Synthesis of 2-[2-(Ethoxymethoxy)phenyl]spiro[cyclopropane-1,2′-indene]-1′,3′-dione

An 1,3-indanedione-derived donor–acceptor cyclopropane, bearing the ethoxymethyl-protected phenolic group at the ortho-position of the donor aryl substituent, has been synthesized using a reaction sequence involving the Knoevenagel condensation of 1,3-indanedione with the corresponding protected salicylaldehyde followed by the Corey–Chaykovsky cyclopropanation of the obtained adduct with dimethylsulfoxonium methylide. The structure of the synthesized cyclopropane was unambiguously proved by single-crystal X-ray diffraction data.

Relieving the stress together: annulation of two different strained rings towards the formation of biologically significant heterocyclic scaffolds

Small carbo- and heterocycles have become versatile building blocks owing to their intrinsic ring strain and ease of synthesis. However, the traditional approaches of heterocycle synthesis involved the combination of one strained-carbocycle or heterocycle with one unsaturated molecule. On the contrary, there is an exciting possibility of combining two different strained rings to furnish varieties of heterocycles, where one of the strained rings can act as a valuable alternative to the unsaturated molecule. These strategies are also useful to access multi-functionalized rings. Despite these distinctive synthetic benefits, this chemistry has not drawn considerable attention of the community. In this minireview, we explicitly choose this topic to reveal the unexplored possibilities with these different strained rings. This minireview provides comprehensive details with the mechanistic rationale about the reactivity of these pairs of small rings when they are allowed to react together in the presence of different Lewis acids. Subsequently, it will also open a new avenue for heterocycle synthesis.

In(OTf)3-Catalyzed Cascade Cyclization for Construction of Oxatricyclic Compounds

A highly diastereoselective cascade cyclization reaction has been developed for establishing a series of oxatricyclic compounds using Chan's diene and simple keto alkynal substrates with only 1 mol % of In(OTf)₃ as the catalyst in 82-92% yields. The potential utility of this synthetic strategy has been demonstrated in model studies for the construction the core structures of 1α,8α:4α,5α-diepoxy-4,5-dihydroosmitopsin and cortistatin A.

Lewis Acid Triggered Vinylcyclopropane-Cyclopentene Rearrangement

We report a mild Lewis acid induced isomerization of donor-acceptor cyclopropanes, containing an alkenyl moiety and diverse electron-withdrawing group(s) at the adjacent positions, into substituted cyclopentenes. We have found that 1,1,2-trisubstituted cyclopent-3-enes were exclusively obtained in yield of 51-99% when cyclopropanes with a 2-substituted alkenyl group as a donor underwent isomerization. For cyclopropanes bearing a trisubstituted alkenyl group either the corresponding cyclopent-3-enes or isomeric cyclopent-2-enes having two acceptor groups at the C(1) atom were formed, with the reaction selectivity being determined by the applied Lewis acid. We have shown that the reactivity of the donor-acceptor cyclopropane increases with the increase of the electron-donating character of (hetero)aromatic group attached to the alkenyl moiety. The synthetic utility of the developed methodology was also demonstrated through the synthesis of polysubstituted cyclopentane and piperidine derivatives.

Lewis acid catalyzed [3 + 2] annulation of ketenimines with donor–acceptor cyclopropanes: an approach to 2-alkylidenepyrrolidine derivatives

C,C,N-Trisubstituted ketenimines undergo [3 + 2] annulation reactions via its CN bond with donor–acceptor cyclopropanes triggered by Sc(OTf)₃ to yield a diversity of 2-alkylidene pyrrolidines.

Stereoselective synthesis of octahydrocyclohepta[c]pyran-6(1H)-one scaffolds through a Prins/alkynylation/hydration sequence

Aldehydes undergo a smooth coupling with (E/Z)-non-3-en-8-yn-1-ol in the presence of 10 mol% of CuX and BF3·OEt2 under mild conditions to produce a novel class of octahydrocyclohepta[c]pyran-6(1H)-one derivatives in good yields with excellent diastereoselectivity through a sequential Prins/alkynylation/hydration. This is the first report on the termination of Prins cyclization with a tethered alkyne.

Metal-free, one-pot, sequential protocol for transforming α,β-epoxy ketones to β-hydroxy ketones and α-methylene ketones

A new sequential, one-pot protocol for transforming 1,3-disubstituted 2,3-epoxy ketones to β-hydroxy ketones and α-methylene ketones has been developed. Reaction of epoxy ketones with boron trifluoride etherate (BF3·OEt2) generates the cationic intermediates by regioselective epoxide ring opening and an acyl shift. Then, a treatment of these cations with 2-aryl-1,3-dimethylbenzimidazolines (DMBIH) results in formation of 1,2-disubstituted 3-hydroxy ketones. DMBIH serves as a hydride donor in the second step of this process. Finally, the β-hydroxy ketones can be converted to 1,2-disubstituted 2-methylene ketones by treatment with methanesulfonic acid or a combination of methanesulfonyl chloride and triethylamine. Importantly, the sequential steps involved in formation of the α-methylene ketone products can be carried out in one pot.