Acid-Catalyzed [3,3]-Sigmatropic Rearrangements ofN-Propargylanilines

Catalytic Enantioselective Synthesis of N-C Axially Chiral N-(2,6-Disubstituted-phenyl)sulfonamides through Chiral Pd-Catalyzed N-Allylation

Recently, catalytic enantioselective syntheses of N-C axially chiral compounds have been reported by many groups. Most N-C axially chiral compounds prepared through a catalytic asymmetric reaction possess carboxamide or nitrogen-containing aromatic heterocycle skeletons. On the other hand, although N-C axially chiral sulfonamide derivatives are known, their catalytic enantioselective synthesis is relatively underexplored. We found that the reaction (Tsuji-Trost allylation) of allyl acetate with secondary sulfonamides bearing a 2-arylethynyl-6-methylphenyl group on the nitrogen atom proceeds with good enantioselectivity (up to 92% ee) in the presence of (S,S)-Trost ligand-(allyl-PdCl)₂ catalyst, affording rotationally stable N-C axially chiral N-allylated sulfonamides. Furthermore, the absolute stereochemistry of the major enantiomer was determined by X-ray single crystal structural analysis and the origin of the enantioselectivity was considered.

Computational and Further Experimental Explorations of the Competing Cascades Following Claisen Rearrangements of Aryl Propargyl Ethers: Substituent Effects on Reactivity and Regioselectivity

We report a computational investigation of two reaction cascades occurring following the Claisen rearrangements of aryl propargyl ethers to the alternate ortho positions in unsymmetrical reactants. Our computations explain how substituents influence reactivity and regioselectivity. Rearrangement to the substituted ortho carbon leads to a tricyclo[3.2.1.0]octane core, while rearrangement to an unsubstituted ortho carbon leads to a benzopyran. Density functional theory with ωB97X-D indicates that these reactions involve rate-determining Claisen rearrangements followed by subsequent reaction cascades of the Claisen rearrangement products depending on the presence or absence of a substituent at the ortho carbon.

An atom efficient route to N-aryl and N-alkyl pyrrolines by transition metal catalysis

The synthesis of N-aryl, N-tosyl, and N-alkyl pyrrolines from allyl alcohols and amines has been developed. The reaction sequence includes a palladium-catalyzed allylation step in which non-manipulated allyl alcohol is used to generate the diallylated amine in good to excellent yield. An excess of allyl alcohol was necessary for efficient diallylation of the amine, where the excess alcohol could be recycled three times. For aryl and tosyl amines, Pd[P(OPh)(3)](4) was used and for benzyl and alkyl amines a catalytic system comprising Pd(OAc)(2), P(n)Bu(3), and BEt(3) was used. Both the electronic properties and the steric influence of the amine affected the efficiency of the allylation. The isolated diallylated amines were transformed into their corresponding pyrrolines by ring-closing metathesis catalyzed by (H(2)IMes)(PCy(3))Cl(2)RuCHPh in good to excellent yield. A one-pot reaction was developed in which aniline was transformed into the corresponding pyrroline without isolating the diallylated intermediate. This one-pot reaction was successfully scaled-up to 1 mL of aniline in which the N-phenyl pyrroline was isolated in 95% yield. The versatility of the reaction in which 3-methyl-1-phenyl pyrroline was prepared in two-steps was demonstrated.

Rhodium(I)-catalyzed synthesis of indoles: amino-Claisen rearrangement of N-propargylanilines

Mild and facile preparations of 2-substituted or 2,3-disubstituted indole compounds were achieved by RhH(CO)(Ph(3)P)(3) (4-10 mol %)-catalyzed reaction of N-propargylanilines in hexafluoroisopropyl alcohol (HFIP). The formation of indoles was proven to be derived from an o-allenylaniline intermediate, which was generated by the Rh(I)-catalyzed amino-Claisen rearrangement of N-propargylanilines. The catalytic system is also available for the one-pot synthesis of indoles by reacting N-alkylaniline (1 equiv) with propargyl bromide (1.3 equiv) in the presence of K(2)CO(3) (3 equiv) in HFIP. The active catalyst was proven to be [Rh(CO)(Ph(3)P)(2)]OCH(CF(3))(2) generated in situ from RhH(CO)(Ph(3)P)(3) and HFIP. The structure of [Rh(CO)(Ph(3)P)(2)]OCH(CF(3))(2) was confirmed by single-crystal X-ray crystallographic analysis.

Practical synthesis of 7-prenylindole

7-Prenylindole is a useful building block for natural product and natural product analogue synthesis. While there have been several past syntheses of 7-prenylindole, none of them is very practical for its preparation on scale. Using an aza-Claisen rearrangement as the key step, 7-prenylindole has been prepared in four steps from indoline in 62% overall yield.

Cu(I)-catalyzed direct addition and asymmetric addition of terminal alkynes to imines

A Cu(I)-catalyzed direct addition of alkynes to imines was developed. The process is simple and provides a diverse range of propargylamines in high enantiomeric excess and good yield both in water and in toluene. The absolute configuration of such addition products has been determined by x-ray crystallography.

New Synthesis of Propargylic Amines from 2-(Bromomethyl)aziridines. Intermediacy of 3-Bromoazetidinium Salts

A new, efficient, and straightforward synthesis provides propargylamines in high overall yields (64-77%) by transformation of 1-(arylmethyl)-2-(bromomethyl)aziridines into N,N-di(arylmethyl)-N-(2-propynyl)amines via N-(2,3-dibromopropyl)amines and N-(2-bromo-2-propenyl)amines. The conversion of N-(2,3-dibromopropyl)amines into N-(2-bromo-2-propenyl)amines is based on a novel analogue of the Hofmann elimination. A Yamaguchi-Hirao alkylation, a Sonogashira coupling, or a hydroarylation reaction further functionalized these propargylamines toward potentially interesting compounds for medicinal and agrochemical use.

Total syntheses of demethylasterriquinone B1, an orally active insulin mimetic, and demethylasterriquinone A1

Two total syntheses of the unsymmetrical bis-indolylquinone natural product demethylasterriquinone B1 (also known as L-783,281) have been accomplished. The first exploits a known base-promoted condensation of indoles with bromanil, which stops at monoaddition using the sterically hindered 2-isoprenylindole. This permits addition of the second indole, 7-prenylindole, which gives both meta- and para-substituted bis-indolylquinone products. This regiochemical control problem was solved by extension of a method we recently developed for acid-promoted addition of indoles to 2,5-dichlorobenzoquinone. Under our original mineral acid conditions, reaction of 2-isoprenylindole with dichlorobenzoquinone fails, but it succeeds with 3-bromo-2,5-dichlorobenzoquinone using acetic acid as the promoter. The regiochemistry established in such selectively bromine-substituted quinones can be exploited in Stille couplings. As a model system, the synthesis of demethylasterriquinone A1 was accomplished using as the key step a Stille coupling of a 2,5-dibromobenzoquinone with an (N-isoprenylindol-3-yl)tin, producing the para-substituted bis-indolylquinone exclusively. Use of a (7-prenylindole)tin in coupling with a bromo-2,5-dichloro-4-indolylbenzoquinone gives the demethylasterriquinone B1 precursor. The dihaloquinone products of these indole/quinone coupling processes can be hydrolyzed to the dihydroxyquinone natural products. Demethylasterriquinone B1 is of high recent interest as a small molecule insulin mimetic with oral anti-diabetic activity in mice.

Total Synthesis of (-)-Muscoride A

The recently isolated cyanobacterium metabolite muscoride A was synthesized in 15 steps and in 4.3% overall yield. Novel structural features of this peptide antibiotic include the presence of a threonine-derived bioxazole core and an N-(1,1-dimethyl)allyl ("reverse prenyl") valine residue. In the context of our synthesis, efficient new strategies for the preparation of these segments were developed. The synthesis of two epimers of muscoride A allowed the unambiguous assignment of the relative and absolute configuration of the natural product by NMR and optical rotation analyses.