Enantioselective total syntheses of (+)- and (−)-ottelione A and (+)- and (−)-ottelione B. Absolute configuration of the novel, biologically active natural products

Enantioselective Synthesis of the 1,3-Dienyl-5-Alkyl-6-Oxy Motif: Method Development and Total Synthesis

The 1,3-dienyl-5-alkyl-6-oxy motif is widely found in various types of bioactive natural products. However, present synthesis is mainly non-asymmetric which relied upon different olefination or transition metal-catalyzed cross-coupling reactions using enantioenriched precursors. Herein, based upon a newly developed enantioselective α-alkylation of conjugated polyenoic acids, a variety of 1,3-dienyl-5-alkyl-6-oxy motif (with E-configured internal olefin) was generated as the corresponding α-adducts in a highly enantioselective and diastereoselective manner. Utilizing 1,3-dienyl-5-alkyl-6-oxy motif as key intermediates, we further demonstrated their synthetic potential by expedient total syntheses of three types of natural products (glutarimide antibiotics, α-pyrone polyketides and Lupin alkaloids) within 4-7 steps.

Enantioselective Total Synthesis of Otteliones A and B, Novel and Powerful Antitumor Agents from the Freshwater Plant Ottelia alismoides

Otteliones A and B isolated from the freshwater plant Ottelia alismoides have attracted significant attention because of their potential as novel anticancer agents. In this review four independent enantioselective total syntheses and one formal synthesis of these natural products are presented with particular focus on their methodology and strategy.

Synthesis and antiproliferative activities of ottelione a analogues

Through the syntheses of its C-1 desvinyl, C-7 methylene, C-7 exocyclic ethylidene, and various C-3 phenylmethyl analogues, the structure-activity relationship of antimitotic ottelione A (4) against tubulin and various cancer cells was established. The results indicated that compound 4 was a colchicine-competitive inhibitor and that the C-1 vinyl group is unnecessary for its potency, whereas the C-7 exocyclic double bond is essential, possibly because of its irreversible interaction with tubulin. Further optimization of the substituents on the phenylmethyl group at the C-3 position generated compound 10g with a 3'-fluoro-4'-methoxyphenylmethyl substituent, which was 6-38-fold more active against MCF-7, NCI-H460, and COLO205 cancer cells relative to 4. Results from in vitro tubulin polymerization assay confirmed the potency of compounds 4, 10g, and 11a.

Enantioselective total synthesis of otteliones A and B

Enantioselective total synthesis of otteliones A and B was accomplished. The key steps are radical cyclization of an alpha-iodoketone to construct the cis-hydrindanone skeleton and Suzuki-Miyaura coupling to incorporate the aromatic group. (+)-Ottelione A was converted to (-)-ottelione B on treatment with NaOH in THF.

Studies on chemo- and diastereo-selectivity of the Diels–Alder reactions of sulfinyltoluquinones with cyclopentadiene

Sulfinyltoluquinones (2a–2c) were submitted to thermal or catalyzed [4+2] cycloaddition reactions with cyclopentadiene. For p-tolylsulfinyltoluquinones (2b) and (2c), almost complete C2–C3-chemo- and unlike-diastereoselectivity was achieved by catalysis with ZnBr2, yielding adducts 6. Under thermal conditions, Diels–Alder reaction took place at the C5–C6 double bonds of quinones 2a–2c, generating mixtures of diastereoisomeric like- and unlike-adducts 4.

Enantioselective Total Synthesis of (+)-Ottelione A, (−)-Ottelione B, (+)-3-epi-Ottelione A and Preliminary Evaluation of Their Antitumor Activity

Enantioselective total synthesis of (+)-ottelione A (1) and (-)-ottelione B (2), novel and potent antitumor agents from a freshwater plant, and (+)-3-epi-ottelione A (3), the earlier proposed stereostructure of 1, was efficiently achieved starting from the known tricyclic compound 10. The synthesis involved the following key steps: i) coupling reactions of aldehydes 8 and 9 with the aromatic portion 7 (8+7-->15 and 9+7-->27), ii) base-induced hemiacetal-opening/epimerization reactions of the cyclic hemiacetals 6 and 27 (6-->17 and 27 a-->26 a), and iii) Corey-Winter's reductive olefination of the cyclic thiocarbonates 21 and 36 (21-->22 and 36-->37). The present total synthesis fully established the absolute configuration of these natural products. The cell growth inhibition profile, COMPARE analysis, and tubulin inhibitory assay of (+)-3-epi-ottelione A (3) and its O-acetyl derivative 24 demonstrated that these unnatural substances could be prominent lead compounds for the development of anticancer agents with a novel mode of action.

Total Synthesis and Absolute Configuration of Otteliones A and B, Novel and Potent Antitumor Agents from a Freshwater Plant

[reaction: see text] The first enantioselective total synthesis of otteliones A and B, biologically important and structurally novel natural products, has been successfully achieved. This total synthesis fully confirms the absolute configuration of these natural products.