Search

The reactions proceed with significant levels of regioselectivity for both the ring-opening and O-functionalization steps.

Two-fold Suzuki–Miyaura cross-couplings of cyclic diarylborinic acids are described. This novel annulation method enables the synthesis of benzo-fused heterocycles from dihaloarenes or gem-dibromoolefins.

The combination of a boronic acid and a Lewis base, both employed in substoichiometric amounts, allows for the regioselective silylation of cis-diol groups in alkylpyranoside substrates.

Halogen bonds are noncovalent interactions in which covalently bound halogens act as electrophilic species. The utility of halogen bonding for controlling self-assembly in the solid state is evident from a broad spectrum of applications in crystal engineering and materials science. Until recently, it has been less clear whether, and to what extent, halogen bonding could be employed to influence conformation, binding or reactivity in the solution phase. This tutorial review summarizes and interprets solution-phase thermodynamic data for halogen bonding interactions obtained over the past six decades and highlights emerging applications in molecular recognition, medicinal chemistry and catalysis.

An overview of the applications of boronic and borinic acids in catalysis is presented. Taking advantage of the Lewis acidity of trivalent boron and the reversible covalent interactions of organoboron acids with OH groups, diverse modes of catalytic reactivity have been achieved. Interactions with carbonyl compounds enable acceleration of addition and cycloaddition processes, whereas binding of their enol tautomers can lead to organoboron-catalyzed aldol and related reactions. Binding of organoboron acids to hydroxyl and carboxyl OH groups has been employed as a mode of electrophilic activation for catalysis of substitution, cycloaddition, rearrangement, and elimination reactions. By altering the nature of the interaction with the organoboron acid catalyst, activation of OH groups as pronucleophiles is also possible, resulting in catalyst-controlled methods for regioselective functionalization of diols and carbohydrate derivatives. Applications of organoboron acids in bifunctional and assisted catalysis a...

The efficient synthesis of oligosaccharides requires methods for regioselective manipulation of hydroxyl groups in monosaccharide derivatives. Catalysis represents a potentially general solution to this problem, and over the past decade, the development of catalyst-controlled methods for regioselective functionalization of carbohydrates has intensified. This Short Review highlights the range of catalysts and catalytic mechanisms that may be exploited to alter the reactivity of hydroxyl groups in carbohydrate substrates. 1 Introduction 2 Intrinsic Trends in the Reactivity of Carbohydrate Hydroxyl Groups 2.1 Steric Control 2.2 Electronic Control 3 Enzyme Catalysis 4 Phase-Transfer Catalysis 5 Lewis Base Catalysis 5.1 Peptide-Based Catalysts 5.2 Modified DMAP Catalysts 5.3 Chiral Amines 6 Lewis Acid Catalysis 6.1 Tin(IV) Reagents 6.2 Transition-Metal Reagents 6.3 Lanthanum(III) Reagents 6.4 Multistep One-Pot Methods 6.5 Organoboron Reagents 7 Conclusions and Outlook

The method can be generally applied to the monotosylation of pyranoside derivatives bearing three secondary hydroxyl groups.

Silver-promoted ionization of α-halocarbonyl compounds generates the corresponding α-carbonyl carbocations which smoothly react with simple (het)arenes as nucleophiles to give diarylcarbonyl compounds, e.g. (III), (V)/(VI) and (XII).

Reversible covalent interactions of organoboron compounds are exploited as the basis for regioselective borinic acid-catalyzed acylations of polyols. This catalytic protocol enables differentiation of the secondary OH groups of a wide range of carbohydrate derivatives with diverse acid chloride and chloroformate reagents, using a structurally simple diarylborinic acid-derived catalyst.

Zinc trifluoromethanesulfonate promotes efficient condensations of anilines with squarate esters, providing access to symmetrical and unsymmetrical squaramides in high yields from readily available starting materials. Efficient access to electron-deficient diaryl squaramides has enabled a systematic investigation of the colorimetric anion-sensing behavior of a p-nitro-substituted squaramide. Its behavior differs in dramatic and unexpected ways from that of structurally similar p-nitroaniline-based ureas, an effect that highlights the remarkable differences in acidity between the squaramide and urea functional groups. Computational studies illustrating the enhanced hydrogen bond donor ability and acidity of squaramides in comparison to ureas are presented.

Lewis acid catalyzed Ritter reaction of α-oxo tosylates, trifluoroacetates and alcohols with nitriles under optimized conditions affords trisubstituted oxazole products in good to excellent yields.

The non-8-enoate anion undergoes losses of the elements of C3H6, C4H8 and C6H12 on collisional activation, The mechanisms of these processes have been elucidated by a combination of product ion and labelling (H-2 and C-13) studies, together with a neutralisation reionisation mass spectrometric study. These studies allow the following conclusions to be made. (i) The loss of C3H6 involves cyclisation of the enolate anion of non-8-enoic acid to yield the cyclopentyl carboxylate anion and propene. (ii) The loss of 'C4H8' is a charge-remote process (one which proceeds remote from the charged centre) which yields the pent-4-enoate anion, butadiene and dihydrogen. This process co-occurs and competes with complex H scrambling. (iii) The major loss of 'C6H12' occurs primarily by a charge-remote process yielding the acrylate anion, hexa-1,5-diene and dihydrogen, but in this case no H scrambling accompanies the process. (iv) It is argued that the major reason why the two charge-remote processes occur in preference to anion-induced losses of but-l-ene and hex-l-ene from the respective 4- and 2-anions is that although these anions are formed, they have alternative and lower energy fragmentation pathways than those involving the losses of but-l-ene and hex-l-ene; viz. the transient 4-anion undergoes facile proton transfer to yield a more stable anion, whereas the 2-(enolate) anion undergoes preferential cyclisation followed by elimination of propene [see (i) above].

The ion PhCO2––CHPh, upon collision activation, undergoes competitive losses of CO and CO2 of which the former process produces the base peak of the spectrum. Product ion and substituent effect (Hammett) studies indicate that PhCO2––CHPh cyclises to a deprotonated hydroxydiphenyloxirane which ring opens to PhCOCH(O–)Ph. This anion then undergoes an anionic 1,2-Wittig type rearrangement {through [PhCO– (PhCHO)]} to form Ph2CHO– and CO. The mechanism of the 1,2-rearrangement has been probed by an ab initio study [at MP4(SDTQ)/6-31++G(d,p) level] of the model system HCOCH2O– → MeO– + CO. The analogous system RCO2––CHPh (R = alkyl) similarly loses CO, and the migratory aptitudes of the alkyl R groups in this reaction are But > Me > Et ≈Pri). This trend correlates with the order of anion basicities (i.e. the order of ΔGoacid values of RH), supporting the operation of an anion migration process. The loss of CO2 from PhCO2––CHPh yields Ph2CH– as the anionic product: several mechanistic scenarios are possible, one of which involves an initial ipso nucleophilic substitution.

Interactions between pyruvic acids and diphenylborinic acid form the basis of an efficient, direct, boron-catalyzed aldol reaction that takes place in water at room temperature with low catalyst loadings. Both boronic and borinic acids function as catalysts, with the latter demonstrating particularly high activity. A wide range of aldehydes, including enolizable species, may be employed, delivering useful isotetronic acid derivatives in high yields.

Chiral (salen)Al complex 1a catalyzes the highly enantioselective conjugate addition of carbon- and nitrogen-based nucleophiles to acyclic alpha,beta-unsaturated ketones. This methodology is tolerant of substantial variation of the ketone structure, providing access to a wide range of useful chiral building blocks in high yield and enantiomeric excess. Synthetic manipulations of the conjugate addition products are demonstrated, including the straightforward preparation of beta-amino ketones and highly enantioenriched carbo- and heterocyclic compounds.

This article describes three distinct strategies by which stereochemically complex molecules are synthesized and the ways asymmetric catalysis can impact on all three. The development of general methods to prepare synthetically useful building blocks leads to an expanded “chiral pool” of potential starting materials for asymmetric synthesis. The possibility of discovering new reactions to access new types of building blocks is particularly attractive and serves to help define the frontiers of the field. Asymmetric catalysis can also be applied to diastereoselective synthesis such that the stereochemistry of the catalyst, and not that of the substrate, determines the relative configuration of the product. Finally, in reactions where multiple stereocenters are generated simultaneously or in tandem, catalyst and substrate control can operate in a complementary manner to achieve one of many possible stereochemical outcomes selectively.

The scope of the rhodium-catalyzed asymmetric ring opening reaction of oxabenzonorbornadiene has been extended to include phenolic nucleophiles. The enanatioenriched, functionalized dihydronaphthalene products are highly valuable intermediates for which no other practical methods of preparation are available. A new catalyst system has been developed which allows the use of less reactive o-halophenols. The utility of these products has been demonstrated through their application in the synthesis of benzofuran polycyclic materials.