Lewis Acid Assisted Diels-Alder Reaction with Regio- and Stereoselectivity: Anti-1,4-Adducts with Rigid Scaffolds and Their Application in Explosives Sensing

Synthesis of a Constitutional Isomer of Armeniaspirol A, Pseudoarmeniaspirol A, via Lewis Acid-Mediated Rearrangement

The natural product armeniaspirol possesses a unique spirocyclic N,O-ketal in an α,β-dichloro-α,β-unsaturated lactam scaffold that has proved challenging to synthesize. Herein, we characterize the oxidative chlorination of pyrrole-2-carboxylate derivatives that rapidly generates this scaffold. The scope of this oxidation was extended to a series of esters and amides. Pyrrole-2-ketones could not be converted into the lactam due to an oxidative fragmentation. This result was unexpected since chloro-armeniaspirol has been synthesized via oxidative chlorination of a pyrrole-2-ketone. Examination of this successful oxidation showed that the desired scaffold was accessed due to intramolecular trapping from the neighboring free phenol, preventing fragmentation. Using the product of methyl N-methyl pyrrole-2-carboxylate oxidation 7b, we attempted to access the natural product armeniaspirol 2; however, an unanticipated Lewis acid-mediated rearrangement led to formation of a constitutional isomer, pseudoarmeniaspirol A 1. A small panel of pseudoarmeniaspirol analogues was synthesized and evaluated for antibiotic activity, inhibition of the targets of armeniaspirol, ClpXP and ClpYQ, and protonophore activity. While pseudoarmeniaspirol shows antibiotic activity, it does not target ClpXP or ClpYQ and has less protonophore activity than the natural product.

Effect of Novel Deep Eutectic Solvents on the Endo/Exo Ratio of Diels-Alder Reactions at Room Temperature

The Diels-Alder reaction is a prototypical example of a thermally allowed [4 + 2] cycloaddition with good control over the regio- and stereochemical outcomes. Therefore, Diels-Alder reactions in which adjacent stereocenters are generated at the two ends of the newly formed single bonds imply two different possible stereochemical outcomes. In cases where the dienophile has a single electron-withdrawing substituent, the outcome can often be predicted by applying the known "endo rule". Furthermore, the use of chiral eutectic solvents in asymmetric synthesis has become a novel tool to maintain sustainability in organic synthesis. In the present work, a set of recyclable and sustainable bio-based deep eutectic solvents (DESs) was designed using hydrogen bond acceptors (HBAs) with a chiral center. These compounds were used in their racemic and enantiomerically enriched forms to prepare DESs with lactic acid (LA), glycerol (Gly), and ethylene glycol, which act as hydrogen bond donors (HBDs) in the corresponding eutectic mixture. These DESs were used as solvents to study the reaction between cyclopentadiene and ethyl acrylate or butyl acrylate in typical [4 + 2] cycloadditions. The best yields and endo-selectivity were achieved using LA as the HBD in the eutectic mixtures. The results and adduct ratios obtained show that these DESs were able to improve both reaction yields and selectivity when compared to those observed in organic solvents or ionic liquids. Moreover, the reaction products (adducts) were easily recovered with diethyl ether from the reaction mixture, where they appeared as an upper layer.

Diels-Alder reactions and electrophilic substitutions with atypical regioselectivity enable functionalization of terminal rings of anthracene

Reversing the regioselectivity of the renowned Diels-Alder reaction by overriding the usual thermodynamic and kinetic governing factors has always been a formidable challenge to synthetic organic chemists. Anthracenes are well-known to undergo [4 + 2]-cycloadditions with dienophiles at their 9,10-positions (central ring) over 1,4-positions (terminal ring) guided by the relative aromatic stabilization energy of the two possible products, and also by harboring the largest orbital coefficients of the highest occupied molecular orbital (HOMO) at the 9,10-positions. We, herein, report a 1,4-selective [4 + 2]-cycloaddition strategy of 9,10-unsubstituted anthracenes by installing electron-donating substituents on the terminal rings which is heretofore unprecedented to the best of our knowledge. The developed synthetic strategy does not require any premeditated engagement of the 9,10-positions either with any sterically bulky or electron-withdrawing substituents and allows delicate calibration of the regioselectivity by modulating the electron-donating strength of the substituents on the terminal rings. Likewise, the regioselective functionalization of the terminal anthracene ring in electrophilic substitution reactions is demonstrated. A mechanistic rationale is offered with the aid of detailed computational studies, and finally, synthetic applications are presented.

Metal-Free Dehydrogenative Diels-Alder Reactions of Prenyl Derivatives with Dienophiles via a Thermal Reversible Process

An efficient dehydrogenative Diels-Alder reaction of prenyl derivatives with dienophiles has been developed. The reaction exhibits broad substrate scope and provides efficient access to cyclohexene derivatives with good to excellent yields. A reasonable mechanism involving a metal-free thermal reversible process is proposed.

Iptycenes with an acridinone motif developed through [4+2] cycloaddition of tethered naphthalene and iminoquinone via a radical reaction

A new class of iptycenes was developed by combining 2-(naphthalen-1-yl)anilines and p-benzoquinones through copper(ii)-mediated radical cyclisation.