*2.1. General Experimental Procedures*

Optical rotations were measured on a Perkin-Elmer 343 polarimeter (Perkin Elmer, Waltham, MA, USA). UV spectra were recorded on a Shimadzu UV-1601PC spectrometer (Shimadzu Corporation, Kyoto, Japan) in methanol. CD spectra were measured with a Chirascan-Plus CD spectrometer (Leatherhead, UK) in methanol. NMR spectra were recorded in CDCl3, acetone-*d<sup>6</sup>* and DMSO-*d6*, on a Bruker DPX-300 (Bruker BioSpin GmbH, Rheinstetten, Germany), a Bruker Avance III-500 (Bruker BioSpin GmbH, Rheinstetten, Germany) and a Bruker Avance III-700 (Bruker BioSpin GmbH, Rheinstetten, Germany) spectrometer, using TMS as an internal standard. HRESIMS spectra were measured on a Maxis impact mass spectrometer (Bruker Daltonics GmbH, Rheinstetten, Germany). Microscopic examination and photography of fungal cultures were performed with Olympus CX41 microscope equipped with an Olympus SC30 digital camera. Detailed examination of ornamentation of the fungal conidia was performed using scanning electron microscopy (SEM) EVO 40.

Low-pressure liquid column chromatography was performed using silica gel (60/100 µm, Imid Ltd., Krasnodar, Russia) and Gel ODS-A (12 nm, S—75 um, YMC Co., Ishikawa, Japan). Plates precoated with silica gel (5–17 µm, 4.5 cm × 6.0 cm, Imid Ltd., Russia) and silica gel 60

RP-18 F254S (20 cm × 20 cm, Merck KGaA, Darmstadt, Germany) were used for thin-layer chromatography. Preparative HPLC was carried out on an Agilent 1100 chromatograph (Agilent Technologies, Santa Clara, CA, USA) with an Agilent 1100 refractometer (Agilent Technologies, Santa Clara, CA, USA) and a Shimadzu LC-20 chromatograph (Shimadzu USA Manufacturing, Canby, OR, USA) with a Shimadzu RID-20A refractometer (Shimadzu Corporation, Kyoto, Japan) using YMC ODS-AM (YMC Co., Ishikawa, Japan) (5 µm, 10 mm × 250 mm), YMC ODS-AM (YMC Co., Ishikawa, Japan) (5 µm, 4.6 mm × 250 mm) and Hydro-RP (Phenomenex, Torrance, CA, USA) (4 µm, 250 mm × 10 mm) columns.

#### *2.2. Fungal Strain*

The brown algae samples were collected in Vostok Bay (Sea of Japan) in sterile plastic bags. Before use, they were stored in a freezer at −18 ◦C. Isolation of fungi from algae samples was carried out by the plate method using Tubaki agar medium. The fungus was isolated into a pure culture by transferring the inoculum from a Petri dish onto a slant wort agar, where it was further stored. Microscopic examination of the strain was performed using Olympus CX41.

For DNA isolation, a fungus culture grown at 25 ◦C for 7 days was used. Isolation of genomic DNA was carried out using a commercial DNA kit (DNA-Technology Ltd., Moscow, Russia) in accordance with the protocol. Amplification and sequencing of the ITS genes were performed using ITS1 and ITS4 gene-specific primers [16]. The obtained sequence was compared with the GenBank sequence dataset and registered under accession number OL477331.

#### *2.3. Cultivation of Fungus*

The fungus was cultured at 22 ◦C for three weeks in 60 × 500 mL Erlenmeyer flasks, each containing rice (20.0 g), yeast extract (20.0 mg), KH2PO<sup>4</sup> (10 mg) and natural seawater from the Marine Experimental Station of PIBOC, Troitsa (Trinity) Bay, Sea of Japan (40 mL).

#### *2.4. Extraction and Isolation*

At the end of the incubation period, the mycelia and medium were homogenized and extracted with EtOAc (1 L). The obtained extract was concentrated to dryness. The residue (17.5 g) was dissolved in H2O–EtOH (4:1) (100 mL) and extracted successively with *n*-hexane (0.2 L × 3), EtOAc (0.2 L × 3) and BuOH (0.2 L × 3). After evaporation of the ethyl acetate layer, the residual material (5.5 g) was subjected to column chromatography on silica gel, which was eluted with a gradient of *n*-hexane in ethyl acetate (100:1, 95:5, 90:10, 80:20, 70:30, 50:50). Fractions of 250 mL were collected and combined on the basis of TLC (silica gel, toluene–isopropanol 6:1 and 3:1, *v/v*).

The fractions of *n*-hexane-EtOAc (95:5, 80 mg) and *n*-hexane-EtOAc (90:10, 200 mg) were separated on a Gel ODS-A column (1.5 cm × 8 cm), which was eluted by a step gradient from 40% to 80% CH3OH in H2O (total volume 1 L), to afford subfractions I and II. Subfraction I (40% CH3OH, 146 mg) was purified by RP HPLC on a YMC ODS-AM column eluted with CH3OH-H2O (90:10) and then with CH3OH-H2O (60:40) to yield **2** (1.8 mg) and **4** (6.0 mg). Subfraction II (60% CH3OH, 110 mg) was purified by RP HPLC on a YMC ODS-AM column eluted with CH3OH-H2O (80:20) and then with CH3OH-H2O (55:45) to yield **1** (7 mg).

The *n*-hexane-EtOAc (80:20, 470 mg) fraction was separated on a Gel ODS-A column (1.5 cm × 8 cm), which was eluted with a step gradient from 40% to 80% CH3OH in H2O (total volume 1 L) to give subfraction III. Subfraction III (40% CH3OH, 250 mg) was separated by RP HPLC on a YMC ODS-AM column eluting with CH3OH-H2O (90:10) and then with CH3OH-H2O (60:40) to yield **6** (58 mg).

After evaporation of the BuOH layer, the residual material (0.98 g) was passed through a silica column (3 cm × 14 cm), which was separated in the same way as the ethyl acetate extract.

The *n*-hexane-EtOAc (50:50, 252 mg) fraction was purified by RP HPLC on a YMC ODS-A column eluted with CH3OH-H2O (45:1055) and then with CH3CN-H2O (25:75) to yield **3** (3.9 mg) and **5** (5.2 mg).

#### *2.5. Spectral Data*

Acrucipentyn A (**1**): amorphous solids; [α]<sup>D</sup> <sup>20</sup> – 36.0 (*<sup>c</sup>* 0.09 CH3OH); CD (*<sup>c</sup>* 9.6 <sup>×</sup> <sup>10</sup>−<sup>4</sup> , CH3OH), λmax (∆ε) 210 (−0.97), 266 (−0.14) nm, see Supplementary Figure S1; UV (CH3OH) *λ*max (log *ε*) 271 (2.54), 256 (2.50) and 224 (3.77) nm, see Supplementary Figure S7; <sup>1</sup>H and <sup>13</sup>C NMR data, see Tables 1 and 2, Supplementary Figures S13–S22; HRESIMS *m*/*z* 229.0625 [M – H]– (calcd. for C11H14ClO3, 229.0637, ∆5.0 ppm), 253.0599 [M + Na]<sup>+</sup> (calcd. for C11H15ClO3Na, 253.0602, ∆1.2 ppm).


**Table 1.** <sup>1</sup>H NMR data (*δ* in ppm, *J* in Hz) for compounds (**1**–**6**).

<sup>a</sup> Chemical shifts were measured at 700.13 MHz in DMSO-d6. <sup>b</sup> Chemical shifts were measured at 700.13 MHz in acetone-d6. <sup>c</sup> Chemical shifts were measured at 500.13 MHz in DMSO-d6. <sup>d</sup> Chemical shifts were measured at 500.13 MHz in CDCl3.

Acrucipentyn B (**2**): amorphous solids; [α]<sup>D</sup> <sup>20</sup> +46.2 (*<sup>c</sup>* 0.09 CH3OH); CD (*<sup>c</sup>* 8.7 <sup>×</sup> <sup>10</sup>−<sup>4</sup> , CH3OH), λmax (∆ε) 223 (+0.87) nm, see Supplementary Figure S2; UV (CH3OH) *λ*max (log *ε*) 224 (3.93), 200 (3.41) and 194 (3.51) nm, see Supplementary Figure S8; <sup>1</sup>H and <sup>13</sup>C NMR data, see Tables 1 and 2, Supplementary Figures S23–S29; HRESIMS *m*/*z* 229.0634 [M – H]– (calcd. for C11H14ClO3, 229.0637, ∆1.4 ppm), 253.0601 [M + Na]<sup>+</sup> (calcd. for C11H15ClO3Na, 253.0602, ∆0.5 ppm).


**Table 2.** <sup>13</sup>C NMR data (*δ* in ppm) for compounds **1**–**6**.

<sup>a</sup> Chemical shifts were measured at 176.04 MHz in DMSO-d6. <sup>b</sup> Chemical shifts were measured at 75.47 MHz in acetone-d6. <sup>c</sup> Chemical shifts were measured at 125.77 MHz in DMSO-d6. <sup>d</sup> Chemical shifts were measured at 125.77 MHz in CDCl3.

Acrucipentyn C (**3**): amorphous solids; [α]<sup>D</sup> <sup>20</sup> –127.9 (*<sup>c</sup>* 0.07 CH3OH); CD (*<sup>c</sup>* 1.2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> , CH3OH), λmax (∆ε) 223 (−0.89), 297 (−0.11), 305 (−0.12) nm, see Supplementary Figure S3; UV (CH3OH) *λ*max (log *ε*) 270 (3.06), 251 (2.63) and 223 (3.70) nm, see Supplementary Figure S9; <sup>1</sup>H and <sup>13</sup>C NMR data, see Tables 1 and 2, Supplementary Figures S30–S35; HRESIMS *m*/*z* 229.0631 [M – H]– (calcd. for C11H14ClO3, 229.0637, ∆2.5 ppm), 253.0595 [M + Na]<sup>+</sup> (calcd. for C11H15ClO3Na, 253.0602, ∆2.7 ppm).

Acrucipentyn D (**4**): amorphous solids; [α]<sup>D</sup> <sup>20</sup> – 94.3 (*<sup>c</sup>* 0.06 CH3OH); CD (*<sup>c</sup>* 8.8 <sup>×</sup> <sup>10</sup>−<sup>4</sup> , CH3OH), λmax (∆ε) 210 (−3.73), 227 (−1.44), 238 (−1.17) nm, see Supplementary Figure S4; UV (CH3OH) *λ*max (log *ε*) 259 (3.90), 226 (3.56) and 198 (3.80) nm, see Supplementary Figure S10; <sup>1</sup>H and <sup>13</sup>C NMR data, see Tables 1 and 2, Supplementary Figures S36–S42; HRESIMS *m*/*z* 227.0471 [M – H]– (calcd. for C11H12ClO3, 227.0480, ∆4.0 ppm), 251.0441 [M + Na]<sup>+</sup> (calcd. for C11H13ClO3Na, 251.0445, ∆1.6 ppm).

Acrucipentyn E (**5**)*:* amorphous solids; [α]<sup>D</sup> <sup>20</sup> –60.5 (*<sup>c</sup>* 0.09 CH3OH); CD (*<sup>c</sup>* 8.8 <sup>×</sup> <sup>10</sup>−<sup>4</sup> , CH3OH), λmax (∆ε) 199 (−5.11), 226 (−1.19), 237 (−0.83) nm, see Supplementary Figure S5; UV (CH3OH) *λ*max (log *ε*) 260 (4.22), 226 (3.77) and 198 (4.10) nm, see Supplementary Figure S11; <sup>1</sup>H and <sup>13</sup>C NMR data, see Tables 1 and 2, Supplementary Figures S43–S49; HRESIMS *m*/*z* 227.0470 [M – H]– (calcd. for C11H12ClO3, 227.0480, ∆4.5 ppm), 251.0441 [M + Na]<sup>+</sup> (calcd. for C11H13ClO3Na, 251.0445, ∆1.6 ppm).

Acrucipentyn F (**6**): amorphous solids; [α]<sup>D</sup> <sup>20</sup> +40.3 (*<sup>c</sup>* 0.11 CH3OH); CD (*<sup>c</sup>* 1.0 <sup>×</sup> <sup>10</sup>−<sup>3</sup> , CH3OH), λmax (∆ε) 199 (−4.56), 226 (−0.72), 259 (+1.13) nm, see Supplementary Figure S6; UV (CH3OH) *λ*max (log *ε*) 259 (4.05), 226 (3.59) and 201 (3.88) nm, see Supplementary Figure S12; <sup>1</sup>H and <sup>13</sup>C NMR data, see Tables 1 and 2, Supplementary Figures S50–S56; HRESIMS *m*/*z* 191.0704 [M – H]– (calcd. for C11H11O3, 191.0714, ∆5.2 ppm), 215.0677 [M + Na]<sup>+</sup> (calcd. for C11H12O3Na, 215.0679, ∆0.8 ppm).

#### *2.6. Preparation of Acetonides of* **1a** *and* **4a**

To a DMFA solution of **1** (4.0 mg) 2,2-dimethoxypropane (0.5 mL) and catalyst *p*toluenesulfonic acid (0.8 mg) at room temperature were added and the solution was stirred for 24 h. After the evaporation of the solvent, the product was dissolved in MeOH and purified by reversed-phase HPLC (YMC ODS-A column) eluted with CH3CN-H2O (50:50) to yield the acetonide product **1a** (1.5 mg). Compound **4** (4.0 mg) was treated similarly and yielded the acetonide product **4a** (1.4 mg).

Acetonide of acrucipentyn A (**1a**): amorphous solids; <sup>1</sup>H NMR (Acetone-*d*6, 500.13 MHz) δ: 5.20 (1H, s, H-4a0 ), 5.19 (1H, s, H-4b0 ), 4.37 (1H, t, *J =* 4.3 Hz, H-1), 4.20 (1H, dd, *J =* 8.3;

4.8 Hz, H-2), 4.11 (1H, m, H-4), 4.00 (1H, dd, *J* = 8.3; 2.6 Hz, H-3), 3.50 (1H, dt, *J =* 12.2; 4.4 Hz, H-6), 2.03 (1H, m, H-5b), 1.97 (1H, dt, *J =* 13.7; 4.7 Hz, H-5a), 1.85 (3H, s, H3-5<sup>0</sup> ), 1.47 (3H, s, H3-300), 1.34 (3H, s, H3-200); <sup>13</sup>C NMR (Acetone-*d*6, 125.77 MHz) δ: 128.9 (C-3<sup>0</sup> ), 122.0 (C-40 ), 110.3 (C-100), 90.3 (C-10 ), 84.1 (C-20 ), 80.5 (C-2), 77.4 (C-1), 70.7 (C-4), 67.1 (C-3), 35.0 (C-6), 29.3 (C-300), 27.2 (C-200), 26.5 (C-5), 24.6 (C-50 ), see Supplementary Figures S57–S58; HRESIMS *m*/*z* 293.0912 [M + Na]<sup>+</sup> (calcd. for C14H19ClO3Na, 293.0915, ∆0.9 ppm).

Acetonide of acrucipentyn D (**4a**): amorphous solids; <sup>1</sup>H NMR (Acetone-*d*6, 700.13 MHz) δ: 5.33 (1H, t, *J =* 0.9 Hz, H-4a0 ), 5.31 (1H, t, *J =* 1.7 Hz, H-4b0 ), 6.11 (1H, d, *J =* 3.0 Hz, H-5), 4.63 (1H, dd, *J =* 5.7; 0.9 Hz, H-1), 4.58 (1H, m, H-4), 4.57 (1H, t, *J* = 5.7 Hz, H-2), 4.38 (1H, dd, *J =* 5.6; 3.5 Hz, H-3), 1.91 (3H, t, *J =* 1.4 Hz, H3-50 ), 1.37 (3H, s, H3-200/H3-300), 1.33 (3H, s, H3- 2 <sup>00</sup>/H3-300); <sup>13</sup>C NMR (Acetone-*d*6, 125.77 MHz) δ: 137.7 (C-5), 130.0 (C-6), 129.3 (C-3<sup>0</sup> ), 122.8 (C-40 ), 110.6 (C-100), 91.3 (C-20 ), 87.9 (C-10 ), 77.1 (C-2), 74.6 (C-1), 66.0 (C-4), 62.8 (C-3), 27.8 (C-200/C-300), 26.0 (C-200/C-300), 23.4 (C-50 ), see Supplementary Figures S63–S64; HRESIMS *m*/*z* 291.0758 [M + Na]<sup>+</sup> (calcd. for C14H17ClO3Na, 291.0758, ∆0.9 ppm).

#### *2.7. Preparation of (S)-MTPA and (R)-MTPA Esters of* **1a**

To a pyridine solution of **1a** (0.7 mg) 4-dimethylaminopyridine (a few crystals) and (*S*)-MTPA-Cl (10 µL) at room temperature were added, and the solution was stirred for 24 h. After the evaporation of the solvent, the residue was purified by RP HPLC on a YMC ODS-AM column eluted with CH3CN-H2O (70:30) to afford the (*R*)-MTPA ester (**1a-1**). The (*S*)-MTPA ester (**1a-2**) was prepared in a similar manner using (*R*)-MTPACl. <sup>1</sup>H NMR and COSY data, see Supplementary Figure S59–S62.

(*R*)-MTPA ester of **1a**: <sup>1</sup>H NMR (Acetone-*d*6, 500.13 MHz) δ: 5.56 (1H, brs, H-4), 5.22 (1H, s, H-4a0 ), 5.20 (1H, s, H-4b0 ), 4.32 (1H, brs, H-3), 4.30 (1H, brs, H-1), 4.09 (1H, dd, *J* = 8.1; 4.9 Hz, H-2), 2.80 (1H, m, H-6), 2.24 (1H, t, *J =* 13.7 Hz, H-5a), 2.09 (1H, m, H-5b), 1.84 (3H, s, H3-5<sup>0</sup> ), 1.50 (3H, s, H3-300), 1.33 (3H, s, H3-200). HRESIMS *m/z* 509.1310 [M + Na]<sup>+</sup> (calcd for C24H26ClF3O5, 509.1313)

(*S*)-MTPA ester of **1a**: <sup>1</sup>H NMR (Acetone-*d*6, 500.13 MHz) δ: 5.65 (1H, brs, H-4), 5.24 (1H, s, H-4a0 ), 5.21 (1H, s, H-4b0 ), 4.29 (1H, d, *J =* 7.9 Hz, H-3), 4.44 (1H, t, *J =* 4.1 Hz, H-1), 4.06 (1H, dd, *J* = 8.0; 5.2 Hz, H-2), 3.27 (1H, dt, *J* = 12.8; 4.1 Hz, H-6), 2.33 (1H, t, *J =* 13.4 Hz, H-5a), 2.22 (1H, dt, *J* = 14.6; 4.7 Hz, H-5b), 1.85 (3H, s, H3-50 ), 1.50 (3H, s, H3-300), 1.34 (3H, s, H3-200). HRESIMS *m/z* 509.1307 [M + Na]<sup>+</sup> (calcd for C24H26ClF3O5, 509.1313).
