1,1-Diacyloxy-1-phenylmethanes as versatile N-acylating agents for amines

Synthetic Approaches, Properties, and Applications of Acylals in Preparative and Medicinal Chemistry

Diesters of geminal diols (R-CH(O-CO-R')2, RR'C(OCOR″)2, etc. with R = H, aryl or alkyl) are termed acylals according to IUPAC recommendations (Rule P-65.6.3.6 Acylals) if the acids involved are carboxylic acids. Similar condensation products can be obtained from various other acidic structures as well, but these related "non-classical acylals", as one might call them, differ in various aspects from classical acylals and will not be discussed in this article. Carboxylic acid diesters of geminal diols play a prominent role in organic chemistry, not only in their application as protective groups for aldehydes and ketones but also as precursors in the total synthesis of natural compounds and in a variety of organic reactions. What is more, acylals are useful as a key structural motif in clinically validated prodrug approaches. In this review, we summarise the syntheses and chemical properties of such classical acylals and show what potentially under-explored possibilities exist in the field of drug design, especially prodrugs, and classify this functional group in medicinal chemistry.

Stereoselective synthesis of the spirocyclic core of 13-desmethyl spirolide C using an aza-Claisen rearrangement and an exo-selective Diels-Alder cycloaddition

13-Desmethyl spirolide C is a marine natural product of the cyclic imine class that demonstrates remarkable bioactivity against several biomarkers of Alzheimer's Disease, which renders its [7,6]-spirocyclic imine pharmacophore of significant synthetic interest. This work describes a facile and efficient synthesis of the [7,6]-spirocyclic core of 13-desmethyl spirolide C from inexpensive starting materials, featuring an aza-Claisen rearrangement to simultaneously set both stereocentres of the dimethyl moiety with complete atom economy, and a highly exo-selective Diels-Alder cycloaddition to construct the challenging contiguous tertiary and quaternary stereocentres of the spirocyclic core of 13-desmethyl spirolide C. A comprehensive study of the key Diels-Alder reaction was also performed to evaluate the stereoselectivity and reactivity of various functionalised dienes and protected lactam dienophiles, wherein the first successful exo-selective Diels-Alder cycloaddition to construct spirocyclic structures using a bromodiene and α-exo-methylene dienophiles is reported. This strategy not only establishes a more efficient stereoselective access to the spirocyclic core that can be used for the total synthesis of 13-desmethyl spirolide C, but also serves as a sound platform for convenient preparations of a range of spirocyclic analogues required for a comprehensive biological evaluation of this desirable pharmacophore.

Amine Activation: "Inverse" Dipeptide Synthesis and Amide Function Formation through Activated Amino Compounds

A copper(II)/HOBt-catalyzed procedure for the synthesis of dipeptides and "general" amides has been developed using microwave irradiation to considerably hasten the reaction. As an alternative to using traditional carboxylic acid activation, the method relies on the use of N-acyl imidazoles as activated amino partners. By doing so, a nonconventional way to reach dipeptides and amides has been proposed through the challenging and less studied N → C direction synthesis. A series of dipeptides and "general" amides have been successfully synthesized, and the applicability of the method has been illustrated in gram-scale syntheses. The mild reaction conditions proposed are completely adequate for couplings in the presence of sensitive amino acids, affording the products without detectable racemization. Furthermore, experimental observations prompted us to propose a plausible reaction pathway for the couplings.

[4-(3,4-Dimethoxyphenyl)-3,6-dimethyl-2-phenyl-3,4-dihydroquinolin-1(2H)-yl)](furan-2-yl)methanone

A N-(2,4-diaryltetrahydroquinolin-1-yl) furan-2-carboxamide derivative, [4-(3,4-dimethoxyphenyl)-3,6-dimethyl-2-phenyl-3,4-dihydroquinolin-1(2H)-yl)](furan-2-yl)methanone, was synthesized in a two-step procedure from p-toluidine, benzaldehyde, and trans-methyl-isoeugenol as commercial starting reagents through a sequence of Povarov cycloaddition reaction/N-furoylation processes. The structure of the compound was fully characterized by IR, 1H, 13C-NMR, and X-ray diffraction data. Such types of derivatives are known as relevant therapeutic agents exhibiting potent anticancer, antibacterial, antifungal, anti-inflammatory, and immunological modulator properties.

Recent Advances in Cross-Couplings of Functionalized Organozinc Reagents

Cross-couplings involving organozinc reagents usually require a Pd-catalyst (Negishi cross-coupling), however, uncatalyzed cross-couplings of zinc organometallics proceed well in the absence of transition-metal catalysts with reactive electrophiles such as benzal 1,1-diacetates, benzhydryl acetates, and iminium trifluoroacetates. Organozinc compounds also undergo C–N bond formation with O-benzoylhydroxylamines or organic azides in the presence of cobalt- or iron-catalysts. Highly diastereoselective and enantioselective cross-couplings can be readily performed with room-temperature configurationally stable alkylzinc species, producing diastereoselectively and enantiomerically enriched products. Finally, highly regioselective magnesiations of functionalized arenes and heteroarenes undergo Negishi (after transmetalation with ZnCl2) or Cu-catalyzed cross-couplings.

1 Introduction

2 Uncatalyzed Cross-Couplings of Organozinc Reagents with Highly Electrophilic Partners

3 Iron- and Cobalt-Catalyzed Aminations using Organozinc Reagents

4 Stereo- and Regioselective Cross-Couplings of Organozinc Reagents

5 Conclusion

Selective Synthesis of Acylated Cross-Benzoins from Acylals and Aldehydes via N-Heterocyclic Carbene Catalysis

The utility of acylals as building blocks for selective cross-benzoin synthesis was explored in this study. The synthesis of α-acetoxyketones (O-acyl cross-benzoins) was achieved via selective N-heterocyclic carbene-catalyzed cross-benzoin reactions using acylals as aldehyde equivalents. Thus, the combination of ortho-substituted phenyl acylals and aromatic/aliphatic aldehydes as coupling substrates using bicyclic triazolium salts as precatalysts and potassium carbonate as a base in THF at reflux temperature selectively yielded O-acyl cross-benzoins.