A unified approach for divergent synthesis of contiguous stereodiads employing a small boronyl group

Copper-Catalyzed Diastereoselective Borylative Allylation of Alkenyl Sulfones and Base-Controlled Synthesis of Skipped Dienes

Divergent functionalization of α,β-unsaturated sulfones under copper catalysis is reported. Diastereoselective borylative allylation of alkenyl sulfones was achieved with a copper catalyst, allyl phosphate, bis(pinacolato)diboron, and LiOMe. In this method, the addition of a copper-boryl complex to alkenyl sulfone and subsequent allylic substitution rendered boroallylated products in good yield with excellent syn-diastereoselectivity. In contrast, a stronger base (KOt-Bu) promoted the reaction further toward the formation of (E)-skipped dienes, through elimination of the sulfonyl and boryl groups.

The Aldol-Tishchenko Reaction of Butanone, Cyclobutanone and a 3-Pentanone Derived Sulfinylimine and DFT Calculations of the Stereo-determining Step

Herein, we present a highly diastereoselective method to furnish acyclic 3-amino-1,5-diol derivatives using a tandem double-aldol-Tishchenko protocol (dr up to >99 : 1) using a butanone derived sulfinylimine. In most cases only 1 diastereomer predominates, from a possible 16. The reaction is also regioselective. In addition, the highly challenging cyclobutanone and 3-pentanone derivatives are also amenable to a double-aldol-Tishchenko reaction, although the dr values are modest. Despite that, clean single diastereomers can be isolated, which should prove very useful in medicinal chemistry and other areas. Detailed DFT calculations support the observed stereoselectivities in all cases, providing a rationale for the excellent dr values in the butanone series and the moderate values for the 3-pentanone class.

anti-Selective synthesis of β-boryl-α-amino acid derivatives by Cu-catalysed borylamination of α,β-unsaturated esters

A copper-catalysed regio- and diastereoselective borylamination of α,β-unsaturated esters with B₂pin₂ and hydroxylamines has been developed to deliver acyclic β-boryl-α-amino acid derivatives with high anti-diastereoselectivity (up to >99 : 1), which is difficult to obtain by the established methods. A chiral phosphoramidite ligand also successfully induces the enantioselectivity, giving the optically active β-borylated α-amino acids. The products can be stereospecifically transformed into β-functionalised α-amino acids, which are of potent interest in medicinal chemistry.

Catalysis with Diboron(4)/Pyridine: Application to the Broad-Scope [3 + 2] Cycloaddition of Cyclopropanes and Alkenes

In contrast to the extensive but non-recyclable use of tetraalkoxydiboron(4) compounds as stoichiometric reagents in diverse reactions, this article reports an atom-economical reaction using a commercial diboron(4) as the catalyst. The key to success was designing a catalytic cycle for radical [3 + 2] cycloaddition involving a pyridine cocatalyst to generate from the diboron(4) catalyst and reversibly mediate the transfer of boronyl radicals. In comparison with known [3 + 2] cycloaddition with transition metal-based catalysts, the current reaction features not only metal-free conditions, inexpensive and stable catalysts, and simple operation but also remarkably broadened substrate scope. In particular, previously unusable cyclopropyl ketones without an activating group and/or alkenes with 1,2-disubstitution and 1,1,2-trisubstitution patterns were successfully used for the first time. Consequently, challenging cyclopentane compounds with various levels of substitution (65 examples, 57 new products, up to six substituents at all five ring atoms) were readily prepared in generally high to excellent yield and diastereoselectivity. The reaction was also successfully applied in concise formal synthesis of an anti-obesity drug and building natural product-like complex bridged or spirocyclic compounds. Mechanistic experiments and computational investigation support the proposed radical relay catalysis featuring a pyridine-assisted boronyl radical catalyst. Overall, this work demonstrates the first approach to use tetraalkoxydiboron(4) compounds as catalysts and may lead to the development of new, green, and efficient transition metal-like boron-catalyzed organic reactions.

Enantioselective synthesis of cyclic α-aminoboronates via copper-catalyzed dearomative borylation of 4-quinolinols

A highly enantioselective and regioselective dearomative borylation of 4-quinolinols was developed using a Cu(I)/(R,R)-Ph-BPE catalyst for efficient synthesis of unprecedented heterocyclic α-amino boronates, a new class of compounds potentially relevant to drug discovery, in generally excellent yields and enantioselectivities. The products were also useful intermediates for highly functionalized tetrahydroquinolines and cyclic α-aminoboronate derivatives.

Stereoselective Installation of Five Contiguous Stereogenic Centers in a Double Aldol-Tishchenko Cascade and Evaluation of the Key Transition State through DFT Calculation

The stereoselective formation of 5 contiguous chiral centers in a single pot reaction is demonstrated using an aldol, aldol–Tishchenko reaction of N-tert-butyl sulfinimines. One diastereoisomer (from 32 possibilities) predominates, and a series of cyclic and acyclic 3-amino-1,5-diol derivatives are synthesized in good yields (up to 80%) and excellent diastereoselectivities (up to >98:2 dr). Investigations support two reversible aldol steps, and multiple intermediates which are funnelled through a remarkably selective, irreversible, Tishchenko reduction, in a Curtin–Hammett phenomenon. DFT calculations using a disolvated (THF) model reveal the factors controlling stereoselectivity in the final irreversible Tishchenko step.

Prediction of NHC-catalyzed chemoselective functionalizations of carbonyl compounds: a general mechanistic map

Generally, N-heterocyclic carbene (NHC) complexed with carbonyl compounds would transform into several important active intermediates, i.e., enolates, Breslow intermediates, or acylazolium intermediates, which act as either a nucleophile (Nu) or an electrophile (E) to react with the other E/Nu partner. Hence, the key to predicting the origin of chemoselectivity is to compute the activity (i.e., electrophilic index ω for E and nucleophilic index N for Nu) and stability of the intermediates and products, which are suggested in a general mechanistic map of these reactions. To support this point, we selected and studied different cases of the NHC-catalyzed reactions of carbonyl compounds in the presence of a base and/or an oxidant, in which multiple possible pathways involving acylazolium, enolate, Breslow, and α,β-unsaturated acylazolium intermediates were proposed and a novel index ω + N of the E and Nu partners was employed to exactly predict the energy barrier of the chemoselective step in theory. This work provides a guide for determining the general principle behind organocatalytic reactions with various chemoselectivities, and suggests a general application of the reaction index in predicting the chemoselectivity of the nucleophilic and electrophilic reactions.

A novel index ω + N can be used to predict the chemoselectivity according to the general NHC-catalyzed reaction mechanism.

Facile synthesis of the daphnane and tigliane framework by semi-flow tube-based-bubbling photooxidation and diastereoselective conjugate addition

The common [5-7-6] tricyclic framework in tigliane and daphnane natural diterpenes containing five contiguous stereocenters has been synthesized in 9 steps from readily available starting materials in a completely stereocontrolled manner.