Total Synthesis of Cytospolide D and Its Biomimetic Conversion to Cytospolides M, O, and Q

Tandem Macrolactone Synthesis: Total Synthesis of (-)-Exiguolide by a Macrocyclization/Transannular Pyran Cyclization Strategy

Tetrahydropyran-containing macrolactones were synthesized by integrating Meyer-Schuster rearrangement, macrocyclic ring-closing metathesis, and transannular oxa-Michael addition under gold and ruthenium catalysis. Single-step access to a variety of 14- to 20-membered macrolactones containing a tetrahydropyran ring was possible from readily available linear precursors in good yields and with moderate to excellent diastereoselectivity. A 13-step synthesis of (-)-exiguolide, an anticancer marine macrolide, showcased the feasibility of our tandem reaction sequence for macrolactone synthesis and also demonstrated the power of transannular reactions for rapid assembly of the tetrahydropyran rings of the target natural product.

The Tetrahydrofuran Motif in Polyketide Marine Drugs

Oxygen heterocycles are units that are abundant in a great number of marine natural products. Among them, marine polyketides containing tetrahydrofuran rings have attracted great attention within the scientific community due to their challenging structures and promising biological activities. An overview of the most important marine tetrahydrofuran polyketides, with a focused discussion on their isolation, structure determination, approaches to their total synthesis, and biological studies is provided.

The Role of Total Synthesis in Structure Revision and Elucidation of Decanolides (Nonanolides)

Ten-membered lactones are commonly observed structures of natural products. They are mostly fungal metabolites, which often act as plant pathogens, but recently ten-membered lactones were identified as pheromones of frogs and termites. Although modern spectroscopic methods are nowadays routinely used to elucidate the structure of natural products, structural assignments of ten-membered lactones often remain incomplete or are surprisingly often erroneous. Most errors concern the absolute configuration. The examples discussed in this chapter demonstrate that enantioselective total synthesis is not only an efficient tool for corroborating or revising a proposed structure, but that the synthesis of different stereoisomers as references for gas chromatographic investigations can be a vital part of the structure elucidation process if only minute amounts of material are available. As a method of outstanding importance for the synthesis of ten-membered lactones olefin metathesis has emerged. Most of the examples discussed herein use one or more olefin metathesis reactions as key steps.

Synthesis of Cytospolide Analogues and Late-State Diversification Thereof

The cytospolides are a novel group of fungal secondary metabolites first described in 2011. Although all 17 natural derivatives share the same C-14 polyketide backbone, they exhibit a fairly broad structural diversity regarding their oxygenation and acetylation pattern as well as macrolide structure, e.g., monocyclic nonanolide core or bicyclic ring systems with a bridging THF ring. In the present work, the prospects for an extension of the structural diversity of cytospolides have been investigated. On the basis of a previously established synthesis of cytospolide D, a modified route to a truncated analogue carrying an alkyne instead of the natural n-pentyl side chain has been developed. In a bioinspired approach the so-derived cytospolide D alkyne analogue has been further converted to bicyclic and THF ring containing derivatives with a different backbone architecture. Finally, Sonogashira couplings or Huisgen-Sharpless click reactions have been used for late-stage diversifications. Thus, a set of 15 novel and structurally diverse natural product derivatives has been synthesized in a highly efficient manner.

Oxidative Deprotection of p-Methoxybenzyl Ethers via Metal-Free Photoredox Catalysis

An efficient and greener deprotection method for p-methoxybenzyl (PMB) ethers using a metal-free visible light photoredox catalyst and air and ammonium persulfate as the terminal oxidants is presented. Various functional groups and protecting groups were tolerated in the developed method to achieve good to excellent yields in short reaction times. Significantly, the developed method was compatible with PMB ethers derived from primary, secondary, and tertiary alcohols and a gram-scale reaction. Mechanistic studies support a proposed reaction mechanism that involves single electron oxidation of the PMB ether.

Total Synthesis of Cytospolide Q

A flexible and convergent strategy for the stereoselective total synthesis of bioactive marine natural product cytospolide Q has been developed. The key features of this synthesis include Evans anti-aldol reaction for the installation of C-2 and C-3 stereocenters and cycloetherification via epoxide opening followed by concomitant lactonization for the construction of tetrahydrofuran and γ-butyrolactone scaffolds. This synthetic study also revealed that protected oxygenated functionality (methyl ester or benzyl ether) at C-1 position participated readily in epoxide opening.

Studies Directed toward the Stereoselective Synthesis of Cytospolide E

Our exhaustive effort toward the total synthesis of cytotoxic marine nonanolide cytospolide E has been detailed. To achieve this synthesis, we have explored both the ring-closing metathesis and lactonization-based macrocyclization strategies using a variety of precursors. Unfortunately, none of them provided the desired product. The ring-closing metathesis approach provided mainly the macrocycle with Z-olefin, whereas the macrolactonization strategy culminated in 8-epi-9-epi-cytospolide E following the regioselective formation of a 10-membered macrocycle over a 9-membered macrocycle.