5-Aminopyrazoles as Dienophiles in the Inverse Electron Demand Diels−Alder Reactions of 2,4,6-Tris(ethoxy- carbonyl)-1,3,5-triazine: Syntheses of Pyrazolopyrimidines

Inverse Electron Demand Diels-Alder-Type Heterocycle Syntheses with 1,2,3-Triazine 1-Oxides: Expanded Versatility

1,2,3-Triazine 1-oxides are remarkably effective substrates for inverse electron demand Diels-Alder reactions. Formed from vinyldiazoacetates via reaction with tert-butyl nitrite, these stable heterocyclic compounds undergo clean nucleophilic addition with amidines to form pyrimidines, with β-ketocarbonyl compounds and related nitrile derivatives to form polysubstituted pyridines and with 3/5-aminopyrazoles to form pyrazolo[1,5-a]pyrimidines, in high yield. These practical reactions are rapid at room temperature, are base catalyzed, and offer a diversity of structural modifications.

Triazines: Syntheses and Inverse Electron-demand Diels-Alder Reactions

Triazines are an important class of six-membered aromatic heterocycles possessing three nitrogen atoms, resulting in three types of regio-isomers: 1,2,4-triazines (a-triazines), 1,2,3-triazines (v-triazines), and 1,3,5-triazines (s-triazines). Notably, the application of triazines as cyclic aza-dienes in inverse electron-demand Diels-Alder (IEDDA) cycloaddition reactions has been established as a unique and powerful method in N-heterocycle synthesis, natural product preparation, and bioorthogonal chemistry. In this review, we comprehensively summarize the advances in the construction of these triazines via annulation and ring-expansion reactions, especially emphasizing recent developments and challenges. The synthetic transformations of triazines are focused on IEDDA cycloaddition reactions, which have allowed access to a wide scope of heterocycles, including pyridines, carbolines, azepines, pyridazines, pyrazines, and pyrimidines. The utilization of triazine IEDDA reactions as key steps in natural product synthesis is also discussed. More importantly, a particular attention is paid on the bioorthogonal application of triazines in fast click ligation with various strained alkenes and alkynes, which opens a new opportunity for studying biomolecules in chemical biology.

Aromatic Heterocycles as Productive Dienophiles in the Inverse Electron-Demand Diels-Alder Reactions of 1,3,5-Triazines

Heterocycles are often found as the structural nucleus in natural products and biological active compounds. Consequently, research toward the discovery and development of novel and efficient synthetic methodologies is of constant interest to organic chemists. Diels-Alder reactions are powerful at forming multiple bonds simultaneously, often with stereoselectivity, and thus are one of the most widely used synthetic methodologies in organic syntheses. Inverse electron-demand Diels-Alder (IEDDA) reactions, a subclass of Diels-Alder reactions, have been developed for the efficient synthesis of various heterocycles, with 1,3,5-triazines used as azadienes. The initial 1,3,5-triazine IEDDA reactions mostly included nonaromatic, electron-rich species such as vinyl ethers, enamines, ynamines, and amidines as dienophiles, producing in high yields pyrimidine derivatives with excellent regioselectivity. We hypothesized that some electron-rich aromatic heterocycles could be studied as potential dienophiles for 1,3,5-triazine IEDDA reactions; 5-aminopyrazoles proved to be productive dienophiles leading to high yields of pyrazolopyrimidines. Subsequently, many studies were reported to investigate the mechanism and scope of this new type of IEDDA reaction. Mechanistically, this new type of IEDDA reaction is quite interesting since it entails two aromatic compounds proceeding through a perceived high energy nonaromatic transition state, leading to a new aromatic compound, a counterintuitive process. Both theoretical and experimental studies provide key insights to this reaction mechanism, with learnings from these studies possibly stimulating unconventional thinking in other areas. Theoretical calculations of these cascade reactions of amino-substituted heterocycles with 1,3,5-triazines indicate that the reactions occur in a stepwise fashion via a highly polar zwitterionic intermediate; elimination of ammonia from IEDDA adducts and subsequent retro Diels-Alder reaction drive the reaction toward the fully aromatized IEDDA products. This amino substituent is critical in determining the regioselectivity and driving the cascade reactions to completion. With regard to reaction scope, many amino-heterocycles such as pyrroles, imidazoles, furans, thiophenes, and indoles all proved to be productive dienophiles for this new IEDDA reaction, leading to various fused-pyrimidines in a single step with moderate to high yields and high regioselectivity. Application of this new IEDDA reaction with 1,3,5-triazines was reported to lead to interesting heterocyclic compounds such as nucleoside natural product nebularine and analogues, as well as fluorine-containing fused pyrimidines with potential for biological activities.Herein, the scope of various aromatic heterocycles as dienophiles in the 1,3,5-triazine IEDDA reaction is reviewed. Moreover, both experimental and theoretical studies of the mechanisms for this interesting cascade IEDDA reaction are discussed. Finally, applications of this new type (aromatic heterocycles as dienophiles with 1,3,5-triazines as azadienes) of IEDDA reaction are also covered.

Clarification of the Mechanism of the Cascade Reactions between Amino-Substituted Heterocycles and 1,3,5-Triazines

The mechanisms of the cascade reactions of amino-substituted imidazoles, pyrroles, and pyrazoles with 1,3,5-triazines have been studied using first principle MP2/6-311++G**//B3LYP/6-31G* calculations in both the gas phase and solution. Calculations indicate that these cascade reactions start with stepwise inverse-electron demand Diels-Alder (IDA) reactions, followed by elimination of ammonia from the IDA adducts, and retro Diels-Alder (RDA) reactions (referred as to the IER path). An alternative IRE path involving a reaction sequence of IDA, RDA, and elimination of ammonia is ruled out since the RDA transition states in this pathway are higher in energy than the reactants by as much as 50 kcal/mol. The preference of the IER over the IRE path is discussed.

A New Strategy toward Indole Alkaloids Involving an Intramolecular Cycloaddition/Rearrangement Cascade

The intramolecular Diels-Alder reaction between an amidofuran moiety tethered onto an indole component was examined as a strategy for the synthesis of Aspidosperma alkaloids. Furanyl carbamate 23 was acylated using the mixed anhydride 26 to provide amidofuran 22 in 68% yield. Further N-acylation of this indole furnished 27 in 88% yield. Cyclization precursors were prepared by removing the carbamate moiety followed by N-alkylation with the appropriate alkyl halides. Large substituent groups on the amido nitrogen atom causes the reactive s-trans conformation of the amidofuran to be more highly populated, thereby facilitating the Diels-Alder cycloaddition. The reaction requires the presence of an electron-withdrawing substituent on the indole nitrogen in order for the cycloaddition to proceed. Treatment of N-allyl-bromoenamide 48 with n-Bu(3)SnH/AIBN preferentially led to the 6-endo trig cyclization product 50, with the best yield (91%) being obtained under high dilution conditions. The initially generated cyclohexenyl radical derived from 48 produces the pentacyclic heterocycle 50 by either a direct 6-endo trig cyclization or, alternatively, by a vinyl radical rearrangement pathway.

An efficient synthesis of pyrrolo[2,3-d]pyrimidines via inverse electron demand diels-alder reactions of 2-amino-4-cyanopyrroles with 1,3,5-triazines

The scope of the inverse electron demand Diels-Alder reaction of 2-amino-4-cyanopyrroles (3a-e) with 1,3,5-triazines (1, 2) is reported. This methodology is suitable for one-pot syntheses of highly substituted and highly functionalized pyrrolo[2,3-d]pyrimidines that are the central heterocyclic nucleus of various nucleoside natural products such as toyocamycin, sangivamycin, and tubercidin.

Concise synthesis and transannular inverse electron demand Diels-Alder reaction of [3](3,6)pyridazino[3](1,3)indolophane. Rapid access to a pentacyclic indoloid system

[reaction: see text] The title compound was synthesized concisely from indole. A 2-fold sequential hydroboration/Suzuki-Miyaura cross-coupling was employed to generate the cyclophane. When heated in N,N-diethylaniline, it underwent a transannular inverse electron demand Diels-Alder (IEDDA) reaction to form a pentacyclic product, which appears to be well suited as a precursor to a variety of indole alkaloids such as strychnine.

Aromatic dienophiles. 1. A theoretical study of an inverse-electron demand Diels-Alder reaction between 2-aminopyrrole and 1,3,5-triazine

This study is devoted to a detailed theoretical study of an inverse-electron demand Diels-Alder reaction (IDA) with 1,3,5-triazine as the diene and 2-aminopyrrole 1A(alpha) as the dienophile, which is a key step in a cascade reaction for the one-pot synthesis of purine analogues. Geometries were optimized with the B3LYP/6-31G* method and energies were evaluated with the MP2/6-311++G** method. This IDA reaction occurs through a stepwise mechanism, where the first step corresponds to the nucleophilic attack of 2-aminopyrrole to triazine to form a zwitterionic intermediate, which is in equilibrium with a neutral intermediate through a hydrogen transfer process, followed by a rate-determining ring-closure step. It is shown that the B3LYP method significantly overestimates the activation energy, whereas the MP2 method offers a reasonable activation barrier of 27.9 kcal/mol in the gas phase. The solvation effect has been studied by the PCM model. In DMSO, the calculated activation energy of the IDA reaction is decreased to 24.0 kcal/mol with a strong endothermicity of 17.4 kcal/mol due to the energy penalty of transforming two aromatic reactants into a nonaromatic IDA adduct. The possible stepwise [2+2] pathway is ruled out based on its higher activation and reaction energies than those of the [4+2] pathway. By comparing the IDA reactions of triazine to 2-aminopyrrole and pyrrole, we address two crucial roles of the alpha-amino substituent in lowering activation and reaction energies and controlling the reaction regiochemistry.