Switching of regioselectivity in base-mediated diastereoselective annulation of 2,3-epoxy tosylates and their N-tosylaziridine analogs with 2-mercaptobenzimidazole

Base-Mediated, Chemo- and Regioselective (4+2) Annulation of Indole-2-carboxamides with 2,3-Epoxy Tosylates toward 1,2-Fused Indoles

Base-mediated [4+2] annulation of indole-2-carboxamides with 2,3-epoxy tosylates has been explored. The protocol delivers 3-substituted pyrazino[1,2-a]indol-1-ones in high yields in diastereoselective fashion, and neither 4-substituted pyrazino[1,2-a]indol-1-ones nor tetrahydro-1H-[1,4]diazepino[1,2-a]indol-1-ones are generated, irrespective of whether the distal epoxide C3 substituent is alkyl or aryl, or the epoxide is cis- or trans-configured. This reaction proceeds in one pot via N-alkylation of the indole scaffold with 2,3-epoxy tosylates, concomitantly followed by 6-exo-selective epoxide-opening cyclization. Notably, the process is chemo- and regioselective with respect to both the starting materials. To our knowledge, the process represents the first successful example of one-pot annulation of indole-based diheteronucleophiles with epoxide-based dielectrophiles.

One-Pot Double Intramolecular Cyclization Approach to Tetralin-Based Cis-Fused Tetracyclic Compounds

Presented herein is a BF₃·OEt₂-mediated, diastereoselective one-pot double cyclization of 4-aryl-2-[(arylthio)methyl]butanals leading to the formation of cis-tetrahydro-6H-naphtho[2,1-c]thiochromenes for the first time. Mechanistically, the formation of the title products involves the one-pot intramolecular Friedel-Crafts hydroxyalkylation/intramolecular Friedel-Crafts alkylation cascade. This synthetic methodology is featured by its high atom economy, broad substrate scope, mild transition-metal-free reaction conditions, capability to assemble two new rings in one pot, and moderate to high yields (up to 94% yield). It was then applied in the synthesis of a thia analogue of brazilane and a chromeno[3,4-c]chromene derivative. Moreover, the methodology was successfully extended to the synthesis of cis-hexahydrobenzo[c]phenanthrenes. Specifically, 1,5-diarylpentan-3-ones were first subjected to the Corey-Chaykovsky reaction, and the resulting epoxides, without being chromatographically isolated, were treated with BF₃·OEt₂ to afford the cyclized products in high yields (up to 84% yield over two steps).

Highly Diastereoselective Synthesis of Dihydro-benzoimidazo-[1,3]-thiazines via Electro-oxidative Selenocyclization of Thioallyl Benzoimidazoles

The current methodology reveals a green and proficient electro-oxidative tandem selenocyclization of thioallyl benzoimidazoles manufacturing selenylated dihydro-benzoimidazo-thiazine derivatives. Both C-Se and C-N bond formation were achieved via this mild protocol which exhibits good functional group tolerability affording an extensive range of substrate scope up to 96% isolated yields. Complete control over the regioselective formation of the six-membered heterocycle and stereoselective construction of the contiguous stereocenters was established. The practical electrochemical method operates in an undivided cell at ambient temperature without using any metal and external chemical oxidant.

Trifluoroethanol-promoted ring-opening cyclization of 4-(2-oxiranylmethoxy)indoles: access to 4,5-fused indoles

Herein, we report that the trifluoroethanol-mediated ring-opening cyclization of readily accessible 4-(2-oxiranylmethoxy)indoles takes place in a diastereoselective and 6-endo fashion to generate pyrano[2,3-e]indol-3-ols in high yields. This regioselective cyclization at the indole C-5 position requires the presence of a π-activating aryl substituent on the reacting epoxide carbon atom, but remains uninfluenced by the electronic nature of the indole-N-substituent. Interestingly, blocking the C-5 position of the indole unit directs the reaction to generate oxepino[4,3,2-cd]indol-3-ols via 7-endo epoxide-arene cyclization.

Hexafluoroisopropanol-Mediated Intramolecular Ring-Opening Cyclization of Indolyl-N-Tethered Epoxides: Tether-Length-Controlled Synthesis of 1,7- and 1,2-Fused Indoles

Despite having the capability to construct benzo-fused heterocycles in complete atom economy and high chemo-, regio-, enantio-, and diastereoselectivities, intramolecular Friedel-Crafts epoxide arene cyclization (IFCEAC) remains underutilized in organic synthesis. The wide adaptation of this powerful Csp²-Csp³ bond-forming reaction, therefore, requires a broad understanding of the substrate scope to better impact heterocycle synthesis. Along this line, we investigated the applicability of IFCEAC for the synthesis of 1,7- and 1,2-fused indoles. In this article, we report the results of our systematic investigation into the scope and limitations of the first examples of the hexafluoro-2-propanol (HFIP)-mediated IFCEAC of readily accessible indolyl-N-tethered epoxides. We observed that the nature and position of the indole and epoxide substituents and the tether length separating these two reacting moieties have strong effects on the cyclization. This mild and transition-metal-free protocol delivered pyrrolo[3,2,1-ij]quinolin-5-ols in moderate to good yields from substrates bearing both a methylene linker that connects the indole and epoxide moieties and an electron-rich indole carbocyclic ring. Notably, the reactions required the presence of a π-activating aryl substituent on the reacting epoxide carbon atom. Interestingly, replacing the methylene tether with an ethylene unit resulted in regioswitching, which delivered the corresponding tetrahydropyrido[1,2-a]indol-8-ols in good to high yields. We could also successfully extend this methodology to pyrrolyl-N-tethered epoxides for a very high-yielding synthesis of tetrahydroindolizin-7-ols.

Regioselectivity of the trifluoroethanol-promoted intramolecular N-Boc-epoxide cyclization towards 1,3-oxazolidin-2-ones and 1,3-oxazinan-2-ones

The intramolecular N-Boc-epoxide cyclization leading to the formation of 1,3-oxazolidin-2-one and 1,3-oxazinan-2-one derivatives has scarcely been reported in the literature. More specifically, the intramolecular cyclization of N-Boc aniline-tethered 2,3-disubstitued epoxides has never been disclosed. Herein, we demonstrate that this reaction could proceed in a diastereoselective fashion in refluxing trifluoroethanol, in the absence of any external promoter or catalyst. Substrates bearing an alkyl group at the C-3 position furnished 1,3-oxazolidin-2-ones in a completely regioselective fashion via 5-exo epoxide ring-opening cyclization, thereby paving the way to synthesize alkyl side chain-bearing analogs of the antidepressant drug toloxatone. On the other hand, replacing the alkyl group with an aryl group resulted in easily separable mixtures of 1,3-oxazolidin-2-ones and 1,3-oxazinan-2-ones, the former being obtained as the major products. Remarkably, a tetralin-bearing substrate underwent fully regioselective 6-endo ring closure to form the corresponding 1,3-oxazinan-2-one. Our present study on the intramolecular ring opening-cyclization of epoxides with a tethered N-Boc group is the most comprehensive to date and features broad substrate scope, mild transition metal-free conditions, excellent functional group tolerance, and scalability.