Structurally Novel Bi- and Tricyclic β-Lactams via [2 + 2] Cycloaddition or Radical Reactions in 2-Azetidinone-Tethered Enallenes and Allenynes

Heterogeneous Photocatalytic Ring Expansion of Cyclic Ketones for the Construction of Medium-Sized Lactams

Construction of medium-sized ring compounds remains challenging in synthetic chemistry. Herein, we describe the synthesis of medium-sized lactams via a photoinduced ring expansion of benzo-fused cyclic ketones using graphitic carbon nitride (g-C3N4) as a photocatalyst. The ring expansion protocol provided an efficient access to 8-10-membered lactams in good yields and displayed good tolerance to a range of functional groups. The mechanism studies revealed that the photochemical reaction proceeds via an intermediary of a nitrogen radical, which is generated through an oxidative hydrogen atom transfer (HAT) process.

Copper-Catalyzed Tandem Cross-Coupling/Thermally Promoted [2 + 2] Cycloaddition of 1,6-Enynes and Diazo Compounds To Assemble Methylenecyclobutane-Fused Ring System

An unprecedented copper-catalyzed tandem reaction of 1,6-enynes with diazo compounds via a cross-coupling/[2 + 2] cycloaddition sequence was reported. A library of methylenecyclobutane-fused ring systems including cyclobuta[b]indolines, cyclobuta[b]benzofuran, benzo[b]cyclobuta[d]thiophene, and bicyclo[3.2.0] structures were obtained in moderate to excellent yields under very mild reaction conditions. The reaction exhibited high proximal-regioselectivity and diastereoselectivity. Moreover, 1,6-allenene has proven to be the key intermediate and proceeds via a thermally promoted [2 + 2] cycloaddition in the absence of copper catalyst.

Mechanistic Aspects and Synthetic Applications of Radical Additions to Allenes

More than 50 years have passed since Haszeldine reported the first addition of a trifluoromethyl radical to an allene; in the intervening years, both the chemistry of allenes and the reactivity of single-electron species have become topics of intense interest. In this Review, we provide an overview of the fundamentals of radical additions to allenes and highlight the emergence of theoretical and experimental evidence that reveals unique reactivity patterns for radical additions to allenes as compared with other unsaturated compounds. Factors capable of exerting control over the chemo-, regio-, and stereoselectivities of the attack of carbon- and heteroatom-based radicals at each of the three potential reactive sites in an allene substrate are described. These include reaction conditions, the nature of the attacking radical, the substitution pattern of the allene, and the length of the linker between the radical center and the proximal allene carbon in the substrate. Cycloaddition reactions between allenes and partners containing π-bonds, which are likely to proceed through radical pathways, are presented to highlight their ability to rapidly access complex polycyclic scaffolds. Finally, the synthetic utility of the products arising from these chemistries is described, including their applications to the construction of complex molecules.

Recent developments in the synthesis of condensed β-lactams

β-Lactams are important heterocycles with diverse pharmacological profiles and have emerged as useful organic synthons.

Cyclization reactions of bis(allenes) for the synthesis of polycarbo(hetero)cycles

This review presents the reactivity of a variety of substituted bis(allenes) under different reaction conditions, showing the high number of complex structures that can be obtained.

Chameleon Reactivity of the Allene Bond of 4-Vinylidene-2-oxazolidinone: Novel Through-Space Conjugative Nucleophilic Addition of Electron-Rich Alkenes and Hetero-Nucleophiles

The Calpha==Cbeta double bond of allene carbamates 1 serves as an electron acceptor similar to the double bond of conjugated enones by means of a through-space interaction with the N--SO2 bond; the carbamate double bond is thus subject to nucleophilic addition for a wide variety of nucleophiles, which proceeds under mild conditions by heating at 70-100 degrees C. Depending on the kind of nucleophiles, 1 displays three different reaction modes: 1) Typically enol ethers and allylsilanes promote 1,3-sulfonyl migration of 1 and undergo the inverse electron demand Diels-Alder reaction with the 1-aza-1,3-butadiene intermediates II thus formed to furnish bicyclic 2-alkoxy-5-sulfonyltetrahydropyridines 2 and 2-silylmethyl-5-sulfonyltetrahydropyridines 3, respectively, with high regio- and stereoselectivity and retention of configuration of the double bonds of these electron-rich alkenes; 2) silanes (RnSiH4-n, n=1-3) and thiols deliver the hydride and the thiolate at the Cbeta carbon and promote the 1,3-sulfonyl migration, followed by protonation of the thus-formed carbamate anion (Z)-III to provide, for example, (Z)-4 a and (Z)-4 j, respectively; 3) alcohols simply add to the Calpha==Cbeta double bond and provide (E)-6. Usually, the reaction with alcohols is accompanied by the second pathway, giving rise to, for example, (Z)-4 b in addition to (E)-6 b. Phenol engages in the third pathway and provides (E)-6 g exclusively. Heteroaromatics, such as furans and benzofurans follow the first pathway, however, in a different regioselectivity from enol ethers and allylsilanes, delivering the oxygen atom at the 3-position of 5-sulfonyltetrahydropyridines (2 g and 2 h). Indoles, on the other hand, show a dichotomy, equally enjoying the first and the third pathways and provide mixtures of (E)-7 and (E)-8, respectively.

Indium-Mediated Selective Introduction of a 1,3-Butadien-2-yl Group at the C4-Position in 2-Azetidinones and Application of 1,3-Diene-Tethered 2-Azetidinones in the Diels–Alder Reaction

The reaction of 4-acetoxy-2-azetidinones with organoindium reagents generated in situ from indium and 1,4-dibromo-2-butyne in the presence of LiCl in DMF selectively produced 2-azetidinones which contain a 1,3-butadienyl-2-yl group at the C4-position in good yields. The Diels-Alder reaction of 4-[(1-methylene)prop-2-enyl]-2-azetidinones with a variety of dienophiles provided 2-azetidinones with valuable functional-group-substituted six-membered rings at the C4-position in good yields.

A Highly Regio- and Stereoselective Formation of Bicyclo[4.2.0]oct-5-ene Derivatives through Thermal Intramolecular [2 + 2] Cycloaddition of Allenes

Thermal [2 + 2] cycloaddition of allenes with an additional multiple bond is described. By simply heating the allenenes or allenynes having a three-atom tether in an appropriate solvent such as dioxane or DMF, the distal double bond of the allenic moiety regioselectively participates in the cycloaddition to form bicyclo[4.2.0]oct-5-ene derivatives in good to excellent yields. In all the reactions of allenenes, the olefin geometry was completely transferred to the cycloadducts. While the reaction of terminal allenes afforded bicyclic cyclobutane derivatives as a single isomer, the cycloaddition of some internal allenes with axial chirality yielded a diastereomeric mixture of cycloadducts. These results are in good accordance with the stepwise mechanism through a biradical intermediate with a coplanar allyl radical.

Carbonyl Allenylation/Free Radical Cyclization Sequence as a New Regio- and Stereocontrolled Access to Bi- and Tricyclic β-Lactams

A novel approach to racemic and enantiopure nonconventional fused bi- and tricyclic beta-lactams has been developed by using regio- and stereocontrolled intramolecular free radical reactions in monocyclic 2-azetidinone-tethered allenynes and haloallenes. The access to allene cyclization precursors was achieved by metal-mediated carbonyl allenylation of appropriately substituted 4-oxoazetidine-2-carbaldehydes in an aqueous environment. The tin-promoted radical cyclizations of allene-beta-lactams are totally regioselective for the central allenic carbon, providing bi- and tricyclic beta-lactams containing a seven-membered ring.

A computational investigation on the sequential rearrangement mechanism of 2-allyl-2,4,5-hexatrienaldehyde involving [1,5]-hydrogen migration and 8π-electrocyclization

The sequential rearrangement reaction mechanism of the 2-allyl-2,4,5-hexatrienaldehyde has been studied at the unrestricted Becke three-parameter hybrid functional combined with Lee-Yang-Parr correlation functional (UB3LYP)/6-31G**, Complete Active Space Self-Consistent Field (CAS (8,8))/6-31G**, Configuration Interaction with Single and Double Excitations (CISD)//UB3LYP/6-31G** and the second-order perturbation theory based on the CASSCF reference wave function (CASPT2)//CAS(8,8)/6-31G** levels. Two pathways have been found to be involved for this reaction. The first pathway includes four processes of the rotation of the C3--C4 single bond, the stepwise [2 + 2] cycloaddition, the [1,5]-hydrogen migration, and the ring opening isomerization, while the second pathway undergoes only two processes of the [1,5]-hydrogen migration and the 8pi-electrocyclization. The calculation results indicate that the second pathway is favorable, in good agreement with the recent experimental observation. The whole reaction is stepwise and strong exothermic.

Synthesis of Strained Tricyclic β-Lactams by Intramolecular [2+2] Cycloaddition Reactions of 2-Azetidinone-Tethered Enallenols: Control of Regioselectivity by Selective Alkene Substitution

A convenient metal-free methodology for the preparation of structurally novel, strained tricyclic beta-lactams containing a cyclobutane ring has been developed. The first examples accounting for the intramolecular [2+2] cycloaddition reactions in beta-lactams have been achieved by the thermolysis of 2-azetidinone-tethered enallenols, which have been prepared in aqueous media by regio- and diastereoselective indium-mediated carbonyl allenylation of 4-oxoazetidine-2-carbaldehydes. Notably, the regioselectivity of the cycloaddition can be tuned in the allene component just by a subtle variation in the substitution pattern of the alkene component.

Pd-Cu Bimetallic Catalyzed Domino Cyclization of α-Allenols Followed by a Coupling Reaction: New Sequence Leading to Functionalized Spirolactams

A novel regioselective metal-catalyzed spirocyclization of alpha-allenols-cross coupling (Heck, Sonogashira, and Suzuki) reaction sequence, leading to potentially bioactive spirocyclic lactam derivatives has been developed. Precursors for the tandem spirocyclization-coupling reaction, alpha-allenols 2 a-d were obtained starting from alpha-oxolactams 1 a-c via indium-mediated Barbier-type carbonyl-allenylation reaction in aqueous media by using our previously described methodologies.