A new mode of reactivity of N-methoxy-N-methylamides with strongly basic reagents

Base-mediated homologative rearrangement of nitrogen-oxygen bonds of N-methyl-N-oxyamides

Due to the well known reactivity of C(O)-N functionalities towards canonical C1-homologating agents (e.g. carbenoids, diazomethane, ylides), resulting in the extrusion of the N-centered fragment en route to carbonyl compounds, formal C1-insertions within N-O bonds still remain obscure. Herein, we document the homologative transformation of N-methyl-N-oxyamides - with high tolerance for diverse O-substituents - into N-acyl-N,O-acetals. Under controlled basic conditions, the N-methyl group of the same starting materials acts as a competent precursor of the methylene synthon required for the homologation. The logic is levered on the formation of an electrophilic iminium ion (via N-O heterolysis) susceptible to nucleophilic attack by the alkoxide previously expulsed. The procedure documents genuine chemocontrol and flexibility, as judged by the diversity of substituents placed on both amide and nitrogen linchpins. The mechanistic rationale was validated through experiments conducted on D-labeled materials which unambiguously attributed the origin of the methylene fragment to the N-methyl group of the starting compounds.

Highly Reactive Hydrocarbon Soluble Alkylsodium Reagents for Benzylic Aroylation of Toluenes using Weinreb Amides

Deaggregating the alkyl sodium NaCH₂ SiMe₃ with polydentate nitrogen ligands enables the preparation and characterisation of new, hydrocarbon soluble chelated alkylsodium reagents. Equipped with significantly enhanced metalating power over their organolithium counterparts, these systems can promote controlled sodiation of weakly acidic benzylic C-H bonds from a series of toluene derivatives under mild stoichiometric conditions. This has been demonstrated through the benzylic aroylation of toluenes with Weinreb amides, that delivers a wide range of 2-arylacetophenones in good to excellent yields. Success in isolating and determining the structures of key organometallic intermediates has provided useful mechanistic insight into these new sodium-mediated transformations.

Synthesis of 13C/19F/2H labeled indoles for use as tryptophan precursors for protein NMR spectroscopy

Synthesis of indoles labeled with 13C-1H and 13C-19F spin pairs is described. All syntheses utilize inexpensive carbon-13C dioxide as the 13C isotope source. Ruthenium-mediated ring-closing metathesis is the key step in construction of the 13C containing indole carbocycle. Fluorine is introduced via electrophilic fluorination at the 7-position and via palladium-mediated cross-coupling at the 4-position. Indole and fluoroindoles are viable tryptophan precursors for in vivo protein expression. We show that they are viable also in in vitro protein synthesis using standard E. coli S30 extracts. Incorporation of the synthesized 13C-1H and 13C-19F spin pair labeled tryptophans into proteins enables high-resolution and high-sensitivity nuclear magnetic resonance (NMR) spectroscopy.

Dealkoxylation of N-alkoxyamides without an external reductant driven by Pd/Al cooperative catalysis

Lewis acid-assisted palladium-catalysed dealkoxylation of N-alkoxyamides has been realised in the absence of an external reductant.

Synthesis of benzo[a]carbazoles via cyanide-catalyzed imino-Stetter reaction/Friedel–Crafts reaction sequence

A new sequential protocol for the synthesis of benzo[a]carbazoles was developed via the cyanide-catalyzed imino-Stetter reaction followed by Friedel–Crafts reaction.

The Hancock Alkaloids (-)-Cuspareine, (-)-Galipinine, (-)-Galipeine, and (-)-Angustureine: Asymmetric Syntheses and Corrected 1H and 13C NMR Data

The asymmetric syntheses of all members of the Hancock alkaloid family based upon a 2-substituted N-methyl-1,2,3,4-tetrahydroquinoline core are delineated. The conjugate addition of enantiopure lithium N-benzyl- N-(α-methyl- p-methoxybenzyl)amide to 5-( o-bromophenyl)- N-methoxy- N-methylpent-2-enamide is used to generate the requisite C-2 stereogenic center of the targets, while an intramolecular Buchwald-Hartwig coupling is used to form the 1,2,3,4-tetrahydroquinoline ring. Late-stage diversification completes construction of the C-2 side chains. Thus, (-)-cuspareine, (-)-galipinine, (-)-galipeine, and (-)-angustureine were prepared in overall yields of 30%, 28%, 15%, and 39%, respectively, in nine steps from commercially available 3-( o-bromophenyl)propanoic acid in all cases. Unambiguously corrected ¹H and ¹³C NMR data for the originally isolated samples of (-)-cuspareine, (-)-galipinine, and (-)-angustureine are also reported, representing a valuable reference resource for these popular synthetic targets.

A Diverse and Versatile Regiospecific Synthesis of Tetrasubstituted Alkylsulfanylimidazoles as p38α Mitogen-Activated Protein Kinase Inhibitors

An alternative strategy for the synthesis of 1-aryl- and 1-alkyl-2-methylsulfanyl-4-(4-fluorophenyl)-5-(pyridin-4-yl)imidazoles as potential p38α mitogen-activated protein kinase inhibitors is reported. The regioselective N-substitution of the imidazole ring was achieved by treatment of α-aminoketones with different aryl or alkyl isothiocyanates. In contrast to previously published synthesis routes starting from 2-amino-4-methylpyridine, the presented route is characterized by a higher flexibility and a lower number of steps. This strategy was also applied to access 1-alkyl-2-methylsulfanyl-5-(4-fluorophenyl)-4-(pyridin-4-yl)imidazoles in six steps starting from 2-chloro-4-methylpyridine.

The dual reactivity of Weinreb amides applied to the late-stage divergent functionalisation of meso pyrrolidines

The dual reactivity of Weinreb amides was exploited to prepare diversified symmetrical and dissymmetrical 2,5-disubstituted pyrrolidines from simple building blocks.

Barbier Continuous Flow Preparation and Reactions of Carbamoyllithiums for Nucleophilic Amidation

An ambient temperature continuous flow method for nucleophilic amidation and thioamidation is described. Deprotonation of formamides by lithium diisopropylamine (LDA) affords carbamoyllithium intermediates that are quenched in situ with various electrophiles such as ketones, allyl bromides, Weinreb and morpholino amides. The nature of the reactive lithium intermediates and the thermodynamics of the metalation were further investigated by ab initio calculations and kinetic experiments.

Direct asymmetric syntheses of chiral aldehydes and ketones via N-acyl chiral auxiliary derivatives including chiral Weinreb amide equivalents

This article covers N-acyl chiral auxiliary-based approaches to the asymmetric synthesis of enantiopure aldehydes and ketones. The use of diastereoisomerically pure N-acyl derivatives of chiral auxiliaries (including chiral Weinreb amide equivalents) and their conversion to the corresponding enantiopure aldehydes and ketones in a single synthetic operation by treatment with a hydride reducing agent or an organometallic reagent, respectively, are highlighted.

Efficient preparation of α-ketoacetals

The Weinreb amides 2a,b were prepared from the α,α-dimethoxyacetic acids 1c,d. A number of representative nucleophilic additions (RMgX and RLi) on 2 afforded α-ketoacetals 3a–j in 70–99% yield. These compounds represent a versatile arrangement of functional groups of significant synthetic value, as demonstrated in the synthesis of (±)-salbutamol.

Preparation of bifunctional isocyanate hydroxamate linkers: Synthesis of carbamate and urea tethered polyhydroxamic acid chelators

Two novel bifunctional N-methylhydroxamate-isocyanate linkers 20 and 21 were prepared in good yield and high purity from the corresponding amine salts using a biphasic reaction with phosgene. The facile ring opening reaction of N-Boc lactams using the anion of O-benzylhydroxylamine gave the protected amino hydroxamates 6a and 6c in good yields. The selective methylation of the hydroxamate nitrogen in the presence of the N-Boc group in these intermediates could be readily accomplished. The utility of the linkers was clearly demonstrated by the synthesis of the carbamate-tethered trishydroxamic acid 27 and the urea-tethered 29.

Asymmetric synthesis of anti-α-substituted β-amino ketones from sulfinimines

Previously unknown, enantiopure, β-amino ketones were prepared in modest yield by addition of lithium reagents to N-sulfinyl anti-α-substituted β-amino Weinreb amides. Grignard reagents failed to add to these Weinreb amides in contrast to the syn-α-substituted isomers which did. The anti-α-substituted β-amino Weinreb amides were prepared by addition of LiN(OMe)Me to the corresponding N-sulfinyl anti-α-substituted β-amino esters because α-alkylation of N-sulfinyl β-amino Weinreb amide enolates resulted in poor diastereoselectivities.

Stereoselective synthesis of 2-dienyl-substituted piperidines using an η4-dienetricarbonyliron complex as the stereocontrolling element in a double reductive amination cascade

In the presence of NaBH(OAc)(3), a 1,5-keto-aldehyde, contained within a side-chain of an eta(4)-dienetricarbonyliron complex, undergoes a double reductive amination sequence with a series of primary amines, to provide the corresponding piperidine products in good to excellent yield. The dienetricarbonyliron complex functions as a powerful chiral auxiliary in this cascade process, exerting complete control over the stereoselectivity of the reaction, with the formation of a single diastereoisomeric product. The sense of stereoinduction has been confirmed by X-ray crystallography. Removal of the tricarbonyliron moiety can be effected with CuCl(2) to afford the corresponding 2-dienyl-substituted piperidine in excellent yield. Attempted extension of this cyclisation strategy to the corresponding azepane ring system using a 1,6-keto-aldehyde as the cyclisation precursor was unsuccessful; in this case, the reaction stopped after a single reductive amination on the aldehyde to provide an acyclic keto-amine product.

Alkyl, Unsaturated, (Hetero)aryl, and N-Protected α-Amino Ketones by Acylation of Organometallic Reagents

Stable and easily accessible N-acylbenzotriazoles, derived from a variety of aliphatic, unsaturated, (hetero)aromatic, and N-protected alpha-amino carboxylic acids, were reacted with Grignard and heteroaryllithium reagents to afford corresponding ketones in good to excellent yields.

OSW saponins: facile synthesis toward a new type of structures with potent antitumor activities

[reaction: see text] OSW saponins, featuring a 16beta,17alpha-dihydroxycholest-22-one aglycon and an acylated beta-D-xylopyranosyl-(1-->3)-alpha-L-arabinopyranosyl residue attached to the 16-hydroxyl group, have recently been discovered from a group of lily plants, which show potent antitumor activities with a novel mechanism of action. This paper describes an aldol approach to the stereoselective construction of the 16alpha,17alpha-dihydroxycholest-22-one structure from 16alpha-hydroxy-5-androsten-17-ones and propionates. Elaboration of the aldol adducts toward OSW-1, involving installation of the isoamyl ketone side chain, inversion of the 16-hydroxyl configuration, and selective protection of the C22-oxy function, has been explored and accomplished. In particular, the present route was found convenient for the synthesis of OSW saponin analogues with a C22-ester side chain. Thus, the 23-oxa-analogue of OSW-1 (40) was prepared starting from the industrial dehydroisoandrosterone (1) in a linear eight-step sequence and in 26% overall yield. Analogues with a variety of modified side chains were prepared, via aldol condensation with propionates of varying length, thiopropionate, and acetate (for preparation of 68-75) or via aminolysis of the 22,16-lactone 26 (for preparation of the 23-N-analogues). Cross metathesis (CM) reaction was also found feasible for modification at the final stage from C22-allyl ester 70. Valuable structure-activity relationships (SAR), together with the practical synthetic approach, have thus been provided to set a new stage for further studies on this new type of antitumor structures.

Palladium-Catalyzed Intermolecular α-Arylation of Zinc Amide Enolates under Mild Conditions

The intermolecular alpha-arylation and vinylation of amides by palladium-catalyzed coupling of aryl bromides and vinyl bromides with zinc enolates of amides is reported. Reactions of three different types of zinc enolates have been developed. The reactions of aryl halides occur in high yields with isolated Reformatsky reagents generated from alpha-bromo amides, with Reformatsky reagents generated in situ from alpha-bromo amides, and with zinc enolates generated by quenching lithium enolates of amides with zinc chloride. This use of zinc enolates, instead of alkali metal enolates, greatly expands the scope of amide arylation. The reactions occur at room temperature or 70 degrees C with bromoarenes containing cyano, nitro, ester, keto, fluoro, hydroxyl, or amino functionality and with bromopyridines. Moreover, the reaction has been developed with morpholine amides, the products of which are precursors to ketones and aldehydes. The arylation of zinc enolates of amides was conducted with catalysts bearing the hindered pentaphenylferrocenyl di-tert-butylphosphine (Q-phos) or the highly reactive, dimeric, Pd(I) complex [[P(t-Bu)3]PdBr]2.

Nitrenium ion azaspirocyclization-spirodienone cleavage: a new synthetic strategy for the stereocontrolled preparation of highly substituted lactams and N-hydroxy lactams

[reaction: see text] Although 1,4-cyclohexadienes 2, obtained through the Birch reduction of arenes 1, have found widespread use as masked beta-oxo carbonyl synthons 3, the possibility that 2,5-cyclohexadienones 5 might also be employed to the same end has been overlooked despite their ready availability. As part of our ongoing investigation of the synthetic chemistry of nitrenium ions, we have developed a novel and efficient strategy for the stereoselective preparation of di- and trisubstituted azetidinone, pyrrolidinone, and piperidinone derivatives, which features the ozonolytic cleavage of azaspirocyclic 2,5-cyclohexadienones 12. For example, ozonolysis of spirodienone 12c in CH2Cl2 and reductive workup with dimethyl sulfide generated unstable beta-formyl ester 21, whereas cleavage in MeOH followed by reduction with thiourea led to hemiacetal 22. While both 21 and 22 partially decompose upon exposure to silica gel, they can be trapped in situ, with a variety of weakly basic nucleophiles, to usefully substituted products. The requisite spirodienone substrates are readily accessible through the nitrenium ion cyclization of alkyl omega-arylhydroxamates 10, which proceeds with moderate to high diastereoselectivity.

Evaluation of Solution and Solid-Phase Approaches to the Synthesis of Libraries of α,α-Disubstituted-α-acylaminoketones

Solid phase, solution, and hybrid approaches to the synthesis of small focused libraries of alpha,alpha-disubstituted-alpha-acylaminoketones have been explored. Solution and hybrid approaches that used support-bound reagents and scavenger resins were the most productive.

Asymmetric synthesis and applications of beta-amino Weinreb amides: asymmetric synthesis of (S)-coniine

Conjugate addition of lithium (S)-N-benzyl-N-alpha-methylbenzylamide to a range of alpha, beta-unsaturated Weinreb amides proceeds with high levels of diastereoselectivity (>95% de). The beta-amino Weinreb amide products may be transformed into beta-amino ketones via reactions with Grignard reagents, while treatment with DIBAL-H furnishes beta-amino aldehydes. Trapping of the aldehyde via Wadsworth-Emmons reaction and subsequent manipulation offers an efficient route to homochiral delta-amino acid derivatives and 2-substituted piperidines. The application of this methodology for the synthesis of (S)-coniine is demonstrated.

Stanna-Brook Rearrangement of Carboxylic Acid Derivatives. Synthetic Utility and Mechanistic Studies

[reaction: see text] The reaction of R(3)SnLi with carboxylic acid derivatives proceeds through a novel, very fast stanna-Brook rearrangement that generates alpha-alkoxyorganolithium compounds as intermediates. The outcome of these reactions depends on the nature of the carboxyl derivatives. Reaction of R(3)SnLi with ester derivatives gives rise to coupled products through a novel C-C bond formation reaction. Experimental evidence of the detailed reaction mechanism is provided.

Synthesis and reactions of 2-substituted ethyl N-alkylmalonylhydroxamic acids

Alkylation of O-silylated N-alkylmalonylhydroxamic acids provides a method for the synthesis of 2-substituted N-alkylmalonyl hydroxamic acids. The substituent at C-2 does not materially change the chemistry of the alpha-lactam intermediates produced from them. They can be converted to unsymmetric ureas and hydantoins in high yields. The addition of unsaturated substituents at C-2 is used to produce cyclic ureas containing medium rings via RCM reactions.

N-acyl-5,5-dimethyloxazolidin-2-ones as latent aldehyde equivalents

A study of the properties of N-hydrocinnamoyl- derivatives of 5,5-dimethyloxazolidin-2-one, 4,4-dimethyloxazolidin-2-one and oxazolidin-2-one upon hydride reduction with DIBAL-H demonstrates that the 5,5-dimethyl-group is essential for inhibition of endocyclic nucleophilic attack. For instance, treatment of N-hydrocinnamoyl-5,5-dimethyloxazolidin-2-one with DIBAL-H results in the selective formation of the stable N-1'-hydroxyalkyl derivative which may be regarded as a masked hydrocinnamaldehyde equivalent, as treatment under basic conditions affords the parent aldehyde in excellent yield. Treatment of N-hydrocinnamoyl-4,4-dimethyloxazolidin-2-one with DIBAL-H under identical conditions affords a complex mixture of products, including the formate ester product of endocyclic cleavage. As an alternate strategy, DIBAL-H reduction of straight chain and branched N-acyl-5,5-dimethyloxazolidin-2-one derivatives, followed by a Horner-Wadsworth-Emmons reaction affords alpha,beta-unsaturated esters in good yields. Branching alpha- to the exocyclic carbonyl in N-acyl-oxazolidinones inhibits DIBAL-H reduction, but this can be overcome by precomplexation with ZnCl2, with subsequent fragmentation generating either the corresponding aldehyde or alpha,beta-unsaturated esters. The addition of ZnCl2 has been shown to increase the diastereoselectivity observed in Wadsworth-Horner-Emmons reactions of lithiated phosphonates.

Optimization of alpha-acylaminoketone ecdysone agonists for control of gene expression

Fifteen new alpha-acylaminoketones were prepared by four different routes in an initial effort to optimize the potency of these compounds as ecdysone agonists. The compounds were assayed in mammalian cells expressing the ecdysone receptors from Bombyx mori (BmEcR) and Choristoneura fumiferana (CfEcR) for their ability to cause expression of a reporter gene downstream of an ecdysone response element. A new alpha-acylaminoketone was identified which had activity equal to that of the standard dibenzoylhydrazine ecdysone agonist GS()-E in the assay based on CfEcR.

Synthesis and SAR of alpha-acylaminoketone ligands for control of gene expression

A lead discovery library and a follow-up focused library of alpha-acylaminoketones were designed based on known dibenzoylhydrazine ecdysone agonists, including GS(TM)-E. The compounds were assayed in mammalian cells expressing the ecdysone receptor from Bombyx mori for their ability to cause expression of a reporter gene downstream of an ecdysone response element. The most potent alpha-acylaminoketones were comparable to GS(TM)-E in this assay.