Asymmetric Sulfonium Ylide Mediated Cyclopropanation: Stereocontrolled Synthesis of (+)‐LY354740

The reaction of ester‐stabilized sulfonium ylides with cyclopentenone to give (+)‐ 5 ((1S,5R,6S)‐ethyl 2‐oxobicyclo[3.1.0]hexane‐6‐carboxylate), an important precursor to the pharmacologically important compound (+)‐LY354740, has been studied using chiral sulfides operating in both catalytic (sulfide, Cu(acac)₂, ethyl diazoacetate, 60 °C) and stoichiometric modes (sulfonium salt, base, room temperature). It was found that the reaction conditions employed had a major influence over both diastereo‐ and enantioselectivity. Under catalytic conditions, good enantioselectivity with low diastereoselectivity was observed, but under stoichiometric conditions low enantioselectivity with high diastereoselectivity was observed. When the stoichiometric reactions were conducted at high dilution, diastereoselectivity was reduced. This indicated that base‐mediated betaine equilibration was occurring (which is slow relative to ring closure at high dilution). Based on this model, conditions for achieving high enantioselectivity were established as follows: use of a preformed ylide, absence of base, hindered ester (to reduce ylide‐mediated betaine equilibration), and low concentration. Under these conditions high enantioselectivity (95 % ee) was achieved, albeit with low diastereocontrol. Our model for selectivity has been applied to other sulfonium ylide mediated cyclopropanation reactions and successfully accounts for the diastereoselectivity observed in all such reported reactions to date.     

Readily Synthesized Chiral Sulfides as Reagents for Asymmetric Epoxidation

Chiral oxathianes were designed, synthesized, and successfully used for asymmetric sulfur ylide mediated epoxidation. A considerable emphasis has been placed upon the design of sulfides with suitable architecture in a small number of steps (three or four). The use of (4aR,6S,8aR)‐6‐isopropenyl‐8a‐methyloctahydro‐1,4‐benzoxathiane in asymmetric epoxidation resulted in good diastereo‐ and enantioselectivity in the formation of stilbene oxide, and (2S,6S)‐2‐allyl‐2,3,3,6‐tetramethyl‐1,4‐oxathiane produced even better results. Moderate to good diastereoselectivities with essentially complete enantioselectivities were observed in the formation of alkyl–aryl‐, vinyl–aryl‐, and propargyl–aryl‐substituted epoxides. The selectivities were rationalized and supported by density functional theory calculations.     

Tandem Cross‐Coupling/Spirocyclization/Mannich‐Type Reactions of 3‐(2‐Isocyanoethyl)indoles with Diazo Compounds toward Polycyclic Spiroindolines

Tandem reactions of Pd‐catalyzed cross‐coupling of 3‐(2‐isocyanoethyl)indoles with diazoacetates and subsequent spirocyclization/Mannich‐type reaction have been developed to assemble polycyclic spiroindoline skeletons. Formation of spiroindolenines has been proven as the crucial step for the following Mannich‐type cyclization reaction. Accordingly, a novel approach on chiral phosphoric acid catalyzed Mannich‐type cyclization toward the formation of diastereomerically and enantiomerically enriched pentacyclic spiroindolines has been established. Moreover, the products of the reaction are versatile building blocks in synthetic chemistry, as demonstrated by the synthesis of the key framework of aspidosperma and kopsia alkaloids.     

Asymmetric Substitutions of 2‐Lithiated N‐Boc‐piperidine and N‐Boc‐azepine by Dynamic Resolution

Proton abstraction of N‐tert‐butoxycarbonyl‐piperidine (N‐Boc‐piperidine) with sBuLi and TMEDA provides a racemic organolithium that can be resolved using a chiral ligand. The enantiomeric organolithiums can interconvert so that a dynamic resolution occurs. Two mechanisms for promoting enantioselectivity in the products are possible. Slow addition of an electrophile such as trimethylsilyl chloride allows dynamic resolution under kinetic control (DKR). This process occurs with high enantioselectivity and is successful by catalysis with substoichiometric chiral ligand (catalytic dynamic kinetic resolution). Alternatively, the two enantiomers of this organolithium can be resolved under thermodynamic control with good enantioselectivity (dynamic thermodynamic resolution, DTR). The best ligands found are based on chiral diamino‐alkoxides. Using DTR, a variety of electrophiles can be used to provide an asymmetric synthesis of enantiomerically enriched 2‐substituted piperidines, including (after Boc deprotection) the alkaloid (+)‐β‐conhydrine. The chemistry was extended, albeit with lower yields, to the corresponding 2‐substituted seven‐membered azepine ring derivatives.     

5‐(Alkynyl)dibenzothiophenium Triflates: Sulfur‐Based Reagents for Electrophilic Alkynylation

The synthesis of a series of 5‐(alkynyl) dibenzothiophenium triflates, prepared from dibenzo[b,d]thiophene 5‐oxide and the corresponding trimethylsilyl‐substituted alkynes is reported. Their structures were determined by X‐ray crystallography, and their reactivities as electrophilic alkynylation reagents evaluated. Their broad substrate scope and functional‐group tolerance illustrate their potential to become an alternative to the broadly used EBX reagents. Isotope labeling studies reveal that alkynyldibenzothiophenium salts may undergo attack by nucleophiles at either the α‐ or β‐carbon atom depending on the nature of their substitution pattern. Subsequent elimination of the dibenzothiophene unit and 1,2‐migration of one of the groups (in case of β‐attack) affords the desired alkynes.     

On the Scope of Trimethylaluminium‐Promoted 1,2‐Additions of ArZnX Reagents to Aldehydes

A practical asymmetric 1,2‐addition of functionalised arylzinc halides to aromatic and aliphatic aldehydes is described by the use of aminoalcohol catalysis in the presence of AlMe₃. The process is simple to carry out, uses only commercially available reagents/ligands and provides moderate to good (80–96 % ee) enantioselectivities for a wide range of substrates. Either commercial ArZnX reagents or those prepared in situ from low cost aryl bromides can be used. In the latter case electrophilic functional groups are tolerated (CO₂Et, CN). The reaction relies on rapid exchange between ArZnX and AlMe₃ to generate mixed organometallic species that lead to the formation of a key intermediate that is distinctly different from the classic “anti” transition states of Noyori. NMR monitoring and related experiments have been used to probe the validity of the proposed selective transition state.     

[CpRu]‐Catalyzed Carbene Insertions into Epoxides: 1,4‐Dioxene Synthesis through SN1‐Like Chemistry with Retention of Configuration

Rather than lead to the usual deoxygenation pathway, metal carbenes derived from α‐diazo‐β‐ketoesters undergo three‐atom insertions into epoxides using a combination of 1,10‐phenanthroline and [CpRu(CH₃CN)₃][BAr_F] as the catalyst. Original 1,4‐dioxene motifs are obtained as single regio‐ and stereoisomers. A perfect syn stereochemistry (retention, e.r. up to 97:3) is observed for the ring opening, which behaves as an S_N1‐like transformation.     

Direct Synthesis of Chiral Allenoates from the Asymmetric CH Insertion of α‐Diazoesters into Terminal Alkynes

The asymmetric C? H insertion of α‐diazoesters into 1‐alkynes was achieved using chiral cationic guanidinium salts and copper(I) complexes. Optically active 2,4‐disubstituted allenoates were generated under mild reaction conditions from various α‐diazoesters and 1‐alkynes in high yield (up to 99 %) and enantioselectivity (up to 97:3 e.r.). Control experiments excluded the possibility of an asymmetric isomerization of alkynoates.