(R)- and (S)-4-TIPS-3-butyn-2-ol. Useful Precursors of Chiral Allenylzinc and Indium Reagents

Advances Toward Amphidinolides C, F and U: Isolations, Synthetic Studies and Total Syntheses

Amphidinolides C, F, and U, including C2-C4 analogs, are highly cytotoxic marine macrolides, mainly isolated from dinoflagellates of the genus Amphidinium. All these polyketides share a 75 % or more similar structure, highlighted by a macrolactone ring, at least one trans-2,5-substituted-THF motif and a characteristic polyenic side chain. From their isolation and absolute configurational assignment, the total synthesis of these marine macrolides represented an intense challenge to the organic synthesis community over the last 15 years, with around 14 research groups engaged in this inspiring task. In the first part of this review, we present the different approaches to the isolation and characterization of these natural products, including the most recent analogs, which may cast doubt on the biogenetic origin of these compounds. The various synthetic approaches to the total synthesis of C, F, and U amphidinolides are presented in a second part, focusing on key reactions and/or innovative strategies. The review concludes in a third section summarizing the successful approaches leading to the total synthesis of one of the members of this amphidinolide subfamily.

Concise diastereoselective synthesis of calcaripeptide C via asymmetric transfer hydrogenation/Pd-induced chiral allenylzinc as a key reaction

The synthesis of the cyclodepsipeptide calcaripeptide C was accomplished with an overall yield of 10.7% by a catalytic asymmetric transfer hydrogenation (ATH) together with Marshall's allenylation as pivotal reactions.

A concise formal total synthesis of lactimidomycin

An ene–yne coupling/alkyne reduction tandem ring-closing reaction was employed for the formal total synthesis of the anti-tumor natural product lactimidomycin.

General strategy for synthesis of C-19 methyl-substituted sarpagine/macroline/ajmaline indole alkaloids including total synthesis of 19(S),20(R)-dihydroperaksine, 19(S),20(R)-dihydroperaksine-17-al, and peraksine

A detailed account of the development of a general strategy for synthesis of the C-19 methyl-substituted alkaloids including total synthesis of 19(S),20(R)-dihydroperaksine-17-al (1), 19(S),20(R)-dihydroperaksine (2), and peraksine (6) is presented. Efforts directed toward the total synthesis of macrosalhine chloride (5) are also reported. Important to success is the sequence of chemical reactions which include a critical haloboration reaction, regioselective hydroboration, and controlled oxidation (to provide sensitive enolizable aldehydes at C-20). In addition, the all-important Pd-catalyzed α-vinylation reaction has been extended to a chiral C-19 alkyl-substituted substrate for the first time. Synthesis of the advanced intermediate 64 completes an improved formal total synthesis of talcarpine (26) and provides a starting point for synthesis of macroline-related alkaloids 27-31. Similarly, extension of this synthetic strategy in the ring A oxygenated series should provide easy access to the northern hemisphere 32b of the bisindoles angustricraline, alstocraline, and foliacraline (Figure 4 ).

Studies toward the unique pederin family member psymberin: full structure elucidation, two alternative total syntheses, and analogs

Two synthetic approaches to psymberin have been accomplished. A highly convergent first generation synthesis led to the complete stereochemical assignment and demonstrated that psymberin and irciniastatin A are identical compounds. This synthesis featured a diastereoselective aldol coupling between the aryl fragment and a central tetrahydropyran core and a novel one-pot procedure to convert an amide, via intermediacy of a sensitive methyl imidate, to the N-acyl aminal reminiscent of psymberin. The highlights of the second generation synthesis include an efficient iridium-catalyzed enantioselective bisallylation of neopentyl glycol and a stepwise Sonogashira coupling/cycloisomerization/reduction sequence to construct the dihydroisocoumarin unit. The two synthetic avenues were achieved in 17-18 steps (longest linear sequence, ~14-15 isolations) from 3 fragments prepared in 7-8 (first generation) and 3-8 (second generation) steps each. This convergent approach allowed for the preparation of sufficient amounts of psymberin (~ 0.5 g) for follow-up biological studies. Meanwhile, our highly flexible strategy enabled the design and synthesis of multiple analogs, including a psymberin-pederin hybrid, termed psympederin, that proved crucial to a comprehensive understanding of the chemical biology of psymberin and related compounds that will be described in a subsequent manuscript.

Enantioselective synthesis of anti- and syn-homopropargyl alcohols via chiral Brønsted acid catalyzed asymmetric allenylboration reactions

Chiral Brønsted acid catalyzed asymmetric allenylboration reactions are described. Under optimized conditions, anti-homopropargyl alcohols 2 are obtained in high yields with excellent diastereo- and enantioselectivities from stereochemically matched aldehyde allenylboration reactions with (M)-1 catalyzed by the chiral phosphoric acid (S)-4. The syn-isomers 3 can also be obtained in good diastereoselectivities and excellent enantioselectivities from the mismatched allenylboration reactions of aromatic aldehydes using (M)-1 in the presence of the enantiomeric phosphoric acid (R)-4. The stereochemistry of the methyl group introduced into 2 and 3 is controlled by the chirality of the allenylboronate (M)-1, whereas the configuration of the new hydroxyl stereocenter is controlled by the enantioselectivity of the chiral phosphoric acid catalyst used in these reactions. The synthetic utility of this methodology was further demonstrated in highly diastereoselective syntheses of a variety of anti, anti-stereotriads, the direct synthesis of which has constituted a significant challenge using previous generations of aldol and crotylmetal reagents.

Efficient syntheses of 25,26-dihydrodictyostatin and 25,26-dihydro-6-epi-dictyostatin, two potent new microtubule-stabilizing agents

The dictyostatins are powerful microtubule-stabilizing agents that have shown antiproliferative activity against a variety of human cancer cell lines. Two highly active analogs of dictyostatin, 25,26-dihydrodictyostatin and 25,26-dihydro-6-epi-dictyostatin, were prepared by a new streamlined total synthesis route. Three complete carbon fragments were prepared to achieve maximum convergency. These were coupled by a Horner–Wadsworth–Emmons reaction sequence and an esterification. A late stage Nozaki–Hiyama–Kishi reaction was then used to form the 22-membered macrolide. The stereoselectivity of this reaction depended on the configurations of the nearby stereocenter at C6.

Total synthesis of chivosazole F

The first synthesis of the highly biologically active chivosazole F is described. It features an intramolecular Stille coupling for the macrolactone formation and thereby circumvents the problem of isomerization associated with the tetraene segment. Additionally, the synthesis confirms the structure which has been proposed based solely on a combination of NMR/computational methods and genetic analysis.

Polymerization of substituted acetylenes and features of the formed polymers

This review summarizes the recent advances of substituted polyacetylene chemistry, including new polymerization catalysts and the properties and functions of the polymers.

Enantioselective synthesis of a diastereomer of iriomoteolide-1a. What is the correct structure of the natural product?

An enantioselective approach to a diastereomer of iriomoteolide-1a is described. Highlighted is a SmI(2)-mediated intramolecular reductive cyclization approach to complex cyclic hemiketals. An acetylide-chloroformate coupling strategy is also featured. Our results show that the structures of iriomoteolide-1a-1c require careful reassessment.

Palladium- and copper-catalyzed 1,4-additions of organozinc compounds to conjugated aldehydes

Conjugated aldehydes undergo smooth Pd(OAc)2.PPh3- or Me2CuCNLi2-catalyzed 1,4-addition of dialkylzinc reagents. [reaction: see text].

Titanocene(II)-promoted, one-pot, three-component coupling of thioacetals, alkynyl sulfones, and carbonyl compounds: highly stereoselective formation of tert-homopropargyl alcohols

Titanocene alkylidene complexes, generated by desulfurizative titanation of thioacetals with Cp2Ti[P(OEt)3]2, reacted with alkynyl methyl sulfones to produce organotitanium species, which gave tert-homopropargyl alcohols with high diastereoselectivity on treatment with aromatic and alpha,beta-unsaturated ketones.

Total Synthesis of Iejimalide A−D and Assessment of the Remarkable Actin-Depolymerizing Capacity of These Polyene Macrolides

A concise and convergent total synthesis of the highly cytotoxic marine natural products iejimalide A-D (1-4) is reported, which relies on an effective ring-closing metathesis (RCM) reaction of a cyclization precursor containing no less than 10 double bonds. Because of the exceptional sensitivity of this polyunsaturated intermediate and its immediate precursors toward acid, base, and even gentle warming, the assembly process hinged upon the judicious choice of protecting groups and the careful optimization of all individual transformations. As a consequence, particularly mild protocols for Stille as well as Suzuki reactions of elaborate coupling partners have been developed that hold considerable promise for applications in other complex settings. Moreover, a series of non-natural "iejimalide-like" compounds has been prepared, differing from the natural lead in the polar head groups linked to the macrolide's N-terminus. With the aid of these compounds it was possible to uncover the hitherto unknown effect of iejimalide and analogues on the actin cytoskeleton. Their capacity to depolymerize this microfilament network rivals that of the latrunculins which constitute the standard in the field. Structural modifications of the peptidic terminus in 2 are thereby well accommodated, without compromising the biological effects. The iejimalides hence constitute an important new class of probe molecules for chemical biology in addition to their role as promising lead structures for the development of novel anticancer agents.

Chiral allylic and allenic metal reagents for organic synthesis

This account traces the evolution of our work on the synthesis of chiral allylic and allenic organometal compounds of tin, silicon, zinc, and indium and their application to natural product synthesis over the past quarter century.