Direct and highly regioselective and enantioselective allylation of β-diketones

An Iterative Phosphate Tether Mediated Approach for the Synthesis of Complex Polyol Subunits

A pot-economical approach to advanced polyol subunits is reported. The key reactions involved are iterative use of a phosphate tether-mediated one-pot sequential RCM/CM/H2 with subsequent utilization of either a regio-/diasteroselective cuprate addition or a Pd-catalyzed reductive allylic transposition. This method highlights the asymmetric synthesis of 12 complex polyol subunits in 4-6 one-pot sequential operations with a total of 12-14 reactions, of which 4-5 are catalytic, with minimal workup and purification procedures.

Synthesis and Optical Resolution of Configurationally Stable Zwitterionic Pentacoordinate Silicon Derivatives

Stereogenic silicon centres in functionalised tetracoordinated organosilanes generally exhibit very high configurational stability under neutral conditions. This stability drops completely when higher coordination states of the silicon centre are reached due to rapid substituent exchange. Herein we describe the synthesis of chiral and neutral pentacoordinate silicon derivatives with high configurational stability. The zwitterionic nature of these air- and water-tolerant species allows for the first time their direct and efficient optical resolution using chiral HPLC techniques. By means of this method, pentacoordinate silicon compounds exhibiting high Si-inversion have been obtained as single enantiomers. A rationalisation of the enantiomerisation pathways has been also carried out using DFT calculations.

Lewis Superacidic Catecholato Phosphonium Ions: Phosphorus-Ligand Cooperative C-H Bond Activation

A series of catecholato phosphonium ions, including the first stable bis(catecholato)-substituted derivatives, are isolated and fully characterized. The cations rank among the most potent literature-known Lewis acids on the Gutmann-Beckett and ion affinity scales. In contrast to halogenated or multiply charged phosphorus cations, Lewis superacidity is imparted by structural constraints, as disclosed by energy decomposition analysis. The modular access provides a tunable scaffold while maintaining extreme affinity, demonstrated by the synthesis of a chiral Lewis superacid. The combination of electrophilic phosphorus and basic oxygen substituents leverages new reactivity modes by phosphorus-ligand cooperativity. With this, a phosphorus-mediated C-H bond activation is accomplished.

Enantioselective Syntheses of (Z)-6'-Boryl-anti-1,2-oxaborinan-3-enes via a Dienylboronate Protoboration and Asymmetric Allylation Reaction Sequence

The enantioselective synthesis of 6'-boryl-anti-1,2-oxaborinan-3-enes is reported. A Cu-catalyzed highly stereoselective 1,4-protoboration of 1,1-bisboryl-1,3-butadiene is developed to generate (E)-α,δ-bisboryl-crotylboronate. The chiral phosphoric-acid-catalyzed asymmetric allylboration of aldehydes with the boron reagent produces 6'-boryl-anti-1,2-oxaborinan-3-enes with excellent Z-selectivities and enantioselectivities. The product contains a vinyl and alkyl boronate unit that can directly participate in a variety of subsequent transformations.

Computational Design of Enhanced Enantioselectivity in Chiral Phosphoric Acid-Catalyzed Oxidative Desymmetrization of 1,3-Diol Acetals

A general method for the highly enantioselective desymmetrization of 2-alkyl-substituted 1,3-diols is presented. A combination of computational and experimental studies has been utilized to understand the origin of the stereocontrol of oxidative desymmetrization of 1,3-diol benzylideneacetals. DFT calculations demonstrate that the acetal protecting group is highly influential for high enantioselectivity, and a simple but effective new protecting group has been designed. The desymmetrization reactions proceed with high enantioselectivity for a variety of substrates. Moreover, the reaction conditions are also shown to be effective for desymmetrization of 2,2-dialkyl-substituted 1,3-diols, which provides chiral products bearing acyclic all-carbon quaternary stereocenters. The method has been applied to the formal synthesis of indoline alkaloids.

Bis(catecholato)silanes: assessing, rationalizing and increasing silicon's Lewis superacidity

Although bis(catecholato)silanes have been known for several decades, their substantial Lewis acidity is not yet well described in the literature. Herewith, the synthesis and characterization of multiple substituted bis(catecholato)silanes and their triethylphosphine oxide, fluoride and chloride ion adducts are reported. The Lewis acidity of bis(catecholato)silanes is assessed by effective (Gutmann-Beckett, catalytic efficiency), global (theoretical and relative experimental fluoride (FIA) and chloride (CIA) ion affinities) and intrinsic (electrophilicity index) scaling methods. This comprehensive set of experimental and theoretical results reveals their general Lewis acidic nature and provides a consistent Lewis acidity trend for bis(catecholato)silanes for the first time. All experimental findings are supported by high-level DLPNO-CCSD(T) based thermochemical data and the Lewis acidity is rationalized by complementary chemical bonding analysis tools. Against the common belief that inductive electron withdrawal is the most important criterion for strong Lewis acidity, the present work highlights the decisive role of π-back bonding effects in aromatic ring systems to enhance electron deficiency. Thus, bis(perbromocatecholato)silane is identified and synthesized as the new record holder for silicon Lewis superacids.

Parallel Kinetic Resolution of Unsymmetrical Acyclic Aliphatic syn-1,3-Diols

Disclosed is a mild, reliable, and enantioselective catalytic parallel kinetic resolution of unsymmetrical acyclic aliphatic syn-1,3-diol derived acetals mediated by chiral phosphoric acid. This method provides stereoselective access to a variety of syn-1,3-diols as valuable building blocks with high enantioselectivity. Moreover, this mild system allows for site-selective protection of optically pure syn-1,3-diols in excellent regioselectivity.

One-Pot Domino Friedel-Crafts Acylation/Annulation between Alkynes and 2-Methoxybenzoyl Chlorides: Synthesis of 2,3-Disubstituted Chromen-4-one Derivatives

A highly regioselective synthesis of 2,3-disubstituted chromen-4-one derivatives is accomplished from readily available internal alkynes and 2-methoxybenzoyl chlorides. The reaction proceeds via a domino intermolecular Friedel-Crafts acylation/intramolecular vinyl carbocation trapping (or oxa-Michael addition)/demethylation reaction sequence. This Lewis acid promoted method features relatively mild reaction conditions to synthesize a variety of 2,3-disubstituted chromen-4-one derivatives in one pot with up to 93% yield. The chromen-2-one (coumarin) product was obtained when 2,6-dimethoxybenzoyl chloride was used as a starting material via an electrophilic aromatic substitution/rearrangement process.

A thiocyanopalladation/carbocyclization transformation identified through enzymatic screening: stereocontrolled tandem C-SCN and C-C bond formation

Herein we describe a formal thiocyanopalladation/carbocyclization transformation and its parametrization and optimization using a new elevated temperature plate-based version of our visual colorimetric enzymatic screening method for reaction discovery. The carbocyclization step leads to C-SCN bond formation in tandem with C-C bond construction and is highly stereoselective, showing nearly absolute 1,2-anti-stereoinduction (5 examples) for substrates bearing allylic substitution, and nearly absolute 1,3-syn-stereoinduction (16 examples) for substrates bearing propargylic substitution. Based upon these high levels of stereoinduction, the dependence of the 1,2-stereoinduction upon cyclization substrate geometry, and the generally high preference for the transoid vinyl thiocyanate alkene geometry, a mechanistic model is proposed, involving (i) Pd(ii)-enyne coordination, (ii) thiocyanopalladation, (iii) migratory insertion and (iv) β-elimination. Examples of transition metal-mediated C-SCN bond formation that proceed smoothly on unactivated substrates and allow for preservation of the SCN moiety are lacking. Yet, the thiocyanate functionality is of great value for biophysical chemistry (vibrational Stark effect) and medicinal chemistry (S,N-heterocycle construction). The title transformation accommodates C-, O-, N- and S-bridged substrates (6 examples), thereby providing the corresponding carbocyclic or heterocyclic scaffolds. The reaction is also shown to be compatible with a significant range of substituents, varying in steric and electronic demand, including a wide range of substituted aromatics, fused bicyclic and heterocyclic systems, and even biaryl systems. Combination of this new transformation with asymmetric allylation and Grubbs ring-closing metathesis provides for a streamlined enantio- and diastereoselective entry into the oxabicyclo[3.2.1]octyl core of the natural products massarilactone and annuionone A, as also evidenced by low temperature X-ray crystal structure determination. Utilizing this bicyclic scaffold, we demonstrate the versatility of the thiocyanate moiety for structural diversification post-cyclization. Thus, the bridging vinyl thiocyanate moiety is smoothly elaborated into a range of derivative functionalities utilizing transformations that cleave the S-CN bond, add the elements of RS-CN across a π-system and exploit the SCN moiety as a cycloaddition partner (7 diverse examples). Among the new functionalities thereby generated are thiotetrazole and sulfonyl tetrazole heterocycles that serve as carboxylate and phosphate surrogates, respectively, highlighting the potential of this approach for future applications in medicinal chemistry or chemical biology.

Stereoselective Multicomponent Reactions Using Zincate Nucleophiles: β-Dicarbonyl Synthesis and Functionalization

A general strategy for conjugate addition-C-acylation that enables the synthesis of enantioenriched β-dicarbonyl compounds is described. A novel method for derivatizing these adducts by stereo- and site-selective zinc-catalyzed addition of alkyllithium reagents is also reported. These reactions can be performed in tandem to achieve an enantio- and diastereoselective four-component coupling. The in situ generation of weakly basic lithium zincate species is central to the success of all three transformations.

A multicomponent approach for the preparation of homoallylic alcohols

The generation of transient boronic acid species followed by their subsequent trapping with aldehydes as electrophiles to yield homoallylic alcohols, in a multicomponent and metal-free fashion.

Here we report the in situ generation of transient allylic boronic species, by reacting TMSCHN₂ and E-vinyl boronic acids, followed by their subsequent trapping with aldehydes as electrophiles to yield homoallylic alcohols. This metal-free reaction was initially discovered by the use of a flow chemistry approach to generate a variety of homoallylic alcohols in a straightforward fashion and then transferred to a batch protocol.

Toward a more step-economical and scalable synthesis of spongistatin 1 to facilitate cancer drug development efforts

An efficient, step-economical, and scalable synthesis of a diene-bearing AB spiroketal fragment of spongistatin 1, and a demonstration of its efficient coupling to an aldehyde derived from silylformylation of a homopropargyl alcohol to produce the entire complex C(13)-C(17) linker region are described. The scalability of the synthesis of the AB spiroketal fragment was demonstrated by the preparation of 34.5 grams by one chemist in ~60 workdays, and more than 40 grams overall. With this material in hand and having established a method for its efficient coupling to the CD fragment, we have set the stage for the rapid synthesis and evaluation of a series of analogs of the CD spiroketal.

Divergent synthesis of functionalized carbocycles through organosilane-directed asymmetric alkyne-alkene reductive coupling and annulation sequence

An organosilane-directed alkyne-alkene reductive coupling of readily available propargylsilanes is used to access densely functionalized chiral allylsilanes. The divergent reactivity of the allylsilanes can be controlled to afford a range of novel carbocyclic ring systems through an intramolecular allylation, [3+2] annulation, and Sakurai-like homodimerization.

Complex fragment coupling by crotylation: A powerful tool for polyketide natural product synthesis

The first examples of the use of crotylation as a stereocontrolled complex fragment coupling strategy are described. Asymmetric aldehyde isoprenylation provides access to 2-substituted-1,3-butadienes that may be subjected to highly regio- and stereoselective 1,4 hydrosilylation with trichlorosilane. After complexation with a chiral diamine, the 2-sub-stituted-cis-crotylsilanes may be employed in highly diastereoselective Sc(OTf)3-catalyzed aldehdye crotylation reactions.