Brønsted acid mediated intramolecular cyclopropane ring expansion/[4 + 2]-cycloaddition

A cascade reaction of 3-hydroxy-2-phenylisoindolin-1-one and cyclopropyl ketone has been developed via BrØnsted acid-promoted ring-opening/intramolecular cross-cycloaddition/[4 + 2]-cycloaddition process.

A General Protocol for Radical Anion [3+2] Cycloaddition Enabled by Tandem Lewis Acid Photoredox Catalysis

A method for intermolecular [3+2] cycloaddition between aryl cyclopropyl ketones and alkenes involving the combination of Lewis acid and photoredox catalysis is reported. In contrast to other more common methods for [3+2] cycloaddition, these conditions operate using a broad range of both electron-rich and electron-deficient reaction partners. The critical factors predicting the success of these reactions is the redox potential of the cyclopropyl ketone and the ability of the alkene to stabilize a key radical intermediate.

C–O Ring Construction: The Reisman Synthesis of (–)-Acetylaranotin

The Prins cyclization is a powerful approach for the construction of oxygen-containing heterocycles. B.V. Subba Reddy at the Indian Institute of Technology has reported (Tetrahedron Lett. 2012, 53, 3100) an approach to 2,6-dioxabicyclo[3.2.1]octanes 2 by way of a tandem Prins reaction/intramolecular acetalization of the diol 1 and a variety of aldehydes. Christine L. Willis of the University of Bristol utilized (Angew. Chem. Int. Ed. 2012, 51, 3901) nontraditional γ, δ-unsaturated alcohols 3 for a Prins-type strategy to access bicyclic heterocycles 5, while Zhenlei Song of Sichuan University employed (Angew. Chem. Int. Ed. 2012, 51, 5367) a bis(silyl) homoallylic alcohol 7 in the synthesis of structures such as 8, corresponding to the B ring of the bryostatins. In a mechanistically related process, the conversion of unsaturated ketones 9 to tetrahydropyranyl products 11 by treatment with a boronic acid 10 and triflic anhydride was described (Org. Lett. 2012, 14, 1187) by Aurelio G. Csáky at the Universidad Complutense in Spain. A powerful approach to heterocycles is via the ring expansion of smaller, and especially strained, ring systems. Jon T. Njardarson of the University of Arizona has been exploring such strategies and has reported (Angew. Chem. Int. Ed. 2012, 51, 5675) the conversion of vinyl oxetanes to dihydropyrans via catalysis by transition metals or Brønsted acids. The use of a chiral catalyst such as 13 allowed for the enantioselective conversion of divinyl oxetane 12 to enantioenriched dihydropyran 14. Meanwhile, Amir H. Hoveyda at Boston College and Richard R. Schrock at MIT have developed (J. Am. Chem. Soc. 2012, 134, 2788) a highly reactive and stereoselective catalyst for the ring-opening/cross-metathesis of several ring systems such as 15 with enol ethers. Notably, reactions occur rapidly (e.g., 10 min) using as little as 0.15 mol% catalyst. An alkynyl cyclopropyl ketone such as 17 can be converted (Angew. Chem. Int. Ed. 2012, 51, 4112) to products 18 by treatment with a gold/silver catalyst mixture, as shown by Zhongwen Wang at Nankai University. Notably, the oxabicyclic ring structure contained within 18 is present in a diversity of natural product structures.