Metal‐Free Dehydrogenative Diels–Alder Reactions of 2‐Methyl‐3‐Alkylindoles with Dienophiles: Rapid Access to Tetrahydrocarbazoles,Carbazoles, and Heteroacenes

An unprecedented strategy for in situ generation of indole‐based ortho‐quinodimethanes (oQDMs) from 2‐methyl‐3‐alkylmethylindoles by either a metal‐free DDQ‐ or BQ‐mediated dehydrogenative process was developed. These oQDMs were trapped by electron‐deficient dienophiles to provide a facile approach to synthetically valuable tetrahydrocarbazoles, carbazoles, and hetereoacenes. The salient features of this transformation include direct C(sp³)? H bond functionalizations, readily available starting materials, metal‐free conditions, high efficiency, operational simplicity, and ease of scale‐up.     

A Tunable and Enantioselective Hetero‐Diels–Alder Reaction Provides Access to Distinct Piperidinoyl Spirooxindoles

The active complexes of chiral N,N′‐dioxide ligands with dysprosium and magnesium salts catalyze the hetero‐Diels–Alder reaction between 2‐aza‐3‐silyloxy‐butadienes and alkylidene oxindoles to selectively form 3,3′‐ and 3,4′‐piperidinoyl spirooxindoles, respectively, in very high yields and with excellent enantioselectivities. The exo ‐selective asymmetric cycloaddition successfully regaled the construction of sp³‐rich and highly substituted natural‐product‐based spirooxindoles supporting many chiral centers, including contiguous all‐carbon quaternary centers.     

Synthesis of 3,4,5‐Trisubstituted Isoxazoles from Morita–Baylis–Hillman Acetates by an NaNO2/I2‐Mediated Domino Reaction

An efficient NaNO₂/I₂‐mediated one‐pot transformation of Morita–Baylis–Hillman (MBH) acetates into alkyl 3‐nitro‐5‐(aryl/alkyl)isoxazole‐4‐carboxylates is described. In a cascade event, initial Michael addition of NaNO₂ to the MBH acetate furnishes the allylnitro intermediate which undergoes I₂‐catalyzed oxidative α‐C H nitration of the nitromethyl subunit followed by [3+2] cycloaddition to afford the title compounds. Structural elaborations of these highly substituted isoxazoles by S_NAr reactions and hydrogenolysis allows access to useful products.