Stereoselective synthesis of cyclic ethers via the palladium-catalyzed intramolecular addition of alcohols to phosphono allylic carbonates

Palladium-Catalyzed [7 + 5] and Higher-Order Annulations of Oxa-1,n-dipoles: Synthesis of 12- to 15-Membered Lactones

Macrolactones are commonly found in natural products and pharmaceuticals. Herein, we present Pd-catalyzed [7 + 5] and higher-order annulations between unprecedented oxa-1,n-dipoles (n = 7-9) with active enol lactones. This protocol enables the rapid synthesis of complex 12 to 15-membered lactones bearing internal E-alkenes. Density functional theory calculations have revealed the favorable reaction pathway and identified coordination interactions between active enol lactones and palladium centers of π-allyl dipole species as key factors in manipulating these dipoles in annulations.

Recent Advances in the Synthesis of Diarylheptanoids

In the quest for synthesizing biologically important natural products, medicinal chemists embark on an endless journey. This review focuses on the reports published towards the syntheses of diarylheptanoids, classifying them into linear, tetrahydropyran, diarylether, and biphenyl categories. The synthesis methods for each class from 2013 to 2023 are discussed, providing a comprehensive overview of the advancements in the field. Representative natural product examples are highlighted for each category. The review emphasizes the importance of diarylheptanoids in the realms of chemistry and medicine, showcasing their potential as valuable compounds for medicinal and synthetic chemists.

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.

Convergent Total Synthesis of (-)-Cyclopamine

A concise and enantioselective total synthesis of the Veratrum alkaloid cyclopamine is disclosed. This highly convergent synthesis with a 16-step longest linear sequence (LLS) was enabled by a de novo synthesis of the trans-6,5-heterobicycle via a strain-inducing halocyclization process, a key Tsuji-Trost cyclization to construct the fully substituted, spirocyclic THF motif with exquisite diastereocontrol, and a late-stage ring-closing metathesis (RCM) reaction to forge the central tetrasubstituted olefin.

Highly Stereospecific Cyclizations of Homoallylic Silanols

We demonstrate that di-tert-butylsilanols are competent nucleophiles for the intramolecular interception of palladium π-allyl species. In these reactions, allyl ethyl carbonates are the best precursors for the formation of palladium π-allyl intermediates, and [(Cinnamyl)PdCl]₂/BINAP is superior to other Pd salt/ligand framework combinations. Our optimized protocol is compatible with a variety of silanol substrates. Importantly, the cyclization is perfectly stereospecific, proceeding via an anti-syn mechanism, which stands in contrast to reported analogous reactions of alcohols and phenols, known to proceed via an anti-anti mechanism. The alkenes in the product dioxasilinanes serve as blank slates for further functionalization.

O-Allylated Pudovik and Passerini Adducts as Versatile Scaffolds for Product Diversification

The palladium-catalyzed O-allylation of α-hydroxyphosphonates and α-hydroxyamides obtained from Pudovik and Passerini multicomponent reactions has allowed interesting and highly straightforward access to a variety of building blocks for product diversification. These post-functionalizations include a selective base- or ruthenium hydride-mediated isomerization/Claisen rearrangement cascade and a ring-closing metathesis that allows access to a variety of diversely functionalized phosphono-oxaheterocycles.

Synthesis of Phostones via the Palladium-Catalyzed Ring Opening of Epoxy Vinyl Phosphonates

The reaction of epoxy aldehydes with tetraethyl methylenediphosphonate gave γ,δ-epoxy vinyl phosphonates. The palladium-catalyzed addition of primary alcohols gave the monoprotected diols as single diastereoisomers. The trans- and cis-epoxides lead to opposite ( syn and anti) diastereoisomers of the addition products. The alkene of the vinyl phosphonates was subjected to hydrogenation, and the resulting saturated phosphonates underwent base-catalyzed cyclization to give phostones with a very high diastereoselectivity in the formation of the new chiral center at the phosphorus atom.

Recent advances and applications of p-toluenesulfonylmethyl isocyanide (TosMIC)

TosMIC, a unique and multifunctional synthetic reagent, reacts with a wide spectrum of synthons to deliver novel scaffolds.

Ru-catalyzed asymmetric hydrogenation of δ-keto Weinreb amides: enantioselective synthesis of (+)-Centrolobine

An efficient asymmetric hydrogenation of δ-keto Weinreb amides catalyzed by a Ru-Xyl-SunPhos-Daipen bifunctional catalyst has been achieved.

Selective Tsuji–Trost type C-allylation of hydrazones: a straightforward entry into 4,5-dihydropyrazoles

Tsuji–Trost type C-allylation access to 4,5-dihydropyrazoles.

Trapping Hemiacetals with Phosphono Substituted Palladium π-Allyl Complexes for the Synthesis of Substituted Cyclic Ethers

Oxidation of hydroxy substituted phosphono allylic carbonates gave the aldehyde substituted phosphonates in good yield. Stereospecific palladium (0)-catalyzed cyclization in the presence of methanol or water gave acetal tetrahydrofuran and tetrahydropyran vinyl phosphonate products derived from hemiacetal trapping. The tetrahydrofuran acetals undergo Lewis acid catalyzed addition of nucleophiles to give diastereoisomeric mixtures of substituted tetrahydrofurans.

2-Arylcyclopropylmethanol as a substitute for homoallyl aryl alcohol in the construction of cis-2,6-disubstituted tetrahydropyran: synthesis of (±)-centrolobine

The application of 2-arylcyclopropylmethanols as substitutes to homoallyl aryl alcohols and their reactions with aliphatic aldehydes in the presence of SnCl4 in CH2Cl2 leads to an efficient Prins cyclization to generate cis-2,6-disubstituted tetrahydropyrans in high yields. The reaction is free from 2-oxonia-Cope rearrangement. This protocol was used to synthesize (±)-centrolobine in an overall 84% yield over three steps. The protocol holds promise for scaffold generation for medicinal chemistry exploitation.

Facile base-mediated redox transformation: an efficient strategy for the synthesis of α-acyloxyphosphoryl compounds

An efficient one-pot synthesis of α-acyloxyphosphoryl compounds from aldehydes and hydrogen phosphoryl compounds has been developed using a facile base-mediated redox strategy. This redox transformation is applicable to synthesize a wide range of valuable α-acyloxyphosphoryl compounds with high atom- and step-economic efficiency.

Relay cross metathesis reactions of vinylphosphonates

Dimethyl (β-substituted) vinylphosphonates do not readily undergo cross metathesis reactions with Grubbs catalyst and terminal alkenes. However, the corresponding mono- or diallyl vinylphosphonate esters undergo facile cross metathesis reactions. The improved reactivity is attributed to a relay step in the cross metathesis reaction mechanism.

Synthesis of non-racemic α-hydroxyphosphonates via asymmetric phospho-aldol reaction

It has been more than 50 years since the first phospho-aldol reactions of dialkyl phosphites were reported. These efficient P-C bond-forming reactions have become the cornerstone of methods for the synthesis of α-hydroxyphosphonates and, by numerous available substitution reactions, the synthesis of other α- and γ-substituted phosphonates and phosphonic acids. Much of the interest in α- and γ-substituted phosphonates and phosphonic acids has been stimulated by reports of their biological activity, which is often dependent upon their absolute and relative stereochemistry. In this chapter, we review diastereoselective and enantioselective additions of dialkyl phosphites to aldehydes and ketones, otherwise called the phospho-aldol, Pudovik or Abramov reactions.

An expeditious total synthesis of both diastereoisomeric lipid dihydroxytetrahydrofurans from Notheia anomala

Short, high yielding syntheses of both diastereomers of the naturally occurring oxylipids 1 and 2 using a combination of organocatalytic hydroxylation of an aldehyde, alkene cross metathesis, and palladium(0) catalyzed cyclization chemistry (six-step process) are reported. Furthermore, the influence of the catalyst on the cross metathesis reaction of the homoallylic 1,2-diol has been studied in detail.

Synthesis of the C(18)-C(34) fragment of amphidinolide C and the C(18)-C(29) fragment of amphidinolide F

A convergent synthesis of the C(18)-C(34) fragment of amphidinolide C and the C(18)-C(29) fragment of amphidinolide F is reported. The approach involves the synthesis of the common intermediate tetrahydrofuranyl-β-ketophosphonate via cross metathesis, Pd(0)-catalyzed cyclization, and hydroboration-oxidation. The β-ketophosphonate was coupled to three side chain aldehydes using a Horner-Wadsworth-Emmons (HWE) olefination reaction to give dienones, which were reduced with l-selectride to give the fragments of amphidinolide C and F.

Enantioselective construction of cis-2,6-disubstituted dihydropyrans: total synthesis of (-)-centrolobine

This paper presents a simple and efficient route to chiral cis-6-substituted 2-(2-hydroxyethyl)-5,6-dihydro-2H-pyrans, a versatile chiral building block. The strategy is based on three key transformations: enantioselective hetero-Diels-Alder (HDA) reaction of aldehyde with Danishefsky's diene, selective reduction of carbonyl function, and Claisen or related rearrangement. The synthetic utility of the methodology is illustrated by total synthesis of antibiotic (-)-centolobine.