Titanium Enolates Research Articles

4(S)-Benzyl-1,3-thiazolidin-2-one a Novel Chiral Auxiliary for Asymmetric Aldol Coupling through Titanium Enolates

The chlorotitanium enolate of N-propionyl-4(S)-benzyl-1,3-thiazolidin-2-one, was condensed with aryl aldehydes, in good diastereoselectivity to give the ‘Evans syn’ aldol (73:27 - 97:3), using equimolar amounts of titanium tetrachloride (1.5 equiv), and N,N-diisopropylethylamine (DIPEA). The facial selectivity in the aldol additions probably involves a non-chelated transition state. In all aldol reactions, the presence of a minor product, the ‘non-Evans syn’ aldol, was obtained and confirmed by X-ray diffraction analysis of a single-crystal compound containing the mixture of diastereoisomers. The chiral auxiliary in these 1,3-thiazolidin-2-one aldol derivatives can be easily removed by nucleophilic species through acyl substitution. Resumen. El enolato de clorotitanio de N-propionil-4(S)-bencil-1,3-tiazolidin-2-ona, fue condensado con arilaldehídos, con buena diastereoselectividad produciendo el aldol “syn Evans” (73:27 - 97:3), utilizando cantidades equimolares de tetracloruro de titanio (1.5 equiv) y N,N-diisopropiletilamina (DIPEA). La selectividad facial en las adiciones aldólicas probablemente implica un estado de transición no-quelatado. En todas las reacciones aldólicas, la presencia de un producto minoritario, él aldol “non-Evans”, fue obtenido y confirmado por el análisis de difracción de rayos-X de monocristal de una mezcla de los diastereoisómeros. El auxiliar quiral en estos derivados de 1,3-tiazolidin-2-onas puede ser removido fácilmente por especies nucleofílicas a través de la sustitución del grupo acilo.

Titanium(iv) enolates of cyclic ketones – stereoselective addition of cyclododecanone to aromatic aldehydes

The application of the “soft enolization technique” led to the first synthesis of syn aldols from cyclododecanone titanium(iv) enolates and aromatic aldehydes.

Highly diastereoselective asymmetric syn-aldol reactions of (R)-(N-tosyl)phenylalaninol propionate-derived titanium enolate and bidentate aromatic and aliphatic aldehydes

Asymmetric aldol reactions of ester-derived titanium enolates of (R)-(N-tosyl)phenylalaninol propionate and bidentate aromatic aldehydes has been investigated. Substrate scope of this aldol reaction was examined with a variety of bidentate aromatic aldehydes to provide syn-aldol products with high diastereoselectivity and isolated yields. Both enantiomers of N-(tosyl)phenylalaninol are readily prepared from commercially available and optically active phenylalaninol. Mild saponification removed the chiral auxiliary and furnished optically active β-hydroxy carboxylic acids.

Titanium-Mediated Domino Cross-Coupling/Cyclodehydration and Aldol-Addition/Cyclocondensation: Concise and Regioselective Synthesis of Polysubstituted and Fused Furans.

Titanium enolates, in situ-generated from readily available ketones and titanium tetraisopropoxide, undergo domino cross-coupling/cyclodehydration or domino Aldol-addition/cyclocondensation with α-chloroketones to provide synthetically valuable furan derivatives. The domino process tolerates a variety of cyclic and acyclic ketones and chloroketones, producing polysubstituted furans and bi-, tri-, and tetracyclic fused furans.

Amphidinolides F and C2: An Odyssey in Total Synthesis.

Amphidinolides F, C, C2, and C3 are marine natural products isolated from dinoflagellates Amphidinium species. They share the same macrolactone core, with the difference between them residing at the side chain level. A predominant feature of these amphidinolides is the presence of two trans-THF rings inside the macrolactone core, which is thought to be built by C-glycosylation with titanium enolate of N-acetyl oxazolinethiones. Thus, the original strategy for their total synthesis was based on the assembly of three main fragments corresponding to C1-C9, C10-C19, and C20-C29 or C20-C34 disconnections. Whereas synthesis of all fragments was successful, the C-glycosylation reaction between C19 and C20 turned out to be an issue. Therefore, a second route was designed. The new disconnection between C17 and C18 was based on a sulfone addition and a desulfonylation sequence. Our convergent strategy allowed the total synthesis of amphidinolide F and enabled a new unifying route toward the synthesis of amphidinolides C, C2, and C3 using a late-stage divergent approach. Although there were unsatisfying yields at some critical steps, our work culminated into the first total synthesis of amphidinolide C2.

A stepwise lactol carbocyclisation to bridged ethers via a keto–acetal cascade

Lactol carbocyclisations provide a succinct method of constructing the oxabicyclo[3.2.1]octane scaffold, a motif present in various natural products of medicinal interest. Lactols containing an unsaturated ketone or ester were prepared by olefin cross-metathesis; an electrophilic alkene derived from methyl vinyl ketone underwent concomitant terminal α-methylenation and oxa-Michael addition to give a bridged lactol which then underwent oxygen-to-carbon transposition in the presence of titanium (IV) chloride giving the desired unsaturated carbocyclic seven-membered bridged ether via a novel dehydrative cascade considered to involve titanium enolates.

Stereoselective Alkylation of Chiral Titanium(IV) Enolates with tert-Butyl Peresters.

Here, we present a new stereoselective alkylation of titanium(IV) enolates of chiral N-acyl oxazolidinones with tert-butyl peresters from Cα-branched aliphatic carboxylic acids, which proceeds through the decarboxylation of the peresters and the subsequent formation of alkyl radicals to produce the alkylated adducts with an excellent diastereoselectivity. Theoretical calculations account for the observed reactivity and the outstanding stereocontrol. Importantly, the resultant compounds can be easily converted into ligands for asymmetric and catalytic transformations.

Total synthesis of Cinncassin A1

AbstractIn this communication, we report the first total synthesis of the phenolic natural product Cinncassin A1, which was recently isolated from the twigs of the cinnamon plant (Cinnamomum cassia). The novel and concise synthesis strategy involves a highly diastereoselective titanium enolate homocoupling and a permanganate‐mediated oxidative lactonization reaction as key steps. The structure of the compound was verified by single‐crystal X‐ray crystallography.

The Dianion of Dehydroacetic Acid: A Direct Synthesis of Pogopyrone A

Dehydroacetic acid was converted into a silyl enol ether and titanium enolate. These reacted effectively with aldehydes and N-bromosuccinimide. Oxidation of the adduct with benzaldehyde afforded pogopyrone A in excellent overall yield.

Synthesis of 8-Oxabicyclo[3.2.1]octanes by TiCl4-Mediated Reactions of 3-Alkoxycyclobutanones and Allenylsilanes

1-Substituted allenylsilanes reacted with 3-alkoxycyclobutanones in the presence of TiCl4 to afford 8-oxabicyclo[3.2.1]octan-3-ones stereoselectively. Nucleophilic attack of allenylsilanes to a 1,4-zwitterionic intermediate formed from 3-alkoxycyclobutanones and TiCl4­ followed by 1,2-silyl migration, five-membered cyclization with an alkoxy group, and seven-membered cyclization of titanium enolate was proposed. Deuteration and one-pot Peterson olefination suggested that alkyl titanium species were formed after cyclization to 8-oxabi­cyclo[3.2.1]octane skeletons.

Stereoselective Oxidation of Titanium(IV) Enolates with Oxygen

A novel approach to synthesize enantiomerically pure α-hydroxy carboxylic derivatives is reported. A highly stereoselective oxidation of titanium(IV) enolates from chiral N-acyloxazolidinones is performed with oxygen under simple experimental conditions that do not require any reducing steps. The success of this approach depends on the biradical character of titanium(IV) enolates.

Total Synthesis of the Marine Macrolide Amphidinolide F.

A new and efficient convergent approach toward the synthesis of amphidinolide F is described through the assembly of three fragments. The two trans-tetrahydrofurans were built by a diastereoselective C-glycosylation with titanium enolate of bulky N-acetyloxazolidinethiones. The side chain was inserted by a Liebeskind-Srogl cross-coupling reaction. A sulfone condensation/desulfonylation sequence, a Stille cross-coupling, and a macrolactonization were applied to connect the fragments.

Substrate‐Controlled Michael Additions of Titanium Enolates from Chiral α‐Benzyloxy Ketones to Conjugated Nitroalkenes

Lewis acid mediated substrate‐controlled reactions of the titanium(IV) enolates of chiral α‐benzyloxy ketones with conjugated nitroalkenes give 2,4‐anti‐4,5‐syn Michael adducts in good yields and diastereomeric ratios. The supplementary Lewis acid plays a key role in the outcome of these transformations, probably as a consequence of the formation of dimetallic enolates that increase the reactivity of the enolate and direct the approach of the nitroalkene. Importantly, the most appropriate Lewis acid depends on the electrophilic partner: TiCl4 is the most suitable Lewis acid for β‐aryl‐nitroalkenes, whereas the best results for β‐alkyl‐nitroalkenes are obtained with SnCl4. Finally, the nitro group of the resultant compounds can be converted into the corresponding amine, oxime, and nitrile groups under mild conditions, which permits the synthesis of a variety of enantiomerically pure derivatives in excellent yields.

Experimental and Computational Evidence of the Biradical Structure and Reactivity of Titanium(IV) Enolates.

Quantum chemical calculations have unveiled the unexpected biradical character of titanium(IV) enolates from N-acyl oxazolidinones and thiazolidinethiones. The electronic structure of these species therefore involves a valence tautomerism consisting of an equilibrium between a closed shell (formally Ti(IV) enolates) and an open shell, biradical, singlet (formally Ti(III) enolates) electronic states, whose origin is to be basically found in changes of the Ti-O distance. Spectroscopic studies of the intermediate species lend support to such a model, which also turns out to be crucial for a better understanding of the overall reactivity of titanium(IV) enolates. In this context, a thorough computational analysis of the radical addition of titanium(IV) enolates from N-acyl oxazolidinones to TEMPO has permitted us to suggest an entire mechanism, which accounts for the experimental details and the diastereoselectivity of the process. All together, this evidence highlights the relevance of biradical intermediates from titanium(IV) enolates and may be a useful contribution to the foundations of a more insightful comprehension of the structure and reactivity of titanium(IV) enolates.

Stereoselective Alkylation of Imines and its Application towards the Synthesis of β‐Lactams

Abstract(S)‐4‐Isopropyl‐1‐[(R)‐1‐phenylethyl]imidazolidin‐2‐one was evaluated as a chiral auxiliary for asymmetric acetate and propionate Mannich‐type reactions, by generation of the titanium enolates, affording excellent yields and stereoselectivities. The application of the auxiliary was exemplified in the stereoselective synthesis of the β‐lactam antihyperlipidemic drug ezetimibe.

Synthesis of the C3–C7 fragment of tylonolide by the γ-hydroxybutenolide approach

The seven-step synthesis of the C3–C7 fragment from 3-furanmethanol is described. The key step is an asymmetric aldol reaction of a γ-hydroxybutenolide with the titanium enolate of an N-propionyloxazolidinone. The resulting lactone was reduced to give the C4–C6 stereotriad, with further selective reductions to differentiate the two termini and C20 as a protected acetal.

ChemInform Abstract: Titanium Enolate Chemistry at the Beginning of the 21st Century

AbstractReview: [recent developments in the field of titanium(IV)‐mediated aldol, aldol‐like, Mannich, and Michael reactions, radical alkylation, oxidative couplings, and Morita—Baylis—Hillman reactions with special emphasis on the last fifteen years; 146 refs.

Titanium Enolate Chemistry at the Beginning of the 21st Century

AbstractThe aim of the microreview is to summarize the current state of knowledge in the field of titanium enolate chemistry. The authors focus on stereoselective organic syntheses promoted by titanium enolates and analyze developments in the synthetic methodology observed in this area during the last fifteen years.

Intramolecular Diels-Alder (IMDA) Studies toward the Synthesis of Australifungin. Stereocontrol in the Acetate Aldol Reaction of β,β'-Branched Aldehydes.

Studies of australifungin illustrate an enantiocontrolled synthesis of the trans-decalin core 28 via an intramolecular [4π + 2π] cycloaddition. This strategy utilizes the nitroalkene dienophile of 27 as a surrogate ketene equivalent. Stereocontrol at C-2 is critically important for an effective intramolecular Diels-Alder (IMDA) process. Our studies report high asymmetric induction using a nonracemic Duthaler titanium enolate in the acetate aldol reaction with β,β'-branched aldehyde 13 to introduce the required C-2 chirality.

Promotion of a Ti-Mediated Mannich Reaction by a Proton Source.

Low temperature NMR studies revealed that a diastereoselective Mannich reaction between a phenyl oxazolidone-derived titanium enolate and an aromatic aldimine was found to occur only after introduction of a proton source. While various protic additives could be used to promote the transformation, the best results were obtained using AcOH to afford the corresponding Mannich products in high diastereoselectivities and yields.