Efficient Asymmetric Synthesis of 2,6-Disubstituted 2H-Dihydropyransvia a Catalytic Hetero-Diels–Alder/Allylboration Sequence

Multicomponent Hetero-[4 + 2] Cycloaddition/Allylboration Reaction: From Natural Product Synthesis to Drug Discovery

Multicomponent reactions (MCR), transformations employing three or more simple substrates in a single and highly atom-economical operation, are very attractive in both natural product synthesis and diversity-oriented synthesis of druglike molecules. Several popular multicomponent reactions were designed by combining two well-established individual reactions that utilize mutually compatible substrates. In this regard, it is not surprising that the merging of two reactions deemed as workhorses of stereoselective synthesis, the Diels-Alder cycloaddition and carbonyl allylboration, would produce a powerful and highly versatile tandem MCR process. The idea of using 1,3-dienylboronates in [4 + 2] cycloadditions as a means to produce cyclic allylic boronates was first reported by Vaultier and Hoffmann in 1987. In their seminal study, a 1-boronodiene was reacted with electron-poor alkenes, and the intermediate cycloadducts were isolated and added to aldehydes in a separate step leading to α-hydroxyalkylated carbocycles via a highly diastereoselective allylboration reaction. The one-pot three-component variant was realized in 1999 by Lallemand and co-workers, and soon after groups led by Hall and Carboni reported heterocyclic variants of the tandem [4 + 2] cycloaddition/allylboration to prepare α-hydroxyalkylated piperidine and pyran containing compounds, respectively. These classes of heterocycles are ubiquitous in Nature and are important components of pharmaceuticals. This Account summarizes the development and evolution of this powerful multicomponent reaction for accessing nonaromatic heterocycles and its many applications in natural products synthesis and drug discovery. The aza[4 + 2]cycloaddition/allylboration MCR was first optimized in our laboratory using 4-boronylhydrazonobutadienes and N-substituted maleimides, and it was exploited in the preparation of combinatorial libraries of polysubstituted imidopiperidines that feature as many as four elements of chemical diversity. Biological screening of these druglike imidopiperidine libraries unveiled promising bioactive agents such as A12B4C3, the first reported inhibitor of the human DNA repair enzyme, polynucleotide kinase-phosphatase (hPNKP). Related applications of this MCR in target-oriented synthesis also led to total syntheses of palustrine alkaloids. The inverse electron-demand oxa[4 + 2] cycloaddition/allyboration variant can take advantage of Jacobsen's chiral Cr(III)salen catalyst, affording a rare example of catalytic enantioselective MCR, one that provides a rapid access to α-hydroxyalkyl dihydropyrans in high enantio- and diastereoselectivity. This process exploits 3-boronoacrolein pinacolate as the heterodiene with ethyl vinyl ether or various 2-substituted enol ethers, along with a wide variety of aldehydes in the allylation stage. This versatile methodology was deployed in total syntheses of thiomarinol antibiotics, goniodiol and its derivatives, and the complex anticancer macrolide palmerolide A. More recent work from our laboratory centered on the regio- and stereoselective Suzuki-Miyaura cross-coupling of the dihydropyranyl boronates, thus providing a glimpse of the potential for new multicomponent variants that merge hetero[4 + 2] cycloadditions of 1-borylated heterodienes with transition metal-catalyzed transformations. This stereoselective MCR strategy holds great promise for provoking continuing applications in complex molecule synthesis and drug discovery, and is likely to inspire new and innovative MCR-based approaches to nonaromatic heterocycles.

Stereoselective and Regiodivergent Allylic Suzuki-Miyaura Cross-Coupling of 2-Ethoxydihydropyranyl Boronates: Synthesis and Confirmation of Absolute Stereochemistry of Diospongin B

Oxygen-containing heterocycles such as pyrans are a common substructure present in a variety of natural products and pharmaceutical drugs. Highly functionalized 4- and 6-aryl/heteroaryl dihydropyran derivatives are assembled by a highly stereoselective, ligand-controlled regiodivergent sp(3)-sp(2) Suzuki-Miyaura cross-coupling of a 2-ethoxy dihydropyranyl boronate derived from a catalytic enantioselective inverse-electron-demand oxa[4 + 2] cycloaddition. The scope and selectivity of this method were assessed along with an application to a concise total synthesis of the diarylheptanoid natural product diospongin B.

Recent applications of the hetero Diels–Alder reaction in the total synthesis of natural products

The synthetic utility and potential power of the Diels–Alder (D–A) reaction in organic chemistry is evident.

Boron-substituted 1,3-dienes and heterodienes as key elements in multicomponent processes

In the last few years, multicomponent reactions involving boron substituted 1,3-dienes have emerged as important tools in synthetic organic chemistry. The most significant recent results and developments obtained in this area are reported in this review.

Catalytic enantioselective three-component hetero-[4+2] cycloaddition/allylboration approach to alpha-hydroxyalkyl pyrans: scope, limitations, and mechanistic proposal

This article describes the design and optimization of a catalytic enantioselective three-component hetero-[4+2] cycloaddition/allylboration reaction between 3-boronoacrolein, enol ethers, and aldehydes to afford alpha-hydroxyalkyl dihydropyrans. The key substrate, 3-boronoacrolein pinacolate (2) was found to be an exceptionally reactive heterodiene in the hetero-[4+2] cycloaddition catalyzed by Jacobsen's chiral Cr(III) catalyst 1. The scope and limitations of this process were thoroughly examined. The adduct of 3-boronoacrolein pinacolate and ethyl vinyl ether was obtained in high yield and with over 95 % enantioselectivity. This cyclic alpha-chiral allylboronate adds to a very wide variety of aldehyde substrates, including unsaturated aldehydes and alpha-chiral aldehydes to give diastereomerically pure products. Acyclic 2-substituted enol ethers can be employed, in which case the catalyst promotes a kinetically selective reaction that favors Z enol ethers over the E isomers. Surprisingly, 3-boronoacrolein pinacolate was found to be a superior heterodiene than ethyl (E)-4-oxobutenoate, and a mechanistic interpretation based on a possible [5+2] transition state is proposed.

Preparation and Suzuki−Miyaura Coupling Reactions of Tetrahydropyridine-2-boronic Acid Pinacol Esters

[reaction: see text] A study on the conversion of lactam-derived vinyl triflates and phosphates into the corresponding vinyl boronates was carried out. While delta-valerolactam-derived vinyl triflates were successfully converted into 1,4,5,6-tetrahydropyridine-2-boronic acid pinacol ester derivatives by Pd-catalyzed coupling reaction with both bis(pinacolato)diboron and pinacolborane, pyrrolidinone and epsilon-caprolactam derivatives either did not react or were readily reduced. The delta-valerolactam-derived vinyl boronates are thermally stable compounds that efficiently coupled, under Pd catalysis, with structurally diverse aryl and heteroaryl bromides and triflates, vinyl iodides and bromides, and aromatic acid chlorides, to give the corresponding 2-substituted piperidines in good to excellent yields. The number of electrophiles that can virtually be coupled with these new boronic esters makes them very useful reagents for the synthesis of N-heterocyclic compounds.

Catalytic Asymmetric Synthesis of a Potent Thiomarinol Antibiotic

Thiomarinols, isolated from the bacterium Alteromonas rava sp. nov. SANK 73390, are potent marine antibiotics that possess both Gram-positive and Gram-negative activity. The first total synthesis of a member of the thiomarinol class of marine antibiotics was achieved in a remarkable global yield of 22% (from 3-boronoacrolein pinacolate). The highlight of this synthesis is the efficient catalytic enantio-, regio-, E/Z-, and diastereoselective three-component inverse electron demand Diels-Alder/allylboration sequence. This key operation provides a rare example of an enantioselective HDA reaction involving acyclic 2-substituted enol ethers, and it featured an unusual but fortuitous kinetic selection that favored the requisite Z-dienophile.

A Two-Step Reaction Sequence for the Syntheses of Tetrahydronaphthalenes

[Reaction: see text] A cobalt(I)-catalyzed Diels-Alder reaction of a boron-functionalized enyne is the key step in a two-step reaction cascade interconnecting four simple starting materials to obtain polycyclic multifunctionalized products in good yields and with a very high degree of diastereoselectivity.