Pericyclic Reactions of Azafulvenium Methides Bearing Internal Dipolarophiles - Synthesis of Chromene and Chromane Derivatives

Catalyst- and Additive-Free Methodical Ring Expansion Protocol to Access Benzooxepino-Fused Pyrroles

An efficient metal-free, additive-free synthetic method was developed to access benzooxepino-fused pyrrole derivatives from alkynyl substituted aziridines. In this organic transformation, two new C-C bonds were formed via initial cleavage of C-C bond of aziridine ring by in situ generated azomethine ylides. Moderate to excellent yields of benzooxepino-fused pyrroles were obtained atom economically in the presence of t-BuOH in one-pot.

Chemistry of aza- and diazafulvenium methides in heterocyclic synthesis

Aza- and diazafulvenium methide systems are versatile building blocks for the synthesis of pyrroles and pyrazoles. These extended dipoles participate in sigmatropic [1,8]H shifts and 1,7-electrocyclizations giving vinyl pyrroles and pyrazoles. Under flash vacuum pyrolysis conditions these heterocycles undergo interesting rearrangements. Aza- and diazafulvenium methides can be intercepted by dipolarophiles. Derivatives with carboxylate groups at C-4 and/or C-5 act exclusively as 1,7-dipoles affording products resulting from the addition across the 1,7-positions. These 1,7-cycloadducts include chlorin and bacteriochlorin type macrocycles as well as steroidal analogues, compounds with relevance in medicinal chemistry. In contrast with this chemical behavior, 5-trifluoromethylazafulvenium methides can participate in both 1,7- and 1,3-dipolar cycloadditions. The generation and reactivity of benzodiazafulvenium methides is also discussed.

Synthesis of chiral hexacyclic steroids via [8π + 2π] cycloaddition of diazafulvenium methides

16-Dehydropregnenolone acetate reacted with diazafulvenium methides to afford novel chiral 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused steroids.