Highly Stereoselective Synthesis of 2,5-Disubstituted 3-Vinylidene Tetetrahydrofurans via Prins-Type Cyclization

Chiral Phosphoric-Acid-Catalyzed Cascade Prins Cyclization

Asymmetric Prins cyclization of in situ generated quinone methides and o-aminobenzaldehyde has been developed with chiral phosphoric acid as an efficient catalyst. This unconventional method provides a facile access to diverse functionalized trans-fused pyrano-/furo-tetrahydroquinoline derivatives in excellent yield and with excellent diastereo- and enantioselectivities (up to 99% yield and 99% ee). Mechanistic studies suggested that the three adjacent tertiary stereocenters were constructed through the sequential formation of C-O, C-C, and C-N bonds.

Exploitation of Cyclopropane Carbaldehydes to Prins Cyclization: Quick Access to ( E)-Hexahydrooxonine and Octahydrocyclopenta[ b]pyran

A single-step TiX₄-mediated Prins-type cyclization of cyclopropane carbaldehydes with 3-buten-1-ol for the highly stereoselective construction of relatively strained ( E)-hexahydrooxonines is reported. Switching the alcohol to 3-butyn-1-ol prompted a similar route, augmented by another cyclization within a nine-membered ring to afford a bicyclized product (4,4-dihalo-5-aryloctahydrocyclopenta[ b]pyran). Easy transformation of the resulting geminal dihalide to a vinyl halide and a ketone further supplemented the substance of this approach.

Intramolecular alkyne–dithiolium cycloaddition: a joint experimental and DFT mechanistic study

The intramolecular alkyne–dithiolium cycloaddition reactions were found to occur through either a concerted or stepwise pathway, depending on the conjugation degree of the alkynyl unit.

Electronic effects on a one-pot aromatization cascade involving alkynyl-Prins cyclization, Friedel–Crafts alkylation and dehydration to tricyclic benzo[f]isochromenes

A short, efficient cyclization/aromatization cascade affords tricyclic 1,4-dihydro-2H-benzo[f]isochromenes. Electronic factors suggest concerted alkynyl-Prins and Friedel–Crafts reactions in the sequence.

Prins cyclization of α-bromoethers under basic conditions

α-Bromoethers have been found to undergo Prins-type cyclization under basic conditions and without the need to add a promoter. The products are those derived from a Markovnikov addition on the pendant alkene. However, the stereochemistry and even the structure of the products sometimes differ from those expected with the classical Lewis-acid-catalyzed Prins reaction of acetals.

[2-(4-Chlorophenyl)-5-phenyloxolan-3-yl](cyclopentenyl)methanone

In the title compound, C₂₂H₂₁ClO₂, the oxolane ring adopts a twisted conformation. The dihedral angles between the mean plane of the oxolane ring and the mean planes of the 4-chloro­phenyl, phenyl and cyclo­pentenyl rings are 71.81 (18), 76.9 (18) and 82.08 (18)°, respectively.

How easy are the syntheses of allenes?

This short review highlights some of the recent important developments on the synthesis of allenes and its applications on the synthesis of some allenic natural products and allenic-based optoelectronic materials.

Gold-catalyzed reactions of 1,5- and 1,6-enynes with carbonyl compounds: cycloaddition vs. metathesis

Gold and rings: The gold(I)-catalyzed addition of aldehydes to 1,6-enynes gives 1,3-dienes, by a cycloaddition/fragmentation process. 1,5-Enynes react with aldehydes and ketones by the 5-endo-dig pathway to give the corresponding cycloadducts.

An Improved Catalyst System for the Iron-Catalyzed Intermolecular Ring-Expansion Reactions of Epoxides

A considerable improvement is reported in the iron-catalyzed ring-expansion reactions of epoxides generating tetrahydrofuran derivatives by formal insertion of an alkene. Optimization of the catalyst system revealed that a preformed [Fe(salen)] complex minimizes the formation of polymerization side-products so that increased yields of intermolecular reactions were obtained. However, more importantly, the scope of the reaction could also be enlarged considerably. The iron-catalyzed ring-expansion reaction can now be applied to some styrene oxide derivatives, acting as radical donors, as well as to a wide variety of acceptor-substituted acyclic alkenes and cyclic dienes that act as radical acceptors. The use of unsymmetrical radical acceptors led to interesting questions concerning the regiochemistry of the reactions. The conservation of the stereochemistry of the starting materials in the products was investigated through a study of the reactions of E- and Z-configured acceptor-substituted double bonds. The reactions of fumaric and maleic esters were performed and the ratios of diastereomeric and regioisomeric products were determined.

5-Exocyclic Products, 2,3,5-Trisubstituted Tetrahydrofurans via Prins-Type Cyclization

[reaction: see text] 5-Exocyclic products, 2,3,5-trisubstituted tetrahydrofurans, were synthesized from homopropargylic alcohols with terminally substituted alkynes and various aldehydes via Prins-type cyclization. It is of interest that the exocyclic vinyl cation generated as a result of Prins-type cyclization could be trapped as a vinyl triflate when CH2Cl2 was used as a solvent, whereas in ethereal solution the vinyl cation underwent hydrolysis to give the corresponding ketone product.