Evolution of a Synthetic Strategy toward the Syntheses of Bis-tetrahydroisoquinoline Alkaloids.

ConspectusThe bis-tetrahydroisoquinoline (bis-THIQ) natural products represent a medicinally important class of isoquinoline alkaloids that exhibit broad biological activities with particularly potent antitumor properties, as exemplified by the two U.S. FDA approved molecules trabectidin and lurbinectedin. Accordingly, other members within the bis-THIQ family have emerged as prime targets for synthetic chemists, aiming to innovate an orthogonal chemical production of these compounds. With the ability of these complementary strategies to reliably and predictably manipulate molecular structures with atomic precision, this should allow the preparation of synthetic derivatives not existing in nature as new drug leads in the development of novel medicines with desired biological functions.Beyond the biological perspective, bis-THIQ natural products also possess intricate and unique structures, serving as a source of intellectual stimulation for synthetic organic chemists. Within our laboratory, we have developed an integrated program that combines reaction development and target-directed synthesis, leveraging the architecturally complex molecular framework of bis-THIQ natural products as a driving force for the advancement of novel reaction methodologies. In this Account, we unveil our synthetic efforts in a comprehensive story, describing how our synthetic strategy toward bis-THIQ natural products, specifically jorunnamycin A and jorumycin, has evolved over the course of our studies through our key transformations comprising (a) the direct functionalization of isoquinoline N-oxide to prepare the bis-isoquinoline (bis-IQ) intermediate, (b) the diastereoselective and enantioselective isoquinoline hydrogenation to forge the pentacyclic skeleton of the natural product, and (c) the late-stage oxygenation chemistry to adjust the oxidation states of the A- and E-rings. First, we detail our plan in utilizing the aryne annulation strategy to prepare isoquinoline fragments for the bis-THIQ molecules. Faced with unpromising results in the direct C-H functionalization of isoquinoline N-oxide, we lay out in this Account our rationale behind the design of each isoquinoline coupling partner to overcome these challenges. Additionally, we reveal the inspiration for our hydrogenation system, the setup of our pseudo-high-throughput screening, and the extension of the developed hydrogenation protocols to other simplified isoquinolines.In the context of non-natural bis-THIQ molecules, we have successfully adapted this tandem coupling/hydrogenation approach in the preparation of perfluorinated bis-THIQs, representing the first set of electron-deficient non-natural analogues. Finally, we include our unsuccessful late-stage oxygenation attempts prior to the discovery of the Pd-catalyzed C-O cross-coupling reaction. With this full disclosure of the chemistry developed for the syntheses of bis-THIQs, we hope our orthogonal synthetic tactics will provide useful information and serve as an inspiration for the future development of bis-THIQ pharmaceuticals.

Visible Light-Induced One-Pot Carbonylation of Alkyl Halides with Aryl Formates.

Described herein is the development of a visible-light-driven carbonylation of alkyl halides. The exploitation of visible light to activate Pd complexes and the use of formates to serve the dual role of a CO surrogate and a phenoxide source allow the preparation of esters in moderate to good yields. Its relatively mild reaction conditions and the ability to perform this transformation without direct handling of toxic CO gas provide a practical means to access esters from alkyl halides.

Recent Advances in the Total Synthesis of the Tetrahydroisoquinoline Alkaloids (2002-2020).

The tetrahydroisoquinoline (THIQ) natural products constitute one of the largest families of alkaloids and exhibit a wide range of structural diversity and biological activity. Ranging from simple THIQ natural products to complex trisTHIQ alkaloids such as the ecteinascidins, the chemical syntheses of these alkaloids and their analogs have been thoroughly investigated due to their intricate structural features and functionalities, as well as their high therapeutic potential. This review describes the general structure and biosynthesis of each family of THIQ alkaloids as well as recent advancements of the total synthesis of these natural products from 2002 to 2020. Recent chemical syntheses that have emerged harnessing novel, creative synthetic design, and modern chemical methodology will be highlighted. This review will hopefully serve as a guide for the unique strategies and tools used in the total synthesis of THIQ alkaloids, as well as address the longstanding challenges in their chemical and biosynthesis.

Syntheses of 3-Aryl Tetrahydroisoquinolines via an Intermolecular [4 + 2] Cycloaddition of Sultines with Imines.

The development of an intermolecular aza-Diels-Alder (DA) cycloaddition of sultines and imines is reported. By exploiting sultines as o-quinodimethane precursors and aryl imines as dienophiles in the presence of Cu(OTf)2, an aza-DA reaction proceeds to provide a wide variety of 3-aryl tetrahydroisoquionlines in moderate to excellent yield (up to 89%). The synthetic utility of these products was demonstrated in the preparation of tetracyclic N-heterocycles, including a tetrahydroprotoberberine skeleton.

Iridium-catalyzed asymmetric trans-selective hydrogenation of 1,3-disubstituted isoquinolines.

The development of the first asymmetric trans-selective hydrogenation of 1,3-disubstituted isoquinolines is reported. Utilizing [Ir(cod)Cl]2 and a commercially available chiral Josiphos ligand, a variety of differentially substituted isoquinolines are hydrogenated to produce enantioenriched trans-tetrahydroisoquinolines in good yield with high levels of enantioselectivity. Directing group studies demonstrate that the hydroxymethyl functionality at the C1 position is critical for hydrogenation to favor the trans-diastereomer. Preliminary mechanistic studies reveal that non-coordinating chlorinated solvents and halide additives are crucial to enable trans-selectivity.

The Enantioselective Synthesis of Eburnamonine, Eucophylline, and 16'-epi-Leucophyllidine.

A synthetic approach to the heterodimeric bisindole alkaloid leucophyllidine is disclosed herein. An enantioenriched lactam building block, synthesized through palladium-catalyzed asymmetric allylic alkylation, served as the precursor to both hemispheres. The eburnamonine-derived fragment was synthesized through a Bischler-Napieralski/hydrogenation approach, while the eucophylline-derived fragment was synthesized by Friedländer quinoline synthesis and two sequential C-H functionalization steps. A convergent Stille coupling and phenol-directed hydrogenation united the two monomeric fragments to afford 16'-epi-leucophyllidine in 21 steps from commercial material.

Iridium-Catalyzed Enantioselective and Diastereoselective Hydrogenation of 1,3-Disubstituted Isoquinolines.

The development of a general method utilizing a hydroxymethyl directing group for asymmetric hydrogenation of 1,3-disubstituted isoquinolines to provide chiral 1,2,3,4-tetrahydroisoquinolines is reported. The reaction, which utilizes [Ir(cod)Cl]2 and a commercially available chiral xyliphos ligand, proceeds in good yield with high levels of enantioselectivity and diastereo-selectivity (up to 95% ee and >20:1 dr) on a range of differentially substituted isoquinolines. Directing group studies demonstrate that the hydroxymethyl functional group at the C1-position is more efficient at enabling hydrogenation than other substituents, although high levels of enantioselectivity were conserved across a variety of polar and non-polar functional groups. By utilizing the generated chiral ß-amino alcohol as a functional handle, the synthetic utility is further highlighted via the synthesis of 1,2-fused oxazolidine, oxazolidinone, and morpholinone tetrahydroisoquinolines in one step. Additionally, a non-natural analog of the tetrahydroprotoberberine alkaloids was successfully synthesized.

Enantioselective synthesis of highly oxygenated acyclic quaternary center-containing building blocks via palladium-catalyzed decarboxylative allylic alkylation of cyclic siloxyketones.

The development of a palladium-catalyzed enantioselective decarboxylative allylic alkylation of cyclic siloxyketones to produce enantioenriched silicon-tethered heterocycles is reported. The reaction proceeds smoothly to provide products bearing a quaternary stereocenter in excellent yields (up to 91% yield) with high levels of enantioselectivity (up to 94% ee). We further utilized the unique reactivity of the siloxy functionality to access chiral, highly oxygenated acyclic quaternary building blocks. In addition, we subsequently demonstrated the utility of these compounds through the synthesis of a lactone bearing vicinal quaternary-trisubstituted stereocenters.

Reactions of (Arylimido)vanadium(V)-Trialkyl Complexes with Phenols: Effects of Arylimido Ligands and Phenols for Formation of the Vanadium Phenoxides.

Reactions of a series of (arylimido)vanadium(V) trialkyl complexes, V(NAr')(CH2SiMe3)3 (Ar' = C6H5, 2-MeC6H4, 2,6-Me2C6H3, 2,6-Cl2C6H3), with various phenols (ArOH, Ar = 2,6-F2C6H3, 2,6-Cl2C6H3, 2,6-Me2C6H3, 2,6- i Pr2C6H3, 2- t BuC6H4, 2,6- t Bu2C6H3; 1.0 equiv) affording V(NAr')(CH2SiMe3)2(OAr) were conducted in C6D6 at 25 °C, and the effects of both arylimido ligands and phenols on the substitution rate were explored. Sterically hindered arylimido ligands showed lower reactivity, and the reaction proceeded in the order: Ar' = 2,6-Me2C6H3 < 2,6-Cl2C6H3 < 2-MeC6H4 < C6H5. This order is somewhat different from that obtained from the chemical shifts in V(NAr')(CH2SiMe3)3 in the 51V NMR spectra. The conversions with various disubstituted phenols increased in the order: 2,6- i Pr2C6H3OH < 2,6-Me2C6H3OH < 2,6-Cl2C6H3OH < 2,6-F2C6H3OH, irrespective of the kind of arylimido ligands. The reactions of V(NAr')(CH2SiMe3)3 with 2,6- t Bu2C6H3OH (1.0 or 3.0 equiv) did not take place even upon heating at 60 °C. These results suggest that the reactions proceed via coordination of ArOH toward vanadium, and the reactivity is highly dependent on steric bulk of both the arylimido ligand and the phenol.

Cycloadditions of Oxacyclic Allenes and a Catalytic Asymmetric Entryway to Enantioenriched Cyclic Allenes.

The chemistry of strained cyclic alkynes has undergone a renaissance over the past two decades. However, a related species, strained cyclic allenes, especially heterocyclic derivatives, have only recently resurfaced and represent another class of valuable intermediates. We report a mild and facile means to generate the parent 3,4-oxacyclic allene from a readily accessible silyl triflate precursor, and then trap it in (4+2), (3+2), and (2+2) reactions to provide a variety of cycloadducts. In addition, we describe a catalytic, decarboxylative asymmetric allylic alkylation performed on an α-silylated substrate, to ultimately permit access to an enantioenriched allene. Generation and trapping of the enantioenriched cyclic allene occurs with complete transfer of stereochemical information in a Diels-Alder cycloaddition through a point-chirality, axial-chirality, point-chirality transfer process.

Concise total syntheses of (-)-jorunnamycin A and (-)-jorumycin enabled by asymmetric catalysis.

The bis-tetrahydroisoquinoline (bis-THIQ) natural products have been studied intensively over the past four decades for their exceptionally potent anticancer activity, in addition to strong Gram-positive and Gram-negative antibiotic character. Synthetic strategies toward these complex polycyclic compounds have relied heavily on electrophilic aromatic chemistry, such as the Pictet-Spengler reaction, that mimics their biosynthetic pathways. Herein, we report an approach to two bis-THIQ natural products, jorunnamycin A and jorumycin, that instead harnesses the power of modern transition-metal catalysis for the three major bond-forming events and proceeds with high efficiency (15 and 16 steps, respectively). By breaking from biomimicry, this strategy allows for the preparation of a more diverse set of nonnatural analogs.

Nickel-catalyzed enantioselective allylic alkylation of lactones and lactams with unactivated allylic alcohols.

The first nickel-catalyzed enantioselective allylic alkylation of lactone and lactam substrates to deliver products bearing an all-carbon quaternary stereocenter is reported. The reaction, which utilizes a commercially available chiral bisphosphine ligand, proceeds in good yield with a high level of enantioselectivity (up to 90% ee) on a range of unactivated allylic alcohols for both lactone and lactam nucleophiles. The utility of this method is further highlighted via a number of synthetically useful product transformations.

Crystal structures of trans-acetyl-dicarbon-yl(η(5)-cyclo-penta-dien-yl)(di-methyl-phenyl-phosphane)molybdenum(II) and trans-acetyl-dicarbon-yl(η(5)-cyclo-penta-dien-yl)(ethyl-diphenyl-phosphane)molybdenum(II).

The title compounds, [Mo(C5H5)(COCH3)P(CH3)2(C6H5)(CO)2], (1), and [Mo(C5H5)(COCH3)P(C2H5)(C6H5)2)(CO)2], (2), have been prepared by phosphine-induced migratory insertion from [Mo(C5H5)(CO)3(CH3)]. Both complex mol-ecules exhibit a four-legged piano-stool geometry with trans-disposed carbonyl ligands along with Mo-P bond lengths and C-Mo-P angles that reflect the relative steric pressure of the respective phosphine ligand. The structure of compound (1) exhibits a layered arrangement parallel to (100). Within the layers mol-ecules are linked into chains along [001] by non-classical C-H⋯O inter-actions between the acetyl ligand of one mol-ecule and the phenyl and methyl phosphine substituents of another. In the structure of complex (2), a chain motif of centrosymmetrical dimers is found along [010] through C-H⋯O inter-actions.