Synthesis of 9,11-Secosteroids Pinnisterol E, Glaciasterol B, and 6-Keto-aplidiasterol B

Skeletal Isomerization of Ergosterol-5,8-Peroxide Leading to the Discovery of Unprecedented Ergostanes and Collective Syntheses of 5,6-Epoxysterols and (+)-Sarocladione

Skeletal isomerization of ergosterol peroxide, a primary oxidation product of ergosterol, was investigated under thermal and iron(II)-mediated conditions. Thermal isomerization resulted in not only the isolation of the predicted 7-hydroxy-5,6-epoxides but also the discovery of the unprecedented 7/9/5-ring-fused ergostane for the first time. The iron(II)-mediated isomerization proceeded at ambient temperature, resulting in the formation of the expected 5,6-epoxysterols and a ring-opened bicyclic diketone. The diketone was further converted into novel ergostane under thermal conditions and into (+)-sarocladione under acidic conditions. All transformations from ergosterol to sarocladione, including the isolation of the unstable diketone intermediate, were achieved at ambient temperature, confirming the biosynthetic pathway of sarocladione. Several mushroom ingredients with a 5,6-epoxy group were synthesized stereoselectively from the isomerization products, leading to the confirmation or revision of the structures of natural products. The β-amyloid aggregation inhibitory activity of synthetic sterols was evaluated for the first time to gain insights into the potential for dementia prevention. This study is valuable both for supplying rare sterols found in mushrooms for biological studies and for shedding light on the oxidative metabolic pathways of ergosterol.

Hipposponols A and B, two new 9, 11-secosterols from the marine sponge Hippospongia lachne de Laubenfels

Two new 9,11-secosterols, hipposponols A (1) and B (2), together with five known analogues, aplidiasterol B (3), (3β,5α,6β)-3,5,6-triol-cholest-7-ene (4), (3β,5α,6β,22E)-3,5,6-triol-ergosta-7,22-diene (5), and one pair of inseparable C-24 epimers of (3β,5α,6β,22E)-3,5,6-triol-stigmasta-7,22-diene (6/7), were isolated from the marine sponge Hippospongia lachne de Laubenfels. The structures of isolated compounds were extensively elucidated based on HRESIMS and NMR data. Compounds 2 - 5 showed cytotoxicity against PC9 cells with IC50 values ranging from 34.1 ± 0.9 to 38.9 ± 1.0 µM and compound 4 displayed cytotoxicity against MCF-7 cells with IC50 value of 39.0 ± 0.4 µM.

Marine natural products

Covering: January to the end of December 2022This review covers the literature published in 2022 for marine natural products (MNPs), with 645 citations (633 for the period January to December 2022) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, the submerged parts of mangroves and other intertidal plants. The emphasis is on new compounds (1417 in 384 papers for 2022), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. An analysis of NP structure class diversity in relation to biota source and biome is discussed.

Discovery and Bioinspired Synthesis of Salpratone A

Salpratone A (1), a novel abietane diterpenoid containing a unique cis-fused A/B ring, was isolated from Salvia prattii. Bioactivity studies showed that 1 has potent activity in inhibiting platelet aggregation induced by multiple agonists as well as antithrombotic efficacy in the FeCl3-induced rat in vivo thrombosis model. Furthermore, a bioinspired synthesis of 1 from the abundant natural product ferruginol was achieved in 6 steps with a 22% overall yield. The key steps include a stereoselective allyl oxidation and a subsequent regioselective Meinwald rearrangement.

Divergent and Stereoselective Synthesis of Ustusal A, (-)-Albrassitriol, and Elegansin D

The first synthesis of ustusal A as well as expeditious access to (-)-albrassitriol is described as featuring a singlet oxygen [4 + 2] cycloaddition, achieving the desired stereoselectivity for the 1,4-cis-hydroxyl groups. Transformation of (+)-sclareolide to III followed by a key Horner-Wadsworth-Emmons (HWE) reaction and stereospecific allylic oxidation facilitated the first synthesis of elegansin D. The biological evaluation of these natural products together with seven elegansin D analogues was performed, among which several elegansin D analogues exhibited potential anticancer activity against liver cancer HepG2 cells (IC50 = 11.99-25.58 μM) with low cytotoxicity on normal liver HL7702 cells (IC50 > 100 μM).

Total Synthesis of Aplysiasecosterols A and B, Two Marine 9,11-Secosteroids

The total synthesis of aplysiasecosterols A and B has been accomplished. Key features of the synthesis include the Suzuki-Miyaura coupling of each AB-ring segment and common D-ring segment. The AB-ring segment of aplysiasecosterol B was synthesized by Shi asymmetric epoxidation as a key reaction. The common D-ring segment was constructed by stereoselective hydrogenation and Sharpless asymmetric dihydroxylation as key reactions. This late-stage convergent synthesis, which has rarely been reported in secosteroid synthesis, can be adapted to many 9,11-secosteroids.