Rh(II)-Catalyzed Spirocyclization of α-Diazo Homophthalimides with Cyclic Ethers

α-Carbonyl Rh-Carbenoid Initiated Cascade Assembly of Diazobarbiturates with Alkylidene Pyrazolones for Synthesis of Spirofuropyrimidines

Catalyzed by Rh2(esp)2 (10 mol%) and (±)-BINAP (20 mol%) in DCE at 80 °C, the cascade assembly between diazobarbiturates and alkylidene pyrazolones proceeded readily and produced spiro-furopyrimidines in 38-96% chemical yields. The chemical structure of the prepared spirofuro-pyrimidines was firmly confirmed by X-ray diffraction analysis.

Access to Spiro Bis-β-lactams via a Metal-Free Microwave-Assisted Wolff Rearrangement/Staudinger [2+2] Cycloaddition Cascade Involving 3-Diazotetramic Acids and Imines

Herein, we report the study of the thermally promoted reaction of 3-diazotetramic acids with imines as a rapid route to a novel spiro heterocyclic scaffold, spiro bis-β-lactams (2,6-diazaspiro[3.3]heptane-1,5-diones). The transformation proceeds via metal-free microwave-assisted Wolff rearrangement of the diazo reagent followed by Staudinger [2+2] cycloaddition of the heterocyclic ketenes with Shiff bases. This methodology enables the preparation of diastereomerically pure spiro bis-β-lactams in high yields and provides an avenue for exploring new versions of the privileged β-lactam core for drug design.

Photoinduced Synthesis of Functionalized Oxacyclic Spirooxindoles Via Ring Expansion

A versatile photochemical ring-expansion protocol for the synthesis of oxacyclic spirooxindoles under catalyst-free conditions is described. The reaction is enabled by the use of unstrained O-containing heterocycles with 3-diazoindolin-2-ones under visible-light irradiation. Several synthetic advantages for this method are exhibited, including mild conditions, good functional group tolerance, operational simplicity, and scalability. Mechanistic studies indicate that the transformation may proceed through the formation of oxonium ylide intermediate followed by an ionic cyclization.

Unexpected Ring Contraction of Homophthalic Anhydrides under Diazo Transfer Conditions

An attempted Regitz diazo transfer onto homophthalic anhydride led to the discovery of an unexpected ring contraction, which gave N-sulfonyl phthalide-3-carboxamide derivatives. The reaction is thought to proceed via a [3 + 2] cycloaddition of the substrate's enol form and the azide followed by a two-step fragmentation of the intermediate 1,2,3-triazoline with a loss of the nitrogen molecule.

Unusual highly diastereoselective Rh(II)-catalyzed dimerization of 3-diazo-2-arylidenesuccinimides provides access to a new dibenzazulene scaffold

Formation of unusual unsymmetrical dimers or/and indenes via Rh2(esp)2-catalyzed decomposition of 3-diazo-2-arylidenesuccinimides has been investigated. The reaction proceeded under mild conditions, and its result was shown to strongly depend on the nature of the substituents in the diazo substrate. The new reaction provides access to dibenzoazulenodipyrrole and indenopyrrole derivatives in moderate to high yield. Dibenzoazulenodipyrroles bearing alkyl substituents at the nitrogen atom showed pronounced cytotoxocity against the A549 human lung adenocarcinoma cell line while N-aryl analogs were non-cytotoxic.

Synthesis of Spiro[benzo[d][1,3]oxazine-4,4'-isoquinoline]s via [4+1+1] Annulation of N-Aryl Amidines with Diazo Homophthalimides and O2

Synthesis of spiro[benzo[d][1,3]oxazine-4,4'-isoquinoline]s through a unique [4+1+1] annulation of N-aryl amidines with diazo homophthalimides and O₂ is presented. This unprecedented spirocyclization reaction features readily obtainable substrates, structurally and pharmaceutically attractive products, a cost-free and clean oxygen source, sustainable reaction medium, tolerance of a broad spectrum of functional groups, and an interesting reaction mechanism based on sequential C(sp²)-H/C(sp³)-H bond cleavage and oxygen insertion.

Recent synthetic strategies toward the synthesis of spirocyclic compounds comprising six-membered carbocyclic/heterocyclic ring systems

Spirocyclic compounds fascinate the synthetic chemists due to their privileged ring system and efficacy in drug discovery. Many natural compounds comprise spirocyclic moiety in their skeleton and are effective in pharmaceutical industry. Over the years, many synthetic methodologies have been established for the construction of spirocyclic compounds. In this review, recent synthetic approaches to accessing various spirocompounds comprising six-membered carbocycles/heterocycles have been summarized.

Natural-Like Spirocyclic Δα,β -Butenolides Obtained from Diazo Homophthalimides

α-Diazo homophotalimides were reacted with various propiolic acids on Rh₂ (esp)₂ catalysis. The resulting propiolate esters were transformed into novel, heterocyclic Δ^α,β -spirobutenolides in good to excellent product yields. The approach represents a fundamentally novel entry into natural-like Δ^α,β -spirobutenolides present in many biologically active natural products as well as fully synthetic compounds endowed with diverse biological activities. The Δ^α,β -spirobutenolides thus obtained were shown to inhibit thioredoxin reductase, a selenocysteine enzyme target for cancer. Moreover, for the best compound in the series (TrxR IC₅₀ 1.49±0.08 μM), by using MALDI-TOF mass-spectrometry it was shown that it selectively binds selenocysteine in the presence of a 10-fold excess of cysteine. This validates the new compound as a promising lead for anticancer therapy development.

Spirocyclizations Involving Oxonium Ylides Derived from Cyclic α-Diazocarbonyl Compounds: An Entry into 6-Oxa-2-azaspiro[4.5]decane Scaffold

New types of cyclic diazo compounds capable of Rh(II)-catalyzed spirocyclizations with tetrahydrofuran have been discovered. The formation of the spirocyclic framework is thought to proceed via the formation of Rh(II) carbene species followed by interaction with the Lewis basic oxygen atom of tetrahydrofuran to give oxonium ylide species. The latter evolves predominantly via the Stevens type rearrangement leading to an [n + 1] ring expansion of the tetrahydrofuran moiety, which results in the formation of a medicinally relevant 6-oxa-2-azaspiro[4.5]decane scaffold. The spirocyclization process was often observed in competition with mechanistically distinct C-H insertion into a tetrahydrofuran molecule. This competing process gave compounds based on the 3-(tetrahydrofur-2-yl)pyrrolidine scaffold, which are also relevant from the medicinal chemistry standpoint. These findings enrich the available arsenal of metal-catalyzed spirocyclization methods based on the use of cyclic diazo compounds.

Palladium catalyzed asymmetric allylic alkylation of isoquinolinedione derivatives in the preparation of quaternary carbon stereocenters

A highly enantioselective allylic alkylation of isoquinolinedione derivatives under palladium catalysis was developed in the preparation of quaternary carbon stereocenters. Under standard reaction conditions, excellent yields and enantioselectivities were realized and the products could be transformed into dihydroisoquinolone with vicinal chiral carbon centers or THIQ core structures in short steps with high yields.

Enantioselective synthesis of isoquinoline-1,3(2H,4H)-dione derivatives via a chiral phosphoric acid catalyzed aza-Friedel-Crafts reaction

A highly enantioselective aza-Friedel-Crafts reaction of structurally new ketimines with indoles and pyrrole is developed by using a chiral phosphoric acid as the catalyst. This protocol enables the first enantioselective synthesis of isoquinoline-1,3(2H,4H)-dione derivatives in good to excellent yields (up to 99% yield) and excellent enantioselectivities (up to >99% ee).