Total Synthesis and Stereochemical Proposal of the Hexaene Framework of Bacillaene

The central hexaene core of the labile polyene antibiotic bacillaene bearing three Z-configured olefins was prepared with excellent geometrical purity by an iterative cross-coupling strategy, involving improved syntheses of bimetallic reagents and efficient protocols for stereoselective polyene generation, revealing insights into the origin of bacillaene's instability. Furthermore, a useful method for monitoring polyene iteration by UV-vis shift analyses and the absolute stereochemistry of this core by biosynthetic gene cluster analysis are reported.

Development and Applications of an Amide Linchpin Reagent

Linchpin reagents are building blocks that can be chemoselectively functionalized to afford products with a common, useful functional group. In this work, we describe the development and validation of the first amide linchpin reagent and demonstrate its use as a doubly electrophilic building block for the synthesis of a variety of amides, including challenging classes. The linchpin reagent was first functionalized via rhodium-catalyzed electrophilic amination. Selected masked C-isocyanate products were then further derivatized with Grignard reagents to produce secondary amides, or tertiary amides if an alkylating agent was added subsequently. The success of this sequence relies on fully controlled reactivity at each electrophilic site, first exploiting the weak N-O bond and then, the ability to form the free isocyanate intermediate in situ. The overall transformation proceeds with high chemoselectivity, demonstrating the ability of this new linchpin reagent to form amides through atypical bond construction. Finally, the potential of this reagent as a more broadly applicable NCO linchpin is demonstrated by the formation of lactams and unsymmetrical ureas.

Total Synthesis and Configuration Assignment of (+)-Myxoquaterine-450

Myxoquaterines represent a recently discovered class of natural products with intriguing biological properties. They were isolated from Pendulasporacea albinea MSr 11954 and display a unique structure combining heterocyclic (pyrrole, oxazoline), hexaene, and 2-amino-1,3-diol subunits. We have now synthesized the first example of a myxoquaterine natural product, myxoquaterine-450, in a highly convergent fashion, in which the sensitive hexaene unit was established in the final stages of the synthesis (16 linear steps starting from l-serine). Comparison of the synthetic material with an authentic sample allowed to establish and confirm the constitution and configuration of the natural product.

Total Synthesis of Pentamycin by a Conformationally Biased Double Stille Ring Closure with a Trienyl-bis-stannane

The total synthesis of the potent polyene macrolide antibiotic pentamycin was accomplished by an expedient strategy involving a highly stereoselective assembly of the polyol segment in combination with an adventurous double Stille cross-coupling with a trienyl-bis-stannane closing the macrolactone and installing the sensitive pentaene fragment. Presumably, this remarkable linchpin insertion is enhanced by the linear hydrogen bonding skeleton of the polyol substrate. Further key features include a tailored Rychnovsky alkylation of cyanohydrin acetonides and elaborate Krische couplings to set the characteristic hydroxyl and hydroxymethyl bearing centers with excellent selectivity and yield. The total synthesis unequivocally confirms the full relative and absolute stereochemistry of this polyketide, including a previously uncertain hydroxyl bearing center.

Stereoselective Synthesis of Highly Substituted 1,3-Dienes via "à la carte" Multifunctionalization of Borylated Dendralenes

Despite the high relevance of 1,3-dienes, stereoselective methods to access tetrasubstituted conjugated dienes are still scarce. We here report an efficient and modular approach that provides access to multifunctional tetrasubstituted 1,3-dienes with excellent levels of regio- and stereoselectivity. This methodology is based on a tetracomponent reaction between a borylated dendralene, an organolithium reagent and two different electrophiles. Mechanistic studies reveal that this transformation proceeds through a regio- and stereoselective carbolithiation/electrophilic trapping of an in situ formed dendralenic boron-ate complex, followed by a stereoretentive halodeborylation. The ease in which complex structural dienes can be accessed and their synthetic versatility highlight the importance and utility of this method.

Ruthenium-catalysed oxidative coupling of vinyl derivatives and application in tandem hydrogenation

The first ruthenium-catalyzed oxidative homo- and cross-coupling of exclusive vinyl derivatives giving highly valued 1,3-diene building blocks is reported. In situ ruthenium-catalyzed hydrogenation afforded relevant adipic acid ester derivatives.

Synthesis of Symmetrical and Nonsymmetrical Polyenes by Iterative and Bidirectional Palladium-Catalyzed Cross-Coupling Reactions

Bifunctional unsaturated reagents designed to undergo palladium-catalyzed cross-coupling reactions with complementary polyenyl connective fragments are highly useful for the undoubtedly challenging synthesis of polyenes. The current toolkit of building blocks for the bidirectional formation of Csp² -Csp² single bonds of polyenes includes homo-bisfunctionalized reagents with equal or unequal reactivity (due to steric and/or electronic factors), and hetero-bisfunctionalized counterparts containing either two different nucleophiles, two electrophiles or one of these functionalities and a latent nucleophile that can be unmasked when desired. The combination of these bifunctional linchpin reagents using tactics that modulate the reactivity of each terminus in order to achieve the required connection have streamlined the synthesis of polyenes of great complexity using (iterative) cross-coupling methods for Csp² -Csp² bond formation. Reaction conditions for the Pd-catalyzed cross-coupling reactions are mild and functional-group-tolerant, and therefore these protocols allow to construct the polyene structures using shorter unsaturated reactants with the desired geometries, since in general the products preserve the stereochemical information of the connected cross-coupling partners.

Synthesis of Allene-Containing Apocarotenoids by Cross-Coupling Strategy

The stereocontrolled total synthesis of the allene and carbonyl conjugated apocarotenoids, paracentrone and 19-hexanoyloxyparacentrone 3-acetate, was achieved by sequential cross-coupling reactions using boronic acid ester and iodine- or tin-substituted C5 dienes, which were the building blocks for the elongation of the conjugated polyene systems at both terminals.

Bidirectional Hiyama-Denmark Cross-Coupling Reactions of Bissilyldeca-1,3,5,7,9-pentaenes for the Synthesis of Symmetrical and Non-Symmetrical Carotenoids

The construction of the carotenoid skeleton by Pd-catalyzed Csp² -Csp² cross-coupling reactions of symmetrical and non-symmetrical 1,10-bissilyldeca-1,3,5,7,9-pentaenes and the corresponding complementary alkenyl iodides has been developed. Reaction conditions for these bidirectional and orthogonal Hiyama-Denmark cross-coupling reactions of bisfunctionalized pentaenes are mild and the carotenoid products preserve the stereochemical information of the corresponding oligoene partners. The carotenoids synthesized in this manner include β,β-carotene and (3R,3'R)-zeaxanthin (symmetrical) as well as 9-cis-β,β-carotene, 7,8-dihydro-β,β-carotene and β-cryptoxanthin (non-symmetrical).

Palladium-Catalyzed α-Allylation of Secondary Nitroalkanes with Allylic Alcohols and Strategic Exploitation of Seebach's Reagent for the Total Synthesis of (±)-Adalinine

A method is reported for the catalytic direct coupling of allylic alcohols and nitroalkanes. In the allylation process, the synergistic action of palladium complexes and titanium(IV) alkoxide facilitates the formation of nitronate and π-allylpalladium intermediate. In the cases of reluctant allylations, typically with sterically demanding nitroalkanes, we found the addition of substoichiometric amount of DBU greatly facilitates the desired transformation. We also accomplished a total synthesis of (±)-adalinine through a homoallyl nitroalkane derived from Seebach's reagent.

A Simple Procedure for the Synthesis of β-Hydroxyallenamides via Homoallenylation of Aldehydes

A simple one-pot synthesis of β-hydroxyallenamides is reported. This procedure entails chemo- and regioselective hydroboration of 3-en-1-ynyl-sulfonylamides with Cy₂BH followed by homoallenylation of aldehydes to yield β-hydroxyallenamides (up to 94% yield and >20:1 dr). Controlled synthesis of up to three continuous stereochemical elements was realized. Density functional theory (DFT) calculations suggest a concerted Zimmerman-Traxler chair-like transition state. Initial results suggest that enantio- and diastereoselective synthesis of β-hydroxyallenamides with optically active hydroboration reagents is viable.

Accessing 1,3-Dienes via Palladium-Catalyzed Allylic Alkylation of Pronucleophiles with Skipped Enynes

An unprecedented palladium-catalyzed allylic alkylation of pronucleophiles with unactivated skipped enynes has been developed. This method provides a straightforward access to a wide array of 1,3-dienes without the need to preinstall leaving groups or employ extra oxidants. The reaction exhibited high atom economy, good functional group tolerance, excellent regioselectivities, and scalability. With D₂O as cosolvent, deuterium could be incorporated in high efficiency.

Acid Chlorides as Formal Carbon Dianion Linchpin Reagents in the Aluminum Chloride-Mediated Dieckmann Cyclization of Dicarboxylic Acids

The development of acid chlorides as formal dianion linchpin reagents that enable access to cyclic 2-alkyl- and 2-acyl-1,3-alkanediones from dicarboxylic acids is described herein. Mechanistic experiments relying on ¹³C-labeling studies confirm the role of acid chlorides as carbon dianion linchpin reagents and have led to a revised reaction mechanism for the aluminum(III)-mediated Dieckmann cyclization of dicarboxylic acids with acid chlorides.

Regio- and stereoselective synthesis of conjugated trienes from silaborated 1,3-enynes

Palladium-catalyzed silaboration of 1,3-enynes followed by Suzuki–Miyaura coupling with alkenyl iodides and Hiyama coupling with aryl iodides provides access to 1,3,5-trienes with defined regio- and stereochemistry.

1-(Triethoxysilyl)buta-1,3-dienes-New Building Blocks for Stereoselective Synthesis of Unsymmetrical (E,E)-1,4-Disubstituted 1,3-dienes

A convenient methodology for the highly stereoselective synthesis of unsymmetrical (1E,3E)-1,4-disubstituted 1,3-dienes based on palladium-catalyzed Hiyama cross-coupling reaction of 1-(triethoxysilyl)-substituted buta-1,3-dienes with aryl iodides is reported.