Highly Enantioselective Preparation of Tertiary Alcohols and Amines by Copper-Mediated Diastereoselective Allylic SN2′ Substitutions

Stereoselective Sc(OTf)3 -Catalyzed Aldol Reactions of Disubstituted Silyl Enol Ethers of Aldehydes with Acetals

Facile and modular access to stereodefined disubstituted aldehyde-derived silyl enol ethers allowed their successful application in a stereoselective aldol reaction affording the products with excellent yields and diastereomeric ratios. The counter-intuitive stereochemical behavior of this Mukaiyama-aldol reaction is accounted for by a non-classical open transition state.

Harnessing the Power of the Asymmetric Grignard Synthesis of Tertiary Alcohols: Ligand Development and Improved Scope Exemplified by One-Step Gossonorol Synthesis

A series of N-substituted cyclohexyldiaminophenolic ligands for the asymmetric Grignard synthesis of tertiary alcohols is reported. The 2,5-dimethylpyrrole-decorated ligand led to improved enantioselectivities and broadened the scope of the methodology. As an exemplar, we report an unprecedented highly selective one-step synthesis of gossonorol in 93% ee, also constituting the shortest formal syntheses of natural products boivinianin B and yingzhaosu C.

Iron-Catalyzed Reductive Vinylation of Tertiary Alkyl Oxalates with Activated Vinyl Halides

We present herein a rare and efficient method for the creation of vinylated all carbon quaternary centers via Fe-catalyzed cross-electrophile coupling of vinyl halides with tertiary alkyl methyl oxalates. The reaction displays excellent functional group tolerance and broad substrate scope, which allows cascade radical cyclization and vinylation to afford complex bicyclic and spiral structural motifs. The reaction proceeds via tertiary alkyl radicals, and the putative vinyl-Br/Fe complexation appears to be crucial for activating the alkene and enabling a possibly concerted radical addition/C-Fe forming process.

An entry to non-racemic β-tertiary-β-amino alcohols, building blocks for the synthesis of aziridine, piperazine, and morpholine scaffolds

The synthesis of α-dialkyl-substituted non-racemic allyl alcohols and their transformation into enantiomerically enriched 1,1-dialkylated 1,2-aminoalcohols, aziridines, morpholines and piperazines is reported.

5-Carbohydrazide and 5-carbonylazide of pyrazolo[3,4-b]pyridines as reactive intermediates in the synthesis of various heterocyclic derivatives

5-Carbohydrazides and 5-carbonylazides of pyrazolo[3,4- b]pyridines are used to synthesize new heterocyclic derivatives. Some unexpected behaviors are observed in the reactions of the above two species. The structures of the obtained compounds are proved by spectroscopic studies together with elemental and X-ray structure analyses.

Zirconocene-Mediated Selective C-C Bond Cleavage of Strained Carbocycles: Scope and Mechanism

Several approaches using organozirconocene species for the remote cleavage of strained three-membered ring carbocycles are described. ω-Ene polysubstituted cyclopropanes, alkylidenecyclopropanes, ω-ene spiro[2.2]pentanes, and ω-ene cyclopropyl methyl ethers were successfully transformed into stereodefined organometallic intermediates, allowing an easy access to highly stereoenriched acyclic scaffolds in good yields and, in most cases, excellent selectivities. DFT calculations and isotopic labeling experiments were performed to delineate the origin of the obtained chemo- and stereoselectivities, demonstrating the importance of microreversibility.

The Curtius rearrangement: mechanistic insight and recent applications in natural product syntheses

The Curtius rearrangement is a versatile reaction in which a carboxylic acid can be converted to an isocyanate through an acyl azide intermediate under mild conditions. The resulting stable isocyanate can then be readily transformed into a variety of amines and amine derivatives including urethanes and ureas. There have been wide-ranging applications of the Curtius rearrangement in the synthesis of natural products and their derivatives. Also, this reaction has been extensively utilized in the synthesis and application of a variety of biomolecules. In this review, we present mechanistic studies, chemical methodologies and reagents for the synthesis of isocyanates from carboxylic acids, the conversion of isocyanates to amines and amine derivatives, and their applications in the synthesis of bioactive natural products and their congeners.

The synthesis of non-racemic β-alkyl-β-aryl-disubstituted allyl alcohols and their transformation into allylamines and amino acids bearing a quaternary stereocenter

Two approaches to chiral β-alkyl-β-aryl allyl alcohols and their transformation into non-racemic allylamines and amino acid derivatives bearing a quaternary stereocenter are described.

Sequential Ruthenium Catalysis for Olefin Isomerization and Oxidation: Application to the Synthesis of Unusual Amino Acids

How can you use a ruthenium isomerization catalyst twice? A ruthenium-catalyzed sequence for the formal two-carbon scission of allyl groups to carboxylic acids has been developed. The reaction includes an initial isomerization step using commercially available ruthenium catalysts followed by in situ transformation of the complex to a metal-oxo species, which is capable of catalyzing subsequent oxidation reactions. The method enables enantioselective syntheses of challenging α-tri- and tetrasubstituted α-amino acids including an expedient total synthesis of the antiepileptic drug levetiracetam.

The Synthesis of Chiral β,β-Diaryl Allylic Alcohols and Their Use in the Preparation of α-Tertiary Allylamines and Quaternary α-Amino Acids

An approach to nonracemic β,β-diarylsubstituted allyl alcohols is described. Their synthesis starts from l-lactic acid-derived propargyl alcohol, which is submitted to sequential Sonogashira/Suzuki or Sonagashira/Stille coupling reactions. Both approaches enable the synthesis of either (Z)- or (E)-allylic alcohols regarding the order of introducing coupling agents. The obtained allyl alcohols were applied in the synthesis of nonracemic α-tertiary allylamines via stereocontrolled cyanate-to-isocyanate sigmatropic rearrangement reactions of the corresponding allyl carbamates. The stereoselectivity of the process is controlled by the geometry of the double bond of the starting allyl derivative. As demonstrated, a rearrangement of (S,Z)-allyl carbamates provides (S)-teriary allylamines, whereas the transformation (S,E)-isomers leads to (R)-allylamines.

Cross-coupling of cyclopropanols: concise syntheses of indolizidine 223AB and congeners

A new synthetic method for indolizidine or pyrrolizidine alkaloids based on readily available and attractively functionalized cyclopropanols, as exemplified in concise syntheses of indolizidine (-)-223AB, its 3-epimer, (-)-indolizidine 239AB, and (-)-indolizidine 239CD, is reported. This work highlights the applications of SN2' alkylation and C-acylation of cyclopropanols to meet stereochemical challenges in natural product synthesis. Also included is diastereoselective cyclization of the resulting aminoallene adduct for bicyclic ring formation.

Allylic substitution for construction of a chiral quaternary carbon possessing an aryl group

Phenylcopper reagents derived from 2:1 PhMgBr/Cu(acac)2 and 3:1:1 PhMgBr/Cu(acac)2/ZnI2 were found to be highly reactive and regioselective in the allylic substitution of γ,γ-disubstituted secondary allylic picolinates designed for construction of a quaternary carbon, whereas the previous 2:1 ArMgBr/CuBr·Me2S reagent and that with ZnX2 were unsuccessful. The generality of the ArMgBr/Cu(acac)2 reagent was examined with enantiomerically enriched allylic picolinates, which furnished quaternary carbons with high efficiency in >92% regioselectivity and >97% chirality transfer. Two cyclohexanes with a quaternary carbon were synthesized by using these reagents.

Tandem application of C-C bond-forming reactions with reductive ozonolysis

Several variants of reductive ozonolysis, defined here as the in situ generation of aldehydes or ketones during ozonolytic cleavage of alkenes, are demonstrated to work effectively in tandem with a number of C-C bond-forming reactions. For reactions involving basic nucleophiles (1,2-addition of Grignard reagents, Wittig or Horner-Emmons olefinations, and directed aldol reactions of lithium enolates), the one-pot process offers a rapid and high-yielding alternative to traditional two-step protocols.

Design of structurally rigid trans-diamine-based Tf-amide organocatalysts with a dihydroanthracene framework for asymmetric conjugate additions of heterosubstituted aldehydes to vinyl sulfones

Asymmetric conjugate addition of α-heterosubstituted aldehydes such as α-amido and α-alkoxy aldehydes to vinyl sulfone was effected under the influence of structurally rigid trans-diamine-based Tf-amido organocatalyst (S,S)-2 with a dihydroanthracene framework to furnish α,α-dialkyl(amido)aldehydes and α,α-dialkyl(alkoxy)aldehydes with high enantioselectivity. The chiral efficiency of the structurally unique catalyst (S,S)-2 is apparent in comparison with (S,S)-1 and (S,S)-4 with similar functionality.

Copper-catalyzed gamma-selective allyl-alkyl coupling between allylic phosphates and alkylboranes

Copper-catalyzed allyl-alkyl coupling between allylic phosphates and alkylboranes, prepared by hydroboration of alkenes with 9-BBN-H, takes place with complete gamma- and E-selectivities and with preferential 1,3-anti stereochemistry. The reaction tolerates various functional groups in both the allylic phosphate and alkylborane. Catalytic mechanisms involving transmetalation between a trialkyl(alkoxo)borate and a copper(I) complex to form an alkylcopper(I) species are proposed.

Palladium-catalyzed gamma-selective and stereospecific allyl-aryl coupling between acyclic allylic esters and arylboronic acids

Reactions between acyclic (E)-allylic acetates and arylboronic acids in the presence of a palladium catalyst prepared from Pd(OAc)(2), phenanthroline (or bipyridine), and AgSbF(6) (1:1.2:1) proceeded with excellent gamma-selectivity to afford allyl-aryl coupling products with E-configuration. The reactions of alpha-chiral allylic acetates took place with excellent alpha-to-gamma chirality transfer with syn stereochemistry to give allylated arenes with a stereogenic center at the benzylic position. The reaction tolerated a broad range of functional groups in both the allylic acetates and the arylboronic acids. Furthermore, gamma-arylation of cinnamyl alcohol derivatives afforded gem-diarylalkane derivatives containing an unconjugated alkenic substituent. The synthetic utility of this method was demonstrated by its utilization in an efficient synthesis of (+)-sertraline, an antidepressant agent. The observed gamma-regioselectivity and E-1,3-syn stereochemistry were rationalized based on a Pd(II) mechanism involving transmetalation between a cationic mono(acyloxo)palladium(II) complex and arylboronic acid, and directed carbopalladation followed by syn-beta-acyloxy elimination. The results of stoichiometric reactions of palladium complexes related to possible intermediates were fully consistent with the proposed mechanism.

A unified strategy for the stereospecific construction of propionates and acetate-propionates relying on a directed allylic substitution

Flexible friends: A new strategy that relies on o-DPPB-directed allylic substitution has been implemented for the flexible and stereospecific construction of major polyketide and isoprenoid structural elements (see scheme; o-DPPB=ortho-diphenylphosphanyl benzoate; PG=protecting group).