Melanogenesis-Inhibitory and Cytotoxic Activities of Diarylheptanoids from Acer nikoense Bark and Their Derivatives

Nine cyclic diarylheptanoids, 1-9, including two new compounds, i.e., 9-oxoacerogenin A (8) and 9-O-β-D-glucopyranosylacerogenin K (9), along with three acyclic diarylheptanoids, 10-12, and four phenolic compounds, 13-16, were isolated from a MeOH extract of the bark of Acer nikoense (Aceraceae). Acid hydrolysis of 9 yielded acerogenin K (17) and D-glucose. Two of the cyclic diarylheptanoids, acerogenin A (1) and (R)-acerogenin B (5), were converted to their ether and ester derivatives, 18-24 and 27-33, respectively, and to the dehydrated derivatives, 25, 26, 34, and 35. Upon evaluation of compounds 1-16 and 18-35 for their inhibitory activities against melanogenesis in B16 melanoma cells, induced with α-melanocyte-stimulating hormone (α-MSH), eight natural glycosides, i.e., six diarylheptanoid glycosides, 2-4, 6, 9, and 12, and two phenolic glycosides, 15 and 16, exhibited inhibitory activities with 24-61% reduction of melanin content at 100?μM concentration with no or almost no toxicity to the cells (88-106% of cell viability at 100?μM). In addition, when compounds 1-16 and 18-35 were evaluated for cytotoxic activity against human cancer cell lines, two natural acyclic diarylheptanoids, 10 and 11, ten ether and ester derivatives, 18-22 and 27-31, and two dehydrated derivatives, 34 and 35, exhibited potent cytotoxicities against HL60 human leukemia cell line (IC(50) 8.1-19.3?μM), and five compounds, 10, 11, 20, 29, and 30, against CRL1579 human melanoma cell line (IC(50) 10.1-18.4?μM).     

Cytotoxic Activities of Amino Acid‐Conjugate Derivatives of Abietane‐Type Diterpenoids against Human Cancer Cell Lines

Nine amino acid conjugate derivatives, each 2 – 10 and 12 – 20 , were prepared from abietic acid ( 1 ) and dehydroabietic acid ( 11 ), respectively, and they were evaluated for their cytotoxicities against four human cancer cell lines, i.e., leukemia (HL60), lung (A549), stomach (AZ521), and breast (SK‐BR‐3). All compounds showed cytotoxicities against HL60 with IC₅₀ values in the range of 2.3–37.3 μM . In addition, most of the derivatives exhibited moderate cytotoxicities against the other cancer cell lines. Among the derivatives, methyl N‐[18‐oxoabieta‐8,11,13‐trien‐18‐yl]‐L ‐tyrosinate ( 19 ) exhibited potent cytotoxic activities against four cancer cell lines with IC₅₀ values of 2.3 (HL60), 7.1 (A549), 3.9 (AZ521), and 8.1 μM (SK‐BR‐3). Furthermore, all derivatives were shown to possess high selective cytotoxic activities for leukemia cells, since they exhibited only weak cytotoxicities against normal lymphocyte cell line RPMI1788.     

Synthesis of Lanostane‐Type Triterpenoid N‐Glycosides and Their Cytotoxicity against Human Cancer Cell Lines

Seventeen lanostane‐type triterpenoid derivatives ( 2 – 18 ), including 11N‐glycosides ( 8 – 18 ), were synthesized from the natural triterpenoid, lanosterol ( 1 ), and were evaluated for their cytotoxicity against the human cancer cell lines, HL‐60, A549, and MKN45, as well as the normal human lung cells, WI‐38. Among them, N‐β‐d ‐2‐acetamido‐2‐deoxyglucoside ( 10 ) showed cytotoxicity against HL‐60, A549, MKN45, and WI‐38 cells (IC₅₀ 0.0078 – 2.8 μm ). However, N‐β‐d ‐galactoside ( 12 ) showed cytotoxicity against HL‐60 and MKN45 cells (IC₅₀ 0.0021 – 4.0 μm ), but not the normal WI‐38 cells. Furthermore, Western blot analysis suggested that 12 induces apoptosis by activation of caspases‐3, 8, and 9. These results will be useful for the synthesis of other tetracyclic triterpenoids or steroid N‐glycosides to increase their cytotoxicity and apoptosis‐inducing activities.     

Cytotoxic and Apoptosis‐Inducing Activities of Steviol and Isosteviol Derivatives against Human Cancer Cell Lines

Seventeen steviol derivatives, i.e., 2 – 18 , and 19 isosteviol derivatives, i.e., 19 – 37 , were prepared from a diterpenoid glycoside, stevioside ( 1 ). Upon evaluation of the cytotoxic activities of these compounds against leukemia (HL60), lung (A549), stomach (AZ521), and breast (SK‐BR‐3) cancer cell lines, nine steviol derivatives, i.e., 5 – 9 and 11 – 14 , and five isosteviol derivatives, i.e., 28 – 32 , exhibited activities with single‐digit micromolar IC₅₀ values against one or more cell lines. All of these active compounds possess C(19)‐O‐acyl group, and among which, ent‐kaur‐16‐ene‐13,19‐diol 19‐O‐4′,4′,4′‐trifluorocrotonate ( 14 ) exhibited potent cytotoxicities against four cell lines with IC₅₀ values in the range of 1.2–4.1 μM . Compound 14 induced typical apoptotic cell death in HL60 cells upon evaluation of the apoptosis‐inducing activity by flow‐cytometric analysis. These results suggested that acylation of the 19‐OH group of kaurane‐ and beyerane‐type diterpenoids might be useful for enhancement of their cytotoxicities with apoptosis‐inducing activity.     

Cytotoxic and Apoptosis‐Inducing Activities,and Anti‐Tumor‐Promoting Effects of Cyanogenated and Oxygenated Triterpenes

Two of each semisynthetic lanostane‐ and cycloartane‐type triterpenes with a cyano‐enone functionality, i.e., 13 and 18 , and 23 and 28 , respectively, sixteen of their synthetic intermediates, 9 – 12, 14 – 17, 19 – 22 , and 24 – 27 , along with seven semisynthetic oxygenated triterpene acetates, 29 – 35 , and eight natural hydroxy triterpenes, 1 – 8 , were evaluated for their cytotoxic activities against leukemia (HL60), lung (A549), stomach (AZ521), and breast (SK‐BR‐3) cancer cell lines. One natural triterpene, 8 , and ten semisynthetic triterpenes, 9, 13, 15, 18, 23, 25, 28, 29, 32 , and 33 , exhibited potent cytotoxicities against one or more cell lines with IC₅₀ values in the range of 1.4–9.9 μM . Two lanostane‐type triterpenes with a cyano‐enone functionality, 3‐oxolanosta‐1,8,24‐triene‐2‐carbonitrile ( 13 ) and 3‐oxolanosta‐1,8‐diene‐2‐carbonitrile ( 18 ), induced apoptosis in HL60 cells, as observed by membrane phospholipid exposure in flow cytometry. Western blot analysis showed that 13 and 18 significantly reduced procaspases‐3, ‐8, and ‐9, and increased cleaved caspases‐3, ‐8, and ‐9. These findings indicated that compounds 13 and 18 induced apoptosis in HL60 cells via both the mitochondrial and the death receptor‐mediated pathways. In addition, upon evaluation of the inhibitory effects on Epstein? Barr virus early antigen (EBV‐EA) activation induced with 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) in Raji cells, seven natural triterpenes, 1 – 6 and 8 , and ten semisynthetic triterpenes, 9, 10, 14, 15, 19, 20, 24, 25, 29 , and 30 , exhibited inhibitory effects which were higher than that of β‐carotene, a vitamin A precursor studied widely in cancer‐chemoprevention animal models.     

Anti-inflammatory and anti-tumor-promoting effects of 5-deprenyllupulonol C and other compounds from Hop (Humulus lupulus L.)

A new phloroglucinol derivative, 5‐deprenyllupulonol C ( 1 ), along with four other phloroglucinol derivatives, 2 – 5 , five chalcones, 6 – 10 , four flavanones, 11 – 14 , two flavonol glycosides, 15 and 16 , and five triterpenoids, 17 – 21 , were isolated from the female inflorescence pellet extracts of hop (Humulus lupulus L.). Upon evaluation of these compounds against the Epstein? Barr virus early antigen (EBV‐EA) activation induced by 12‐O‐tetradecanoylphorbol 13‐acetate (TPA) in Raji cells, twelve compounds, i.e., 1 – 4, 11 – 14, 17 – 19 , and 21 , showed potent inhibitory effects on EBV‐EA induction, with IC₅₀ values in the range of 215–393 mol ratio/32 pmol TPA. In addition, eleven compounds, i.e., 1 – 4, 6, 11, 12, 14, 17, 18 , and 20 , were found to inhibit TPA‐induced inflammation (1 μg/ear) in mice, with ID₅₀ values in the range of 0.13–1.06 μmol per ear. Further, lupulone C ( 2 ) and 6‐prenylnaringenin ( 14 ) exhibited inhibitory effects on skin‐tumor promotion in an in vivo two‐stage mouse‐skin carcinogenesis test based on 7,12‐dimethylbenz[a]anthracene (DMBA) as initiator and with TPA as promoter.     

Cytotoxic and Nitric Oxide Production‐Inhibitory Activities of Limonoids and Other Compounds from the Leaves and Bark of Melia azedarach

Nine limonoids, 1 – 9 , one apocarotenoid, 11 , one alkaloid, 12 , and one steroid, 13 , from the leaf extract; and one triterpenoid, 10 , five steroids, 14 – 18 , and two flavonoids, 19 and 20 , from the bark extract of Melia azedarach L. (Chinaberry tree; Meliaceae) were isolated. Among these compounds, three compounds, 4 – 6 , were new, and their structures were established as 3‐deacetyl‐28‐oxosalannolactone, 3‐deacetyl‐28‐oxosalanninolide, and 3‐deacetyl‐17‐defurano‐17,28‐dioxosalannin, respectively, on the basis of extensive spectroscopic analyses and comparison with literature data. All of the isolated compounds were evaluated for their cytotoxic activities against leukemia (HL60), lung (A549), stomach (AZ521), and breast (SK‐BR‐3) cancer cell lines. 3‐Deacetyl‐4′‐demethyl‐28‐oxosalannin ( 3 ) against HL60 and AZ521 cells, and methyl kulonate ( 10 ) against HL60 cells exhibited potent cytotoxicities with IC₅₀ values in the range of 2.8–5.8 μM . In addition, upon evaluation of compounds 1 – 13 against production of nitric oxide (NO) in mouse macrophage RAW 264.7 cells induced by lipopolysaccharide (LPS), seven, i.e., trichilinin B ( 1 ), 4 , ohchinin ( 7 ), 23‐hydroxyohchininolide ( 8 ), 21‐hydroxyisoohchininolide ( 9 ), 10 , and methyl indole 3‐carboxylate ( 12 ), inhibited production of NO with IC₅₀ values in the range of 4.6–87.3 μM with no, or almost no, toxicity to the cells (IC₅₀ 93.2–100 μM ). Western blot analysis revealed that compound 7 reduced the expression levels of the inducible NO synthase (iNOS) and COX‐2 proteins in a concentration‐dependent manner. Furthermore, compounds 5, 6, 13 , and 18 – 20 exhibited potent inhibitory effects (IC₅₀ 299–381 molar ratio/32 pmol TPA) against Epstein? Barr virus early antigen (EBV‐EA) activation induced by 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) in Raji cell line.     

Bioactive Diphenyl Ether Derivatives from a Gorgonian‐Derived Fungus Talaromyces sp.

Three new diphenyl ether derivatives, talaromycins A–C ( 1 – 3 , resp.), together with six known analogs, 4 – 9 , were isolated from a gorgonian‐derived fungus, Talaromyces sp. The structures of the new compounds were determined by analysis of extensive NMR spectroscopic data. All of the isolated metabolites, 1 – 9 , were evaluated for their cytotoxic and antifouling activities. Compound 4 exhibited pronounced cytotoxicity against the tested human cell lines with the IC₅₀ values ranging from 4.3 to 9.8 μM . Compounds 3, 5, 8 , and 9 showed potent antifouling activities against the larval settlement of the barnacle Balanus amphitrite with the EC₅₀ values ranging from 2.2 to 4.8 μg/ml.     

Triterpenoids and Flavonoids from the Leaves of Astragalus membranaceus and Their Inhibitory Effects on Nitric Oxide Production

Four new cycloartane triterpenes, named huangqiyegenins V and VI and huangqiyenins K and L ( 1 – 4 , resp.), together with nine known triterpenoids, 5 – 13 , and eight flavonoids, 14 – 21 , were isolated from a 70%‐EtOH extract of Astragalus membranaceus leaves. The structures of the new compounds were elucidated by detailed spectroscopic analyses, and the compounds were identified as (9β,11α,16β,20R,24S)‐11,16,25‐trihydroxy‐20,24‐epoxy‐9,19‐cyclolanostane‐3,6‐dione ( 1 ), (9β,16β,24S)‐16,24,25‐trihydroxy‐9,19‐cyclolanostane‐3,6‐dione ( 2 ), (3β,6α,9β,16β,20R,24R)‐16,25‐dihydroxy‐3‐(β‐D ‐xylopyranosyloxy)‐20,24‐epoxy‐9,19‐cyclolanostan‐6‐yl acetate ( 3 ), and (3β,6α,9β,16β,24E)‐26‐(β‐D ‐glucopyranosyloxy)‐16‐hydroxy‐3‐(β‐D ‐xylopyranosyloxy)‐9,19‐cyclolanost‐24‐en‐6‐yl acetate ( 4 ). All isolated compounds were evaluated for their inhibitory activities against LPS‐induced NO production in RAW264.7 macrophage cells. Compounds 1 – 3, 14, 15 , and 18 exhibited strong inhibition on LPS‐induced NO release by macrophages with IC₅₀ values of 14.4–27.1 μM .     

Glycosidic Inhibitors of Melanogenesis from Leaves of Passiflora edulis

A new flavonoid glycoside, chrysin 6‐C‐β‐rutinoside (chrysin α‐L ‐rhamnopyranosyl‐(1→6)‐C‐β‐glucopyranoside; 2 ), and two new triterpene glycosides, (31R)‐31‐O‐methylpassiflorine ( 7 ) and (31S)‐31‐O‐methylpassiflorine ( 8 ), along with 14 known glycosides, including three flavonoid glycosides, 1, 3 , and 4 , six triterpene glycosides, 5, 6 , and 9 – 12 , three cyano glycosides, 13 – 15 , and two other glycosides, 16 and 17 , were isolated from a MeOH extract of the leaves of Passiflora edulis (passion flower; Passifloraceae). The structures of new compounds were elucidated on the basis of extensive spectroscopic analysis and comparison with literature data. Upon evaluation of compounds 1 – 17 against the melanogenesis in the B16 melanoma cells induced with α‐melanocyte‐stimulating hormone (α‐MSH), three compounds, isoorientin ( 1 ), 2 , and (6S,9R)‐roseoside ( 17 ), exhibited inhibitory effects with 37.3–47.2% reduction of melanin content with no, or almost no, toxicity to the cells (90.8–100.2% cell viability) at 100 μM . Western blot analysis showed that compound 2 reduced the protein levels of MITF, TRP‐1, and tyrosinase, in a concentration‐dependent manner while exerted almost no influence on the level of TRP‐2, suggesting that this compound inhibits melanogenesis on the α‐MSH‐stimulated B16 melanoma cells by, at least in part, inhibiting the expression of MITF, followed by decreasing the expression of TRP‐1 and tyrosinase. In addition, compounds 1 – 17 were evaluated for their inhibitory effects against the Epstein? Barr virus early antigen (EBV‐EA) activation induced by 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) in Raji cells.     

Three New and Other Limonoids from the Hexane Extract of Melia azedarach Fruits and Their Cytotoxic Activities

A defatted fraction obtained from the hexane extract of the fruits of Melia azedarach L. (chinaberry tree; Meliaceae) exhibited cytotoxic activities against leukemia (HL60), lung (A549), stomach (AZ521), and breast (SK‐BR‐3) cancer cell lines with IC₅₀ values in the range of 2.9–21.9 μg/ml. Three new limonoids, 3‐deacetyl‐4′‐demethylsalannin ( 5 ), 3‐deacetyl‐28‐oxosalannin ( 14 ), and 1‐detigloylohchinolal ( 17 ), along with 16 known limonoids, 1 – 4, 6 – 13, 15, 16, 18 , and 19 , and one known triterpenoid, 20 , were isolated from the fraction. The structures of new compounds were elucidated on the basis of extensive spectroscopic analyses and comparison with literature. These compounds were evaluated for their cytotoxic activities against the four cancer cell lines mentioned above. 3‐Deacetyl‐4′‐demethyl‐28‐oxosalannin ( 16 ), which exhibited potent cytotoxicity against AZ521 (IC₅₀ 3.2 μM ) cells, induced typical apoptotic cell death in AZ521 cells upon evaluation of the apoptosis‐inducing activity by flow cytometry. This work provided, furthermore, valuable information on the structural features of limonoids of the fruits and/or seeds of Melia azedarach and related Meliaceae plants, M. toosendan and Azadirachta indica.     

Cytotoxic and Melanogenesis‐Inhibitory Activities of Limonoids from the Leaves of Azadirachta indica (Neem)

Seventeen limonoids (tetranortriterpenoids), 1 – 17 , including three new compounds, i.e., 17‐defurano‐17‐(2,5‐dihydro‐2‐oxofuran‐3‐yl)‐28‐deoxonimbolide ( 14 ), 17‐defurano‐17‐(2ξ‐2,5‐dihydro‐2‐hydroxy‐5‐oxofuran‐3‐yl)‐28‐deoxonimbolide ( 15 ), and 17‐defurano‐17‐(5ξ‐2,5‐dihydro‐5‐hydroxy‐2‐oxofuran‐3‐yl)‐2′,3′‐dehydrosalannol ( 17 ), were isolated from an EtOH extract of the leaf of neem (Azadirachta indica). The structures of the new compounds were elucidated on the basis of extensive spectroscopic analyses and comparison with literature. Upon evaluation of the cytotoxic activities of these compounds against leukemia (HL60), lung (A549), stomach (AZ521), and breast (SK‐BR‐3) cancer cell lines, seven compounds, i.e., 1 – 3, 12, 13, 15 , and 16 , exhibited potent cytotoxicities with IC₅₀ values in the range of 0.1–9.9 μM against one or more cell lines. Among these compounds, cytotoxicity of nimonol ( 1 ; IC₅₀ 2.8 μM ) against HL60 cells was demonstrated to be mainly due to the induction of apoptosis by flow cytometry. Western blot analysis suggested that compound 1 induced apoptosis via both the mitochondrial and death receptor‐mediated pathways in HL60 cells. In addition, when compounds 1 – 17 were evaluated for their inhibitory activities against melanogenesis in B16 melanoma cells, induced with α‐melanocyte‐stimulating hormone (α‐MSH), seven compounds, 1, 2, 4 – 6, 15 , and 16 , exhibited inhibitory activities with 31–94% reduction of melanin content at 10 μM concentration with no or low toxicity to the cells (82–112% of cell viability at 10 μM ). All 17 compounds were further evaluated for their inhibitory effects against the Epstein? Barr virus early antigen (EBV‐EA) activation induced by 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA) in Raji cells.     

Biological Activities of Triterpenoids and Phenolic Compounds from Myrica cerifera Bark

Seven triterpenoids, 1  –  7 , two diarylheptanoids, 8 and 9 , four phenolic compounds, 10  –  13 , and three other compounds, 14  –  16 , were isolated from the hexane and MeOH extracts of the bark of Myrica cerifera L. (Myricaceae). Among these compounds, betulin ( 1 ), ursolic acid ( 3 ), and myricanol ( 8 ) exhibited cytotoxic activities against HL60 (leukemia), A549 (lung), and SK‐BR‐3 (breast) human cancer cell lines (IC₅₀ 3.1 – 24.2 μm ). Compound 8 induced apoptotic cell death in HL60 cells (IC₅₀ 5.3 μm ) upon evaluation of the apoptosis‐inducing activity by flow cytometric analysis and by Hoechst 33342 staining method. Western blot analysis on HL60 cells revealed that 8 activated caspases‐3, ‐8, and ‐9 suggesting that 8 induced apoptosis via both mitochondrial and death receptor pathways in HL60. Upon evaluation of the melanogenesis‐inhibitory activity in B16 melanoma cells induced with α‐melanocyte‐stimulating hormone (α‐MSH), erythrodiol ( 7 ), 4‐hydroxy‐2‐methoxyphenyl β‐d ‐glucopyranoside ( 13 ), and butyl quinate ( 15 ) exhibited inhibitory effects (65.4 – 86.0% melanin content) with no, or almost no, toxicity to the cells (85.9 – 107.4% cell viability) at 100 μm concentration. In addition, 8 , myricanone ( 9 ), myricitrin ( 10 ), protocatechuic acid ( 11 ), and gallic acid ( 12 ) revealed potent DPPH radical‐scavenging activities (IC₅₀ 6.9 – 20.5 μm ).     

Two New Chromone Glycosides from the Roots of Saposhnikovia divaricata

Five chromone glycosides were isolated from the water‐soluble portions of 70% EtOH extract of the roots of Saposhnikovia divaricata, including two new chromone glycosides 1 and 2 . The structures of the chromone glycosides were identified as (3′S)‐3′‐O‐β‐d ‐apiofuranosyl‐(1 → 6)‐β‐d ‐glucopyranosylhamaudol ( 1 ), (2′S)‐4′‐O‐β‐d ‐apiofuranosyl‐(1 → 6)‐β‐d ‐glucopyranosylvisamminol ( 2 ), 3′‐O‐glucopyranosylhamaudol ( 3 ), 4′‐O‐β‐d ‐glucopyranosylvisamminol ( 4 ), and 4′‐O‐β‐d ‐glucopyranosyl‐5‐O‐methylvisamminol ( 5 ) on the basis of extensive spectroscopic methods, and the absolute configurations of the new compounds were elucidated by the electronic circular dichroism (ECD) calculation and acid hydrolysis. The cytotoxic activities of the glycosides 1 – 5 against three human cancer cell lines (PC‐3, SK‐OV‐3, and H460) were evaluated. The result showed that compounds 1 – 5 had weak cytotoxic activities against the human cancer cell lines with IC₅₀ values in the range of 48.54 ± 0.80 – 94.25 ± 1.45 μm .     

Limonoids and Flavonoids from the Flowers of Azadirachta indica var. siamensis,and Their Melanogenesis‐Inhibitory and Cytotoxic Activities

A new limonoid, 7‐O‐acetyl‐7‐O‐debenzoyl‐22‐hydroxy‐21‐methoxylimocinin ( 2 ), and two new flavonoids, 3′‐(3‐hydroxy‐3‐methylbutyl)naringenin ( 7 ) and 4′‐O‐methyllespedezaflavanone C ( 9 ), along with nine known compounds, including two limonoids, 1 and 3 , and seven flavonoids, 4 – 6, 8 , and 10 – 12 , were isolated from a MeOH extract of the flowers of Azadirachta indica A.Juss. var. siamensis Valeton (Siamese neem tree; Meliaceae). The structures of new compounds were elucidated on the basis of extensive spectroscopic analysis and comparison with literature data. All of these compounds were evaluated for their melanogenesis‐inhibitory activities in B16 melanoma cells induced with α‐melanocyte‐stimulating hormone (α‐MSH). Compound 2 (16.9% melanin content at 30 μM ), 6‐deacetylnimbin ( 3 ; 49.6% melanin content at 100 μM ), and kaempferide ( 10 ; 41.7% melanin content at 10 μM ) exhibited inhibitory effects with no, or almost no, toxicity to the cells (81.0–111.7% cell viability). In addition, evaluation of their cytotoxic activities against HL60, A549, AZ521, and SK‐BR‐3 human cancer cell lines, isoazadironolide ( 1 ), 4′‐O‐methyl‐8‐prenylnaringenin ( 5 ), euchrestaflavanone A ( 8 ), 9 , and 3‐methoxy‐3′‐prenylnaringenin ( 12 ) revealed potent cytotoxicities against one or more cell lines with IC₅₀ values in the range of 4.5–9.9 μM .     

Melanogenesis‐Inhibitory Activities of Isomeric C‐seco Limonoids and Deesterified Limonoids

Treatment of eight C‐seco limonoids including six of salannin‐type, 1 – 6 , and two of nimbin‐type, 7 and 8 , with a combination of BF₃ · Et₂O and iodide ion yielded the isomeric C‐seco derivatives, i.e., six isosalannins, 1a – 6a , and two isonimbins, 7a and 8a , respectively. Ohchinin ( 1 ) was further subjected to LiAlH₄ reduction which yielded a deesterified trihydroxy limonoid, nimbidinol ( 9 ). In addition, ten limonoids including seven of azadirone‐type, 10 – 16 , and three of gedunin‐type, 17 – 19 , all of which possess no ester functionality in the molecule, were obtained from the neutral fraction of Azadirachta indica seed extract after alkaline hydrolysis. Among the above, twelve compounds, i.e., 1a – 4a , 6a , 9 , 13 – 16 , 18 , and 19 , were new compounds, and their structures were elucidated on the basis of extensive spectroscopic analysis and comparison with literature data. Upon evaluation of all these limonoids for their inhibitory activities against melanogenesis in B16 melanoma cells induced with α‐melanocyte‐stimulating hormone (α‐MSH), five structurally modified limonoids, 3‐deacetyl‐28‐oxosalannin ( 6a ), 9 , 17‐epi‐17‐hydroxynimbocinol ( 14 ), 17‐epi‐17‐hydroxy‐15‐methoxynimbocinol ( 15 ), and 7‐deacetyl‐17‐epinimolicinol ( 18 ), in addition to a natural limonoid, 1 , exhibited potent inhibitory activities with 26 – 66% reduction of melanin content at 100 μm concentration with almost no or low toxicity to the B16 melanoma cells (70 – 99% cell viability at 100 μm ).