Synthesis of beta-substituted alpha-amino acids via Lewis acid promoted enantioselective radical conjugate additions

Primary nitro compounds: progress in the synthesis of isoxazoles by condensation with aldehydes or activated ketones

Among the known strategies directed towards the synthesis of isoxazole derivatives, the reactions of aldehydes with primary nitro compounds deserve a comprehensive treatment, including the historical development as well as the more recent applications. The reactions of aldehydes with primary nitro compounds in a 1 : 2 molar ratio have been shown to lead to isoxazoline-N-oxides or isoxazole derivatives, via β-dinitro derivatives. Several modifications of the process allowed the formation of products bearing substituents at various positions of the heterocyclic ring with control of regioselectivity. Ketones are reported to react with primary nitro compounds, only if activated (β-diketones, α-nitroketones, or strained ketones), to give isoxazole derivatives. Symmetric 2,4-dinitroglutarates formed from aromatic aldehydes and nitroacetate undergo ring closure to form isoxazole derivatives or, according to reaction conditions, 5-hydroxy-6-oxo-4-aryl-6H-1,2-oxazine-3-carboxylates ("oxazinones"), by loss of alcohol instead of water. Isoxazole-4-carbaldehydes are obtained by the reaction of 3-oxetanone with primary nitro compounds.

Dibenzofuran-4,6-bis(oxazoline) (DBFOX). A novel trans-chelating bis(oxazoline) ligand for asymmetric reactions

The trans-chelating bis(oxazoline) ligand (R,R)-4,6-dibenzofurandiyl-2,2'-bis(4-phenyloxazoline) [(R,R)-DBFOX/Ph] coordinates metal ions to give C2-symmetric complexes which effectively catalyze a variety of asymmetric reactions. (R,R)-DBFOX/Ph·Ni(ClO4)2·3H2O, whose crystal structure is octahedral with three aqua ligands, can be stored under air for several months without loss of catalytic activity and promotes highly enantioselective reactions even in the presence of excess amounts of water, alcohols, amines, and acids. The complex shows remarkable chiral amplification in asymmetric Diels-Alder (DA) reactions. This review focuses on enantioselective reactions catalyzed by (R,R)-DBFOX/Ph·metal complexes.

Umpolung Reaction of α-Imino Thioesters and the Subsequent C-C Bond Formation with the Unexpected Alkylthio Rearrangement

An umpolung reaction of the α-imino thioester was examined, and we found that α-imino thioesters were more effective substrates for the umpolung N-alkylation than conventional α-imino esters and they gave N-alkylated amino thioesters in high yields under mild reaction conditions in a short time. A new type of C-C bond formation followed by an unexpected rearrangement of the alkylthio group took place with the unsaturated ketones to afford the β-alkylthio-α-amino thioesters in high yields with good diastereoselectivity.

Unusual reactivity of nitronates with an aryl alkyl carbonate: synthesis of α-amino esters

The monoanions of nitroalkanes are ambident nucleophiles that react with carbonate electrophiles through the oxygen atom. Products arising from reactivity at the carbon atom will yield α-nitro esters, which are precursors for α-amino esters. We demonstrate this in the reactions of nitroalkanes with benzyl phenyl carbonate and DABCO where α-nitro esters are obtained instead of nitrile oxides. The products are readily reduced to α-amino esters. This pathway could be a safe alternative to the Strecker reaction.

Total synthesis of the congested propellane alkaloid (-)-acutumine

The enantioselective total synthesis of (-)-acutumine is described. The synthetic strategy was inspired by the premise that the cyclohexenone ring could be derived from an aromatic precursor. After successful construction of a propellane model system, an initial attempt to prepare the spirocyclic subunit was thwarted by incorrect regioselectivity in a radical cyclization. A second-generation approach involving a radical-polar crossover reaction was successful, and the chemistry developed in the aforementioned model system was then applied to synthesize the natural product. Key reactions included a phenolic oxidation, a diastereoselective ketone allylation utilizing Nakamura's chiral allylzinc reagent, an anionic oxy-Cope rearrangement, an acid-promoted cyclization of a secondary amine onto an α,β-unsaturated ketal, and a regioselective methyl enol etherification of a 1,3-diketone.

Nickel-catalyzed substitution reactions of propargyl halides with organotitanium reagents

A simple and mild catalytic coupling reaction of propargyl halides with organotitanium reagents is reported.

C-H activation: a complementary tool in the total synthesis of complex natural products

The recent advent of transition-metal mediated C-H activation is revolutionizing the synthetic field and gradually infusing a "C-H activation mind-set" in both students and practitioners of organic synthesis. As a powerful testament of this emerging synthetic tool, applications of C-H activation in the context of total synthesis of complex natural products are beginning to blossom. Herein, recently completed total syntheses showcasing creative and ingenious incorporation of C-H activation as a strategic manoeuver are compared with their "non-C-H activation" counterparts, illuminating a new paradigm in strategic synthetic design.

Regioselective base-free intermolecular aminohydroxylations of hindered and functionalized alkenes

Regioselective base-free intermolecular aminohydroxylations of functionalized trisubstituted and 1,1-disubstituted alkenes employing benzoyloxycarbamate 3a and catalytic OsO(4) are described. In all cases, the more substituted alcohol isomer is favored. Sluggish reactions could be promoted by gentle heating, the use of amine ligands, or increased catalyst loadings. A competitive rearrangement was observed with a secondary allylic alcohol substrate. The adducts serve as useful precursors to dehydroamino acids.

Experimental evidence for the all-up reactive conformation of chiral rhodium(II) carboxylate catalysts: enantioselective synthesis of cis-cyclopropane alpha-amino acids

Useful empirical insights onto the enantioinduction process of chiral Rh(II)-carboxylate catalysts are described in the first catalytic asymmetric cyclopropanation of alkenes with alpha-nitro diazoacetophenones. X-ray, solution NMR, and reactivity studies made on these complexes suggest that the level of asymmetric induction strongly depends on their active symmetry, which in turn relies on the nature of the chiral ligands' substituents. The catalyst's 'All Up' reactive conformation resulted in being necessary to obtain good stereoselectivity, and the resulting products are shown to be key intermediates in a concise synthesis of highly enantioenriched cis-cyclopropane alpha-amino acids.

Total synthesis of the antimitotic bicyclic peptide celogentin C

An account of the total synthesis of celogentin C is presented. A right-to-left synthetic approach to this bicyclic octapeptide was unsuccessful due to an inability to elaborate derivatives of the right-hand ring. In the course of these efforts, it was discovered that the mild Braslau modification of the McFadyen-Stevens reaction offers a useful method of reducing recalcitrant esters to aldehydes. A left-to-right synthetic strategy was then examined. The unusual Leu-Trp side-chain cross-link present in the left-hand macrocycle was fashioned via a three-step sequence comprised of an intermolecular Knoevenagel condensation, a radical conjugate addition, and a SmI(2)-mediated nitro reduction. A subsequent macrolactamization provided the desired ring system. The high yield and concise nature of the left-hand ring synthesis offset the modest diastereoselectivity of the radical conjugate addition. Formation of the Trp-His side-chain linkage characteristic of the right-hand ring was then accomplished by means of an indole-imidazole oxidative coupling. Notably, Pro-OBn was required as an additive in this reaction. Detailed mechanistic investigations indicated that Pro-OBn moderates the concentration of NCS in the reaction mixture, thereby minimizing the production of an undesired dichlorinated byproduct. The natural product was obtained after macrolactamization and deprotection. The chemical shifts of the imidazole hydrogen atoms exhibited significant dependence on temperature, concentration, and pH. Antitumor screening indicated that celogentin C inhibits the growth of some cancer cell lines.

Second-generation DBFOX ligands for the synthesis of beta-substituted alpha-amino acids via enantioselective radical conjugate additions

A set of second-generation DBFOX ligands possessing extended aryl or benzyl-type groups was synthesized. The requisite amino alcohols were either commercially available (DBFOX/Bn) or constructed via Sharpless asymmetric aminohydroxylation (DBFOX/Nap, DBFOX/ t-BuPh, DBFOX/Pip) or phase-transfer-catalyzed asymmetric alkylation (DBFOX/MeNap). Complexes of the ligands with Mg(NTf2)2 were evaluated as promoters of enantioselective radical conjugate additions to alpha,beta-unsaturated alpha-nitro amides and esters. Reactions employing the DBFOX/Nap ligand exhibited improved enantioselectivity relative to previously published additions mediated by DBFOX/Ph. However, the relatively modest increase in diastereomeric ratio suggests that our substrate-Lewis acid binding model, which was formulated based on results from DBFOX/Ph-promoted radical conjugate additions, is in need of revision.