A novel method for the synthesis of (Z)-α-selenyl-α,β-unsaturated ketones via acylation of (E)-α-selanylvinylstannanes

The (E)-α-selenylvinylstannanes react with acyl halides in presence of a catalytic amount of Pd(PPh₃)₄ to give the corresponding (Z)-α-selenyl-α,β-unsaturated ketones in good yield.     

1,3-dipolar cycloaddition reactions of azomethine ylides on enantiomerically pure (E)-γ-alkoxy-α,β-unsaturated esters.

Enantiomerically pure (E)-γ-alkoxy-α,β-unsaturated esters were reacted with azomethine ylides obtained from glycine imines in the presence of LiBr and diazabicycloundecene (DBU), to afford tetrasubstituted pyrrolidines with complete regiocontrol and fair to excellent diastereoselectivity (only two diastereoisomers formed in up to 96: 4 diastereoisomeric ratio). The results are compared with those of other 1,3-dipolar cycloadditions, and the origin of stereocontrol is discussed.     

Regioselective SN′ allylic substitution versus 1,4-addition: Synthesis of α-substituted β,γ-unsaturated esters

The reaction of the higher order organocuprate reagents R₂Cu(CN)Li₂-BF₃ with γ-bromo α,β-unsaturated esters gives with very high selectivity α-substituted β,γ-unsaturated esters arising from a SN′ allylic substitution. This reaction allows an easy access to α-silyl β,γ-unsaturated esters.     

Additive Pummerer reactions of vinylic sulfoxides. Synthesis of γ-hydroxy-α,β-unsaturated esters, α-hydroxyketones, and 2-phenylsulfenyl aldehydes and primary alcohols

Treatment of β-monosubstituted vinylic sulfoxides 1 with trifluoroacetic anhydride in dichloromethane gave excellent yields of 1,2-bis(trifluoroacetoxy)thioethers 6. Mildly basic methanolysis of 2-alkyl-substituted 6 gave α-hydroxyaldehydes 11 as monomer-dimer mixtures; similar treatment of the 2-aryl analogues afforded aryl (hydroxymethyl) ketones 12. Compounds 11 underwent Wittig reactions with methoxycarbonylmethylenetriphenylphosphorane to give high yields of γ-hydroxy-α,β-unsaturated esters 13, predominantly as the E-isomers. β-Monosubstituted vinylic sulfoxides 1 possessing a β-aryl group, and β-disubstituted vinylic sulfoxides 3 reacted with trifluoromethanesulfonic anhydride-sodium acetate in acetic anhydride to give 2-(phenylsulfenyl)acylals 14. These gave 2-phenylsulfenyl aldehydes 15 upon basic methanolysis, and the corresponding primary alcohols 16 on reduction with sodium borohydride. Reaction of both geometric isomers of enantiomerically pure vinylic sulfoxide 1o with TFAA gave racemic 6o as a mixture of diastereomers. Reaction of optically pure (E)- and (Z)-1p with trifluoromethanesulfonic anhydride-sodium acetate in acetic anhydride gave acylal 19 in 10.5 and 23% e.e., respectively.     

Synthese stereoselective d'esters α,β- ethyleniques α-methyles Z ou E par la reaction de wittig-horner a partir de phosphonates ou d'oxydes de phosphine

The reaction of α-branched aldehydes with diethyl 1-carbomethoxyethyl phosphonate 1b in THF/nBuLi at low temperature leads stereoselectively to Z α-methyl α,β-unsaturated esters. From a linear aldehyde, the reaction is less stereoselective, while from α,β-unsaturated ones, the E isomers are predominantly formed in good yields. From diphenyl 1-carbomethoxyethyl phosphine oxide 2b, E α-methyl α,β-unsaturated esters are stereoselectively formed either in DMF/tBuOK or in phase transfer conditions, whatever the starting aldehyde is, also in good yields. This reagent can thus advantageously replace the corresponding P-ylid. In all cases, the reaction conditions are determined so that by-products formation is minimized.     

Stereoselectivity of the Diels-Alder Reaction of (E)-γ-Oxo-α,β-Unsaturated Thioesters with Cyclopentadiene

The stereoselectivity of the Diels-Alder reaction of (E)-γ-oxo-α,β-unsaturated thioesters 3a-3d with cyclopentadiene is greatly enhanced in the presence of Lewis acids favoring the endo acyl isomers 4a-4d . In the absence of Lewis acid, Diels-Alder reaction of 3a-3d with cyclopentadiene at 25 °C gave two adducts 4a-4d and 5a-5d in a ratio of 1:1 respectively. In the presence of Lewis acids, Diels-Alder reaction of 3a-3d with cyclopentadiene gave 4a-4d and 5a-5d in ratios of 75-94:25-6 respectively. The stereoelectivity was enhanced to ratios of 95-98:5-2 with lowering the reaction temperature. The stereochemistry of the cycloadducts 4 and 5 was confirmed by iodocyclization. Reaction of the endo-thioester 5c with I₂ in aqueous THF at 0 °C gave the novel methylthio group rearranged product 6c in 80% yield, the first example of iodo-lactonization of endo-thioesters. Reaction of the endo-acyl isomer 4b with I₂ under the same reaction conditions gave an isomeric mixture of 7b and 8b in 1:2 ratio. The stereochemistry of the thioester group in 8b was proved by X-ray single-crystal analysis. The solvent effect on the endo selectivity of (Z)-γ-oxo-α,β-unsaturated thioester 2b was also examined.     

Thermische Lactonisierung von Estern γ-Brom-α, β-ungesättigter Carbonsäuren zu Δα-Butenoliden. Direkte γ-Bromierung von α, β-ungesättigten Säuren

By heating with iron powder at 120–150° some γ-bromo-α, β-unsaturated carboxylic methyl esters, and, less smothly, the corresponding acids, were lactonized to Δ^7alpha;-butenolides with elimination of methyl bromide. The following conversions have thus been made: methyl γ-bromocrotonate ( 1c ) and the corresponding acid ( 1d ) to Δ^α-butenolide ( 8a ), methyl γ-bromotiglate ( 3c ) and the corresponding acid ( 3d ) to α-methyl-Δ^α-butenolide ( 8b ), a mixture of methyl trans- and cis-γ-bromosenecioate ( 7c and 7e ) and a mixture of the corresponding acids ( 7d and 7f ) to β-methyl-Δ^α-butenolide ( 8c ). The procedure did not work with methyl trans-γ-bromo-Δ^α-pentenoate ( 5c ) nor with its acid ( 5d ). Most of the γ-bromo-α, β-unsaturated carboxylic esters ( 1c, 7c, 7e and 5c ) are available by direct N-bromosuccinimide bromination of the α, β-unsaturated esters 1a, 7a and 5a ; methyl γ-bromotiglate ( 3c ) is obtained from both methyl tiglate ( 3a ) and methyl angelate ( 4a ), but has to be separated from a structural isomer. The γ-bromo-α, β-unsaturated esters are shown by NMR. to have the indicated configurations which are independent of the configuration of the α, β-unsaturated esters used; the bromination always leads to the more stable configuration, usually the one with the bromine-carrying carbon anti to the carboxylic ester group; an exception is methyl γ-bromo-senecioate, for which the two isomers (cis, 7e , and trans, 7d ) have about the same stability. The N-bromosuccinimide bromination of the α,β-unsaturated carboxylic acids 1b , 3b , 4b , 5b and 7b is shown to give results entirely analogous to those with the corresponding esters. In this way γ-bromocrotonic acid ( 1 d ), γ-bromotiglic acid ( 3 d ), trans- and cis-γ-bromosenecioic acid ( 7d and 7f ) as well as trans-γ-bromo-Δ^α-pentenoic acid ( 5d ) have been prepared. Iron powder seems to catalyze the lactonization by facilitating both the elimination of methyl bromide (or, less smoothly, hydrogen bromide) and the rotation about the double bond. α-Methyl-Δ^α-butenolide ( 8b ) was converted to 1-benzyl-( 9a ), 1-cyclohexyl-( 9b ), and 1-(4′-picoly1)-3-methyl-Δ^α-pyrrolin-2-one ( 9 c ) by heating at 180° with benzylamine, cyclohexylamine, and 4-picolylamine. The butenolide 8b showed cytostatic and even cytocidal activity; in preliminary tests, no carcinogenicity was observed. Both 8b and 9c exhibited little toxicity.     

Diastereoselective SmI2-mediated cyclisation of δ-oxo-α,β-unsaturated esters to cyclopropanols

In the presence of samarium diiodide and a proton source, δ-oxo-γ,γ-disubstituted-α,β-unsaturated esters readily cyclise with complete diastereocontrol to give anti-cyclopropanol products.     

An efficient method for the stereoselective synthesis of cis-3-substituted prolines: conformationally constrained α-amino acids

An efficient synthesis of enantiomerically pure cis-3-substituted prolines is reported. Key steps involve the stereoselective organocuprate addition to the (E)-α,β-unsaturated ester 1, obtained from the Garner's aldehyde, and expedient oxidation–cyclization sequences.     

Photochemische Reaktionen 111. Mitteilung [1] Zur Photochemie α, β-ungesättigter γ, δ-Epoxyester I: Singulett- versus Triplettreaktivität

On triplet excitation (E)- 2 isomerizes to (Z)- 2 and reacts by cleavage of the C(γ), O-bond to isomeric δ-ketoester compounds ( 3 and 4 ) and 2,5-dihydrofuran compounds ( 5 and 19 , s. Scheme 1). - On singulet excitation (E)- 2 gives mainly isomers formed by cleavage of the C(γ), C(δ)-bond ( 6–14 , s. Scheme 1). However, the products 3–5 of the triplet induced cleavage of the C(γ), O-bond are obtained in small amounts, too. The conversion of (E)- 2 to an intermediate ketonium-ylide b (s. Scheme 5) is proven by the isolation of its cyclization product 13 and of the acetals 16 and 17 , the products of solvent addition to b . - Excitation (λ = 254 nm) of the enol ether (E/Z)- 6 yields the isomeric α, β-unsaturated ε-ketoesters (E/Z)- 8 and 9 , which undergo photodeconjugation to give the isomeric γ, δ-unsaturated ε-ketoesters (E/Z)- 10 . - On treatment with BF₃O(C₂H₅)₂ (E)- 2 isomerizes by cleavage of the C(δ), O-bond to the γ-ketoester (E)- 20 (s. Scheme 2). Conversion of (Z)- 2 with FeCl₃ gives the isomeric furan compound 21 exclusively.     

Substoichiometric TiCl4-mediated vicinal difunctionalization of α,β-acetylenic ketones for the synthesis of β-halo Baylis–Hillman olefins

A substoichiometric amount of titanium tetrachloride was found to be effective to promote and participate in the tandem α-hydroxyalkylation/β-chlorination of α,β-acetylenic ketones in the presence of (n-Bu)₄NI. This method provides the concise synthesis of (E)-β-halo Baylis–Hillman adducts. No β-iodo products were detected when using this combination of halogen sources. The reaction process involves 1,4-addition of chloro anion released from TiCl₄ onto α,β-acetylenic ketones to give TiCl₃–allenolate intermediates followed by the titanium Lewis acid-promoted carbonyl addition. Modest to good yields (53–77%) and excellent E/Z stereoselectivity (>95%) have been obtained for 10 examples.     

Lewis acid promoted aldol additions of α-thiosilylketeneacetals to α-alkoxy aldehydes: diastereoselective synthesis of -α-methylene-β-hydroxy-∂-alkoxy esters.

MgBr₂ mediated addition of Methyl α-methylthio propionate silylketene acetal to α and α,β-alkoxy aldehydes is highly 3,4 -selective (18:1). -α- methylene-β- hydroxy-∂-alkoxy esters (6) and (8) are synthesized.     

Nickel‐Catalyzed Mizoroki–Heck‐ versus Michael‐Type Addition of Organoboronic Acids to α,β‐Unsaturated Alkenes through Fine‐Tuning of Ligands

Various arylboronic acids reacted with activated alkenes in the presence of [Ni(dppe)Br₂], ZnCl₂, and H₂O in CH₃CN at 80 °C to give the corresponding Mizoroki–Heck‐type addition products in good to excellent yields. Furthermore, 1 equivalent of the hydrogenation product of the activated alkene was also produced. By tuning the ligands of the nickel complexes and the reaction conditions, Michael‐type addition was achieved in a very selective manner. Thus, various p‐ and o‐substituted arylboronic acids or alkenylboronic acid reacted smoothly with activated alkenes in CH₃CN at 80 °C for 12 h catalyzed by Ni(acac)₂, P(o‐anisyl)₃, and K₂CO₃ to give the corresponding Michael‐type addition products in excellent yields. However, for m‐substituted arylboronic acids, the yields of Michael‐type addition products are very low. The cause of this unusual meta‐substitution effect is not clear. By altering the solvent or phosphine ligand, the product yields for m‐substituted arylboronic acids were greatly improved. In contrast to previous results in the literature, the present catalytic reactions required water for Mizoroki–Heck‐type products and dry reaction conditions for Michael‐type addition products. Possible mechanistic pathways for both addition reactions are proposed.     

Cyanation of nucleophilic alkynes: easy approach to element-substituted α-cyanoenamines

The reactions of trimethylsilyl cyanide and tributyltin cyanide with ynamines proceeds as a regioselective syn-addition and provide previously unknown β-elementosubstituted α-cyanoenamines as pure E-isomers. The reaction of cyanogen bromide with ynamines as well the hydrocyanation of phosphorus substituted N,N-diethylaminoacetylenes by acetone cyanohydrin also proceeds as regioselective syn-addition, though the initially formed Z-isomers undergo an easy transformation into E-isomers. Cross-coupling reaction of β-bromo-α-cyanoenamine with arylboronic acids were shown to be an easy and convenient approach to α-dialkylaminosubstituted cynnamonitriles.     

The clemmensen reduction of α,β-unsaturated ketones

The deoxygenation of the α,β-unsaturated ketones (1) and (5) under the Clemmensen condition yielded the olefins (2) and (6) along with their respective dimers (3+4) and (8+9). The α , β-unsaturated ketone (13) under similar treatment yielded the olefin (14) in satisfactory yield but the dimer could not be characterized. The deoxygenation of the α,β-unsaturated ketones (10) and (16) under similar con- ditions afforded the olefins (12) and (15) respectively in satisfactory yield along with the rearranged olefins (11) and (17) respectively. Epox-idation of the olefin (17) followed by heating with p-toluenesulfonic acid yielded the ketone (18).     

The first example of α-organoseleno arsonium ylides and its Wittig-type reaction

α-Organoseleno arsonium ylide 3 was synthesized for the first time by a phenylselenenylation-transylidation reaction. It has sufficient activity to carry out the Wittig-type reaction, affording a novel method for the stereoselective synthesis of (Z)-α-phenyl-seleno-α, β-unsaturated esters 6.     

Synthesis of α,β-unsaturated aryl esters via Heck reaction of unsymmetrical aryl tellurides

A variety of α,β-unsaturated aryl esters were prepared by the direct reaction of unsymmetrical aryltellurides and ethyl acrylate, catalyzed by PdCl₂ via a Heck cross-coupling reaction.     

(E)‐,(Z)‐Parallel Preparative Methods for Stereodefined β,β‐Diaryl‐ and α,β‐Diaryl‐α,β‐unsaturated Esters: Application to the Stereocomplementary Concise Synthesis of Zimelidine

Parallel and practical methods for the preparation of both (E)‐ and (Z)‐β‐aryl¹‐β‐aryl²‐α,β‐unsaturated esters 1 and (E)‐ and (Z)‐α‐aryl¹‐β‐aryl²‐α,β‐unsaturated esters 2 are described. These methods involve accessible, robust, stereocomplementary N‐methylimidazole (NMI)‐mediated enol tosylations (14 examples, 70–99 % yield), as well as stereoretentive Suzuki–Miyaura cross‐couplings (36 examples, 64–99 % yield). The highlighted feature of the present protocol is the use of parallel and stereocomplementary approaches to obtain highly (E)‐ and (Z)‐pure products 1 and 2 by utilizing sequential enol tosylations and cross‐coupling reactions. An expeditious and parallel synthesis of (E)‐ and (Z)‐zimelidine ( 3 ), which is a highly representative selective serotonin reuptake inhibitor (SSRI), was performed by utilizing the present methods.     

Addition of trimethylsilylmethylmagnesium chloride to α, β-unsaturated carbonyls

Reactions of α,β-unsaturated carbonyl compounds with Me₃SiCH₂MgCl, prepared from chloromethyltrimethylsilane [1], were examined. Unlike its lithium counterpart (Me₃SiCH₂Li), which adds to α,β-unsaturatedketones in the 1,2-sense, the Grignard reagent afforts γ silanes via a 1,4-addition sequence. This tendency is accentuated by the addition of Cu₂Br₂. Addition of the Grignard reagent to α,β-unsaturated aldehydes gives the simple 1,2-addition products.     

Iridium(I)‐catalyzed C−H Borylation of α,β‐Unsaturated Esters with Bis(pinacolato)diboron

A new process has been developed for the iridium(I)‐catalyzed vinylic C?H borylation of α,β‐unsaturated esters with bis(pinacolato)diboron (B₂pin₂). These reactions proceeded in octane at temperatures in the range of 80–120 °C to afford the corresponding alkenylboronic compounds in high yields with excellent regio‐ and stereoselectivities. The presence of an aryl ester led to significant improvements in the yields of the acyclic alkenylboronates. Crossover experiments involving deuterated substrates as well as a mixture of stereoisomers confirmed that this reaction proceeds via a 1,4‐addition/β‐hydride elimination mechanism. Notably, this reaction was also used to develop a one‐pot borylation/Suzuki–Miyaura cross‐coupling procedure.