Stereochemical Aspects in the Asymmetric Michael Addition of Chiral Imines to Substituted Electrophilic Alkenes

Synthesis and reactivity of hydroindole enelactams leading to densely functionalized scaffolds

The 5-endo-trig radical cyclization of N-benzyl-N-[(2-substituted)cycloalkenyl] trichloroacetamides (tetrasubstituted enamides) using Bu3SnH and AIBN is a reliable synthetic procedure giving access to 3a-methyl- and 3a-methoxycarbonyl enelactams. The substrate-controlled diastereoselective enolate alkylation of these enelactams resulted in the synthesis of a set of 3-substituted derivatives that upon reduction furnished polyfunctionalized cis-octahydroindoles. The latter building blocks, which embody three consecutive stereocenters at C-3, C-3a, and C-7a, were also synthesized through an initial reductive radical cyclization using (carbo-substituted)dichloroacetamides.

Straightforward access to densely substituted chiral succinimides through enantioselective organocatalyzed Michael addition of α-alkyl-cyclic ketones to maleimides

A simple, cheap and unchanged organocatalytic system provides efficient access to densely substituted chiral succinimides bearing QCC–TCC vicinal stereocenters from unreactive α-alkyl cyclic ketones and maleimides.

Stereoselective Michael additions on α-aminoacrylates as the key step to an l-Oic analogue bearing a quaternary stereocenter

A highly stereoselective synthesis of an l-Oic-analogue bearing a tetrasubstituted sterocenter is reported.

Construction of Stereogenic α,α-Disubstituted Cycloalkanones via 1° Amine Thiourea Dual Catalysis: Experimental Scope and Computational Analyses

The mechanistic exploration and an expanded experimental discussion of the organocatalyzed, asymmetric Pfau-d'Angelo reaction by exploiting a bifunctional 1° amine thiourea catalyst system is disclosed. Notable breadth in substrate scope has been demonstrated on both the cyclic ketone moiety and the α,β-unsaturated electrophile. Exploration into the matched and mismatched selectivity of this process with a ketone containing pre-existing stereocenters has been demonstrated. Computational analyses of the reaction mechanism are reported. In concert with kinetic isotope effect (KIE) experiments, these computational results provide a detailed understanding of the likely mechanism, including the aspects of the organocatalyst scaffold that are critical for stereoselectivity.

Concise synthesis of the tricyclic skeleton of crotobarin and crotogoudin via a gold-catalyzed cycloisomerization reaction

A concise synthesis of the tricyclic skeleton of crotobarin and crotogoudin via a gold-catalyzed 1,6-enyne cycloisomerization reaction is reported.

Ring-closing metathesis in the synthesis of BC ring-systems of taxol

BC ring-systems of taxol with different or no protecting group for the C1,C2-diol moiety have been efficiently synthesized. The eight-membered B ring is formed by a ring-closing metathesis reaction (RCM) between the C10 and C11 carbon atoms. The influence of the 1,2-diol protecting group on the RCM reaction has been studied in detail.

Enamides and enecarbamates as nucleophiles in stereoselective C-C and C-N bond-forming reactions

Because the backbone of most of organic compounds is a carbon chain, carbon-carbon bond-forming reactions are among the most important reactions in organic synthesis. Many of the carbon-carbon bond-forming reactions so far reported rely on nucleophilic attack of enolates or their derivatives, because those nucleophiles can be, in general, readily prepared from the corresponding carbonyl compounds. In this Account, we summarize the recent development of reactions using enamide and enecarbamate as a novel type of nucleophile. Despite their ready availability and their intrinsic attraction as a synthetic tool that enables us to introduce a protected nitrogen functional group, enamide and enecarbamate have rarely been used as a nucleophile, since their nucleophilicity is low compared with the corresponding metal enolates and enamines. A characteristic of enamides and enecarbamates is that those bearing a hydrogen atom on nitrogen are relatively stable at room temperature, while enamines bearing a hydrogen atom on nitrogen are likely to tautomerize into the corresponding imine form. Enamides and enecarbamates can be purified by silica gel chromatography and kept for a long time without decomposition. During the investigation of nucleophilic addition reactions using enamides and enecarbamates, it has been revealed that enamides and enecarbamates bearing a hydrogen atom on nitrogen react actually as a nucleophile with relatively reactive electrophiles, such as glyoxylate, N-acylimino ester, N-acylimino phosphonate, and azodicarboxylate, in the presence of an appropriate Lewis acid catalyst. Those bearing no hydrogen atom on nitrogen did not react at all. The products initially obtained from the nucleophilic addition of enamides and enecarbamates are the corresponding N-protected imines, which can be readily transformed to important functional groups, such as ketones by hydrolysis and N-protected amines by reduction or nucleophilic alkylation. In the nucleophilic addition reactions of enamides and enecarbamates to aldehydes, it was unveiled that the reaction proceeds stereospecifically, that is, (E)-enecarbamate gave anti product and (Z)-enecarbamate afforded syn product with high diastereoselectivity (>97/3). This fact can be rationalized by consideration of a concerted reaction pathway via a hydrogen-involved cyclic six-membered ring transition state. In the addition reactions to N-acylimino phosphonates, much higher turnover frequency was observed when enamides and enecarbamates were used as a nucleophile than was observed when silicon enolates were used. When silicon enolates were used, the intermediates bearing a strong affinity for the catalyst inhibited catalyst turnover, resulting in low enantioslectivity because of the dominance of the uncatalyzed racemic pathway. In the case of nucleophilic addition of enamides and enecarbamate, however, a fast intramolecular hydrogen transfer from the enecarbamate nitrogen may prevent the intermediate from trapping the catalyst for a long time, to afford the product with a high enantioselectivity. In conclusion, enamides and enecarbamates, although originally employed as just N-analogues to silicon enolates, have emerged as remarkably useful nucleophiles in a variety of Lewis acid-catalyzed reactions.

Stereochemical analysis of N-cyclohexylidene-N-(1-phenylethyl)amine derivatives

The configurational properties of a series of cyclohexylidene imines are discussed on the basis of their 1H, 13C and 15N NMR spectral data.

Green Chemistry Strategies Using Crystal-to-Crystal Photoreactions: Stereoselective Synthesis and Decarbonylation of trans-α,α‘-Dialkenoylcyclohexanones

This paper describes steps to develop green chemistry strategies to prepare compounds with adjacent quaternary centers by stereospecific photodecarbonylation of crystalline ketones bearing radical-stabilizing alkenyl substituents in their alpha-positions. Crystals of trans-2,6-dimethyl-2,6-di(benzyloxycarbonyl-trans-ethenyl)-cyclohexanone, trans-2, prepared by double Michael addition of benzyl propiolate to 2,6-dimethyl-cyclohexanone iso-butylimine (1), were investigated along with those of the dicarboxylic acid (trans-3) and a dibenzylammonium salt derivative (trans-4) to establish the best substrates for the solid-state reaction. While reactions of the ester and the acid lose CO to give radical combination products with high selectivity at low conversions, the crystalline salt gave a similar product in >97% yield. Explorative reactions carried out under sunlight and in semipreparative (1.5 g) scale highlight the potential of reactions in crystals as a viable strategy for the development of green chemistry.

Easy approach to chiral Michael adducts by Eu+3-catalyzed conjugate addition

Michael addition of enantiopure N-acetoacetyl-oxazolidin-2-ones is shown to take place in the presence of catalytic amounts of Eu(+3) salts and complexes in high yields, very reduced reaction times, and moderate diastereoselectivity. The level of diastereoselectivity can be significantly enhanced by the suitable exploitation of the easy epimerization of the adducts in the presence of silica gel.

Solid-phase synthesis of 1,2,3,4-tetrahydro-2-pyridones via aza-annulation of enamines

An efficient solid-phase approach has been developed to prepare nitrogen heterocycles with a 1,2,3,4-tetrahydro-2-pyridone core via aza-annulation of enamines. Immobilized enamines were prepared from the reaction of primary amines with propynoic acid derivatives or ketones. Aza-annulation reactions were carried out by reacting resin-bound enamines with symmetrical alpha,beta-unsaturated acid anhydrides or alpha,beta-unsaturated acids in the presence of DPPA and TEA. The annulation products were isolated in good to high crude yields. Influence of sterically hindered amines as well as alpha- and beta-substituted acrylic acid derivatives on the annulation reaction was also investigated.

Conjugate additions of 1-propenylphosphonates to metalated Schöllkopf's bis-lactim ether: stereocontrolled access to 2-amino-3-methyl-4-phosphonobutanoic acids

Diastereoselectivity in the conjugate addition of metalated Schöllkopf's bis-lactim ethers 5a-e to (E)- and (Z)-1-propenylphosphonates 4a,b was studied experimentally and theoretically and utilized to achieve a direct and stereocontrolled synthesis of all four diastereoisomers of 2-amino-3-methyl-4-phosphonobutanoic acid, 6a,b and their enantiomers. The relative stereochemistry was assigned from an NMR study of cyclic derivatives 13a,b. According to semiempirical calculations, both in vacuo (PM3) or a dielectric continuum (PM3/COSMO), initial lithium-phosphoryl coordination, without an energy barrier, to form a solvated chelate complex is followed by the rate-determining reorganization to the 1,4-addition product through an eight-membered transition state. The translation of the Z,E geometry into a syn, anti configuration at the adducts originates from an orientational preference in the transition state for a compact disposition of the reaction partners.