The Diels–Alder reactivity of 2-vinylindenes: synthesis of functionalized tetrahydrofluorenes

Brønsted-Acid-Catalyzed Intramolecular Carbonyl–Olefin Reactions: Interrupted Metathesis vs Carbonyl-Ene Reaction

“…In closely related work, Schindler’s group also found that Brønsted-acid catalysts (triflic acid, TfOH ) promote a related but qualitatively different mode of reactivity where carbonyl–olefin ( CO ) substrates are selectively transformed into tetrahydrofluorene ( TDF ) products in a single synthetic step. This transformation is a significant advance over strategies to form TDF that involves a series of transition-metal-catalyzed cycloaddition reactions, , while also avoiding unwanted polymerization and isomerization of olefins. , The TfOH -catalyzed transformation was shown to be applicable to a variety of electronically and sterically functionalized CO substrates, highlighting the potential of this promising synthetic approach …”

Interrupted carbonyl-olefin metathesis via oxygen atom transfer

“…Whereas this multiple bond-forming process selectively yields the tetrahydrofluorene product 9, the presence of a Brønsted acid has previously been reported to be detrimental to many Lewis acidcatalyzed carbonyl-ene and Prins reactions, resulting in undesired polymerization and isomerization of olefins (22). Furthermore, this interrupted carbonyl-olefin metathesis reaction provides access to tetrahydrofluorene product 9 in a single synthetic transformation, whereas current strategies rely on multistep sequences and precious metal-catalyzed cycloisomerizations (23,24). Tetrahydrofluorenes represent key structural elements in materials science (10) (25) and are ubiquitous core structures in biologically active natural products (11) (26).…”

Bis(1-cyanovinyl acetate) Is a Linear Precursor to 3-Oxidopyrylium Ions

“…Since that time, diene 1 has wallowed in relative obscurity. We initially posited that 1 could serve as a masked bisketene equivalent and might undergo a double Diels–Alder cycloaddition reaction with dienes such as vinylindene 2a (Scheme ) to give symmetrical tetrahydrofluorene dimers (e.g., 3 ). , However, upon heating diene 1 with vinylindene 2a , we found, much to our surprise, that diene 1 instead undergoes rearrangement and cycloaddition reactions (with 2 equiv of 2a ) to give a diastereomeric mixture of a bis-adduct (the connectivity of which was confirmed by X-ray crystallographic analysis of the major diastereomer, 4a ) (see the Supporting Information for details). , …”

“…1-(toluene-4-sulfonyl)-3-vinyl-1 H -indole 2m was synthesized according to a literature procedure . 2-Vinyl-1 H -indene 2a , 4-isopropylstyrene 2e , 4-trifluoromethylstyrene 2h , benzyl vinyl ether 2k , 2-bromo-4,5-dimethoxystyrene 2n , and 2-acetoxy-4,5-dibromostyrene 2o were synthesized as described below. All other commercially available reagents were used as purchased (i.e., with stabilizers).…”