[4+2] and [3+2] Annulations of α‐Chloroaldehydes and Dithio‐ esters: Synthesis of 1,4‐Oxathiin‐2(3H)‐ones and 1,3‐Oxathioles

Abstract

The[ 4 + +2] and[ 3 + +2] annulations of achloroaldehydes with dithioesters are reported. In the presence of an N-heterocyclic carbene( NHC) catalyst, the [4+ +2] annulation went smoothly to afford highly substituted1 ,4-oxathiin-2(3H)-ones, while the [3+ +2] annulation was observed to give 1,3-oxathioles in the absence of an NHC.

Recent Developments on the Synthesis of Various Sulfur‐Containing Heterocycles via [3+2]‐ and [4+2]‐Cycloaddition Reactions with Thiocarbonyls

“…With the use of a cinchona alkaloid‐derived catalyst (C3), the six‐membered product (as represented by structure 81 ) could be obtained with up to 83 : 17 er. Ye and co‐worker reported another similar [4+2]‐cycloaddition of dithioesters with various α‐chloroaldehydes in presence of a NHC‐catalyst . In this step‐wise reaction, dithioester behave as the heterodiene system and afforded various 1,4‐oxathiin‐2(3 H )‐ones.…”

“…Ye and co-worker reported another similar [4 + 2]-cycloaddition of dithioesters with various α-chloroaldehydes in presence of a NHC-catalyst. [39] In this stepwise reaction, dithioester behave as the heterodiene system and afforded various 1,4-oxathiin-2(3H)-ones.…”

N‐Heterocyclic Carbene (NHC)‐Catalyzed Transformations Involving Azolium Enolates

“…It may be noted in this context that, recently, Chi and co‐workers employed cyclohexadiene‐1,2‐diimines in a closely related [4+2] annulation for the enantioselective synthesis of dihydroquinoxaline derivatives [35] . Moreover, Ye and co‐workers used α‐ketodithioesters as the diene component for the synthesis of versatile 1,4‐oxathiin‐2(3 H )‐ones [36] …”

“…[35] Moreover, Ye and co-workers used αketodithioesters as the diene component for the synthesis of versatile 1,4-oxathiin-2(3H)-ones. [36] In 2013, Enders and co-workers described the NHCcatalyzed formal [2 + 3] annulation reaction of α-chloroaldehydes with nitrovinyl indoles 64 resulting in the enantioselective synthesis of pyrroloindolones 65 (Scheme 21). [37] The initial Michael addition of the NHC-bound enolate to the electron-poor double bond is followed by the intramolecular lactamization to afford the desired tricyclic product 65 in moderate to good yield and high diastereo-and enantioselectivity.…”

N-Heterocyclic Carbene-Mediated Carbon–Sulfur Bond-Forming Reactions

“…Subsequently, Ye and co-workers reported the [4 + 2] and [3 + 2] cyclizations of α-chloroaldehyde 177 and dithioester 178 (Figure A). The [4 + 2] cyclization provides a series of 1,4-oxathiin-2­(3 H )-ones 180 in the presence of carbene catalysts.…”