A New Co₂(CO)₈-Mediated Tandem [5 + 1]/[2 + 2 + 1]-Cycloaddition Reaction:  A One-Pot Synthesis of Tricyclic δ-Lactones from cis-Epoxy Ene-ynes

Abstract

In the presence of Co2(CO)8 and CO, cis-epoxyalkynes bearing a tether olefin undergo a tandem [5 + 1]/[2 + 2 + 1]-cycloaddition to give tricyclic delta-lactones efficiently in a one-pot operation. The reaction mechanism is proposed to involve a cobalt-coordinated cyclic allene species.

[4 + 2]-Annulation of MBH-Acetates of Acetylenic Aldehydes with Imidazoles/Benzimidazoles To Access Imidazo[1,2-a]pyridines/Benzimidazo[1,2-a]pyridines

“…The necessity of ester group to facilitate the cycloisomerization was also tested by the treatment of enynyl imidazole 10 , prepared from the known enynyl bromide, with DBU in the presence of CuI (10 mol %) in CH 3 CN at 80 °C and observed that the reaction was futile (Scheme ). This result strongly confirms the vital role of the ester group in providing the desired product.…”

Oxidative Aza-Annulation of Enynyl Azides to 2-Keto/Formyl-1H-pyrroles

“…To a stirred solution of the corresponding known alcohol (prepared using the literature protocol) (5.8 mmol) in 10 mL of toluene were added DBU (6.9 mmol) and DPPA (diphenylphosphoryl azide, 6.9 mmol) at 0 °C, and the mixture was stirred at room temperature for 3 h. After completion of the reaction (monitered by TLC), the mixture was diluted with 3 N HCl (10 mL) and extracted with EtOAc (2 × 10 mL). The combined organic layers were dried over anhydrous Na 2 SO 4 and concentrated in vacuo.…”

Dicobaltoctacarbonyl-Mediated Synthesis of Tricyclic 5,6-Diydropyran-2-one Derivatives via Tandem Cycloaddition Reaction between cis-Epoxyalkynes, a Tethered Olefin, and Carbon Monoxide

“…14 By analogy with this cyclization, we envisage that a suitable metal complex should implement a [5 + 1]-cyclization of epoxyalkynes with one CO molecule to generate δ-lactonyl allene species A as depicted in eq 2. Within our continuing interest in the metal-catalyzed coupling reaction of epoxides, we report here a new and efficient Co 2 (CO) 8 -mediated tandem cycloaddition of epoxyalkyne, alkene, and CO to produce tricyclic δ-lactones via a formal [5 + 1]/[2 + 2 + 1]-cycloaddition mode (eq 3) . This unusual reaction is thought to arise from an intramolecular [2 + 2 + 1]-cycloaddition of the hypothetical cyclic allene intermediate A ‘.…”

“…For 1,1,2-trisusbtituted epoxides (R 2 ≠ H), the corresponding Co 2 (CO) 6 -stabilized allene species A ‘ undergoes an alternative [2 + 2]-cycloaddition to form a fused cyclobutane ring. Here we report the details of such substrate-dependent cycloaddition reactions 1 …”