Synthesis of Nonracemic 1,4-Benzoxazines via Ring Opening/Cyclization of Activated Aziridines with 2-Halophenols: Formal Synthesis of Levofloxacin

Regioselective Brønsted acid catalyzed ring opening of aziridines by phenols and thiophenols; a gateway to access functionalized indolines, indoles, benzothiazines, dihydrobenzo-thiazines, benzo-oxazines and benzochromenes

Brønsted acid catalyzed regioselective ring opening of aziridines by phenols and thiophenols have been reported. Involvement of a series of aziridines with a range of phenols and thiophenols offer the generality of the reported protocol. Completion of the reaction at room temperature within very short time brings the uniqueness of the developed technique. To emphasis on the application of the developed methodology, the products have been used for the further synthesis of a range of useful and novel heterocyclic molecules such as indolines, indoles, benzothiazines, dihydrobenzothiazines, benzo-oxazines and benzochromenes.

Rapid Synthesis of 3-Methyleneisoindolin-1-ones via Metal-Free Tandem Reactions of Ester-Functionalized Aziridines

We have disclosed a novel metal-free tandem cyclization reaction for the synthesis of 3-methyleneisoindolin-1-ones starting from ester-functionalized aziridines. This strategy can be effectively promoted by DBU and carboxylic acids. Mechanistically, it involves sequential ring opening of aziridines with carboxylic acids, lactamization, and elimination of carboxylic acids.

Catalytic Aminium Radical-Cation Salt (Magic Blue)-Initiated SN2-Type Nucleophilic Ring-Opening Transformations of Aziridines

A simple and atom economic protocol for the construction of C-X/C-C bonds via catalytic aminium radical-cation salt (Magic Blue)-initiated SN2-type nucleophilic ring-opening transformations of racemic and nonracemic aziridines with different hetero and carbon nucleophiles to afford various amino ethers, thioethers, and amines in up to 99% yield, and with perfect enantiospecificity for some substrates but reduced ee with others (for nonracemic aziridines), is developed. This aminium radical-cation salt-initiated, SN2-type nucleophilic ring-opening strategy, along with various cyclization protocols, is employed to synthesize various biologically significant compounds.

Ring-Opening Cyclization (ROC) of Aziridines with Propargyl Alcohols: Synthesis of 3,4-Dihydro-2H-1,4-oxazines

Activated aziridines react with propargyl alcohols in the presence of Zn(OTf)₂ as the Lewis acid catalyst following an S_N2-type ring-opening mechanism to furnish the corresponding amino ether derivatives. Those amino ethers further undergo intramolecular hydroamination via 6-exo-dig cyclization in the presence of Zn(OTf)₂ as the catalyst and tetrabutylammonium triflate salt as an additive under one-pot two-step reaction conditions. However, for nonracemic examples, ring-opening and cyclization steps were conducted under two-pot conditions. The reaction works well without any additional solvents. The final 3,4-dihydro-2H-1,4-oxazine products were obtained with 13 to 84% yield and 78 to 98% enantiomeric excess (for nonracemic examples).

Substrate-Controlled Product Divergence in Iron(III)-Catalyzed Reactions of Propargylic Alcohols: Easy Access to Spiro-indenyl 1,4-Benzoxazines and 2-(2,2-Diarylvinyl)quinoxalines

We report herein unprecedented cascade reactions of O-propargyl-N-tosyl-amino phenols with 10 mol% FeCl₃ in DCE at room temperature for 0.67-3 h to form spiro-indenyl 1,4-benzoxazines with 38-89 % yield. Replacing the substrates' oxygen atom by a N-tosylimine group followed by treatment with the same catalyst and solvent at 80 °C produced 2-(2,2-diarylvinyl)quinoxalines in 12-20 h with up to 62 % yield. Mechanistic understanding provided an insight into the transformations. The use of simple substrates and an environmentally benign low-cost catalyst, broad substrate scope and tolerance of diverse functional groups makes the methodology inherently attractive.

Synthesis of 1,4-benzoxazines via Y(OTf)3-catalyzed ring opening/annulation cascade reaction of benzoxazoles with propargylic alcohols

A convenient Y(OTf)₃-catalyzed cascade formal [4 + 2] cyclization approach for the formation of 1,4-benzoxazine scaffolds from benzoxazoles and propargyl alcohols through a ring-opening and regioselective ring-closure process has been developed. By using this mild and practical protocol, a broad range of aldehyde-containing 1,4-benzoxazine compounds were prepared in moderate to excellent yields with good functional group tolerance. Mechanistic studies indicated that an S_N1 nucleophilic substitution of benzoxazole with a propargyl cation was involved in this transformation.

One-pot synthesis of 3-trifluoromethylbenzo[b][1,4]oxazines from CF3-imidoyl sulfoxonium ylides with 2-bromophenols

A one-pot two-step fashion for the synthesis of 3-trifluoromethyl-1,4-benzoxazines from CF3-imidoyl sulfoxonium ylides and 2-bromophenols via lithium-bromide-promoted O–H insertion of sulfoxonium ylides and annulation has been demonstrated.

Construction of 4-spiroannulated tetrahydroisoquinoline skeletons via a sequential ring opening of aziridines and Pictet–Spengler reaction

A stepwise cyclization involving sequential ring opening of aziridines and Pictet–Spengler reaction has been developed for the synthesis of 4-spiroannulated tetrahydroisoquinoline compounds (22 examples).

Aziridine used as a vinylidene unit in palladium-catalyzed [2 + 2 + 1] domino annulation

A series of chromone fused methylenecyclopentanes are efficiently constructed in moderate to good yields by Pd-catalyzed [2 + 2 + 1] annulation, in which aziridine is used as a vinylidene unit by cleavage of two C–N bonds for the first time.

One-Pot, Multicomponent, Diastereoselective, Green Synthesis of 3,4-Dihydro-2H-benzo[b][1,4]oxazine Analogues

A novel green and efficient catalyst-free, mild one-pot, multicomponent synthetic strategy has been developed to construct substituted 3,4-dihydro-2H-benzo[b][1,4]oxazine. This reaction proceeds via in situ formation of Schiff-base followed by base mediated alkylation with phenacyl bromide/substituted phenacyl bromide, finally leading to intramolecular cyclization to give a mixture of diastereomers with excellent diastereoselectivity (up to dr = 99:1), which were isolated as a single diastereomer in moderate to excellent yields (41-92%). Besides, this new versatile methodology provides a wide scope for the synthesis of different functionally substituted benzoxazine scaffolds and can be further exploited as building blocks for the synthesis of multifaceted molecular structures, especially for pharmaceutical applications.

A synthetic route to 1,4-disubstituted tetrahydro-β-carbolines and tetrahydropyranoindoles via ring-opening/Pictet–Spengler reaction of aziridines and epoxides with indoles/aldehydes

A route to 1,4-disubstituted tetrahydro-β-carbolines and tetrahydropyrano[3,4-b]indoles in high yields and stereoselectivity via S_N2-type ring opening/Pictet–Spengler reaction of aziridines and epoxides with indoles is described.

One-Pot Synthesis of Enantiopure Spiro[3,4-dihydrobenzo[b][1,4]oxazine-2,3'-oxindole] via Regio- and Stereoselective Tandem Ring Opening/Cyclization of Spiroaziridine Oxindoles with Bromophenols

A highly efficient regio- and stereoselective spiroaziridine ring opening with 2-bromophenols and a subsequent tandem cyclization reaction was developed for the one-pot synthesis of enantiopure 3,4-dihydrospiro[benzo[b][1,4]oxazine-2,3'-oxindole] with excellent enantiopurity (ee up to >99%). It is further extended to asymmetric synthesis of NH-free 3,4-dihydrospiro[benzo[b][1,4]oxazine-2,3'-xindole] retaining the optical activity.

Synthesis of 2-Aminophosphates via SN2-Type Ring Openings of Aziridines with Organophosphoric Acids

The synthesis of 2-aminophosphates is achieved by a S_N2-type ring opening reaction of various N-protected or free aziridines with phosphoric acids in a regiospecific and/or enantiospecific way. A one-pot, two-step procedure is also developed enabling direct access to 2-aminophosphates from olefins without isolation of the aziridine intermediates.

Strategies towards the synthesis of anti-tuberculosis drugs

In this report, we reviewed the strategies towards the synthesis of anti-tuberculosis drugs. They include semisynthetic approaches, resolution based strategies, microbial transformations, solid phase synthesis, and asymmetric synthesis.