Conformationally Constrained β-Amino Acid Derivatives by Intramolecular [2 + 2]-Photocycloaddition of a Tetronic Acid Amide and Subsequent Lactone Ring Opening

A Photochemical Strategy for the Conversion of Nitroarenes into Rigidified Pyrrolidine Analogues

Bicyclic amines are important motifs for the preparation of bioactive materials. These species have well-defined exit vectors that enable accurate disposition of substituents toward specific areas of chemical space. Of all possible skeletons, the 2-azabicyclo[3.2.0]heptane framework is virtually absent from MedChem libraries due to a paucity of synthetic methods for its preparation. Here, we report a modular synthetic strategy that utilizes nitroarenes as flat and easy-to-functionalize feedstocks for the assembly of these sp3-rich materials. Mechanistically, this approach exploits two concomitant photochemical processes that sequentially ring-expand the nitroarene into an azepine and then fold it into a rigid bicycle pyrroline by means of singlet nitrene-mediated nitrogen insertion and excited-state-4π electrocyclization. A following hydrogenolysis provides, with full diastereocontrol, the desired bicyclic amine derivatives whereby the aromatic substitution pattern has been translated into the one of the three-dimensional heterocycle. These molecules can be considered rigid pyrrolidine analogues with a well-defined orientation of their substituents. Furthermore, unsupervised clustering of an expansive virtual database of saturated N-heterocycles revealed these derivatives as effective isosteres of rigidified piperidines. Overall, this platform enables the conversion of nitroarene feedstocks into complex sp3-rich heterocycles of potential interest to drug development.

Aqueous Amine-Tolerant [2+2] Photocycloadditions of Unactivated Olefins

The Kochi-Salomon reaction is the only photochemical [2+2] cycloaddition capable of combining two electronically unactivated olefins into a cyclobutane. Yet, the reaction has remained largely unexplored and suffers many drawbacks, most notably an intolerance to Lewis/Brønsted basic amines and amides. Since these groups are ubiquitous in biologically active pharmaceuticals, an amine-tolerant Kochi-Salomon reaction would greatly facilitate rapid exploration of novel drug scaffolds. Herein, we disclose a transformation that is run in water with the most widely available Cu(II) salts and mineral acids. Furthermore, we apply this methodology to synthesize a variety of amine-containing cyclobutanes, including known and novel pharmacological analogues.

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent aspects of [2 + 2] photocycloaddition chemistry with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in sections 2 and 4 , whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin.

Thermodynamically driven, syn-selective vinylogous aldol reaction of tetronamides

A stereoselective vinylogous aldol reaction of N-monosubstituted tetronamides with aldehydes is described.

Regiospecific Ficini [2 + 2] cycloaddition of ynamides with cyclic isoimidium salts under catalyst-free conditions

The regiospecific [2 + 2] cycloaddition of cyclic isoimidium salts with ynamides is described. This effort led to the development of the first successful example of a catalyst-free, thermally driven Ficini [2 + 2] cycloaddition process of ynamides with α,β-unsaturated carbonyl compounds, giving the stable cyclobutenamides in excellent yields (up to 99%).

2-azapinanes: aza analogues of the enantiomeric pinyl carbocation intermediates in pinene biosynthesis

The enantiomeric 2-azapinanes, aza analogues of the pinyl carbocation intermediates in pinene biosynthesis, were synthesized from (-)- and (+)-cis-pinonic acids. The individual reactions in the 5-step sequence were Beckmann rearrangement of the pinonic acid oximes, cyclization to the N-acetyl lactams, hydrolysis to the NH-lactams, N-methylations, and LiAlH(4) reductions. The anti stereochemistry of the N-methyl groups in the salts with respect to the gem-dimethyl bridge was established by NOE measurements and by X-ray diffraction analysis.

Regioselective [2 + 2]-photocycloaddition reactions of chiral tetronates—influence of temperature, pressure, and reaction medium

The intramolecular [2 + 2]-photocycloaddition of the 1,3-divinyl-2-cyclopentyl tetronates was performed under various conditions to give perfect diastereoselectivity and a regioselectivity of up to 85/15 in the presence of gamma-cyclodextrin by differentiating the two chemically very similar double bonds of .