Solid phase synthesis of fused bicyclic amino acid derivatives via intramolecular Pauson-Khand cyclization: Versatile scaffolds for combinatorial chemistry

Synthesis and Reactivity of Propargylamines in Organic Chemistry

Propargylamines are a versatile class of compounds which find broad application in many fields of chemistry. This review aims to describe the different strategies developed so far for the synthesis of propargylamines and their derivatives as well as to highlight their reactivity and use as building blocks in the synthesis of chemically relevant organic compounds. In the first part of the review, the different synthetic approaches to synthesize propargylamines, such as A³ couplings and C-H functionalization of alkynes, have been described and organized on the basis of the catalysts employed in the syntheses. Both racemic and enantioselective approaches have been reported. In the second part, an overview of the transformations of propargylamines into heterocyclic compounds such as pyrroles, pyridines, thiazoles, and oxazoles, as well as other relevant organic derivatives, is presented.

[Intramolecular cyclization reaction of multiple bonds and its application]

The cycloaddition and cycloisomerization of the allene with an alkyne, alkene, or an additional allene for construction of various monocyclic and bicyclic ring systems has been developed. The characteristic features of these methods using allene functionality instead of a simple alkene or alkyne include the reaction mode that originated from the double function as well as the high efficiency for the constructions of medium-sized rings. Furthermore, asymmetric formal synthesis of (+)-nakadomarin A and total synthesis of (+)-fawcettimine and (+)-lycoposerramine-B based on highly stereoselective Pauson-Khand reaction of alkene-alkynes were completed.

Rh(I)-catalyzed intramolecular [2 + 2 + 1] cycloaddition of allenenes: Construction of bicyclo[4.3.0]nonenones with an angular methyl group and tricyclo[6.4.0.0]dodecenone

The [RhCl(CO)dppp]₂-catalyzed intramolecular carbonylative [2 + 2 + 1] cycloaddition of allenenes was developed to prepare bicyclo[4.3.0]nonenones possessing a methyl group at the ring junction, which is difficult to achieve by the Pauson–Khand reaction of the corresponding enynes. This method also provided a new procedure for the construction of the tricyclo[6.4.0.0^1,5]dodecenone framework in a satisfactory yield.

Dicobalt hexacarbonyl complexes of alkynyl imines in a sequential Staudinger/Pauson-Khand process. A route to new fused tricyclic beta-lactams

Dicobalt hexacarbonyl complexes of alkynyl imines were allowed to react with ketenes via Staudinger reaction. Sequential [2 + 2] cycloaddition/Pauson-Khand reaction led to structurally new fused-tricyclic beta-lactams and fused-azabicyclic cyclopentenones. Chemoselectivity, scope, and limitation of the process were investigated.

Rhodium(I)-Catalyzed Intramolecular Pauson−Khand-Type [2 + 2 + 1] Cycloaddition of Allenenes

[reaction: see text] The novel [RhCl(CO)(2)](2)-catalyzed [2 + 2 + 1] cycloaddition of allenenes leading to the bicyclo[4.3.0]non-1(9)-en-8-one as well as the bicyclo[5.3.0]dec-1(10)-en-9-one skeletons has been developed. This method also provides a new procedure for the construction of the bicyclo[4.3.0]non-1(9)-en-8-one skeleton having an alkyl appendage at the ring juncture, which was hardly attained in a satisfactory yield by the Pauson-Khand reaction of the corresponding enynes.

An efficient protocol for the solid-phase synthesis of malondiamides

A novel and straightforward solid-phase synthesis of malondiamides containing a free nitrogen has been developed. These intermediates, which can be directly obtained in good yield and purity, can be further derivatised. This approach can be used for the synthesis of large split-and-mix-libraries.

Solid-phase synthesis of anandamide analogues

The endocannabinoids are amides and esters of arachidonic acid that can mimic the pharmacological properties of Delta(9)-tetrahydrocannabinol (Delta(9)-THC). Anandamide, the most prominent of the endocannabinoids, has been implicated in both metabolic/physiological roles of the central nervous system, making it an attractive medicinal target. As such, we report the first solid-phase methodology that expedites access to various anandamide analogues. Our synthesis features a repetitive Cu-mediated coupling reaction between terminal alkynes and propargyl halides or allylic halides.

Asymmetric Synthesis of Fused Bicyclic α-Amino Acids Having a Hexahydro-cyclopenta[c]pyridine Skeleton via Intramolecular Pauson−Khand Reaction of 1-Sulfonimidoyl-Substituted 5-Azaoct-1-en-7-ynes

An asymmetric synthesis of fused bicyclic amino acids having a hexahydro-cyclopenta[c]pyridine skeleton and carrying besides an enone structural element a substituent at the beta-position is described. The key steps of the synthesis are a highly selective allylation of N-tert-butylsulfonyl imino ester with bis(allylsulfoximine)titanium complexes and a highly diastereoselective Pauson-Khand cycloaddition of sulfonimidoyl-substituted gamma,delta-unsaturated alpha-amino acid esters carrying a substituent at the beta-position and a propargyl group at the N-atom. The cyclization is accompanied by a reductive cleavage of the sulfoximine group of the primary cyclization product. Surprisingly, the removal of the sulfoximine group proceeds with inversion of the configuration at the S-atom and gives N-methyl-phenylsulfinamide with >/=98% ee. Deprotection of the bicyclic N-tert-butylsulfonyl-protected amino acid ester was accomplished through treatment with CF(3)SO(3)H under anhydrous conditions. The enantio- and diastereomerically pure sulfoximine-substituted gamma,delta-unsaturated alpha-amino acid esters used as starting material were obtained through a highly regio- and diastereoselective allylation of N-tert-butylsulfonyl imino ester with acyclic bis(allylsulfoximine)titanium complexes, described previously.

Novel syntheses of alpha-morpholinoamides from alpha,alpha-dichloroacetamides

Dichloroacetamides, on heating with benzotriazole, morpholine, and triethylamine, produce, in a one-pot reaction, alpha-benzotriazolyl-alpha-morpholinoacetamides 4a,b. Intermediates 4a and 4b react with nucleophiles such as allylsilanes, silyl ethers, and organozinc reagents to afford diverse alpha-morpholinoamides.

Pathway development and pilot library realization in diversity-oriented synthesis: exploring Ferrier and Pauson-Khand reactions on a glycal template

Through a correlation of the ability of small molecules to bind biological macromolecules and their ability to modulate cellular and organismal processes, chemistry can inform biology and vice versa. Diversity-oriented organic synthesis (DOS), which aims to provide structurally complex and diverse small molecules efficiently, can play a key role in such chemical genetic studies. Here we illustrate the trial-and-error experimentation that can refine an initial pathway-planning exercise and result eventually in an effective diversity pathway. By exploring Ferrier and Pauson-Khand reactions on a glycal template, we have developed efficient and stereoselective syntheses of tricyclic compounds. In this pathway, diversity results from the substituents and their spatial relationships about the tricyclic rings. A pilot split-pool library synthesis of 2500 tricyclic compounds highlights the use of planning considerations in DOS and a "one-bead, one-stock solution" technology platform. Additionally, it illustrates a promising synthetic pathway for future chemical genetic studies.

Indium-Mediated Organometallic Reactions in Aqueous Media. Stereoselectivity in the Crotylation of Sulfonimines Bearing a Proximal Chelating Group

Reaction of sulfonimines having proximal chelating groups with crotyl bromide and indium in aqueous media gave alpha-crotylation products stereoselectively with syn-selectivity.

A Versatile Approach to the Solution-Phase Combinatorial Synthesis of Substituted Pyridines: The Cobalt-Catalyzed Cyclotrimerization of Alkynes with a Nitrile

The cobalt-catalyzed trimerization of two alkynes and one nitrile was exploited to produce solution-phase libraries of pyridines. Upon addition of 2 equiv of a CO scavenger (i.e., an amine-N-oxide), the commercially available [CpCo(CO)(2)] proved to be an attractive catalyst, as the required reaction temperatures, side product formation (i.e., carbocycles), and oxygen sensitivity are moderate. Purification of the reaction mixture was performed with an acidic cation-exchange resin.

Catalytic Approach for the Formation of Optically Active Allyl alpha-Amino Acids by Addition of Allylic Metal Compounds to alpha-Imino Esters

A new catalytic enantioselective approach for the formation of allyl alpha-amino acid derivatives by reaction of N-tosyl alpha-imino esters with allyl stannanes and silanes catalyzed by chiral copper(I) complexes has been developed. A series of different BINAP and phosphine-oxazoline (P,N) ligands have, in combination with various Lewis acids, been tested as chiral catalysts for allylation of N-tosyl alpha-imino esters. It has been found that both type of ligands, in combination with copper(I) salts, give highly valuable unsaturated alpha-amino acid derivatives. The reaction has been investigated for different allyl stannanes and silanes, and it has been found that tri-n-butyl allyl stannane gives the best results of the simple allyl compounds tested, leading to gamma,delta-unsaturated alpha-amino acid derivatives in up to 94% yield and with up to 83% ee, which can be improved to be >95% ee by recrystallization. The reaction has also been investigated using different acyclic and cyclic allyl stannanes leading to various types of unsaturated alpha-amino acid derivatives in very high yield (up to 95%) and with up to 98% ee. The stereochemistry and absolute configurations of the allyl alpha-amino acid derivatives have been determined by X-ray analysis, and it is suggested that the reaction takes place as an ene-like reaction.

Transition-metal-mediated reactions in combinatorial synthesis

Transition-metal-mediated reactions have played an important role in expanding the scope of combinatorial chemistry from synthesis of peptidic libraries to synthesis of nonoligomeric, small-molecule libraries. Versatile reactions such as heteroannulations, olefin metathesis and dipolar cycloadditions are used for preparation of libraries of historical, as well as novel, pharmacophores. Transition-metal-mediated reactions have also been used strategically to provide traceless linkers' in solid-phase synthesis.

The search for orally active medications through combinatorial chemistry

The literature of combinatorial chemistry is reviewed with particular attention paid to considerations of absorption, distribution, metabolism and excretion in the design and evaluation of libraries containing drug-like molecules. Published libraries are evaluated in particular for the likelihood that the products would possess oral bioavailability.

Will combinatorial chemistry deliver real medicines?

Over the next decade, the impact of library synthesis will play a major role in shortening the lead optimization phase of drug discovery. The prognosis for combinatorial chemistry to discover fundamentally different new classes of therapeutically active small molecules against some of the more difficult biological targets is less certain. Expectations are high because the technology potentially allows us to sample available drug space by synthesizing all possible small molecule ligands (variously estimated to be between 10(30)-10(50) compounds). Some caution is advised, however, since, despite recent increases in high-throughput screening of substantially greater numbers of synthetic compounds and natural products, we are not routinely finding a plethora of new structures. The outcome may be that combinational chemistry offers us the ability to work faster on finding ligands for well-established tractable targets, such as G-protein-coupled receptors, ion channels or proteases, rather than, say, the more complex protein-protein interactions which from the majority of targets in signal transduction pathways.