TBS-pyrrole as an "universal" reference to quantify artemisinin and structurally-diverse natural products in plants extracts by NMR.

The commercial production of artemisinin and other valuable bioactive natural products depends on their plant sources, which may provide variable amounts of the compound depending on plant variety, the period of the year, abiotic stress and other factors. Therefore, it requires a method for large-scale, low-cost natural product quantification. The standard HPLC and UHPLC methods are accurate but the analysis are costly and require different optimization for structurally-diverse products. An alternative method using NMR with TBS-pyrrole as a novel "universal" reference affords a simple, fast method to quantify many different products. The method is shown with antimalarial artemisinin, whose yield using conventional and novel extraction procedures was determined by standard UHPLC-MS procedures and by our NMR protocol, with similar quantification results. The novel reference compound does not interfere with artemisinin or extract signals, only needs a small amount of the extract, is accurate and operationally simple, and a large volume of samples can be processed in little time. Moreover, bioactive terpenes, steroids, alkaloids, aromatic compounds, and quinones, among others, were quantified in a model vegetal extract with this "universal" reference with excellent accuracy.

Conversion of Hydroxyproline "Doubly Customizable Units" to Hexahydropyrimidines: Access to Conformationally Constrained Peptides.

The efficient transformation of hydroxyproline "doubly customizable units" into rigid hexahydropyrimidine units takes place in good global yields and generates compounds of pharmaceutical interest. In particular, the process can readily provide access to peptidomimetics and peptides with reversed sequences or with valuable turns.

"Cut and Paste" Processes in the Search of Bioactive Products: One-Pot, Metal-free O-Radical Scission-Oxidation-Addition of C, N or P-Nucleophiles.

Hypervalent iodine reagents have been applied in many metal-free, efficient synthesis of natural products and other bioactive compounds. In particular, treatment of alcohols, acetals and acids with hypervalent iodine reagents and iodine results in O-radicals that can undergo a ß-scission reaction. Under these oxidative conditions, derivatives of amino acids, peptides or carbohydrates are converted into cationic intermediates, which can subsequently undergo inter- or intramolecular addition of nucleophiles. Most reported papers describe the addition of oxygen nucleophiles, but this review is focused on the addition of carbon, nitrogen and phosphorous nucleophiles. The resulting products (nucleoside and alkaloid analogs, unnatural amino acids, site-selectively modified peptides) are valuable intermediates or analogs of bioactive compounds.

Structural diversity using amino acid "Customizable Units": conversion of hydroxyproline (Hyp) into nitrogen heterocycles.

The ability of amino acid "customizable units" to generate structural diversity is illustrated by the conversion of 4-hydroxyproline (Hyp) units into a variety of nitrogen heterocycles. After a first common step, where the unit underwent a one-pot decarboxylation-alkylation reaction to afford 2-alkylpyrrolidines with high stereoselectivity, a divergent step was carried out. Thus, the deprotected 4-hydroxy group was used either to initiate a radical scission that afforded aliphatic ß-amino aldehydes, or to carry out an elimination reaction, to give 2-alkyl-2,5-dihydro-1H-pyrroles. In the first case, the amines underwent a tandem reductive amination-cyclization to afford ß-amino-δ-lactams, an efficient rigidifying unit in peptides. Different lactam N-substituents, such as alkylamines, peptides, and alkenyl chains suitable for olefin metathesis were introduced this way. In the second case, the pyrrole derivatives were efficiently converted into alkaloid and iminosugar derivatives in good global yields and with excellent stereoselectivity.

FLTX2: A Novel Tamoxifen Derivative Endowed with Antiestrogenic, Fluorescent, and Photosensitizer Properties.

Tamoxifen is the most widely used selective modulator of estrogen receptors (SERM) and the first strategy as coadjuvant therapy for the treatment of estrogen-receptor (ER) positive breast cancer worldwide. In spite of such success, tamoxifen is not devoid of undesirable effects, the most life-threatening reported so far affecting uterine tissues. Indeed, tamoxifen treatment is discouraged in women under risk of uterine cancers. Recent molecular design efforts have endeavoured the development of tamoxifen derivatives with antiestrogen properties but lacking agonistic uterine tropism. One of this is FLTX2, formed by the covalent binding of tamoxifen as ER binding core, 7-nitrobenzofurazan (NBD) as the florescent dye, and Rose Bengal (RB) as source for reactive oxygen species. Our analyses demonstrate (1) FLTX2 is endowed with similar antiestrogen potency as tamoxifen and its predecessor FLTX1, (2) shows a strong absorption in the blue spectral range, associated to the NBD moiety, which efficiently transfers the excitation energy to RB through intramolecular FRET mechanism, (3) generates superoxide anions in a concentration- and irradiation time-dependent process, and (4) Induces concentration- and time-dependent MCF7 apoptotic cell death. These properties make FLTX2 a very promising candidate to lead a novel generation of SERMs with the endogenous capacity to promote breast tumour cell death in situ by photosensitization.

"Doubly Customizable" Unit for the Generation of Structural Diversity: From Pure Enantiomeric Amines to Peptide Derivatives.

Readily available, low-cost 4R-hydroxy-l-proline (Hyp) is introduced as a "doubly customizable" unit for the generation of libraries of structurally diverse compounds. Hyp can be cleaved at two points, followed by the introduction of new functionalities. In the first cycle, the removal and replacement of the carboxylic group are carried out, followed (second cycle) by the scission of the 4,5-position and manipulation of the resulting chains. In this way, three new chains are generated and can be transformed independently to afford a diversity of products with tailored substituents, such as ß-amino aldehydes, diamines, ß-amino acid derivatives, including N-alkylated ones, or modified peptides. Many of these products are high-profit compounds but, in spite of their commercial value, are still scarce. Moreover, the process takes place with stereochemical control, and either pure R or S isomers can be obtained with small variations of the synthetic route.

Conversion of "Customizable Units" into N-Alkyl Amino Acids and Generation of N-Alkyl Peptides.

An efficient conversion of hydroxyproline "customizable" units into new amino acids with a variety of N-alkyl substituents is described. The process is versatile and can afford valuable N-methyl amino acids and N, O-acetals. In addition, it allows the introduction of N-homoallylic substituents and N-chains with terminal ester, ketone, or cyano groups. These chains could be used for peptide extension or conjugation to other molecules (e.g., by olefin metathesis, peptide ligation, etc.). The transformation is carried out in just two (for R = CH2OAc) or three steps (scission of the pyrrolidine ring, manipulation of the α-chain, and the N-substituent) under mild, metal-free conditions, affording products with high optical purity.

Metal-Free, Site-Selective Peptide Modification by Conversion of "Customizable" Units into ß-Substituted Dehydroamino Acids.

Our site-selective modification of serine or threonine units in peptides allows the generation of ß-substituted dehydroamino acids, which increase peptide resistance to hydrolysis and may improve their biological properties. Both the terminal and internal positions can be modified, and different customizable units can be activated separately. Remarkably, high Z selectivity is achieved, even at internal positions. The conversion involves a one-pot oxidative radical scission/phosphorylation process by using the low-toxicity (diacetoxyiodo)benzene/iodine system as the scission reagent. The resulting α-amino phosphonates undergo a Horner-Wadsworth-Emmons reaction to produce the dehydroamino acid derivatives (in a Z/E ratio of usually >98:2) under mild and metal-free conditions.

Efficient routes to carbon-silicon bond formation for the synthesis of silicon-containing peptides and azasilaheterocycles.

Silasubstitution, where silicon is substituted for carbon at specific sites of the substrate, has become a growing practice in medicinal chemistry. Introducing silicon into bioactive compounds provides slight physical and electronic alterations to the parent compound, which in certain instances could make the substrate a more viable candidate for a drug target. One application is in the field of protease inhibition. Various silane diol isosteres can act as potent inhibitors of aspartic and metalloproteases because of their ability to mimic the high-energy tetrahedral intermediate in peptide bond hydrolysis. In particular, since 1998, the Sieburth group has prepared a number of functionalized peptide silane diol isosteres. In a seminal study, they demonstrated that these molecules can bind to the active site of the enzymes. Inspired by these results, we initiated a study to develop a concise and straightforward route to access highly functionalized silicon diol based peptidomimetic analogs, which we describe in this Account. The synthesis of such analogs is challenging because the dipeptide mimics require the formation of two carbon-silicon bonds as well as two chiral carbon centers. Our first strategy was to assemble the two C-Si bonds from diphenylsilane through an initial regioselective hydrosilylation step of a terminal alkene, followed by lithiation of the formed alkyldiphenylsilane by a simple lithium metal reduction. Subsequent diastereoselective addition of this silyllithium species to a tert-butylsulfinimine provided a rapid method to assemble the dipeptide mimic with stereochemical control at the new chiral carbon center adjacent to the silicon. This strategy worked with a wide range of functional groups. However, there were some limitations with the more elaborate targets. In particular, we needed to exchange the phenyl groups of the diphenylsilane with aryl groups that were more labile under acidic conditions in order to introduce Si-O bonds in the end product. We demonstrated that a variety of Ar(2)SiH(2) compounds with methyl substituents on the aromatic core could effectively undergo hydrosilylation and reductive lithiation with a soluble reducing agent, lithium naphthalenide. The electron-rich aromatic groups were more acid labile and, depending on the conditions, could produce either the silane diol or the silanol. In an alternative strategy, we used a highly regioselective Rh-catalyzed sequential double hydrosilylation to form the two C-Si bonds with a single catalyst. This approach is a more efficient, atom economical way to synthesize a wider range of highly functionalized organosilanes with the added possibility of extending this method into an asymmetric protocol. By this method, various functional groups that were not previously tolerated in the lithiation protocol, including OBn, OAc, furyl, and thiophenes, could now be incorporated. Hydrosilylation of a terminal olefin and a peptide functionalized with an enamide at the C-terminus achieved the desired silane in high yields in a one pot reaction without compromising the stereochemical integrity of the peptide. As an extension of this work, we used these methods to efficiently generate a variety of chiral azasilaheterocycles, including silapiperidines and silaindolizidines.

Reductive lithiation of methyl substituted diarylmethylsilanes: application to silanediol peptide precursors.

Reductive lithiation of methyl-substituted diarylmethylsilanes using lithium naphthalenide represents a practical method for the preparation of the corresponding silyl lithium reagents. Their addition to chiral sulfinimines affords versatile precursors to silanols and silanediols. The replacement of the currently used diphenylsilane motif by a more labile diarylsilane moiety allows the selective hydrolysis of one or two aryl groups by treatment with TFA.

Enamides accessed from aminothioesters via a Pd(0)-catalyzed decarbonylative/ß-hydride elimination sequence.

A facile synthesis of various enamides from aminothioesters via a palladium(0)-catalyzed decarbonylation/ß-hydride elimination is reported. This protocol was applied to mercaptopyridyl C-terminal modified peptides for the generation of enamides without epimerization at stereogenic centers.

Stereocontrolled synthesis of 2-substituted-1,3-azasilaheterocycles.

Chiral alpha-silylsulfinamides, prepared by the treatment of an alkyldiphenylsilane with lithium followed by its addition to a sulfinimine, can be applied to the synthesis of 1,3-azasilaheterocycles as derivatives of cyclic alkaloids. This synthetic route, which involves intramolecular substitution of an amino alcohol or cyclization of an amino acid promoted by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), represents a convenient means for accessing these silicon-containing heterocycles.

Further studies toward the stereocontrolled synthesis of silicon-containing peptide mimics.

Further studies are reported on the utilization of the versatile reaction between chiral sulfinimines and alkyldiphenylsilyl lithium reagents with the goal of preparing a wide range of silanediol-based protease inhibitors. In particular, focus has been placed to demonstrate how a number of genetically encoded amino acid side chains such as serine, threonine, tyrosine, lysine, proline, arginine, aspartate and asparagine might be incorporated into the overall approach. Efforts to apply this synthetic methodology for accessing biologically relevant silanediol dipeptide mimics are also described. This includes the synthesis of a potential inhibitor of the human neutrophil elastase, as well as a diphenylsilane mimic of a hexapeptide fragment of the human islet amyloid polypeptide.

Efficient conversion of carbohydrates into 1-C-alditols: application to the synthesis of chiral gamma-substituted butenolides and bicyclic alkaloid analogues.

Readily available sugar derivatives were transformed in a few steps into valuable, more complex products. The tandem radical scission of carbohydrates-oxidation reaction gave acetoxy acetals, which were converted into a variety of chiral C-alditols in good global yields and excellent 1,2-trans stereoselectivity. The reaction was the key step in the synthesis of hydroxylated gamma-substituted butenolides and bicyclic alkaloid analogues.

One-pot synthesis of acyclic nucleosides from carbohydrate derivatives, by combination of tandem and sequential reactions.

The design of processes which combine tandem and sequential reactions allows the transformation of readily available precursors into high-profit products. This strategy is illustrated by the one-pot synthesis of acyclic nucleosides, which are potential antiviral compounds, from readily available carbohydrates. The reaction conditions are mild, compatible with most functional groups. Depending on the starting sugar, both common and uncommon acyclic chains can be prepared. These polyhydroxylated chains can be combined with different bases to generate diversity.

Synthesis of unnatural amino acids from serine derivatives by beta-fragmentation of primary alkoxyl radicals.

The fragmentation of primary alkoxyl radicals has been scarcely used in synthesis since other competing processes (such as oxidation or hydrogen abstraction) usually predominate. However, when serine derivatives were used as substrates, the scission took place in excellent yields. Tandem scission-allylation, -alkylation, or -arylation reactions were subsequently developed. This one-pot methodology was applied to the synthesis of unnatural amino acids, which are useful synthetic blocks or amino acid surrogates in peptidomimetics.

Short and efficient synthesis of chiral furyl carbinols from carbohydrates.

[reaction: see text] Common sugar derivatives can be transformed in a few steps into chiral-substituted furyl carbinols. The mildness of the reaction conditions and the good yields obtained make this procedure an interesting alternative to the conventional processes.

Selective cleavage of methoxy protecting groups in carbohydrates.

The selective cleavage of methoxy protecting groups next to hydroxy groups is achieved using a radical hydrogen abstraction reaction as the key step. Under the reaction conditions, the hydroxy group generates an alkoxyl radical that reacts with the sterically accessible adjacent methoxy group, which is transformed into an acetal. In the second step, the acetals are hydrolyzed to give alcohols or diols. A one-pot hydrogen abstraction-hydrolysis procedure was also developed. Good yields were usually achieved, and the mild conditions of this methodology were compatible with different functional groups and sensitive substrates such as carbohydrates.

Efficient and selective removal of methoxy protecting groups in carbohydrates.

[reaction: see text] The selective removal from carbohydrate substrates of methoxy protecting groups next to hydroxy groups is reported. On treatment with PhI(OAc)(2)-I(2), the methoxy group is transformed into an easily removable acetal. The mild conditions of this methodology are compatible with many functional groups, and good to excellent yields are usually achieved.

beta-Fragmentation of primary alkoxyl radicals versus hydrogen abstraction: synthesis of polyols and alpha,omega-differently substituted cyclic ethers from carbohydrates.

The beta-fragmentation of primary alkoxyl radicals, described in many cases as low-yielding and plagued by side reactions, can proceed in satisfactory yields using carbohydrate substrates. The only reaction that can compete significantly with the beta-scission is the intramolecular hydrogen abstraction. The ratio of beta-fragmentation to hydrogen abstraction can be varied according to the reaction conditions, the stereochemistry of the substituents (e.g., alpha- or beta-anomeric substituents), and the protecting groups chosen. The carbohydrate substrates are easily prepared, and the mild reaction conditions are compatible with most functional groups. The beta-scission reaction provides an expedient way toward shorter and less common sugar series and also toward alpha,omega-differently substituted cyclic ethers. These units are useful building blocks and are present in many natural products with interesting biological activity.