Hemiacetalmeroterpenoids A–C and Astellolide Q with Antimicrobial Activity from the Marine-Derived Fungus Penicillium sp. N-5

Abstract

Four new compounds including three andrastin-type meroterpenoids hemiacetalmeroterpenoids A-C (1–3), and a drimane sesquiterpenoid astellolide Q (15), together with eleven known compounds (4–14) were isolated from the cultures of the marine-derived fungus Penicillium sp. N-5, while compound 14 was first isolated from a natural source. The structures of the new compounds were determined by analysis of detailed spectroscopic data, and the absolute configurations were further decided by a comparison of the experimental and calculated ECD spectra. Hemiacetalmeroterpenoid A (1) possesses a unique and highly congested 6,6,6,6,5,5-hexa-cyclic skeleton. Moreover, the absolute configuration of compound 14 was also reported for the first time. Compounds 1, 5 and 10 exhibited significant antimicrobial activities against Penicillium italicum and Colletrichum gloeosporioides with MIC values ranging from 1.56 to 6.25 μg/mL.

1. Introduction

Andrastins are meroterpenoids characterized by a 6,6,6,5-tetra-carbocyclic skeleton. They are biogenetically derived from 3,5-dimethylorsellinic acid (DMOA) and farnesyl diphosphate (FPP), synthesized via a mixed polyketide-terpenoid pathway, and usually possess a keto-enol tautomerism at the cyclopentane ring [1,2,3,4]. To date, over 40 andrastins have been reported with multiple potential biological activities, including cytotoxic [5], anti-inflammatory [6], antiproliferative [7] and antimicrobial activity [4]. The complex structures and potential biological activities of andrastins have attracted much attention in recent years [8,9,10].

Marine fungus is known to be a natural source of structurally diverse and biologically active metabolites for drug discovery [11,12,13,14,15,16]. Recently, a series of novel bioactive natural products from marine fungi were reported by our group [17,18,19,20,21,22]. In our ongoing search for new bioactive secondary metabolites from marine fungi, the fungus Penicillium sp. N-5, isolated from the rhizosphere soil of mangrove plant Avicennia marina, led to the isolation of four new compounds, hemiacetalmeroterpenoids A–C (1–3) and astellolide Q (15). Especially, hemiacetalmeroterpenoid A (1) was a new andrastin-type meroterpenoid containing a unique 6,6,6,6,5,5-hexa-cyclic skeleton. Meanwhile, eleven known compounds, including 3-deacetyl-citreohybridonol (4) [23] citreohybridone A (5) [24], 3,5-dimethylorsellinic acid-based meroterpenoid 2 (6) [5], andrastins A–C (7, 10, 13) [25], andrastone C (8) [26], penimeroterpenoid A (9) [4], 23-deoxocitreohybridonol (11) [1], 6α-hydroxyandrastin B (12) [1], and compound V (14) [27] were also obtained from the fungus N-5 (Figure 1). All the isolated compounds were investigated for their antimicrobial activity against two phytopathogenic fungi and four bacterial strains. Herein, we report the isolation, structural characterization and antibacterial activity of these compounds.

2. Results

2.1. Structure Identification

Hemiacetalmeroterpenoid A (1) was obtained as a white powder. It molecular formula was assigned as C₂₆H₃₄O₇ according to HRESIMS analysis at m/z 459.23709 [M+H]⁺ (calcd. 459.23773), indicating ten degrees of unsaturation. In the ¹H NMR spectrum, the signal for one olefinic proton (δ_H 5.63), one methoxyl (δ_H 3.60), one methine (δ_H 1.47), four methylenes (δ_H 1.42, 1.62, 1.85, 1.91, 1.94, 2.11, 2.33 and 2.61) and six methyls (δ_H 1.03, 1.07, 1.19, 1.23, 1.33 and 1.49). The ¹³C NMR data exhibited 26 carbon resonances, including two olefinic carbons for one double bond (δ_C 127.2, 150.1), three carbonyl carbons for two ketone (δ_C 203.7 and 203.8), and one ester carbonyl (δ_C 169.3), one methine (δ_C 49.1), five methylenes (one oxygenated), seven methyls (one oxygenated), eight quaternary carbons (one highly oxygenated: δ_C 99.8) (

Table 1. ¹H NMR (600 MHz) and ¹³C NMR (150 MHz) of 1-3 in CD₃OD.

Position	1		2		3
	δC	δH (J in Hz)	δC	δH (J in Hz)	δC	δH (J in Hz)
1	34.7 (CH2)	1.42, m 2.33, m	35.3 (CH2)	1.12, m 2.19, m	34.8 (CH2)	1.15, m 2.20, m
2	30.2 (CH2)	1.85, m 2.16, m	30.3 (CH2)	1.74, m 2.12, m	30.1 (CH2)	1.76, m 2.14, m
3	99.8 (C)		99.5 (C)		99.5 (C)
4	41.4 (C)		41.3 (C)		41.3 (C)
5	49.1 (CH)	1.47, m	50.9 (CH)	1.33, m	51.4 (CH)	1.21, m
6	19.4 (CH2)	1.62, m 1.91, m	20.5 (CH2)	1.55, m 1.75, m	20.4 (CH2)	1.62, m 1.81, m
7	32.3 (CH2)	1.94, m 2.61, m	32.5 (CH2)	2.07, m 2.76, m	32.9 (CH2)	1.94, m 2.23, m
8	40.2 (C)		42.4 (C)		41.9 (C)
9	150.1 (C)		48.5 (CH)	1.89, t (2.7)	48.6 (CH)	1.98, t (2.7)
10	38.5 (C)		36.3 (C)		36.3 (C)
11	127.2 (CH)	5.63, s	124.2 (CH)	5.42, m	126.0 (CH)	5.60, m
12	77.4 (C)		137.7 (C)		133.7 (C)
13	54.4 (C)		57.5 (C)		60.6 (C)
14	73.4 (C)		69.7 (C)		69.2 (C)
15	203.7 (C)		190.7 (C)		171.9 (C)
16	76.7 (C)		113.5 (C)		131.9 (C)
17	203.8 (C)		201.4 (C)		202.1 (C)
18	7.9 (CH3)	1.19, s	6.6 (CH3)	1.57, s	8.8 (CH3)	1.55, s
19	11.0 (CH3)	1.33, s	18.0 (CH3)	1.18, s	17.4 (CH3)	1.20, s
20	24.2 (CH3)	1.23, s	20.2 (CH3)	1.82, s	19.1 (CH3)	1.75, s
21	74.4 (CH2)	3.55, d (7.6) 4.39, d (8.7)	68.6 (CH2)	3.81, d (9.0) 4.22, d (9.0)	68.5 (CH2)	3.82, d (8.9) 4.21, d (9.0)
22	27.2 (CH3)	1.07, s	27.9 (CH3)	1.04, s	27.9 (CH3)	1.07, s
23	18.9 (CH3)	1.04, s	18.9 (CH3)	1.01, s	18.8 (CH3)	1.03, s
24	25.9 (CH3)	1.49, s	16.7 (CH3)	1.19, s	16.5 (CH3)	1.24, s
25	169.3 (C)		172.6 (C)		170.9 (C)
26	52.5 (CH3)	3.60, s	51.9 (CH3)	3.56,s	52.4 (CH3)	3.59,s
Ac-CH3					21.2 (CH3)	2.36, s
Ac-OCO					167.3 (C)

). The NMR data established a nucleus of meroterpenoid characterized by an andrastin scaffold, structurally similar to the citreohybriddione C (Figures S2 and S3) [28]. Analysis of the ¹H-¹H COSY data led to the identification of two isolated spin-systems of C-1/C-2 and C-5/C-6/C-7. The HMBC from H₂-1 to C-3, C-10, from H₁-5 to C-10, and from H₃-22 to C-3, C-4, C-5, C-23, ring A was formed. The HMBC correlations from H₃-24 to C-7, C-8, C-9, from H₁-11 to C-8, C-10 and from H₂-21 to C-5, C-10 completed ring B. Then, the HMBC cross-peaks from H₂-1, H₂-6 to C-10 and from H₃-23, H₂-7 to C-5 indicated ring A and ring B were fused at C-5 and C-10. Then, HMBC correlations from H₃-24 to C-14, C-15, from H₃-20 to C-11, C-12, C-13, C-17, from H₃-19 to C-12, C-13, C-14, C-17 and from H₃-18 to C-15, C-16, C-17, ring C and ring D were constituted, and they were blended at C-13 and C-14. The HMBC correlations from H₂-6, H₁-11 to C-8, from H₁-11 to C-10, suggested ring B and ring C were tightly connected. In addition, the HMBC correlation from H₃-26 to C-25 implied the presence of a methyl carboxylate. A weak HMBC correlation from H₃-26 to C-14 located the methyl carboxylate at C-14. Except one double bond, three carbonyls and four rings, ten degrees of unsaturation indicated that two new rings were required. According to the HMBC correlation from H₂-21 to C-3 (δ_C 99.8), a 6-membered ring was confirmed between C-1, C-2, C-3, C-10, and C-21. Finally, another new 5-membered ring was formed by intramolecular dehydration of hydroxyl groups at C-12 and C-16. Thus, the planar structure of 1 was established as shown in Figure 2.

The relative configuration of compound 1 was defined by the NOESY correlations. The correlations of H₂-21 with H₃-23, H₃-20 with H₃-19, H₃-24, and H₃-18 with H₃-19, H₃-26 were observed in the NOESY spectrum, which means H₃-18, H₃-19, H₃-20, H₂-21, H₃-23, H₃-24 and H₃-26 were on the same side. The NOESY correlations of H₁-5 with H₃-22 suggested that H₁-5 and H₃-22 were in the opposite face (Figure 3). The absolute configuration of 1 was determined by comparing the calculated ECD spectra generated by the time-dependent density functional theory (TDDFT) for two enantiomers 3R, 5S, 8S, 10S, 12R, 13S, 14R, 16R-1a and 3S, 5R, 8R, 10R, 12S, 13R, 14S, 16S-1b with the experimental one. Finally, the experimental ECD spectrum of 1 was nearly identical to the calculated ECD spectrum for 1a (Figure 4), clearly suggesting the 3R, 5S, 8S, 10S, 12R, 13S, 14R, 16R absolute configuration for 1.

Hemiacetalmeroterpenoid B (2) was isolated as a white powder and had a molecular formula of C₂₆H₃₆O₆, determined by HRESIMS data m/z 445.25772 [M+H]⁺ (calcd. 445.25847) with nine degrees of unsaturation. The ¹H NMR spectrum of 2 displayed the signal for one olefinic proton (δ_H 5.42), one methoxyl (δ_H 3.56), two methines (δ_H 1.33 and 1.89), four methylenes (δ_H 1.12, 1.33, 1.55, 1.75, 2.07, 2.12, 2.19 and 2.76) and six methyls (δ_H 1.01, 1.04, 1.18, 1.19, 1.57 and 1.82). The ¹³C NMR data revealed 26 carbon resonances, involving four olefinic carbons for two double bonds (δ_C 113.5, 124.2, 137.7, 190.7), two carbonyl carbons for one ketone (δ_C 201.4), one ester carbonyl (δ_C 172.6) (Table 1). According to 1D NMR and 2D NMR data, the planar structure of 2 was similar to the co-isolated andrastin B (13). The obvious difference is that the acetyl group at the C-3 position of compound 2 disappears. Meanwhile, the HMBC from H₂-21 to C-3 (δ_C 99.5) also indicated that a new 6-membered ring was formed between C-1, C-2, C-3, C-10 and C-21 (Figure 2).

The NOESY spectrum indicated that H₁-5, H₁-9 and H₃-22 were on the same side based on the correlations of H₁-5 with H₁-9 and H₃-22. On the contrary, it was suggested that H₃-19, H₃-21, H₃-23, H₃-24, and H₃-26 were on the other side based on the NOESY correlations of H₂-21 with H₃-23 and H₃-24, along with H₃-19 with H₃-24 and H₃-26 (Figure 3). Thus, the relative configuration of 2 was determined to be 3R, 5S, 8S, 9R, 10S, 13R and 14R. The absolute configuration of the stereogenic centers in 2 was assigned as 3R, 5S, 8S, 9R, 10S, 13R and 14R by comparing its experimental ECD spectrum with that of the calculated model molecule (Figure 4).

Hemiacetalmeroterpenoid C (3) was also purified as a white powder. The molecular formula was specified as C₂₈H₃₈O₇ (ten degrees of unsaturation) by HRESIMS (m/z 509.25015 [M+Na]⁺), which is 42 mass units higher than that of 2 (Figure S17). Analysis of its NMR data (Table 1) revealed the presence of the same partial structure as that found in compound 2. The only difference was 3 has an additional acetyl fragment. Finally, a weak HMBC correlation from Ac-CH₃ to C-15 suggested that the acetyl fragment was attached to C-15 (Figure 2).

Because compound 3 has the same chiral center as 2, the NOESY correlation and experimental ECD spectrum of compound 3 were in agreement with those of 2 (Figure 3 and Figure 4). Thus, the absolute configuration of 3 was identified as 3R, 5S, 8S, 9R, 10S, 13R and 14R.

Compound 14 was obtained as a yellow powder. Analysis of its ¹H NMR and ¹³C NMR data showed that the planar structure of 14 was the same as compound V, which was the product of the alkaline hydrolysis of parasiticolide A [27]. However, the absolute configuration of compound V was ambiguous.

The relative configuration of 14 was also defined by the NOESY correlation. The correlations of H₃-14 with H₁-5 and H₁-6, and H₂-13 with H₂-15 were found in the NOESY spectrum, which means H₁-5, H₁-6, and H₃-4 were on the same side, and H₂-13 and H₂-15 were on the opposite face (Figure 3). Thus, the absolute configuration of the stereogenic centers in 14 was assigned as 4R, 5R, 6S, 10S by comparing its experimental ECD spectrum with that of the calculated model molecule (Figure 4). Finally, compound 14 was named as astellolide J.

Astellolide Q (15) was also acquired as a yellow powder. It molecular formula was determined as C₁₇H₂₄O₆ according to HRESIMS analysis at m/z 347.14578 [M+Na]⁺ (calcd. 347.14651), indicating six degrees of unsaturation. The ¹H NMR of 15 showed two methyls (δ_H 1.15 and 2.08), four methylenes (δ_H 1.16, 1.45, 1.54, 1.78, 1.91, 2.05, 2.34 and 2.50), one methines (δ_H 1.74) one hydroxymethine (δ_H 4.55) and three hydroxy-methylenes (δ_H 3.34, 3.92, 4.09, 4.33, 4.84 and 5.04). In addition, according to the HSQC data, the ¹³C NMR data showed the presence of 17 carbon signals, including two ester carbonyl carbons (δ_C 173.1, 177.0) and two olefinic carbons (δ_C 124.0, 169.0), one methyl, seven methylenes (three oxygenated), two methines (one oxygenated), two aliphatic quaternary carbons (

Table 2. ¹H NMR (400 MHz) and ¹³C NMR (100 MHz) of 15 in CD₃OD.

Position	δC	δH (J in Hz)	Position	δC	δH (J in Hz)
1	35.3 (CH2)	1.45, m 2.05, m	10	44.2 (C)
2	19.5 (CH2)	1.54, m 1.78, m	11	71.8 (CH2)	5.00, d (17.7) 4.84, d (17.6)
3	37.0 (CH2)	1.16, m 1.91, m	12	177.0 (C)
4	39.3 (C)		13	68.1 (CH2)	4.44, d (11.2) 4.62, d (5.4)
5	56.4 (CH)	1.74, s	14	28.1 (CH3)	1.16, s
6	63.6 (CH)	4.61, d (11.0)	15	65.7 (CH2)	3.76, d (12.0) 4.33, d (11.9)
7	33.0 (CH2)	2.34, d, (18.9) 2.50, d (18.3)	Ac-CH3	20.8 (CH3)	2.08, s
8	124.0 (C)		Ac-OCO	173.1 (C)
9	169.0 (C)

). Analysis of its ¹H NMR and ¹³C NMR data in association with the 2D NMR data established a nucleus of drimane sesquiterpenoid characterized by an astellolide scaffold, structurally similar to the co-isolated compound 14 (Figures S26 and S27). It can be clearly observed that compound 15 has an additional acetyl fragment. Furthermore, the HMBC from H₂-13 to Ac-OCO indicated that the acetyl fragment was linked to C-13 (Figure 3).

Finally, the NOESY correlation and experimental ECD spectrum of compound 15 were identical to those of 14 (Figure 3 and Figure 4). Thus, the absolute configuration of 15 was also assigned as 4R, 5R, 6S, 10S.

2.2. Antimicrobial Assay

Compounds 1–15 were investigated for their antimicrobial activities against two phytopathogenic fungi and four bacterial strains. As shown in

Table 3. Antimicrobial activity of compounds 1–15.

	Microbia	Methicillin-Resistent Staphyococcus aureus (μg/mL) a	Bacillus subtilis (μg/mL) a	Pseudomonas aeruginosa (μg/mL) a	Salmonella typhimurium (μg/mL) a	Penicillium italicum (μg/mL) a	Colletrichum gloeosporioides (μg/mL) a
Compound
1		25	6.25	>50	>50	6.25	6.25
2		>50	>50	25	>50	50	>50
3		>50	>50	>50	>50	50	>50
4		>50	>50	>50	>50	>50	>50
5		50	25	25	>50	1.56	3.13
6		>50	25	50	>50	12.50	25
7		>50	>50	>50	>50	25	25
8		>50	>50	>50	>50	>50	>50
9		>50	>50	>50	>50	>50	>50
10		25	12.50	25	3.13	6.25	6.25
11		>50	>50	>50	>50	>50	>50
12		>50	>50	>50	>50	>50	>50
13		50	>50	>50	>50	50	>50
14		>50	>50	>50	>50	>50	>50
15		>50	>50	>50	>50	25	25
Ampicillin		0.13	0.13	0.07	0.13	-	-
Ketoconazole		-	-	-	-	0.78	0.78

^a: The deviation value of three parallel experiments; -: No test.

, andrastin-type meroterpenoids have better antimicrobial activities against phytopathogenic fungus than against bacteria. Most of all the tested compounds (9 compounds out of total 15 compounds) displayed potent antimicrobial activities (MIC < 50 μg/mL). Among them, compounds 1, 5 and 10 exhibited remarkable antimicrobial activities against Penicillium italicum and Colletrichum gloeosporioides with MIC values of 6.25, 1.56, 6.25 and 6.25, 3.13, 6.25 μg/mL. Moreover, compound 1 showed inhibitory activities against Bacillus subtilis under concentration of 6.25 μg/mL. Compound 10 also displayed significant antimicrobial activity against Salmonella typhimurium with an MIC value of 3.13 μg/mL. Notably, compound 5 revealed potential antimicrobial activity against all the strains, the MIC values were lower than 25 μg/mL.

As for the study of the structure–activity relationship (SAR), it was found that the degree of oxidation at C-21 had different effects on the activities of the compounds. The compound with methyl (10) at C-21 has significantly antimicrobial activity, followed by the aldehyde group (7), and hydroxymethyl (13) was the weakest. In-depth analysis showed that apart from the degree of oxidation at C-21, keto-enol tautomerism at the cyclopentane ring also had obvious influences on the antimicrobial activities of compounds. Compared to compounds 7 and 13 (enol form), compounds 8 and 9 (keto form) showed no activities against all strains (Table 3).

3. Experimental Methods

3.1. General Experimental Procedures

The NMR were tested on a Bruker Avance 600 MHz spectrometer (Karlsruhe, Germany) at room temperature. Optical rotations data were recorded on an MCP300 (Anton Paar, Shanghai, China). UV were tested using a Shimadzu UV-2600 spectrophotometer (Shimadzu, Kyoto, Japan). IR spectra were recorded on IR Affinity-1 spectrometer (Shimadzu, Kyoto, Japan). HR-ESI-MS spectra were tested on a ThermoFisher LTQ-Orbitrap-LC-MS spectrometer (Palo Alto, CA, USA). LC-MS/MS data was performed on a Q-TOF manufactured by Waters and a Waters Acquity UPLC BEH C18 column (1.7 µm, 2.1 × 100 mm). Recoated silica gel plates (Qingdao Huang Hai Chemical Group Co., Qingdao, China, G60, F-254), Column chromatography (CC) and Sephadex LH-20 (Amersham Pharmacia, Stockholm, Sweden) were used to purify the compounds.

3.2. Fungal Material

Fungus N-5 was isolated from the rhizosphere soil of mangrove plant Avicennia marina (collected in October 2021 from Nansha Mangrove National Nature Reserve in Guangdong Province, China). It was identified as Penicillum sp. by the ITS region (deposited in GenBank, accession no ON926808), and fungus N-5 was deposited at Sun Yat-sen University, China.

3.3. Fermentation

The fungus Penicillum sp. N-5 was cultured in one hundred 1000 mL Erlenmeyer flasks at 25 °C for 30 days; these contained autoclaved rice solid-substrate medium composed of 50 g rice and 50 mL 3‰ saline water.

3.4. Extraction and Purification

After incubation, the mycelia and solid rice medium were extracted four times with EtOAc, and 75 g of residue was obtained. Next, the residue was separated by a gradient of petroleum ether/EtOAc from 9:1 to 0:10 (v/v) on silica gel CC and divided into six fractions (Fr.1–Fr.6). Fr. 2 (10 g) was separated to Sephadex LH-20 (methanol) to yield three sub-fractions (SFrs. 2.1–2.3). SFrs.2.3 (1.2 g) was applied to silica gel CC (DCM/MeOH v/v, 100:1) and further purified by reversed-phase (RP) high performance liquid chromatography (HPLC; 90–10% MeCN/H₂O for 25 min) to obtain compounds 1 (5 mg) and 3 (7 mg). Fr. 3 (16 g) was also separated to Sephadex LH-20 (methanol) to yield four sub-fractions (SFrs. 3.1–3.4). SFrs.3.1 (1.6 g) was separated to silica gel CC (DCM/MeOH v/v, 80:1) and further purified by reversed-phase (RP) high performance liquid chromatography (HPLC; 75–25% MeCN/H₂O for 22 min) to yield compounds 2 (11 mg) and 15 (6 mg).

Hemiacetalmeroterpenoid A (1): white powder, m.p. 122.8–124.1 °C; ${\left[\alpha \right]}_{\mathrm{D}}^{25}$-52 (c 0.02, MeOH), UV (MeOH) λ_max (log ε): 206 (2.52) (Figure S33); ECD (MeOH) λ_max (∆ε): 240 (+5.47), 301 (−1.14), 362 (+0.61); ¹H (600 MHz, CD₃OD) and ¹³C NMR (150 MHz, CD₃OD) data, see Table 1; HR-ESI-MS: m/z 459.23709 [M+H]⁺ (calcd. for C₂₆H₃₅O₇, 459.23773).

Hemiacetalmeroterpenoid B (2): white powder, m.p. 106.9–108.4 °C; ${\left[\alpha \right]}_{\mathrm{D}}^{25}$ −56 (c 0.02, MeOH), UV (MeOH) λ_max (log ε): 205 (2.83), 238 (1.36) (Figure S34); ECD (MeOH) λ_max (∆ε): 206 (−11.92), 248 (+3.38), 311 (−1.19); ¹H (600 MHz, CD₃OD) and ¹³C NMR (150 MHz, CD₃OD) data, see Table 1; HR-ESI-MS: m/z 445.25772 [M+H]⁺ (calcd. for C₂₆H₃₇O₆, 445.25847).

Hemiacetalmeroterpenoid C (3): white powder, m.p. 100.8–102.6 °C; ${\left[\alpha \right]}_{\mathrm{D}}^{25}$ −66 (c 0.02, MeOH), UV (MeOH) λ_max (log ε): 205 (2.51), 260 (1.78) (Figure S35); ECD (MeOH) λ_max (∆ε): 206 (−18.31), 240 (+12.72), 310 (−2.18); ¹H (600 MHz, CD₃OD) and ¹³C NMR (150 MHz, CD₃OD) data, see Table 1; HR-ESI-MS: m/z 509.25015 [M+Na]⁺ (calcd. for C₂₈H₃₈O₇Na, 509.25097).

Astellolide J (14): yellow powder, m.p. 202.9–204.5 °C; ${\left[\alpha \right]}_{\mathrm{D}}^{25}$ +16 (c 0.02, MeOH), UV (MeOH) λ_max (log ε): 216 (3.15) (Figure S36); ECD (MeOH) λ_max (∆ε): 203 (−1.82), 225 (+3.27); ¹H (400 MHz, CD₃OD) and ¹³C NMR (100 MHz, CD₃OD) data; HR-ESI-MS: m/z 305.13565 [M+Na]⁺ (calcd. for C₁₅H₂₂O₅Na, 305.13594).

Astellolide Q (15): yellow powder, m.p. 160.1–162.2 °C; ${\left[\alpha \right]}_{\mathrm{D}}^{25}$ +12 (c 0.02, MeOH), UV (MeOH) λ_max (log ε): 218 (3.62) (Figure S37); ECD (MeOH) λ_max (∆ε): 232 (+5.19); ¹H (400 MHz, CD₃OD) and ¹³C NMR (100 MHz, CD₃OD) data, see Table 2; HR-ESI-MS: m/z 347.14578 [M+Na]⁺ (calcd. for C₁₇H₂₄O₆Na, 347.14578).

3.5. ECD Calculation

Firstly, ECD calculations of compounds 1–3 and 14–15 were performed by the Gaussian 09 program and Spartan’14. Next, the conformations with a Boltzmann population (>5%) were selected for optimization and calculation in methanol at B3LYP/6-31+G (d, p). Finally, the ECD spectra were generated by the program SpecDis 1.6 (University of Würzburg, Würzburg, Germany) and drawn by OriginPro 8.0 (OriginLab, Ltd., Northampton, MA, USA) from dipole-length rotational strengths by applying Gaussian band shapes with sigma = 0.30 eV [29,30].

3.6. Bioassays Antimicrobial Activity

Antimicrobial activity assay was performed as previously described in [31,32].

4. Conclusions

In summary, three new andrastin-type meroterpenoids (1–3), one new drimane sesquiterpenoid (15) and one sesquiterpenoid J (14) that was first isolated from a natural source, together with ten known compounds (4–13) were isolated from the cultures of the rhizosphere soil of mangrove plant Avicennia marina fungus Penicillium sp. N-5. Their structures were determined by the analysis of NMR, HR-MS and ECD spectra. All the isolated compounds were investigated for their antimicrobial activities against two phytopathogenic fungi and four bacterial strains. Among them, compounds 1, 5 and 10 exhibited significant inhibition against Penicillium italicum and Colletrichum gloeosporioides with MIC values of 6.25, 1.56, 6.25 and 6.25, 3.13, 6.25 μg/mL. Notably, compound 5 showed potential antimicrobial activity against all the strains and the MIC values were lower than 25 μg/mL. Moreover, andrastin-type meroterpenoid antimicrobial activity against phytopathogenic fungi was reported for the first time.