Formal synthesis of leustroducsin B via Reformatsky/Claisen condensation of silyl glyoxylates

Preparation of an advanced intermediate for the synthesis of leustroducsins and phoslactomycins by heterocycloaddition

A convergent strategy for the synthesis of leustroducsins and phoslactomycins has been designed, relying on the synthesis and the coupling of three main fragments. The central fragment was synthesized via a regio-and stereoselective nitroso Diels–Alder reaction with an enol phosphate, followed by reductive cleavage of the phosphate to the ketone 11b. Coupling studies of this fragment with the lactone fragment was accomplished in a stereoselective fashion through a vinyllithium intermediate. An advanced synthetic intermediate was then obtained after functional group transformation.

Sulfinyl-Mediated Stereoselective Functionalization of Acyclic Conjugated Dienes

The chemo- and stereocontrolled functionalization of conjugated sulfinyl dienes in a cascade process that involves a conjugate addition, diastereoselective protonation and a [2,3]-sigmatropic rearrangement is reported. Enantioenriched 1,4-diol and 1,4-aminoalcohol derivatives are obtained in a very straightforward manner. Further functionalization of these structures, including highly stereoselective epoxidation, dihydroxylation and the stereodivergent synthesis of several polyols in a controlled fashion is described.

A catalyst-controlled switchable reaction of β-keto acids to silyl glyoxylates

A catalyst-controlled switchable reaction of β-keto acids to silyl glyoxylates was developed and the decarboxylative-addition products were generated by using a thiourea catalyst.

Organobase-catalyzed [1,2]-Brook rearrangement of silyl glyoxylates

A highly efficient [1,2]-Brook rearrangement of silyl glyoxylates has been developed using DBU as the organo-base catalyst.

Synthetic Strategies Employed for the Construction of Fostriecin and Related Natural Products

Fostriecin and related natural products present a significant challenge for synthetic chemists due to their structural complexity and chemical sensitivity. This review will chronicle the successful efforts of synthetic chemists in the construction of these biologically active molecules. Key carbon-carbon bond forming reactions will be highlighted, as well as the methods used to install the numerous stereocenters present in this class of compounds.

Chiral carbene-borane adducts: precursors for borenium catalysts for asymmetric FLP hydrogenations

The carbene derived from (1R,3S)-camphoric acid was used to prepare the borane adduct with Piers' borane 7. Subsequent hydride abstraction gave the borenium cation 8. Adducts with 9-BBN and the corresponding (1R,3S)-camphoric acid-derived carbene bearing increasingly sterically demanding N-substituents (R = Me 9, Et 10, i-Pr 11) and the corresponding borenium cations 12-14 were also prepared. These cations were not active as catalysts in hydrogenation, although 9-11 were shown to undergo carbene ring expansion reactions at 50 °C to give species 15-17. The IBOX-carbene precursors 18 and 19 derived from amino alcohols (S)-valinol and (S)-tert-leucinol (R = i-Pr, t-Bu) were used to prepare borane adducts 20-23. Reaction of the carbenes 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-di-iso-propylimidazol-2-ylidene (IPr) 1-benzyl-3-methylimidazol-2-ylidene (IBnMe), 1-methyl-3-phenylimidazol-2-ylidene (IPhMe) and 1-tert-butyl-3-methylimidazol-2-ylidene (ItBuMe) with diisopinocampheylborane (Ipc₂BH) gave chiral adducts: (IMe)(Ipc₂BH) 24, (IPr)(Ipc₂BH) 25, (IBnMe)(Ipc₂BH) 26, (IPhMe)(Ipc₂BH) 27, and (ItBuMe)(Ipc₂BH) 28. Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2]decane adduct of 1,3,4-triphenyl-1H-1,2,3-triazolium, rac-29 and the 9-BBN derivative of (S)-2-amino-2'-methoxy-1,1'-binaphthalene-1,2,3-triazolium 34a/b were also prepared. In catalytic studies of these systems, while several species were competent catalysts for imine reduction, in general, low enantioselectivities, ranging from 1-20% ee, were obtained. The implications for chiral borenium cation catalyst design are considered.

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.

Regioselective synthesis of functionalized dihydroisoquinolines from o-alkynylarylaldimines via the Reformatsky reaction

A novel approach for the synthesis of functionalized 1,2-dihydroisoquinolines from o-alkynylarylaldimines via the Reformatsky reaction without the aid of an external Lewis acid has been described.

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.

A Highly Convergent Total Synthesis of Leustroducsin B

Leustroducsin B exhibits a large variety of biological activities and unique structural features. An efficient and highly convergent total synthesis of Leustroducsin B was achieved in 17 longest linear and 39 total steps by disconnecting the molecule into three fragments having similar levels of complexity. These pieces were connected via a highly efficient chelate-controlled addition of a vinyl zincate to an α-hydroxy ketone and a silicon-mediated cross-coupling. The stereochemistry of the central and western fragments was set catalytically in high yields and excellent de by a zinc-ProPhenol-catalyzed aldol reaction and a palladium-catalyzed asymmetric allylic alkylation.

Advances in tandem reactions with organozinc reagents

A review of recent advances in tandem reactions with organozinc reagents that highlight their utility in organic synthesis.

Remote stereocontrol in the synthesis of acyclic 1,4-diols and 1,4-aminoalcohols from 2-sulfinyl dienes

The highly diastereoselective conjugate addition of alcohols and amines (RXH) to enantiopure 2-sulfinyl dienes renders transient allylic sulfoxides which undergo sulfoxide-sulfenate rearrangement and sulfenate cleavage providing 2-ene-1,4-diols and 2-ene-1,4-aminoalcohols with up to 99:1 dr. The method allows for the generation of two stereocenters in a single synthetic operation with remote chirality transfer of one center into the other.

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.

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.

Identification of phoslactomycin biosynthetic gene clusters from Streptomyces platensis SAM-0654 and characterization of PnR1 and PnR2 as positive transcriptional regulators

Phoslactomycins (PLMs) are inhibitors of protein serine/threonine phosphatase 2A showing diverse and important antifungal, antibacterial and antitumor activity. PLMs are polyketide natural products and produced by several Streptomyces species. The PLMs biosynthetic gene clusters were identified from Streptomyces platensis SAM-0654 and localized in two separate genomic regions, consisting of 27 open reading frames that encode polyketide synthases (PKSs), enzymes for cyclohexanecarboxyl-CoA (CHC-CoA) and ethylmalonyl-CoA (Em-CoA) synthesis, enzymes for post-PKS modifications, proposed regulators, and putative resistance transporters. Bioinformatic analysis and inactivation experiment of regulatory genes suggest that PnR1 and PnR2 are two positive regulators of PLMs biosynthesis. Gene transcription analysis by reverse transcriptase PCR (RT-PCR) of the PLMs gene cluster demonstrated that PnR1 and PnR2 activate the transcription of the structural biosynthetic genes while PnR2 specially governs the transcription of pnR1 in a higher level.

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.