A novel synthesis of silyl enol ethers from α-silylbenzylthiols and carboxylic acid derivatives via C---C bond formation; thermal rearrangement of S-α-silylbenzyl thioesters

A new procedure for the synthesis of silyl enol ethers from S-α-silylbenzyl thioesters without need for either bases or catalysts via C---C bond formation is described. Solutions of S-α-silylbenzyl thioesters were simply heated at 180°C for 24 h in a sealed tube to give silyl enol ethers in good yields with high stereoselectivity. Cyclization of the dipoles generated by thermal rearrangement of the silyl group and elimination of sulfur afforded silyl enol ethers.     

Preparation of silyl enol ethers from acyloin derivatives using silyllithium reagents

A new method for the regioselective preparation of silyl enol ethers from acyloin derivatives using silyllithium reagents has been developed. Both dimethylphenyl- and methyldiphenylsilyllithium were found to be effective, the latter providing greater stereocontrol. The reaction is believed to proceed via Brook rearrangement assisted by expulsion of the adjacent leaving group. A number of acyclic acyloin derivatives were reacted to form the corresponding silyl enol ethers in good to excellent yield.     

Stereoselective synthesis of cycloheptanone derivatives via an intermolecular [5 + 2] cycloaddition reaction

[reaction: see text] A [5 + 2] cycloaddition reaction of a new five-carbon unit was developed on the basis of a dicobalt hexacarbonyl propargyl cation species. Under the influence of EtAlCl(2), [5-benzoyloxy-2-(triisopropylsiloxy)-1-penten-3-yne)]dicobalt hexacarbonyl reacted with enol triisopropylsilyl ethers to yield seven-membered dicobalt acetylene complexes in good yield. The reactions with cyclic enol silyl ethers as well as acyclic enol silyl ethers exhibited remarkably high diastereoselectivity. The cycloadducts can be easily converted into various kinds of cycloheptanone derivatives.     

1,4-silatropy of S-alpha-silylbenzyl thioesters: a convenient route to silyl enol and dienol ethers accompanied by C-C bond formation via thiocarbonyl ylides

A novel convenient method for the generation of thiocarbonyl ylides from readily accessible starting materials and the first synthetic application of in situ generated ylides in the synthesis of silyl enol and dienol ethers, accompanied by C-C bond formation, is described. Under completely neutral conditions without any catalyst or additive, thermal reactions of S-alpha-silylbenzyl thioesters in sealed tubes at 180 degrees C provided silyl enol and dienol ethers in good to excellent yields with high stereoselectivities. This procedure consists of a multistep reaction in a one-pot process, i.e., 1,4-silatropy of S-alpha-silylbenzyl thioesters to give thiocarbonyl ylides, 1,3-electrocyclization of the ylides to give thiiranes, and the extrusion of sulfur from thiiranes to give silyl enol and dienol ethers.     

New methods of sulfonation in the thiophene series

Simple preparative methods of sulfonation of thiophene derivatives by complexes of sulfur trioxide with ethers, amides, trialkyl phosphates, and sulfoxides under mild conditions were developed.The developed methods of synthesis were protected by inventor certificates Nos. 707916 and 914557.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1183–1186, September, 1987.     

Cyclooctanone synthesis via a formal [6+2] cycloaddition reaction of a dicobalt acetylene complex

An efficient formal [6+2] cycloaddition reaction of a new six-carbon unit with enol silyl ether was developed on the basis of a dicobalt hexacarbonyl propargyl cation species. Under the influence of EtAlCl₂, 6-benzoyloxy-2-(triisopropylsilyloxy)-1-hexen-4-yne-dicobalthexacarbonyl reacted with enol triisopropylsilyl ethers to yield 7-(triisopropylsilyloxy)-3-cyclooctyn-1-one-dicobalthexacarbonyl derivatives in good yield. The reactions with cyclic enol silyl ethers as well as acyclic enol silyl ethers exhibited remarkably high diastereoselectivity.     

Tandem carbocupration/oxygenation of terminal alkynes

[chemical reaction: see text]. A direct and general synthesis of alpha-branched aldehydes and their enol derivatives is described. Carbocupration of terminal alkynes and subsequent oxygenation with lithium tert-butyl peroxide generates a metallo-enolate. Trapping with various electrophiles provides alpha-branched aldehydes or stereo-defined trisubstituted enol esters or silyl ethers. The tandem carbocupration/oxygenation tolerates alkyl and silyl ethers, esters, and tertiary amines. The reaction is effective with organocopper complexes derived from primary, secondary, and tertiary Grignard reagents and from n-butyllithium.     

Synthetic Studies on and Mechanistic Insight into [W(CO)5(L)]‐Catalyzed Stereoselective Construction of Functionalized Bicyclo[5.3.0]decane Frameworks

Stereoselective preparation of a variety of synthetically useful functionalized bicyclo[5.3.0]decane derivatives was achieved by tandem cyclization of 3‐siloxy‐1,3,9‐triene‐7‐yne derivatives based on the electrophilic activation of alkynes catalyzed by [W(CO)₅(L)]. The reaction proceeded smoothly under photoirradiation, and various substrates were cyclized to give the corresponding bicyclic compounds with up to four chiral centers stereospecifically. Reactions of siloxydienes with a silyl substituent as an equivalent of a hydroxyl group also proceeded with wide generality to afford silyl‐substituted bicyclo[5.3.0]decanes, which were highly useful as synthetic intermediates. Stereochemical studies concerning the silyl enol ether moiety suggested that two types of reaction pathway for the formation of seven‐membered rings were present. The reaction of (Z)‐enol silyl ethers proceeded through Cope rearrangement of cis‐divinylcyclopropane intermediates, and that of (E)‐enol silyl ethers by 1,4‐addition of the dienyl tungsten species at the position δ to the metal atom. In the reactions of siloxydiene derivatives with silyl substituents, all possible diastereomers could be synthesized stereoselectively by changing the geometry of the silyl enol ether and enyne moieties.     

Studies on Reactivity of Pyridinium Chlorochromate — Iodine System: An Efficient Method for Converting Enol Silyl Ethers into α-Iodo Ketones

A new synthesis of α-iodo carbonyl derivatives is described. They are obtained, in very high yields, through an original and efficient procedure, by reaction of enol silyl ethers with I₂-PCC system.     

Facile synthesis of pyrroles via Rh(II)-catalyzed transannulation of 1-tosyl-1,2,3-triazoles with silyl or alkyl enol ethers

A novel Rh(II)-catalyzed transannulation of 1-tosyl-1,2,3-triazoles with silyl or alkyl enol ethers has been developed, which enables the facile synthesis of substituted pyrroles in a regiocontrollable manner. Moreover, the methodology could be extended to access 3-pyrrolin-2-one derivatives with silyl ketene acetals used as the reaction partner.     

Chiral Zinc(II)‐Catalyzed Enantioselective Tandem α‐Alkenyl Addition/Proton Shift Reaction of Silyl Enol Ethers with Ketimines

A new catalytic asymmetric tandem α‐alkenyl addition/proton shift reaction of silyl enol ethers with ketimines was serendipitously discovered in the presence of chiral N,N′‐dioxide/Zn^II complexes. The proton shift preferentially proceeded instead of a silyl shift after α‐alkenyl addition of silyl enol ether to the ketimine. A wide range of β‐amino silyl enol ethers were synthesized in high yields with good to excellent ee values. Control experiments suggest that the Mukaiyama–Mannich reaction and tandem α‐alkenyl addition/proton shift reaction are competitive reactions in the current catalytic system. The obtained β‐amino silyl enol ethers were easily transformed into β‐fluoroamines containing two vicinal tetrasubstituted carbon centers.     

BF3-promoted cyclization reaction of imines and salicylaldehyde with silyl enol ethers: unexpected formation of dioxaspiro compounds

Unexpected spiro cyclic products were formed from the reaction of imines and salicylaldehyde with silyl enol ethers in the presence of BF₃·OEt₂. Different kinds of dioxaspiro products were afforded depending on the nature of starting materials. Furthermore, salicylaldehyde could also react directly with several silyl enol ethers, giving three products with different spiro cyclic structure under the same reaction conditions.     

Convenient stereoselective synthesis of (Z)-chalcone derivatives from 1,3-diaryl-2-propynyl silyl ethers

Various (Z)-chalcone derivatives were easily synthesized in a stereoselective manner from 1,3-diaryl-2-propynyl silyl ethers by a catalytic reaction using potassium tert-butoxide under very mild conditions after acid treatment.     

The reaction of vinyl ethers with acridine

The reaction of vinyl ethers with acridine gives new acridine derivatives in the presence of an organic ammonium salt and alcohol. An organic ammonium salt such as acridinium chloride affect this reaction. All of the addition products were obtained as dihydroacridinyl derivatives. Moreover, in the reaction of ketene silyl acetal with acridine, the addition products were also obtained as the dihydroacridine derivatives.     

Reaction of cyclooctatetraenyldipotassium with disulfides and thiocyanato derivatives of the thiophene series

In contrast to organometallic compounds of the phenyllithium type, cyclooctatetraenyldipotassium on reaction with disulfides and thiocyanato derivatives of the thiophene series in tetrahydrofuran does not give thio ethers but enters into an electron-transfer reaction and is converted to cyclooctatetraene. Thiophene derivatives form the potassium salts of the corresponding thiols. When insufficient amounts of cyclooctatetraenyldipotassium are present, thiocyanatothiophenes are converted to disulfides. It is shown that the latter can be obtained as a result of the reaction of thienyl mercaptides with thiocyanatothiophenes.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 632–636, May, 1973.     

Enantioselective Synthesis of cis-Decalin Derivatives by the Inverse-Electron-Demand Diels–Alder Reaction of 2-Pyrones

A novel strategy for the synthesis of cis-decalins by an ytterbium-catalyzed asymmetric inverse-electron-demand Diels–Alder reaction of 2-pyrones and silyl cyclohexadienol ethers is reported here. A broad range of synthetically important cis-decalin derivatives with multiple contiguous stereogenic centers and functionalities are obtained in good yields and stereoselectivities. A full set of diastereomeric substituted cis-decalin motifs are readily accessible by tuning the absolute configurations of substituted silyl cyclohexadienol ethers (R or S) as well as the ligands (R or S). The synthetic potential is showcased by the enantioselective total synthesis of 4-amorphen-11-ol, and further demonstrated by the first total synthesis of cis-crotonin.     

A Chiral N,N′‐Dioxide–ZnII Complex Catalyzes the Enantioselective [2+2] Cycloaddition of Alkynones with Cyclic Enol Silyl Ethers

A highly efficient enantioselective [2+2] cycloaddition between alkynones and cyclic enol silyl ethers was developed by using a chiral N,N′‐dioxide‐zinc(II) complex as a catalyst. This method functions well for a variety of terminal alkynes as well as cyclic enol silyl ethers, with good to excellent enantioselectivity (up to 97 % ee). This is also the first successful example for the catalytic enantioselective [2+2] cycloaddition of internal alkynes with cyclic enol silyl ethers to give fully substituted cyclobutenes. Meanwhile, the desired cyclobutene product can easily be transformed into fused cyclobutane derivatives.     

Two Reaction Mechanisms via Iminium Ion Intermediates: The Different Reactivities of Diphenylprolinol Silyl Ether and Trifluoromethyl‐Substituted Diarylprolinol Silyl Ether

The reactions of α,β‐unsaturated aldehydes with cyclopentadiene in the presence of diarylprolinol silyl ethers as catalyst proceed via iminium cations as intermediates, and can be divided into two types; one involving a Michael‐type reaction (type A) and one involving a cycloaddition (type B). Diphenylprolinol silyl ethers and trifluoromethyl‐substituted diarylprolinol silyl ethers, which are widely used proline‐type organocatalysts, have been investigated in this study. As the LUMO of the iminium ion derived from trifluoromethyl‐substituted diarylprolinol silyl ether is lower in energy than that derived from diphenylprolinol silyl ether, as supported by ab initio calculations, the trifluoromethyl‐substituted catalyst is more reactive in a type B reaction. The iminium ion from an α,β‐unsaturated aldehyde is generated more quickly with diphenylprolinol silyl ether than with the trifluoromethyl‐substituted diarylprolinol silyl ether. When the generation of the iminium ion is the rate‐determining step, the diphenylprolinol silyl ether catalyst is the more reactive. Because acid accelerates the generation of iminium ions and reduces the generation of anionic nucleophiles in the Michael‐type reaction (type A), it is necessary to select the appropriate acid for specific reactions. In general, diphenylprolinol silyl ether is a superior catalyst for type A reactions, whereas the trifluoromethyl‐substituted diarylprolinol silyl ether catalyst is preferred for type B reactions.     

Synthesis of 1,3,5-tricarbonyl derivatives by condensation of 1,3-bis(silyl enol ethers) with acid chlorides

A variety of 1,3,5-tricarbonyl derivatives were prepared by reaction of 1,3-bis(silyl enol ethers) with acid chlorides under mild conditions. This includes reactions of both aromatic and aliphatic acid chlorides and bis(acid chlorides). The yields vary depending on the type of acid chloride employed.     

New strategic reactions for organic synthesis: catalytic asymmetric C-H activation alpha to oxygen as a surrogate to the aldol reaction

The C-H activation of silyl ethers by means of rhodium carbenoid-induced C-H insertion represents a very direct method for the stereoselective synthesis of silyl-protected beta-hydroxy esters. The reaction can proceed with very high regio-, diastereo-, and enantioselectivity and represents a surrogate to the aldol reaction. The reaction is catalyzed by the rhodium prolinate complex Rh(2)(S-DOSP)(4). A critical requirement for the high chemoselectivity is the use of donor/acceptor-substituted carbenoids such as those derived from methyl aryldiazoacetates. A range of silyl ethers may be used such as allyl silyl ethers, tetraalkoxysilanes, and even simple trimethylsilyl alkyl ethers. In general, C-H activation preferentially occurs at methylene sites, as the reactivity is controlled by a delicate balance between steric and electronic effects.