Multinuclear enantiopure titanium self‐assembly complexes—synthesis,characterization and application to organic synthesis

Hexa‐ and nonanuclear titanium complexes were obtained by self‐assembly of titanium(IV)‐tert‐butoxide and D ‐mandelic acid. Suitable single crystals of these complexes were characterized by X‐ray structure analysis. When used with these complexes, aldol adducts were isolated with a high degree of regioselectivity in direct aldol additions of aromatic and aliphatic aldehydes to functionalized unsymmetrical ketones. High syn‐diastereoselectivities were obtained in aldol additions of enolizable aldehydes with hydroxyacetone and methoxyacetone. Copyright © 2007 John Wiley & Sons, Ltd.     

Histidine-catalyzed asymmetric aldol addition of enolizable aldehydes: insights into its mechanism

Extensive studies of asymmetric cross-aldol addition between enolizable aldehydes are described and provide a deeper insight into histidine-catalyzed aldol additions. In particular, aspects of enantio- as well as diastereoselectivity of these reactions are discussed. Rules and predictions of configurative outcome are explained by using different transition-state models. These discussions are confirmed by extensive computations.     

Bifunctional Brønsted Base Catalyzes Direct Asymmetric Aldol Reaction of α‐Keto Amides

The first enantioselective direct cross‐aldol reaction of α‐keto amides with aldehydes, mediated by a bifunctional ureidopeptide‐based Brønsted base catalyst, is described. The appropriate combination of a tertiary amine base and an aminal, and urea hydrogen‐bond donor groups in the catalyst structure promoted the exclusive generation of the α‐keto amide enolate which reacted with either non‐enolizable or enolizable aldehydes to produce highly enantioenriched polyoxygenated aldol adducts without side‐products resulting from dehydration, α‐keto amide self‐condensation, aldehyde enolization, and isotetronic acid formation.     

The vinylogous aldol reaction of unsaturated esters and enolizable aldehydes using the novel Lewis acid aluminum tris(2,6-di-2-naphthylphenoxide)

The synthesis of the novel Lewis acid, aluminum tris(2,6-di-2-naphthylphenoxide) (ATNP), and its use in the vinylogous aldol reaction between methyl crotonate and enolizable aldehydes are described. ATNP is related to Yamamoto's Lewis acid, aluminum tris(2,6-diphenylphenoxide) (ATPH), but the 2-naphthyl groups more effectively block the α-position of aldehydes, enabling the selective enolization of crotonate esters in the presence of enolizable aldehydes. Vinylogous aldol reactions then proceed smoothly and in high yields with a variety of substrates.     

Anti-selective direct catalytic asymmetric aldol reaction of thiolactams

An anti-selective direct catalytic asymmetric aldol reaction of thiolactam is described. A soft Lewis acid/hard Br?nsted base cooperative catalyst comprised of mesitylcopper/(R,R)-Ph-BPE exhibited high catalytic performance to produce an anti-aldol product with high stereoselectivity. The highly chemoselective nature of the present catalysis allows for the use of enolizable aldehydes as aldol acceptors. The diverse transformations of the thiolactam moiety highlight the synthetic utility of the present anti-aldol protocol.     

Acid-catalyzed aldol-Meerwein–Ponndorf–Verley-etherification reactions—access to defined configured quaternary stereogenic centers

A novel asymmetric aldol-reduction–etherification process of aliphatic enolizable aldehydes is described. The intermediately formed aldol adducts—β-hydroxyaldehydes—were reduced and transformed into the corresponding 1,3-diol ethers by external secondary alcohols at the same time. Thus, with the help of chiral secondary alcohols an access to optically active 1,3-diol ether is given. Furthermore, asymmetric cross-aldol-Meerwein–Ponndorf reactions of enolizable aldehydes can also be realized under these reaction conditions.     

A kinetically controlled direct aldol addition of α-chloro thioesters via soft enolization

Herein we report that simple α-chloro thioesters undergo soft enolization and direct aldol addition to aldehydes in the presence of MgBr₂·OEt₂ and i-Pr₂NEt. At ?78 °C the reaction proceeds in a kinetically controlled manner giving good diastereoselectivity. Significantly, the transformation is applicable to both enolizable and nonenolizable aldehydes. Moreover, excellent stereoselectivity results when a chiral nonracemic α-hydroxy aldehyde derivative is used. To our knowledge, this is the first report of a kinetically controlled soft enolization-based aldol addition.     

Catalytic enantioselective thioester aldol reactions that are compatible with protic functional groups

This communication reports highly enantioselective and diastereoselective methyl malonic acid half thioester (MeMAHT) aldol reactions that are compatible with protic functional groups and enolizable aldehydes, affording syn S-phenyl thiopropionates.     

A facile and efficient anti-selective four-component direct aldol addition via chemoselective thioester enolate formation

A facile and efficient four-component anti-selective direct aldol addition of thioester enolates has been developed that is fully compatible with enolizable aldehydes and able to be conducted using untreated reagent-grade CH2Cl2 open to the air. The thioester enolates are generated in situ via an acylation/conjugate addition sequence using commercially available PhSLi and acryloyl chloride, thus avoiding prior enolate formation while maintaining complete chemoselectivity. The organosulfur products are convertible into various polyketide-based structures.     

Ni(II) bis(oxazoline)-catalyzed enantioselective syn aldol reactions of N-propionylthiazolidinethiones in the presence of silyl triflates

An enantioselective aldol reaction of N-propionylthiazolidinethione and representative aldehydes is disclosed. The reaction is catalyzed by [Ni(S,S)-t-BuBox](Otf)2. Enolization is effected by 2,6-lutidine, and TMSOTf facilitates catalyst turnover. Syn diastereoselectivities range from 88:12 to 97:3, and enantioselectivities are 90% or greater. Both aromatic and enolizable aliphatic aldehydes are included within the scope of this aldol addition process.     

Potassium Base-Promoted Diastereoselective Synthesis of 1,3-Diols from Allylic Alcohols and Aldehydes through a Tandem Allylic-Isomerization/Aldol–Tishchenko Reaction

This study reports the first base-promoted aldol–Tishchenko reactions of allylic alcohols with aldehydes initiated by allylic isomerization. The reaction enables the diastereoselective synthesis of a variety of 1,3-diols with three contiguous stereogenic centers. Unlike commonly reported systems, our method allows the use of readily available allylic alcohols as nucleophiles instead of enolizable aldehydes and ketones.     

Stereoselective direct amine-catalyzed decarboxylative aldol addition

A stereoselective decarboxylative aldol addition of β- and α-keto acids in the presence of catalytic amounts of amines is described. By the optional deployment of chiral enolizable aldehydes an access to enantiopure configurative defined ketopentoses, ketohexoses, or ketoheptoses is given.     

Primary amine catalyzed aldol reaction of isatins and acetaldehyde

Several cinchona alkaloid-derived chiral primary amines were applied as the catalyst for the cross aldol reaction of isatins with acetaldehyde. With the quinine-derived amine catalyst 3, the desired aldol products were obtained in high yields and good enantioselectivities (up to 93% ee) under the optimized conditions. Although other enolizable aldehydes and ketones may also be applied in this reaction, the ee values obtained are usually low. A mechanism was proposed to account for the formation of the major enantiomer in this reaction.     

Stereoselective aldol additions of achiral ethyl ketone-derived trichlorosilyl enolates

Methods for the preparation of geometrically defined enoxy(trichlorosilanes) derived from ethyl ketone enolates have been developed. The addition of enoxy(trichlorosilanes) (trichlorosilyl enolates) to aldehydes proceeds with good yields in the presence of catalytic amounts of chiral phosphoramides. The reaction of Z-trichlorosilyl enolates to aryl aldehydes affords aldol products with good to excellent diastereo- and enantioselectivities. Phosphoramide-catalyzed aldol additions lacked substrate generality providing modest selectivities with unsaturated and aliphatic aldehydes. In all cases, the phosphoramide-catalyzed aldol addition of E-trichlorosilyl enolates to aldehydes provided good yields with moderate to good stereoselectivities.     

Titanium(IV) alkoxide ligand exchange with alpha-hydroxy acids: the enantioselective aldol addition

Ligand exchange of titanium(IV) alkoxides with alpha-hydroxy acids presents an unexpected and novel approach to enantioselective aldol additions of aldehydes and ketones. The aldol products were isolated in a high degree of syn-diastereoselectivity. High enantioselectivities were observed by using simple optically pure alpha-hydroxy acids in this novel aldol addition.     

Highly selective acetate aldol additions using mesityl-substituted chiral auxiliaries

[reaction: see text] Highly diastereoselective acetate aldol additions using chlorotitanium enolates of mesityl-substituted N-acetyloxazolidinethione and N-acetylthiazolidinethione auxiliaries are reported. These additions proceed in high yields and diastereoselectivities (93:7 to 98:2) for aliphatic, aromatic, and ,-unsaturated aldehydes. Double diastereoselective acetate aldol additions are also reported and are found to proceed in high yields and diastereoselectivities (90:10 to 97:3).     

Organocatalyzed highly enantioselective direct aldol reactions of aldehydes with hydroxyacetone and fluoroacetone in aqueous media: the use of water to control regioselectivity

An organocatalyst prepared from (2R,3R)-diethyl 2-amino-3-hydroxysuccinate and L-proline exhibited high regio- and enantioselectivities for the direct aldol reactions of hydroxyacetone and fluoroacetone with aldehydes in aqueous media. It was found that water could be used to control the regioselectivity. The presence of 20-30 mol% of the catalyst afforded the direct aldol reactions of a wide range of aldehydes with hydroxyacetone to give the otherwise disfavored products with excellent enantioselectivities, ranging from 91 to 99% ee, and high regioselectivities. Aldolizations of fluoroacetone with aldehydes mediated by 30 mol% of the organocatalyst in aqueous media preferentially occurred at the methyl group, yielding products with high enantioselectivities (up to 91% ee); however, these additions took place dominantly at the fluoromethyl group in THF. Optically active 3,5-disubstituted tetrahydrofurans and (2S,4R)-dihydroxy-4-biphenylbutyric acid were prepared by starting from the aldol reaction of hydroxyacetone. Theoretical studies on the role of water in controlling the regioselectivity revealed that the hydrogen bonds formed between the amide oxygen of proline amide, the hydroxy of hydroxyacetone, and water are responsible for the regioselectivity by microsolvation with explicit one water molecule as a hydrogen-bond donor and/or an acceptor.     

One-pot enol silane formation-Mukaiyama aldol reactions: Crossed aldehyde-aldehyde coupling,thioester substrates,and reactions in ester solvents

Trimethylsilyl trifluoromethanesulfonate (TMSOTf) and a trialkylamine base promote both in situ enol silane/silyl ketene acetal formation and Mukaiyama aldol addition reactions between a variety of reaction partners in a single reaction flask. Isolation of the required enol silane or silyl ketene acetal is not necessary. For example, crossed aldol reactions between α-disubstituted aldehydes and non-enolizable aldehydes yield β-hydroxy aldehydes in good yield. In a related reaction, the common laboratory solvent ethyl acetate functions as both an enolate precursor and a green reaction solvent. When thioesters are employed as enolate precursors, high yields for additions to non-enolizable aldehydes are routinely observed.     

gem‐Difluoroolefination of Diaryl Ketones and Enolizable Aldehydes with Difluoromethyl 2‐Pyridyl Sulfone: New Insights into the Julia–Kocienski Reaction

The direct conversion of diaryl ketones and enolizable aliphatic aldehydes into gem‐difluoroalkenes has been a long‐standing challenge in organofluorine chemistry. Herein, we report efficient strategies to tackle this problem by using difluoromethyl 2‐pyridyl sulfone as a general gem‐difluoroolefination reagent. The gem‐difluoroolefination of diaryl ketones proceeds by acid‐promoted Smiles rearrangement of the carbinol intermediate; the gem‐difluoroolefination is otherwise difficult to achieve through a conventional Julia–Kocienski olefination protocol under basic conditions due to the retro‐aldol type decomposition of the key intermediate. Efficient gem‐difluoroolefination of aliphatic aldehydes was achieved by the use of an amide base generated in situ (from CsF and tris(trimethylsilyl)amine), which diminishes the undesired enolization of aliphatic aldehydes and provides a powerful synthetic method for chemoselective gem‐difluoroolefination of multi‐carbonyl compounds. Our results provide new insights into the mechanistic understanding of the classical Julia–Kocienski reaction.     

Catalytic enantioselective aldol additions of alpha-isothiocyanato imides to aldehydes

Highly enantioselective organocatalytic aldol additions of alpha-isothiocyanato imides to aldehydes are reported. These reactions provide convenient access to enantiomerically enriched and protected beta-hydroxy-alpha-amino acids with catalyst loadings as low as 1 mol%.