Total Synthesis of Immunosuppressants: Unified Strategies Exploiting Dithiane Couplings and σ-Bond Olefin Constructions

A case study of the MAC (masked acyl cyanide) oxyhomologation of N,N-dibenzyl-L-phenylalaninal with anti diastereoselectivity: preparation of (2S,3S)-allophenylnorstatin esters

The three-component reaction between a protected α-amino aldehyde, an alcohol and an α-silyloxymalononitrile provides an expedient access to protected α-hydroxy-β-amino acid derivatives. The prototypical process, performed on N-Cbz-phenylalaninal, is known to proceed with syn diastereoselectivity. The present study demonstrates that the diastereoselectivity of the reaction can be inverted, using the rationale of a Felkin-Anh interaction model. Reactions performed on N,N-dibenzyl-L-phenylalaninal proceed with a high anti diastereoselectivity, providing a panel of synthetically useful ester derivatives of (2S,3S)-allophenylnorstatin. The procedure is exploited to accomplish one of the most efficient syntheses of the title compound to date, in 3 steps (66% yield) from N,N-dibenzyl-L-phenylalaninal.

Direct synthesis of 1,3-dithiolanes from terminal alkynes via visible light photoredox catalysis

AA visible light-mediated, metal-free, regioselective dihydrothionation of terminal aromatic as well as heteroaromatic alkynes has been achieved by using Eosin Y as a photoredox catalyst at room temperature. The protocol...

Pot-Economy Autooxidative Condensation of 2-Aryl-2-lithio-1,3-dithianes

The autoxidative condensation of 2-aryl-2-lithio-1,3-dithianes is here reported. Treatment of 2-aryl-1,3-dithianes with n-BuLi in the absence of any electrophile leads to condensation of three molecules of 1,3-dithianes and formation of highly functionalized α-thioether ketones orthothioesters in 51-89% yields upon air exposure. The method was further expanded to benzaldehyde dithioacetals, affording corresponding orthothioesters and α-thioether ketones in 48-97% yields. The experimental results combined with density functional theory studies support a mechanism triggered by the autoxidation of 2-aryl-2-lithio-1,3-dithianes to yield a highly reactive thioester that undergoes condensation with two other molecules of 2-aryl-2-lithio-1,3-dithiane.

Organic synthesis in the Smith Group: a personal selection of a dozen lessons learned at the University of Pennsylvania

The passionate study of the complex and ever-evolving discipline of organic synthesis over more than a four-decade span is certain to elucidate meaningful and significant lessons. Over this period, Amos B. Smith III, the Rhodes-Thompson Professor of Chemistry and Member of the Monell Chemical Senses Center at the University of Pennsylvania, has mentored well over 100 doctoral and masters students, more than 200 postdoctoral associates and numerous undergraduates, in addition to collaborating with a wide spectrum of internationally recognized scholars. His research interests, broadly stated, comprise complex molecule synthesis, the development of new, versatile and highly effective synthetic methods, bioorganic and medicinal chemistry, peptide mimicry chemistry and material science. Each area demands a high level of synthetic design and execution. United by a passion to unlock the secrets of organic synthesis, and perhaps of Nature itself, innumerable lessons have been, and continue to be, learned by the members of the Smith Group. This lead article in a Special Issue of the Journal of Antibiotics affords an opportunity to share some of those lessons learned, albeit a small selection of personal favorites.

One-pot sequential 1,2-addition, Pd-catalysed cross-coupling of organolithium reagents with Weinreb amides

An efficient sequential 1,2-addition/cross-coupling of Weinreb amides with two organolithium reagents is reported. This synthetic approach allows access to a wide variety of functionalized ketones in a modular way and in excellent yields.

Rhodium-catalyzed denitrogenative thioacetalization of N-sulfonyl-1,2,3-triazoles with disulfides: an entry to diverse transformation of terminal alkynes

An efficient and useful rhodium-catalyzed denitrogenative thioacetalization of N-sulfonyl-1,2,3-triazoles has been developed for the first time. The protocol uses readily available N-sulfonyl-1,2,3-triazoles and diaryl disulfides as the starting materials. The corresponding hydrolytic and reductive products with thioacetals were obtained in good to excellent yields, and the reactions were carried out easily under mild conditions with tolerance of some functional groups. Furthermore, the generated thioacetals could be transformed into some useful compounds. Therefore, the present method provides a novel and valuable strategy for the diverse transformation of alkynes.

An enantioselective total synthesis of Sch-725674

An enantioselective total synthesis of Sch-725674 using dithiane alkylation, cross metathesis reaction, Yamaguchi macrolactonization and a substrate controlled stereoselective reduction as key steps is described.

Photocatalytic direct intermolecular addition of 1,3-dithianes

A direct intermolecular radical addition of 1,3-dithianes to alkenes has been accomplished via visible-light mediated photocatalysis.

Reversed-Polarity Synthesis of Diaryl Ketones through Palladium-Catalyzed Direct Arylation of 2-Aryl-1,3-dithianes

An umpolung approach to the synthesis of diaryl ketones has been developed based on in situ generation of acyl anion equivalents and their catalytic arylation. This method entails the base promoted palladium catalyzed direct C-H arylation of 2 The resulting 2,2-diaryl-1,3-dithianes with aryl bromides. Use of MN(SiMe3)2 (M=Li, Na) base results in reversible deprotonation of the weakly acidic dithiane. In the presence of a Pd(NiXantphos)-based catalyst and aryl bromide, cross-coupling of the metallated 2-aryl-1,3-dithiane takes place under mild conditions (2 h at rt) with yields as high as 96%. The resulting 2,2-diaryl-1,3-dithianes were converted into diaryl ketones by either molecular iodine, N-bromo succinimide (NBS) or Selectfluor in the presence of water. The dithiane arylation/hydrolysis can be performed in a one-pot procedure to yield a good to excellent yields. This method is suitable for rapid and large-scale synthesis of diaryl ketones. A one-pot preparation of anti-cholesterol drug fenofibrate (TriCor®) has been achieved on 10.0 mmol scale in 86% yield.

A total synthesis of spirastrellolide A methyl ester

A concise total synthesis of spirastrellolide A methyl ester (1 a, R(1) =Me) as the parent compound of a series of highly cytotoxic marine macrolides is disclosed, which exploits and expands the flexibility of a synthesis plan previously developed by our group en route to the sister compound spirastrellolide F methyl ester (6 a, R(1) =Me). Key to success was the masking of the signature Δ(15,16) -bond of 1 a as a C16-carbonyl group until after the stereogenic center at C24 had been properly set by a highly selective hydrogenation of the C24 exo-methylene precursor 66. Conformational control over the macrocyclic frame allowed the proper stereochemical course to be dialed into this reduction process. The elaboration of the C16 ketone to the C15-C16 double bond was accomplished by a chemoselective alkenyl triflate formation followed by a palladium-catalyzed hydride delivery. The role of the ketone at C16 as a strategic design element is also evident up-stream of the key intermediate 66, the assembly of which hinged upon the addition of the polyfunctionalized dithiane 37 to the similarly elaborate aldehyde fragment 46. Other crucial steps of the total synthesis were an alkyl-Suzuki coupling and a Yamaguchi lactonization that allowed the Northern and the Southern sector of the target to be stitched together and the macrocyclic perimeter to be forged. The lateral chain comprising the remote C46 stereocenter was finally attached to the core region by a modified Julia-Kocienski olefination. The preparation of the individual building blocks led to some methodological spin-offs, amongst which the improved procedure for the N-O-bond cleavage of isoxazolines by zero-valent molybdenum and the ozonolysis of a double bond in the presence of other oxidation-prone functionality are most noteworthy. Preliminary biological data suggest that the entire carbon framework, that is the macrocyclic core plus the lateral chain, might be necessary for high cytotoxicity.

From nature to the laboratory and into the clinic

Natural products possess a broad diversity of structure and function, and they provide inspiration for chemistry, biology, and medicine. In this review article, we highlight and place in context our laboratory's total syntheses of, and related studies on, complex secondary metabolites that were clinically important drugs, or have since been developed into useful medicines, namely amphotericin B (1), calicheamicin gamma(1)(I) (2), rapamycin (3), Taxol (4), the epothilones [e.g., epothilones A (5) and B (6)], and vancomycin (7). We also briefly highlight our research with other selected inspirational natural products possessing interesting biological activities [i.e., dynemicin A (8), uncialamycin (9), eleutherobin (10), sarcodictyin A (11), azaspiracid-1 (12), thiostrepton (13), abyssomicin C (14), platensimycin (15), platencin (16), and palmerolide A (17)].

Total synthesis of potent antitumor agent (-)-lasonolide A: a cycloaddition-based strategy

A detailed account of the enantioselective total synthesis of (-)-lasonolide A is described. Our initial synthetic route to the top tetrahydropyran ring involved Evans asymmetric alkylation as the key step. Initially, we relied on the diastereoselective alkylation of an alpha-alkoxyacetimide derivative containing an alpha' stereogenic center and investigated such an asymmetric alkylation reaction. Although alkylation proceeded in good yield, the lack of diastereoselectivity prompted us to explore alternative routes. Our subsequent successful synthetic strategies involved highly diastereoselective cycloaddition routes to both tetrahydropyran rings of lasonolide A. The top tetrahydropyran ring was constructed stereoselectively by an intramolecular 1,3-dipolar cycloaddition reaction. The overall process constructed a bicyclic isoxazoline, which was later unravelled to a functionalized tetrahydropyran ring as well as a quaternary stereocenter present in the molecule. The lower tetrahydropyran ring was assembled by a Jacobsen catalytic asymmetric hetero-Diels-Alder reaction as the key step. The synthesis also features a Lewis acid catalyzed epoxide opening to form a substituted ether stereoselectively.

Total synthesis studies on macrocyclic pipecolic acid natural products: FK506, the antascomicins and rapamycin

This chapter derives its inspiration from the challenges presented to total synthesis chemists, by a particular group of macrocyclic pipecolic acid natural products. Although there is considerable emphasis on the completed syntheses of the main characters (FK506 (1), the antascomycins (4 and 5) and rapamycin (7)), the overall complexity of the molecular problem has stimulated a wealth of new knowledge, including the development of novel strategies and the invention of new synthetic methods. The ingenious and innovative approaches to these targets have enabled new generations of analogues, and provided material to further probe the biology of these fascinating molecules. With pharmaceutical application as an immunosuppressant, as well as potential use for the treatment of cancer and neurodegenerative diseases, this family of natural products continues to inspire new and interesting science while providing solutions to healthcare problems of the world.

Biologically active Phytophthora mating hormone prepared by catalytic asymmetric total synthesis

A Phytophthora mating hormone with an array of 1,5-stereogenic centers has been synthesized by using our recently developed methodology of catalytic enantioselective conjugate addition of Grignard reagents. We applied this methodology in a diastereo- and enantioselective iterative route and obtained two of the 16 possible stereoisomers of Phytophthora hormone alpha1. These synthetic stereoisomers induced the formation of sexual spores (oospores) in A2 mating type strains of three heterothallic Phytophthora species, P. infestans, P. capsici, and P. nicotianae but not in A1 mating type strains. The response was concentration-dependent, and the oospores were viable. These results demonstrate that the biological activity of the synthetic hormone resembles that of the natural hormone alpha1. Mating hormones are essential components in the sexual life cycle of a variety of organisms. For plant pathogens like Phytophthora, sexual reproduction is important as a source of genetic variation. Moreover, the thick-walled oospores are the most durable propagules that can survive harsh environmental conditions. Sexual reproduction can thus greatly affect disease epidemics. The availability of synthetic compounds mimicking the activity of Phytophthora mating hormone will be instrumental for further unravelling sexual reproduction in this important group of plant pathogens.

Asymmetric imino aza-enamine reaction catalyzed by axially chiral dicarboxylic acid: use of arylaldehyde N,N-dialkylhydrazones as acyl anion equivalent

Synthetic utility of arylaldehyde N,N-dialkylhydrazones as a practical acyl anion equivalent could be exploited for the first time in the asymmetric imino aza-enamine reaction catalyzed by axially chiral carboxylic acid.

Stereocontrolled and Convergent Total Synthesis of Amphidinolide T3

Stereocontrolled and convergent total synthesis of amphidinolide T3 has been described. A retrosynthetic scheme was constructed that led to the recognition of readily available and enantiomerically related compounds as starting materials for the total synthesis of amphidinolide T3. Thus, the two key building blocks 6 and 7 were defined as subtargets and synthesized in optically active forms. The C1-C12 fragment 6 was derived from commercially available D-glutamic acid or its synthetically equivalent (R)-5-hydroxymethyltetrahydrofuran-2-one 16 as starting material involving highly diastereoselective asymmetric allylation as a key step. The C13-C21 fragment 7 was efficiently synthesized in high yield through the dithiane coupling of the segment 10 and iodide 11, followed by subsequent deprotection and Petasis olefination. Eventually, assembly of the fragment aldehyde 6 and dithiane 7 along with C-C bond formation, a two-step oxidation-reduction sequence, selective macrolactonization, and functional transformation furnished the convergent total and formal synthesis of amphidinolide T3 and T4, and this approach also provides a flexible and practical synthesis of amphidinolide T macrolides.

A General, Convergent Strategy for the Construction of Indolizidine Alkaloids: Total Syntheses of (−)-Indolizidine 223AB and Alkaloid (−)-205B

N-Toluenesulfonyl aziridines comprise effective second electrophiles in the solvent controlled three-component linchpin union of silyl dithianes for the stereocontrolled convergent elaboration of protected 1,5-amino alcohols. This tactic, in conjunction with a one-flask sequential cyclization, constitutes an effective general strategy for the construction of indolizidine and related alkaloids, illustrated here with the total syntheses of (-)-indolizidine 223AB (1) and alkaloid (-)-205B (2).

Chemoselective Thioacetalization in Water: 3-(1,3-Dithian-2-ylidene)pentane- 2,4-dione as an Odorless, Efficient, and Practical Thioacetalization Reagent

A chemoselective thioacetalization utilizing 3-(1,3-dithian-2-ylidene)pentane-2,4-dione as a novel nonthiolic, odorless 1,3-propanedithiol equivalent catalyzed by p-dodecylbenzenesulfonic acid in water has been developed.

Selective methodologies for the synthesis of biologically active piperidinic compounds

The synthesis of optically active substituted piperidines has been achieved by using four different methodologies. The first one is an intramolecular nucleophilic displacement of activated alcohol moieties that was used to build up the piperidine ring of (-)-prosophylline and (-)-slaframine, and the second one is a ring-closing metathesis of unsaturated amines which was employed in the synthesis of (+)-sedamine and 4a,5-dihydrostreptazoline. The third methodology is the alpha-functionalization of N-Boc piperidines which was particularly useful in the synthesis of argatroban, and the fourth one is a ring expansion of prolinols to 3-chloropiperidines or 3-hydroxypiperidines which was utilized to synthesize (-)-paroxetine, (-)-pseudoconhydrine, the piperidine ring of (-)-velbanamine and (+)-zamifenacin.

A unified approach to polyene macrolides: synthesis of candidin and nystatin polyols

Polyene macrolide antibiotics are naturally occurring antifungal agents. Members of this class include amphotericin B, which has been used widely to treat systemic fungal infections. A general synthetic strategy has been devised to prepare polyol chains associated with the polyene macrolides. Cyanohydrin acetonide alkylations were used to assemble the carbon skeleton, and a simple modification of the strategy allowed an advanced intermediate to be converted to either the candidin polyol or the nystatin polyol. The candidin polyol was further elaborated to a protected candidin aglycone. This strategy will be applicable to other members of the polyene macrolide natural products.

Evolution of dithiane-based strategies for the construction of architecturally complex natural products

Umpolung-based strategies play a significant role in organic synthesis. Particularly important are 1,3-dithiane linchpins, which serve as convenient acyl anion equivalents. The general synthetic accessibility and impressive reactivity of 1,3-dithianes have thus led to widespread application. Since the late 1970s, dithianes have featured prominently in our program directed toward the synthesis of complex natural and unnatural products, both for effective union of advanced fragments and for multicomponent linchpin couplings. In this Account, we present the evolution of dithiane chemistry in our laboratory.

A unified approach to the tedanolides: total synthesis of (+)-13-deoxytedanolide

A unified approach for the construction of the potent marine antitumor agents (+)-tedanolide (1) and (+)-13-deoxytedanolide (2) is described. Highlights of the synthetic strategy include the development of a versatile bifunctional dithiane-vinyl iodide linchpin, the unorthodox use of the Evans-Tishchenko reaction, and a late-stage high-risk stereocontrolled introduction of the C(18,19) epoxide to achieve a total synthesis of (+)-13-deoxytedanolide (2).