Synthesis of the C13–C29 fragment of leiodolide A: allylic asymmetric induction on the stereochemical course of iodolactonization

Synthesis of Leiodolide A Macrolactone

A convergent route toward the synthesis of leiodolide A (1) is described. Our studies explored reactions of the indium chloride-induced transmetalation of allylic stannane 32 for nucleophilic addition with nonracemic aldehyde 15. The stereoselective formation of the all-syn stereotriad was rationalized by an in situ isomerization to produce the Z-allylindium reagent for subsequent anti-Felkin addition. The inversion of C17 stereochemistry led to an effective π-allyl Stille cross coupling utilizing Z-alkenylstannane 11b. The Horner-Wadsworth-Emmons reaction provides macrolactone 37 which exhibits discrepancies as compared with reported NMR data for the purported leiodolide A.

Studies Toward the Synthesis of Leiodolide A

Studies have described a highly convergent plan toward the synthesis of leiodolide A (1), a potent cytotoxic sponge metabolite. The enantiocontrolled preparation of aldehyde 6 is achieved with the application of several advances in methodology for the synthesis of substituted 1,3-oxazoles. Efforts have examined the halogen dance reaction, the selectivity of Stille cross coupling reactions of 4-bromo-1,3-oxazoles, and nucleophilic displacement of the 2-phenylsulfonyl substituent with organolithium reagents as preparatively useful reactions. These techniques have facilitated the efficient synthesis of 6 from the starting bromide 12, alkenylstannane 16, and the primary nonracemic alcohol 25.

Phosphine-Catalyzed Crossed Rauhut-Currier-Type Coupling and [3 + 2]-Cycloaddition of 2-Alkylidene-3-oxindoles with Alkenes and Allenes to Access β,γ-Unsaturated Enones and Polycyclic Oxindoles

Dihydroazepino[1,2-a]indole diones are tricyclic N-acyl-2-alkylidene-3-oxindole enones that readily engage in tertiary phosphine-catalyzed intermolecular coupling reactions with acceptor-substituted alkenes. In these reactions, the tricyclic α-substituted enones undergo an α-alkylation with concomitant formation of a quaternary stereocenter, as well as the installation of a new double bond within the seven-membered azepane ring. The organocatalytic reaction constitutes a special case of the crossed intermolecular Rauhut-Currier reaction as the presence of the α-substituent in the enones prohibits the formation of an α,β-unsaturated product, but instead, skipped β,γ-unsaturated enones are obtained. With allene carboxylates as alternative coupling partners, the crossed Rauhut-Currier-type reaction competes with a regioselective allene to enone [3 + 2]-cycloaddition, and the product selectivity can be controlled by the choice of the phosphine organocatalyst.

High-Throughput Screening of a Marine Compound Library Identifies Anti-Cryptosporidium Activity of Leiodolide A

Cryptosporidium sp. are apicomplexan parasites that cause significant morbidity and possible mortality in humans and valuable livestock. There are no drugs on the market that are effective in the population most severely affected by this parasite. This study is the first high-throughput screen for potent anti-Cryptosporidium natural products sourced from a unique marine compound library. The Harbor Branch Oceanographic Institute at Florida Atlantic University has a collection of diverse marine organisms some of which have been subjected to medium pressure liquid chromatography to create an enriched fraction library. Numerous active compounds have been discovered from this library, but it has not been tested against Cryptosporidium parvum. A high-throughput in vitro growth inhibition assay was used to test 3764 fractions in the library, leading to the identification of 23 fractions that potently inhibited the growth of Cryptosporidium parvum. Bioassay guided fractionation of active fractions from a deep-sea sponge, Leiodermatium sp., resulted in the purification of leiodolide A, the major active compound in the organism. Leiodolide A displayed specific anti-Cryptosporidium activity at a half maximal effective concentration of 103.5 nM with selectivity indexes (SI) of 45.1, 11.9, 19.6 and 14.3 for human ileocecal colorectal adenocarcinoma cells (HCT-8), human hepatocellular carcinoma cells (Hep G2), human neuroblastoma cells (SH-SY5Y) and green monkey kidney cells (Vero), respectively. The unique structure of leiodolide A provides a valuable drug scaffold on which to develop new anti-Cryptosporidium compounds and supports the importance of screening natural product libraries for new chemical scaffolds.