Telluronium Salts Mediated Aziridination of Chiral N-tert-Butylsulfinylimines: Highly Stereoselective Synthesis of Optically Active Vinylaziridines

Regio-, Site- and Stereo-Selective Aziridination of Conjugated Dienes Enabled by Palladium/Copper/Iodide/Oxygen Cooperation

Vinylaziridines are important building blocks in organic chemistry, especially in the synthesis of nitrogen-containing heterocycles. The direct and efficient transfer of an appropriate nitrogen source to readily accessible conjugated dienes is a notable methodology. The Pd-catalyzed oxidative 1,2-difunctionalization of conjugated dienes through a π-allyl-palladium species should be an ideal method for the selective synthesis of vinylaziridines. However, this method faces the challenge of regioselectivity, often resulting in 1,4-difunctionalization instead. In this study, we developed a Pd-catalyzed aerobic 1,2-difunctionalization of conjugated dienes via a π-allyl-palladium species to achieve regio-, site- and stereo-selective aziridination under the synergistic effects of PdII, CuI, I-, and O2. The π-allyl palladium species formed in the system undergoes an unusual iodination process, leading to the formation of an allyl iodide intermediate. Subsequently, the vinylaziridine is obtained through intramolecular SN2' substitution of the allyl iodide.

1,2-Additions on Chiral N-Sulfinylketimines: An Easy Access to Chiral α-Tertiary Amines

Chiral α-tertiary amines, a motif present in α,α-disubstituted α-amino acids, in a wide range of natural products, and many drugs and drug candidates, are important targets in organic chemistry. Among the possible strategies, 1,2-addition to chiral N-sulfinyl­ketimines is one of the best routes to form chiral α-tertiary amines with a high level of stereoselectivity. In this review, we focus first on the addition of organometallic reagents or other nucleophiles as enols or ylides to chiral N-sulfinylketimines. Then secondly we cover a selection of applications of these additions in the synthesis of valuable biologically active compounds.

1 Introduction

2 1,2-Addition Reaction Methodologies

2.1 Organolithium Reagent Additions

2.2 Grignard Additions

2.3 Organozinc Reagent Additions

2.4 Organoindium Reagent Additions

2.5 Organoboron Reagent Additions

2.6 Strecker Reactions

2.7 Palladium-Catalyzed Reactions

2.8 Enols, Enolates, and Other Deprotonated Reagent Additions

2.9 Ylide Additions

2.10 Heteroatom Nucleophiles

2.11 Miscellaneous Reactions

3 Applications to the Synthesis of Biologically Active Molecules

4 Conclusions

Construction of N-Alkyl- and N-Arylaziridines from Unprotected Amines via C-H Oxidative Amination Strategy

A copper-promoted intramolecular C-H oxidative amination reaction between secondary amine (N-H) and C(sp³)-H at the benzylic position of azaarenes or α-position of ketones for the synthesis of aziridine derivatives has been developed. Moreover, a practical annulation of electron-deficient vinylarenes with an unprotected primary alkyl amine by a Yb(OTf)₃-CuI relay system has also been reported. The reactions were carried out with oxygen as the sole oxidant to give the N-alkyl- and N-arylaziridines in good yields.

Synthetic approach to skeletally diverse nitrogen heterocycles from dicyano-2-methylenebut-3-enoates

Divergent synthesis of three types of azaheterocycles, including vinylaziridines, pyrroline and 2-aminopyridines, has been achieved through [2 + 1], formal-[4 + 1] and [5 + 1] annulations.

tert-Butanesulfinimines: structure, synthesis and synthetic applications

Since Ellman's seminal works, over the past ten years tert-butanesulfinimines have proved to be useful chiral amino intermediates for organic synthesis. Through highly stereoselective reactions, amongst which reductions, nucleophilic 1,2-additions and ylide condensations, a broad range of nitrogen-containing compounds has been synthesized. Although the stereoselectivity levels are high in most cases, the sense of the stereoinduction is generally not predictable. The object of this critical review is to present the models proposed to rationalize the stereochemical outcome of the reactions involving tert-butanesulfinimines and to point out an obvious lack of homogeneity amongst them (128 references).

Enantio- and diastereoselectivities in chiral sulfur ylide promoted asymmetric aziridination reactions

Density functional theory investigation on the factors controlling enantio- and diastereoselection in asymmetric aziridination reaction by the addition of chiral bicyclic sulfur ylides to substituted aldimines is presented. High levels of enantioselection are predicted toward the formation of (2S,3S)-cis and (2R,3S)-trans aziridines by the addition of stabilized ylide (R = COMe) respectively to SO2Me and CO2Me protected aldimines. Similarly, high %ee is predicted for the formation of (2S,3R)-cis aziridines from semistabilized (R = Ph) ylide. Moderate to high levels of diastereoselectivity is noticed as well. The present study highlights that a correct prediction on extent of enantioselection requires the knowledge of the activation barriers for elementary steps beyond the initial addition step. In the case of stabilized ylides the ring-closure (or elimination of sulfur compound) is found to be crucial in controlling enantio- and diastereoselection. A cumulative effect of electronic as well as other weak interactions is identified as factors contributing to the relative energies of transition states leading to enantio- and diastereomeric products for the stabilized ylide addition to aldimines. On the contrary, steric control appears quite dominant with semistabilized ylide addition. With the smallest substituent on ylide (R = Me), high enantioselectivity is predicted for the formation of (2R,3R)-trans aziridines although the %de in this case is found to be very low.

Ylide-initiated michael addition-cyclization reactions beyond cyclopropanes

[Reaction: see text]. Ylides are nucleophiles that bear a unique leaving group, L n M, and can attack aldehydes, ketones, imines, and electron-deficient alkenes. Over the course of the reaction, they react with CX (X = C, N, O, etc.) double bonds to form betaine or oxetane intermediates, which further eliminate the heteroatom-containing group in one of two ways to give the corresponding olefination or cyclization product. Since the discovery of the Wittig reaction, ylide olefination has developed as one of the most useful approaches in constructing carbon-carbon double bonds. These reactions provide unambiguous positioning of the C-C double bond and good stereoselectivity. Researchers have also widely used ylides for the synthesis of small ring compounds such as epoxides, cyclopropanes, and aziridines. However, the use of ylides to prepare larger cyclic structures was very limited. This Account outlines our recent work on ylide-initiated Michael addition/cyclization reactions. By altering the heteroatoms and the ligands of the ylides, we have modulated the reactivity of ylides. These modified ylides provide easy access to diverse cyclic compounds with the ability to control regioselectivity, chemoselectivity, diastereoselectivity, and enantioselectivity. Reactions using these ylides produce the structural components of many biologically active compounds and valuable intermediates in organic synthesis. Allylic telluronium and sulfonium ylides can react with alpha,beta-unsaturated esters, ketones, amides, and nitriles to afford multisubstituted vinylcyclopropanes with high selectivities. Telluronium allylides react with aromatic N-phenyl aldimines to give trans-vinylaziridines and with chiral N- tert-butylsulfinylimines to afford the optically active cis-2-substituted vinylaziridines, both with high diastereoselectivities. We also used sulfonium and telluronium allylides to prepare vinylepoxides. In addition, ylides are good reagents for the synthesis of five-membered heterocyclic compounds. By treatment of stable camphor-derived sulfur ylides with alpha-ylidene-beta-diketones, we obtained multisubstituted dihydrofurans with high diastereo- and enantioselectivities. Ammonium salts derived from cinchonidine and cinchonine react smoothly with 3-aryl and 3-heteroaryl-2-nitro acrylates, affording both enantiomers of isoxazoline N-oxides with up to 99% ee. Ylides can initiate tandem cyclizations for the synthesis of chromenes, bicyclic compounds, and cyclohexadiene epoxides. Varying the choice of base allows access to 2 H-chromenes and 4 H-chromenes from 3-(2-(bromomethyl)phenoxy)acrylates via a tandem ylide Michael addition/elimination/substitution reaction. Phosphines can catalyze an intramolecular ylide [3 + 2] annulation constructing bicyclo[ n.3.0] ring systems with three contiguous stereogenic centers. The reaction of crotonate-derived sulfur ylides with alpha,beta-unsaturated ketones affords cyclohexadiene epoxides with excellent diastereoselectivities (>99/1) in good to high yields. Using a camphor-derived sulfonium salt, we have produced asymmetric cyclohexadiene epoxides with high ee's. Overall, these results illustrate the versatility and tunability of ylides for the preparation of cyclic systems containing more than three atoms.

Asymmetric Synthesis of Aziridines by Reduction ofN-tert-Butanesulfinyl α-Chloro Imines

Reduction of (RS)-N-tert-butanesulfinyl alpha-halo imines afforded chiral aziridines in good to excellent yields. Upon reduction of (RS)-N-tert-butanesulfinyl alpha-halo imines with NaBH4 in THF, in the presence of 10 equiv of MeOH, (RS,S)-beta-halo sulfinamides were formed in excellent yield (up to 98%) with very good stereoselectivity (>98:2). Simple treatment of the latter (RS,S)-beta-halo-tert-butanesulfinamides with KOH afforded the corresponding (RS,S)-N-(tert-butylsulfinyl)aziridines in quantitative yields. On the contrary, its epimer, (RS,R)-N-(tert-butylsulfinyl)aziridine was synthesized by switchover of the reducing agent from NaBH4 to LiBHEt3. (RS,R)-N-(tert-Butylsulfinyl)aziridines were synthesized in good yields (up to 85%) and diastereoselectivity (up to 92:8) by reduction of (RS)-N-tert-butanesulfinyl alpha-halo imines with LiBHEt3 in dry THF and subsequent treatment with KOH. All chiral aziridines were obtained as a single diastereomer after recrystallization (overall yield up to 91%) or after flash chromatography.

Use of α-Chlorinated N-(tert-Butanesulfinyl)imines in the Synthesis of Chiral Aziridines

[reaction: see text] Reaction of chiral alpha-chloro tert-butanesulfinyl aldimines with Grignard reagents efficiently afforded beta-chloro N-sulfinamides in high diastereomeric excess. The latter compounds were cyclized toward the corresponding chiral aziridines in a high-yielding one-pot reaction or after separate treatment with base. The diastereoselectivity obtained in the newly synthesized beta-chloro sulfinamides is explained via the coordinating ability of the alpha-chloro atom with magnesium resulting in the opposite stereochemical outcome as generally observed for nonfunctionalized N-sulfinyl imines.

Direct synthesis of chiral aziridines from N-tert-butyl-sulfinylketimines

The direct preparation of a range of variously substituted chiral tert-butylsulfinylketimines was achieved in good yield (41-90%), with relatively rapid reaction times (4-15 hours); their synthetic application was examined through the reaction with the ylides derived from trimethylsulfonium iodide and S-allyl tetrahydrothiophenium bromide, affording convenient access to a diverse range of highly substituted chiral aziridines in up to 78% yield and > 90% d.e.

Enantioselective synthesis of allenic esters via an ylide route

Pseudo-C2-symmetric chiral phosphorus ylides have been designed and synthesized for the enantioselective preparation of allenic esters, and up to 92% ee has been achieved.