton magnificum is used extensively in African ethno-medicine for the treatment of various diseases and, most commonly, the treatment of snake bites [135]. Its structure is shown in Figure 19. Its source and biological activity are summarized in Table 16.

Hybrids of Chromones with Other Classes of Secondary Metabolites
This is an interesting category, as the chromone skeleton is conjugated to another high molecular weight compound, as shown in the following subclasses.

Hybrids of Furano-Chromones with Cycloartane Triterpenes
This subclass of compounds is a hybrid of cycloartane triterpene and chromone. The reported compounds were isolated from the rhizomes of Cimicifuga foetida. The compounds 157-165 are shown in Figure 20. The sources and the reported biological activities (if any) are summarized in Table 17.

Hybrids of Chromones with Other Classes of Secondary Metabolites
This is an interesting category, as the chromone skeleton is conjugated to another high molecular weight compound, as shown in the following subclasses.

Hybrids of Furano-Chromones with Cycloartane Triterpenes
This subclass of compounds is a hybrid of cycloartane triterpene and chromone. The reported compounds were isolated from the rhizomes of Cimicifuga foetida. The compounds 157-165 are shown in Figure 20. The sources and the reported biological activities (if any) are summarized in Table 17.

Hybrids of Chromones with Secoiridoids
There are only two compounds ( Figure 21) belonging to this class, sessilifoside (166) and 7 -O-β-D-glucopyranosylsessilifoside (167). Both compounds were isolated from the roots of Neonauclea sessilifolia roots [41]. The authors did not report biological activities for these compounds.  There are only two compounds ( Figure 21) belonging to this class, sessilifoside ( and 7''-O-β-D-glucopyranosylsessilifoside (167). Both compounds were isolated from roots of Neonauclea sessilifolia roots [41]. The authors did not report biological activ for these compounds.

R 166
H 167 Glu

Chromone Alkaloids Aminoglycosides
This category includes compounds 168-192. Compounds 168-180 were repo from a strain of Streptomyces, isolated from a soil sample. These compounds show antimicrobial activity against Gram-positive bacteria, as well as a potent antitumor ac ity. Conversely, compounds 181-183 were isolated from Saccharothrix species, w compounds 184-192 were reported from Actinomycete and exhibited antitumor and timicrobial activities [137]. Compounds 168-192 are shown in Figures 22 and 23. sources and the reported biological activities (if any) are summarized in Table 18.

UV Features
Most of the published work on chromones show several strong bands in the range of 200-320 nm [150,151]. In contrast to chromone, the pyrone ring of 4-chromanone contains no double bond. The ultraviolet absorption spectra of chromones and chromanones are summarized in Table 19 [150]. The UV spectrum of chromones in alcohol shows two strong bands at λ max 245 and 299 nm [152][153][154]. Some data reported three bands at λ max 245, 303 and 297 nm [150]. 2methyl-5,7-dihydroxy chromone shows bands at λ max 250, 255, 295 and 325 nm, meanwhile 2-methyl-5-hydroxy-7-O-glycosyl chromone shows bands at λ max 248, 255 and 290 nm [154]. The presence of an electron attracting group at C-2 resulted in a bathochromic shift in all bands [151]. The information gained from applying spectral shift reagents with flavonoids can be also applied to chromones. In the case of AlCl 3 , a bathochromic shift of 20-70 nm, which is non-reversible with acids, indicates a free hydroxyl group at position 5. Meanwhile, a bathochromic shift with NaOAc can be diagnostic for the presence of a free 7-hydroxyl group [154,155].

IR Features
Carbonyl region: The IR carbonyl stretching frequency for a chromone is observed at 1640~1660 cm −1 , which is slightly higher than that of δ-pyrone (1650 cm −l ) but is much lower than that of coumarins (1720-1740 cm −l ) [25,153]. Despite that the OH group attached to C-5 of the chromone nucleus chelates strongly with the CO group, this intramolecular H-bonding has only a slight bathochromic effect on the CO stretching frequency [156]. All 5-hydroxychromones possess three significant maxima in the 1580-1700 cm −1 region. The two higher frequencies are intense at 1660 and 1630 cm −1 , with a constant wavenumber separation of 34 ( ± 5) cm −1 in both carbon tetrachloride and chloroform.
Hydroxyl region: The IR hydroxyl stretching vibration for a chromone was observed at 2500-3650 cm −1 . A strong chelation in 5-hydroxychromones does not produce a considerable bathochromic shifts in both the OH and CO stretching bands [156].
Chelated 5-hydroxychromones produce no absorption maxima in the 3300-3600 cm −1 region, but a weak absorption envelope extends from 2400 to 3300 cm −1 . The entire envelope is associated with various stretching modes of the chelated 5-OH group [156]. For 7-hydroxychromones, a steric buttressing effect is observed when the 7-OH group is flanked by a bulky substituent in the ortho-position (6 or 8). The free OH band appears as a doublet centered at 3615 cm −1 , the separation of the components being~26 cm −1 . When a prenyl moiety is located in the ortho-position to the 7-OH group, an intramolecular OH interaction occurs, resulting in two OH stretching frequencies. When a 7-OH group is flanked by an OMe group, intense intramolecularly bonded OH stretching frequencies are found at~3513 and 3517 cm −1 , respectively [156]. The 2-hydroxymethyl group exhibits a free stretching frequency at ≈3615 cm −1 . At concentrations higher than 0.15 M, a broad-bonded OH frequency at 3400 cm −1 occurs due to intermolecular H-bonding, and it consequently disappears on dilution [156].
Molecules 2021, 26, x FOR PEER REVIEW 33 of 58 associated with various stretching modes of the chelated 5-OH group [156]. For 7hydroxychromones, a steric buttressing effect is observed when the 7-OH group is flanked by a bulky substituent in the ortho-position (6 or 8). The free OH band appears as a doublet centered at 3615 cm −1 , the separation of the components being ~26 cm −1 . When a prenyl moiety is located in the ortho-position to the 7-OH group, an intramolecular OH interaction occurs, resulting in two OH stretching frequencies. When a 7-OH group is flanked by an OMe group, intense intramolecularly bonded OH stretching frequencies are found at ~3513 and 3517 cm −1 , respectively [156]. The 2-hydroxymethyl group exhibits a free stretching frequency at ≈3615 cm −1 . At concentrations higher than 0.15 M, a broad-bonded OH frequency at 3400 cm −1 occurs due to intermolecular Hbonding, and it consequently disappears on dilution [156].
Naturally occurring chromones often bear a hydroxyl or methoxy group at C-5 and/or C-7 and a methyl group at C-2 and/or C-5 [153]. The C-5 methyl is usually observed in 6-C and 8-C glycosides. In aprotic solvents such as DMSO-d6, the chelated 5-OH is detected as a singlet at δH 12.57; meanwhile, the 7-OH is detected at δH 10.00, as in compound 56 [31]. The C-2 methyl in Schumaniofioside A 7 can be detected at δH 2.33 (3H, s) [17]. Meanwhile, those located at C-5, can be detected more downfield at δH 2.64 (3H, s) as in 79 [78].
For the phenyl part of the benzo-δ-pyrone skeleton, the protons show chemical shift and coupling constant values similar to those observed for protons in substituted benzenes.
Naturally occurring chromones often bear a hydroxyl or methoxy group at C-5 and/or C-7 and a methyl group at C-2 and/or C-5 [153]. The C-5 methyl is usually observed in 6-C and 8-C glycosides. In aprotic solvents such as DMSO-d 6 , the chelated 5-OH is detected as a singlet at δ H 12.57; meanwhile, the 7-OH is detected at δ H 10.00, as in compound 56 [31]. The C-2 methyl in Schumaniofioside A 7 can be detected at δ H 2.33 (3H, s) [17]. Meanwhile, those located at C-5, can be detected more downfield at δ H 2.64 (3H, s) as in 79 [78].
For the phenyl part of the benzo-δ-pyrone skeleton, the protons show chemical shift and coupling constant values similar to those observed for protons in substituted benzenes.

13 C-NMR Features
For better understanding of the differences in chemical shifts related to the substituents on the chromone moiety, we preferred to add the 13 C-NMR data in Tables 20-40. For the numbering of the skeleton, the following figure ( Figure 25) gives few examples for the numbering system of the skeleton with multiple substituents. Briefly, the basic chromone nucleus was assigned numbers 1-10. In the case of a substitution at C-2, numbers 11, 12 . . . etc. were given to the substituents, followed by substitution at C-3 and so on. Sugar moiety, and substituents attached to it, were assigned numbers 1 , 2 , . . . and then 1 , 2 , . . . etc. For better understanding, the following figure shows representative examples for the numbering system. Some complicated structures have their own numbering system, shown on them within the review.                     C  111  112  113  114  115  116  117  119  120  121  122   4  ---------71.3  71.2