Facile synthesis of optically pure 1,2-diaryl (and 1-Alkyl-2-aryl) ethyl and propylamines

Synthesis of Indolines via Base-Mediated C-H Activation and Defluorinative C-N Coupling, with no Need for Transition-Metals

Syntheses of (partially) aromatic nitrogen heterocycles increasingly rely on transition-metal catalyzed C-C- and C-N-cross-coupling reactions. Here we describe a different approach to the synthesis of indolines by a domino C(sp3)-H activation, 1,2-addition, and defluorinative SNAr-cyclization sequence to provide the target 1,2-diarylindolines (1,2-diaryl-2,3-dihydroindoles) from ortho-fluorinated methyl-arenes and N-aryl imines (benzylidene anilines) in a cyclocondensation that is mediated by potassium hexamethyldisilazide (KHMDS) as base exclusively. This transition-metal-free process via C-H and C-F bond activation provides a one-step entry into a wide array of indoline scaffolds (43 examples, up to 96 % yield). This privileged substructure is common in natural products and pharmaceuticals alike, and cannot be accessed by traditional condensation reactions.

Recent Advances in π-Stacking Interaction-Controlled Asymmetric Synthesis

The π-stacking interaction is one of the most important intramolecular and intermolecular noncovalent interactions in organic chemistry. It plays an important role in stabilizing some structures and transition states in certain reactions via both intramolecular and intermolecular interactions, facilitating different selectivities, such as chemo-, regio-, and stereoselectivities. This minireview focuses on the recent examples of the π-stacking interaction-controlled asymmetric synthesis, including auxiliary-induced asymmetric synthesis, kinetic resolution, asymmetric synthesis of helicenes and heterohelicenes, and multilayer 3D chiral molecules.

Stereocontrolled Addition of Scrambling ortho-Sulfinyl Carbanions: Easy Access to Homopropargylamines and α-Allenylamines

An unprecedented behavior of ortho-sulfinylpropargyl carbanions in the presence of optically active sulfinylimines affords two different families of compounds: this peculiar chemodivergency is importantly affected by the nature of the employed base, and assisted by the configuration of the electrophile, displaying no alteration in the stereocontrol of both reactions. α-Allenylamines are formed exclusively, using R-sulfinyl aldimines as electrophiles, while homopropargylamines result when S-sulfinyl aldimines are employed.

Use of lithiated chiral o-sulfinylbenzyl carbanions for the one-pot building of linear fragments containing up to four connected stereocenters

The reaction of o-sulfinylbenzyl carbanions with prochiral Michael acceptors, such as differently sized cycloalkenones, proceeded with high levels of stereoselectivity, generating molecules containing up to three asymmetric carbon centers in just one synthetic step. All these reactions involved the use of either a proton or an acylating reagent as the final electrophile. Furthermore, the trapping of the enolate resulting from Michael addition with prochiral electrophiles, such as aldehydes or N-sulfonylimines, allowed the highly stereoselective synthesis of densely functionalized compounds containing four chiral centers in just a one-pot sequence, the stereochemical outcome of the sequence being controlled by the sulfinyl auxiliary.

Asymmetric synthesis of (S)-(-)-xylopinine. Use of the sulfinyl group as an ipso director in aromatic SE

Optically pure (S)-(-)-xylopinine 2 was prepared in three steps in 52% overall yield. Thus, condensation of the carbanion derived from (S)-4 with the (S)-(E)-sulfinylimine 5 gave a 2:1 mixture of tetrahydroisoquinolines 6a and 6b, differing only in configuration at sulfur. N-Desulfinylation of this mixture gave the diastereomeric sulfoxides which, without separation, were converted into (S)-(-)-xylopinine (2) with loss of the sulfinyl moieties under Pictet-Spengler conditions. This unprecedented ipso electrophilic substitution of a sulfinyl group may have synthetic implications beyond that described in this work.

π–π Stacking versus Steric Effects in Stereoselectivity Control: Highly Diastereoselective Synthesis ofsyn-1,2-Diarylpropylamines

N-Arylarylideneamines react with sulfinylbenzyl carbanions derived from 2-(p-tolylsulfinyl)toluenes (S)-1 and (S)-2, affording epimeric mixtures at C1 of 1,2-diarylethyl- and 1,2-diarylpropylamine derivatives. The sulfinyl group completely controls the configuration at C2 in the reactions of (S)-2. The configuration at C1 depends on the electron density of the ring adjacent to the iminic carbon atom which is modulated by pi-pi stacking interactions with the ring joined to the carbanionic centre. The stereoselectivity was controlled by modifying the acceptor character of the arylideneamine ring with appropriate substituents, the formation of the highly selective (R) configuration at C1 being made possible by electron-donating groups. N-(2,4,6-Trimethoxyphenyl)arylideneamines have been shown to be suitable starting materials for the preparation of (R)-1,2-diarylethyl- and (1R,2S)-1,2-diarylpropylamines (syn epimers) in a highly stereoselective manner.

Efficient trans-Selectivity in the Cyclocondensation of (S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde with Activated Dienes Catalyzed by Yb(OTf)3

Reactions of (S)-2-[2-(p-tolylsulfinyl)phenyl]acetaldehyde 1 with Danishefsky's and related dienes took place in the presence of Yb(OTf)3 in a completely stereoselective manner, mediated by a remote sulfinyl group (1,5-asymmetric induction), to afford the corresponding 2,3-dihydro-4H-pyran-4-ones. These reactions followed a stepwise mechanism, as was corroborated by isolation of the corresponding intermediates, with a high level of trans-selectivity for 4-methyl-substituted dienes. Treatment of the adducts with Raney Ni provided concomitant cleavage of the C-S bond and reduction of the conjugated carbonyl grouping.

Highly Stereoselective Benzylation of N-Sulfinylketimines

The benzylation of N-sulfinyl ketimines with 2-(p-tolylsulfinyl)ethylbenzene and LDA afford t-alkylamines in good yields. The configuration at each one of the new chiral centers simultaneously created in this reaction is controlled by the configuration of the sulfinyl groups at the nucleophile and electrophile, respectively. Thus, the reactions of the (S)-sulfoxide 6 with the N-(S)-sulfinylketimines 3 only yield the anti diastereoisomers 18, whereas the syn diastereoisomers 19 are exclusively formed in reactions of (S)-6 with N-(R)-sulfinylketimines 3. After a two-step desulfinylation process ((i) TFA, (ii) Ra-Ni), this reaction provides a procedure for synthesizing any epimer of alpha,alpha-dibranched beta-alkylarylamines in optically pure form by choosing the configuration of the starting materials. A similar behavior is observed for carbanions derived from the O-protected 2-(p-tolylsulfinyl) benzyl alcohol 7 thus allowing the synthesis of the optically pure anti- and syn-1,2-amino alcohols containing a chiral quaternary carbon adjacent to the nitrogen.

The synthesis of enantiomerically pure 4-substituted [2.2]paracyclophane derivatives by sulfoxide-metal exchange

A general strategy for the synthesis of enantiomerically pure 4-substituted [2.2]paracyclophanes from a common sulfoxide precursor is described.

Remote Stereocontrol by Sulfinyl Groups: Reduction of δ-Ketosulfoxides

The reduction of delta-ketosulfoxides constitutes the first evidence of the efficiency of the sulfinyl group to control the stereoselectivity of 1,5-asymmetric induction processes. The use of DIBAL/Yb(OTf)3 or L-Selectride as the reducing agents provides delta-hydroxysulfoxides with the opposite configuration at the hydroxylic carbon in a highly stereoselective way.

Synthesis of Enantiopure Sulfinimines (Thiooxime S-Oxides) Catalyzed by Yb(OTf)3 from p-Toluenesulfinamide and Aldehydes in Mild Reaction Conditions

Enantiomerically pure sulfinimines as important building blocks in the asymmetric synthesis of amine derivatives are prepared in good to excellent yields from chiral p-toluenesulfinamide with aromatic, heteroaromatic, and aliphatic aldehydes. The unprecedented feature of the reported procedure is that the formation of the sulfinimines was achieved by the catalytic action of Yb(OTf)(3) in THF at room temperature. The reaction conditions were also applicable to Ellman's sulfinimines.

A General Method for the Preparation of N-Sulfonyl Aldimines and Ketimines

[Reaction: see text] A simple procedure to obtain N-sulfonyl imines involving the condensation of carbonyl compounds with p-tolyl or tert-butyl sulfinamides followed by oxidation with m-CPBA of the resulting N-sulfinylimines is reported. The method is applicable to aldehydes (aliphatics and aromatics) and ketones (diaryl, dialkyl, and aryl alkyl), even those containing enolizable protons. It also does not affect C=N or C=C double bonds and does not epimerize alpha-stereogenic centers.