Super silyl stereo-directing groups for complete 1,5-syn and -anti stereoselectivities in the aldol reactions of beta-siloxy methyl ketones with aldehydes

Design, synthesis and evaluation of bioactivity of peptidomimetics based on chloroalkene dipeptide isosteres

Ample biologically active peptides have been found, identified and modified for use in drug discovery to date. However, several factors, such as low metabolic stability due to proteolysis and non-specific interactions with multiple off-target molecules, might limit the therapeutic use of peptides. To enhance the stability and/or bioactivity of peptides, the development of "peptidomimetics," which mimick peptide molecules, is considered to be idealistic. Hence, chloroalkene dipeptide isosteres (CADIs) was designed, and their synthetic methods have been developed by us. Briefly, in a CADI an amide bond in peptides is replaced with a chloroalkene structure. CADIs might be superior mimetics of amide bonds because the Van der Waals radii (VDR) and the electronegativity value of a chlorine atom are close to those of the replaced oxygen atom. By a developed method of the "liner synthesis", N-tert-butylsulfonyl protected CADIs can be synthesized via a key reaction involving diastereoselective allylic alkylation using organocopper reagents. On the other hand, by a developed method of the "convergent synthesis", N-fluorenylmethoxycarbonyl (Fmoc)-protected carboxylic acids can be also constructed based on N- and C-terminal analogues from corresponding amino acid starting materials via an Evans syn aldol reaction and the Ichikawa allylcyanate rearrangement reaction involving a [3.3] sigmatropic rearrangement. Notably, CADIs can also be applied for Fmoc-based solid-phase peptide synthesis and therefore introduced into bioactive peptides including as the Arg-Gly-Asp (RGD) peptide and the amyloid β fragment Lys-Leu-Val-Phe-Phe (KLVFF) peptide, which are correlated with cell attachment and Alzheimer's disease (AD), respectively. These CADI-containing peptidomimetics stabilized the conformation and enhanced the potency of the cyclic RGD peptide and the cyclic KLVFF peptide.

Super silyl-based stable protecting groups for both the C- and N-terminals of peptides: applied as effective hydrophobic tags in liquid-phase peptide synthesis

Tag-assisted liquid-phase peptide synthesis (LPPS) is one of the important processes in peptide synthesis in pharmaceutical discovery. Simple silyl groups have positive effects when incorporated in the tags due to their hydrophobic properties. Super silyl groups contain several simple silyl groups and play an important role in modern aldol reactions. In view of the unique structural architecture and hydrophobic properties of the super silyl groups, herein, two new types of stable super silyl-based groups (tris(trihexylsilyl)silyl group and propargyl super silyl group) were developed as hydrophobic tags to increase the solubility in organic solvents and the reactivity of peptides during LPPS. The tris(trihexylsilyl)silyl group can be installed at the C-terminal of the peptides in ester form and N-terminal in carbamate form for peptide synthesis and it is compatible with hydrogenation conditions (Cbz chemistry) and Fmoc-deprotection conditions (Fmoc chemistry). The propargyl super silyl group is acid-resistant, which is compatible with Boc chemistry. Both tags are complementary to each other. The preparation of these tags requires less steps than previously reported tags. Nelipepimut-S was synthesized successfully with different strategies using these two types of super silyl tags.

A one-pot diastereoselective synthesis of 1,3-diols and 1,3,5-triols via cascade reactions of arylalkynyl Grignard reagents with enol esters

An efficient cascade reaction has been developed to synthesize a series of 1,3-diols and 1,3,5-triols via reactions of arylalkynyl Grignard reagents with enol esters. The stereoselectivity of reactions and the molecular configurations of the products were confirmed by nuclear magnetic resonance, X-ray diffraction, and high-performance liquid chromatography analysis. A possible reaction mechanism was analyzed with the results indicating that it proceeded through a 1,2-addition/rearrangement and reverse O-acylation to produce the 1,3-diol and via C-acylation to form the 1,3,5-triol.

Catalytic strategies towards 1,3-polyol synthesis by enantioselective cascades creating multiple alcohol functions

This review highlights the different enantioselective catalyst-controlled cascades creating multiple alcohol functions through the formation of several carbon-carbon bonds. Through subsequent simple derivatization, these strategies ensure the rapid preparation of 1,3-polyols. Thanks to the use of efficient metal- or organo-catalysts, these cascades enable the selective assembly of multiple substrates considerably limiting operations and waste generation. For this purpose, several mono- or bi-directional approaches have been devised allowing successive C-C bond-forming events. The considerable synthetic economies these cascades enable have been demonstrated in the preparation of a wide variety of complex bioactive natural products, notably polyketides.

Regio- and Stereoselective Synthesis of Fully Substituted Silyl Enol Ethers of Ketones and Aldehydes in Acyclic Systems

The regio- and stereoselective preparation of fully substituted and stereodefined silyl enol ethers of ketones and aldehydes through an allyl-Brook rearrangement is reported. This fast and efficient method proceeds from a mixture of E and Z isomers of easily accessible starting materials.

Computer-Assisted Synthetic Planning: The End of the Beginning

Exactly half a century has passed since the launch of the first documented research project (1965 Dendral) on computer-assisted organic synthesis. Many more programs were created in the 1970s and 1980s but the enthusiasm of these pioneering days had largely dissipated by the 2000s, and the challenge of teaching the computer how to plan organic syntheses earned itself the reputation of a "mission impossible". This is quite curious given that, in the meantime, computers have "learned" many other skills that had been considered exclusive domains of human intellect and creativity-for example, machines can nowadays play chess better than human world champions and they can compose classical music pleasant to the human ear. Although there have been no similar feats in organic synthesis, this Review argues that to concede defeat would be premature. Indeed, bringing together the combination of modern computational power and algorithms from graph/network theory, chemical rules (with full stereo- and regiochemistry) coded in appropriate formats, and the elements of quantum mechanics, the machine can finally be "taught" how to plan syntheses of non-trivial organic molecules in a matter of seconds to minutes. The Review begins with an overview of some basic theoretical concepts essential for the big-data analysis of chemical syntheses. It progresses to the problem of optimizing pathways involving known reactions. It culminates with discussion of algorithms that allow for a completely de novo and fully automated design of syntheses leading to relatively complex targets, including those that have not been made before. Of course, there are still things to be improved, but computers are finally becoming relevant and helpful to the practice of organic-synthetic planning. Paraphrasing Churchill's famous words after the Allies' first major victory over the Axis forces in Africa, it is not the end, it is not even the beginning of the end, but it is the end of the beginning for the computer-assisted synthesis planning. The machine is here to stay.

Didecarboxylative Iron-Catalyzed Bidirectional Aldolization towards Diversity-Oriented Ketodiol Synthesis

1,3-Acetonedicarboxylic acid was selectively activated by Fe(acac)3 , providing a synthetic platform for rapid synthesis of keto-3,3'-diols. The bidirectional aldol reaction was efficient for challenging aliphatic aldehydes, providing a rapid route to potentially bioactive complex structures.

Regioselective chromatic orthogonality with light-activated metathesis catalysts

The ability to selectively guide consecutive chemical processes towards a preferred pathway by using light of different frequencies is an appealing concept. Herein we describe the coupling of two photochemical reactions, one the photoisomerization and consequent activation of a sulfur-chelated latent olefin-metathesis catalyst at 350 nm, and the other the photocleavage of a silyl protecting group at 254 nm. Depending on the steric stress exerted by a photoremovable neighboring chemical substituent, we demonstrate the selective formation of either five- or six-membered-ring frameworks by light-triggered ring-closing metathesis. The orthogonality of these light-induced reactions allows the initiation of these processes independently and in interchangeable order, according to the wavelength of light used to promote them.

Stereodivergent approach to the avermectins based on "super silyl" directed aldol reactions

A stereodivergent approach to the spiroketal fragment of the avermectins is described. The strategy utilizes a sequence of three aldol reactions directed by the tris(trimethylsilyl)silyl "super silyl" group. Central to this strategy is that each aldol reaction can be controlled to allow access to either diastereomer in high stereoselectivity, thereby affording 16 stereoisomers along the same linear skeleton. The aldol products can be transformed into spiroketals, including an advanced intermediate in the total synthesis of avermectin A1a.

Synthesis of β-hydroxy-α-haloesters through super silyl ester directed syn-selective aldol reaction

Super silyl haloesters including chloro- and bromoacetate were synthesized and utilized for aldol reactions to give syn-β-hydroxy-α-haloacetates in good yields with high diastereoselectivities. β-Hydroxy-α-fluoroacetate was obtained by lithiation of super silyl bromofluoroacetate. Sequential Darzens reactions provided cis-glycidic esters in moderate yields.

β-Siloxy-α-haloketones through highly diastereoselective single and double mukaiyama aldol reactions

Double-action haloketones: A super silyl group enabled the first highly diastereoselective Mukaiyama aldol reactions of α-chloro- and α-fluoroketones with a wide range of aldehydes, providing anti-β-siloxy-α-haloketones. This process is compatible with one-pot double-aldol methodology and allows for rapid access to new halogen-modified polyketide fragments bearing up to four contiguous stereocenters.

Rapid, one-pot synthesis of β-siloxy-α-haloaldehydes

The Mukaiyama cross-aldol reaction of α-fluoro-, α-chloro-, and α-bromoacetaldehyde-derived (Z)-tris(trimethylsilyl)silyl enol ethers is described, furnishing anti-β-siloxy-α-haloaldehydes. A highly diastereoselective, one-pot, sequential double-aldol process is developed, affording novel β,δ-bissiloxy-α,γ-bishaloaldehydes. Reactions are catalyzed by C(6)F(5)CHTf(2) and C(6)F(5)CTf(2)AlMe(2) (0.5-1.5 mol %) and provide access to halogenated polyketide fragments.

Asymmetric aldol reactions of α,β-unsaturated ketoester substrates catalyzed by chiral diamines

Highly efficient asymmetric aldol reactions between α,β-unsaturated keto esters and acyclic ketones catalyzed by chiral diamines are reported. The corresponding products were obtained in excellent yields with excellent enantioselectivities. The absolute configuration for the product was determined by X-ray analysis. A variety of substrates were tolerable in the present catalytic system.

Assigning stereochemistry to single diastereoisomers by GIAO NMR calculation: the DP4 probability

GIAO NMR shift calculation has been applied to the challenging task of reliably assigning stereochemistry with quantifiable confidence when only one set of experimental data are available. We have compared several approaches for assigning a probability to each candidate structure and have tested the ability of these methods to distinguish up to 64 possible diastereoisomers of 117 different molecules, using NMR shifts obtained in rapid and computationally inexpensive single-point calculations on molecular mechanics geometries without time-consuming ab initio geometry optimization. We show that a probability analysis based on the errors in each (13)C or (1)H shift is significantly more successful at making correct assignments with high confidence than are probabilities based on the correlation coefficient and mean absolute error parameters. Our new probability measure, which we have termed DP4, complements the probabilities obtained from our previously developed CP3 parameter, which applies to the case of assigning a pair of diastereoisomers when one has both experimental data sets. We illustrate the application of DP4 to assigning the stereochemistry or structure of 21 natural products that were originally misassigned in the literature or that required extensive synthesis of diastereoisomers to establish their stereochemistry.