Self-consistent synthesis of the squalene synthase inhibitor zaragozic acid C via controlled oligomerization

Rhodium-Catalyzed Asymmetric Arylation-Induced Glycolate Aldol Additions of Silyl Glyoxylates

(Diene)Rh(I) complexes catalyze the stereoselective three-component coupling of silyl glyoxylates, arylboronic acids, and aldehydes to give glycolate aldol products. The participation of Rh-alkoxides in the requisite Brook rearrangement was established through two component Rh-catalyzed couplings of silyl glyoxylates with ArB(OH)2 to give silyl-protected mandelate derivatives. The intermediacy of a chiral Rh-enolate was inferred through enantioselective protonation using a chiral Rh-catalyst. Diastereoselective three-component couplings with aldehydes as terminating electrophiles to give racemic products were best achieved with a bulky aryl ester on the silyl glyoxylate reagent. Optimal enantioselective couplings were carried out with the tert-butyl ester variant using an anisole-derived enantiopure tricyclo[3.2.2.02,4 ]nonadiene ligand.

Stereo-Controlled Synthesis of Vicinal Tertiary Carbinols: Application in the Synthesis of a Diol Substructure of Zaragozic Acid, Pactamycin and Ryanodol

A novel and flexible approach for the stereo-controlled synthesis of vicinal tertiary carbinols is reported. The developed strategy featured a highly diastereoselective singlet-oxygen (O2 1 ) [4+2] cycloaddition of rationally designed cyclohexadienones (derived from oxidative dearomatization of the corresponding carboxylic-acid appended phenol precursors), followed by programmed "O-O" and "C-C" bond cleavage. In doing so, a highly functionalized and versatile intermediate was identified and prepared in synthetically useful quantity as a plausible precursor to access a variety of designed and naturally occurring vicinal tertiary carbinol containing compounds. Most notably, the developed strategy was successfully applied in the stereo-controlled synthesis of advanced core structures of zaragozic acid, pactamycin and ryanodol.

Asymmetric construction of allylicstereogenic carbon center featuring atrifluoromethyl group via enantioselective reductive fluoroalkylation

Emerging as a powerful tool for lead optimization in pharmaceutical research and development, to develop the facile, general protocols that allows the incorporation of fluorine-containing motif in drug candidates has accumulated enormous research interest in recent years. Among these important motifs, the incorporation of strategic motif CF₃ on aliphatic chain especially with the concomitant construction of trifluoromethylated alkanes bearing a CF₃-substituted stereogenic carbon, is of paramount importance. Herein, we disclose an asymmetric nickel-catalyzed reductive trifluoroalkylation of alkenyl halides for enantioselective syntheses of diverse α-trifluoromethylated allylic alkanes, offering a general protocol to access the trifluoromethyl analogue to chiral α-methylated allylic alkanes, one of the most prevalent key components among natural products and pharmaceuticals. Utilities of the method including the application of the asymmetric trifluoroalkylation on multiple biologically active complex molecules, derivatization of transformable alkenyl functionality were demonstrated, providing a facile method in the diversity-oriented syntheses of CF₃-containing chiral drugs and bioactive-molecules.

Synthetic Efforts and Ultimate Limitation to an Asymmetric Achmatowicz Approach Toward EBC-23

An effort toward the total synthesis of the polyketide natural product EBC-23 is reported. The asymmetric approach is convergent and uses a late-stage Claisen-like enolate/acid chloride coupling to establish a key 1,3-diketone intermediate. The 1,3-diketone target is an oxidized form of the hydrated natural product, which fails to spiroketalize. The convergent asymmetric synthesis uses an asymmetric Noyori transfer hydrogenation of a β-furyl ketoester to enantioselectively form a chiral furyl alcohol. An Achmatowicz/Jones/Luche three-step reaction sequence was used to stereoselectively convert the furyl alcohol into the 5-hydroxy-pyran-2-one. The absolute stereochemistry of the 1,3-polyol fragment was established by a Leighton allylation. A subsequent Grubbs cross-metathesis, and Evans acetalation were used to install the 1,3-syn-diol stereochemistry.

Alkylation of lithiated dimethyl tartrate acetonide with unactivated alkyl halides and application to an asymmetric synthesis of the 2,8-dioxabicyclo[3.2.1]octane core of squalestatins/zaragozic acids

(R,R)-Dimethyl tartrate acetonide 7 in THF/HMPA undergoes deprotonation with LDA and reaction at −78 °C during 12–72 h with a range of alkyl halides, including non-activated substrates, to give single diastereomers (at the acetonide) of monoalkylated tartrates 17, 24, 33a–f, 38a,b, 41 of R,R-configuration, i.e., a stereoretentive process (13–78% yields). Separable trans-dialkylated tartrates 34a–f can be co-produced in small amounts (9–14%) under these conditions, and likely arise from the achiral dienolate 36 of tartrate 7. Enolate oxidation and acetonide removal from γ-silyloxyalkyl iodide-derived alkylated tartrates 17 and 24 give ketones 21 and 26 and then Bamford–Stevens-derived diazoesters 23 and 27, respectively. Only triethylsilyl-protected diazoester 27 proved viable to deliver a diazoketone 28. The latter underwent stereoselective carbonyl ylide formation–cycloaddition with methyl glyoxylate and acid-catalysed rearrangement of the resulting cycloadduct 29, to give the 3,4,5-tricarboxylate-2,8-dioxabicyclo[3.2.1]octane core 31 of squalestatins/zaragozic acids. Furthermore, monoalkylated tartrates 33a,d,f, and 38a on reaction with NaOMe in MeOH at reflux favour (≈75:25) the cis-diester epimers epi-33a,d,f and epi-38a (54–67% isolated yields), possessing the R,S-configuration found in several monoalkylated tartaric acid motif-containing natural products.

[Development of Synthetic Strategies for Densely Oxygenated Natural Products: Total Synthesis of Lactacystin and Zaragozic Acid C Using Photochemical C(sp3)-H Functionalization]

This review describes two novel synthetic routes from (S)-pyroglutaminol to (+)-lactacystin, a potent inhibitor of the 20S proteasome and from d-gluconolactone derivative to zaragozic acid C, a potent squalene synthase inhibitor. In lactacystin synthesis, the photoinduced intermolecular C(sp³)-H alkynylation and intramolecular C(sp³)-H acylation chemoselectively and stereoselectively constructed the tetrasubstituted and trisubstituted carbon centers, respectively. In the synthesis of zaragozic acid C, the stereoselective installation of the two contiguous tetrasubstituted carbons was achieved by the photochemical intramolecular C(sp³)-H acylation of a densely oxygenated intermediate.

Stereoselective Tandem Bis-Electrophile Couplings of Diborylmethane

A copper-catalyzed three-component linchpin coupling method for the stereoselective union of readily available epoxides and allyl electrophiles is disclosed. Transformations employ [B(pin)]₂-methane as a conjunctive reagent, resulting in the formation of two C-C bonds at a single carbon center bearing a C(sp³) organoboron functional group. Products are obtained in 42-99% yield, and up to >20:1 dr. The utility of the approach is highlighted by stereospecific transformations entailing allylation, tandem cross coupling, and application to the synthesis 1,3-polyol motifs.

A Simple Tetraminocalix[4]arene as a Highly Efficient Catalyst under "On-Water" Conditions through Hydrophobic Amplification of Weak Hydrogen Bonds

The simple tetraminocalix[4]arene 1, which contains weak H-bond-donor NH₂ groups, is reported to be a highly efficient organocatalyst for the Vinylogous Mukaiyama Aldol Reaction (VMAR) of 2-(trimethylsilyloxy)furan 5 with α-ketoesters 6 a-l under "on-water" conditions owing to the hydrophobic amplification of weak H-bond interactions. The catalytic efficiency of calixarene catalyst 1 was shown to be closely related to its recognition abilities towards the reactants 5 and 6 through a multipoint recognition model. The proposed model provided good explanations for the differences on the reaction rate acceleration and on the stereoselectivity observed with different substrates.

Supramolecular synthons in the gamma-hydroxybutenolides

The supramolecular organization in the solid state of five novel gamma-hydroxybutenolides is described.

11-Step Total Synthesis of Araiosamines

A concise route to a small family of exotic marine alkaloids known as the araiosamines has been developed, and their absolute configuration has been assigned. The dense array of functionality, high polarity, and rich stereochemistry coupled with equilibrating topologies present an unusual challenge for chemical synthesis and an opportunity for innovation. Key steps involve the use of a new reagent for guanidine installation, a remarkably selective C-H functionalization, and a surprisingly simple final step that intersects a presumed biosynthetic intermediate. Synthetic araiosamines were shown to exhibit potency against Gram-positive and -negative bacteria despite a contrary report of no activity.

Silyllithium-Initiated Coupling of α-Ketoamides with tert-Butanesulfinylimines for Stereoselective Synthesis of Enantioenriched α-(Silyloxy)-β-amino Amides

A silyllithium-initiated coupling of α-ketoamides with tert-butanesulfinylimines was developed for the efficient, stereoselective synthesis of enantioenriched α-(silyloxy)-β-amino amides. Nucleophilic addition of silyllithium to α-ketoamides, followed by 1,2-Brook rearrangement, generates nucleophilic enolates, which are then intercepted by chiral imines to provide three-component coupling products. Use of α-ketoamides is critical for achieving high yields and diastereoselectivities in the resulting α-hydroxy-β-amino acid derivatives.

Carbamoyl anion-initiated cascade reaction for stereoselective synthesis of substituted α-hydroxy-β-amino amides

A carbamoyl anion-initiated cascade reaction with acylsilanes and imines allows rapid construction of substituted α-hydroxy-β-amino amides in high yields with excellent diastereoselectivities.

Asymmetric Total Synthesis of the Indole Diterpene Alkaloid Paspaline

An enantioselective synthesis of the indole diterpenoid natural product paspaline is disclosed. Critical to this approach was the implementation of stereoselective desymmetrization reactions to assemble key stereocenters of the molecule. The design and execution of these tactics are described in detail, and a thorough analysis of observed outcomes is presented, ultimately providing the title compound in high stereopurity. This synthesis provides a novel template for preparing key stereocenters in this family of molecules, and the reactions developed en route to paspaline present a series of new synthetic disconnections in preparing steroidal natural products.

Highly enantioselective construction of tertiary thioethers and alcohols via phosphine-catalyzed asymmetric γ-addition reactions of 5H-thiazol-4-ones and 5H-oxazol-4-ones: scope and mechanistic understandings

Phosphine-catalyzed highly enantioselective γ-additions of 5H-thiazol-4-ones and 5H-oxazol-4-ones to allenoates have been developed for the first time. With the employment of amino-acid derived bifunctional phosphines, a wide range of substituted 5H-thiazol-4-one and 5H-oxazol-4-one derivatives bearing heteroatom (S or O)-containing tertiary chiral centers were constructed in high yields and excellent enantioselectivities. The reported method provides facile access to enantioenriched tertiary thioethers/alcohols. The mechanism of the γ-addition reaction was investigated by performing DFT calculations, and the hydrogen bonding interactions between the Brønsted acid moiety of the phosphine catalysts and the "C[double bond, length as m-dash]O" unit of the donor molecules were shown to be crucial in asymmetric induction.

Total synthesis of tetrahydrolipstatin and stereoisomers via a highly regio- and diastereoselective carbonylation of epoxyhomoallylic alcohols

A concise enantioselective synthesis of tetrahydrolipstatin (THL) and seven stereoisomers has been achieved. The synthesis of THL was accomplished in 10 steps and 31% overall yield from an achiral ynone. Key to the success of the approach is the use of a bimetallic [Lewis acid](+)[Co(CO)4](-) catalyst for a late-stage regioselective carbonylation of an enantiomerically pure cis-epoxide to a trans-β-lactone. The success of this route to THL and its stereoisomers also demonstrated the practicality of the carbonylation catalyst for complex molecule synthesis as well as its functional group compatibility.

A convenient enantioselective decarboxylative aldol reaction to access chiral α-hydroxy esters using β-keto acids

We show a convenient decarboxylative aldol process using a scandium catalyst and a PYBOX ligand to generate a series of highly functionalized chiral α-hydroxy esters. The protocol tolerates a broad range of β-keto acids with inactivated aromatic and aliphatic α-keto esters. The possible mechanism is rationalized.

Efficient synthesis of α-quaternary α-hydroxy-β-amino esters via silyl glyoxylate-mediated three-component reactions

An efficient method has been developed for the enantioselective synthesis of fully protected α-quaternary α-hydroxy-β-amino esters via the coupling of three components: aryl Grignard reagents (or methyllithium), silyl glyoxylate, and N-tert-butanesulfinyl imines. This protocol enables successive formation of two C-C bonds and two adjacent chiral carbons with high stereocontrol in a one-pot operation.

"One-pot" reductive lactone alkylation provides a concise asymmetric synthesis of chiral isoprenoid targets

An efficient method, based on nucleophilic addition to lactones followed by modified in situ Clemmensen reduction, provides a short synthetic route to chiral isoprenoid targets. The efficacy of this method has been exemplified through the synthesis of several targets including the commercial fragrance Rosaphen, the side chain of Zaragozic acid C, the cotton leaf sex pheromone, and the side chains of vitamin E.

Diastereoselective synthesis of vicinal tertiary diols

A strategy for the synthesis of differentiated vicinal tertiary diols is described. The key step is a high-yielding, diastereoselective LaCl3·2LiCl-mediated addition of a Grignard or organolithium reagent to ketone 2a. The reaction is believed to proceed via a 1,3-chelated intermediate. One of the adducts has been transformed into a functionalized cyclopentenone resembling the core structure of pactamycin.

Alternaric acid: formal synthesis and related studies

A silyl glyoxylate three-component-coupling methodology has been exploited to achieve a formal synthesis, an analogue to an intermediate in a distinct formal synthetic route, and a third (unique) approach to the natural product alternaric acid. Highlighted in this study is the versatility of silyl glyoxylates to engage a variety of nucleophile and electrophile pairs to provide wide latitude in the approach to complex molecule synthesis.

Constructing molecular complexity and diversity: total synthesis of natural products of biological and medicinal importance

The advent of organic synthesis and the understanding of the molecule as they occurred in the nineteenth century and were refined in the twentieth century constitute two of the most profound scientific developments of all time. These discoveries set in motion a revolution that shaped the landscape of the molecular sciences and changed the world. Organic synthesis played a major role in this revolution through its ability to construct the molecules of the living world and others like them whose primary element is carbon. Although the early beginnings of organic synthesis came about serendipitously, organic chemists quickly recognized its potential and moved decisively to advance and exploit it in myriad ways for the benefit of mankind. Indeed, from the early days of the synthesis of urea and the construction of the first carbon-carbon bond, the art of organic synthesis improved to impressively high levels of sophistication. Through its practice, today chemists can synthesize organic molecules--natural and designed--of all types of structural motifs and for all intents and purposes. The endeavor of constructing natural products--the organic molecules of nature--is justly called both a creative art and an exact science. Often called simply total synthesis, the replication of nature's molecules in the laboratory reflects and symbolizes the state of the art of synthesis in general. In the last few decades a surge in total synthesis endeavors around the world led to a remarkable collection of achievements that covers a wide ranging landscape of molecular complexity and diversity. In this article, we present highlights of some of our contributions in the field of total synthesis of natural products of biological and medicinal importance. For perspective, we also provide a listing of selected examples of additional natural products synthesized in other laboratories around the world over the last few years.

Silyl glyoxylates. Conception and realization of flexible conjunctive reagents for multicomponent coupling

This Perspective describes the discovery and development of silyl glyoxylates, a new family of conjunctive reagents for use in multicomponent coupling reactions. The selection of the nucleophilic and electrophilic components determines whether the silyl glyoxylate reagent will function as a synthetic equivalent to the dipolar glycolic acid synthon, the glyoxylate anion synthon, or the α-keto ester homoenolate synthon. The ability to select for any of these reaction modes has translated to excellent structural diversity in the derived three- and four-component coupling adducts. Preliminary findings on the development of catalytic reactions using these reagents are detailed, as are the design and discovery of new reactions directed toward particular functional group arrays embedded within bioactive natural products.

Three-component coupling approach to trachyspic acid

Three-component coupling of the lithium enolate of t-BuOAc, silyl glyoxylate, and an α,β-unsaturated ketone enables the rapid construction of the trachyspic acid carbon skeleton. A 3,4-disubstituted isoxazole is utilized to mask the C7/C9 dicarbonyl. New enolsilane/nitrile-oxide cycloadditions enable the preparation of various 3,4-disubstituted isoxazoles that are challenging to access by other means.

Diastereoselective self-condensation of dihydroxyfumaric acid in water: potential route to sugars

Jack of all trades: water-soluble salts of DHF underwent self-condensation to afford the threo diastereomer of pentulosonic acid, through differing reaction pathways contingent on the metal salt used. This transformation exemplifies the diverging roles of DHF as a nucleophile (a synthon for α-hydroxyacetyl anion) and an electrophile (an α-carboxyglycolaldehyde equivalent).

Remote stereoinduction in the acylation of fully substituted enolates: tandem Reformatsky/quaternary Claisen condensations of silyl glyoxylates and β-lactones

Reformatsky reagents react sequentially with silyl glyoxylates and β-lactones to give highly functionalized Claisen condensation products. A heretofore undocumented instance of stereochemical 1,4-induction results in efficient transmission of β-lactone stereochemistry to the emerging fully substituted stereocenter. Second-stage transformations reveal that the five heteroatom-containing functionalities embedded within the products are entirely chemo-differentiated, a circumstance that permits rapid assembly of the leustroducsin B core substructure.

Catalytic nucleophilic glyoxylation of aldehydes

Beta-silyloxy-alpha-keto esters are prepared through a cyanide-catalyzed benzoin-type reaction with silyl glyoxylates and aldehydes. The products undergo a dynamic kinetic resolution to provide enantioenriched orthogonally protected alcohols and can be converted to the corresponding beta-silyloxy-alpha-amino esters.

Synthesis at the molecular frontier

Driven by remarkable advances in the understanding of structure and reaction mechanisms, organic synthesis will be increasingly directed to producing bioinspired and newly designed molecules.