Regiochemistry of reactions of α-alkoxyallylphosphine oxide ylides with electrophiles.

Anions $\text{4}$, derived from α-methoxyallylphosphine oxides $\text{3}$, react with electrophiles in a highly regioselective fashion to give products of α-attack or γ-attack depending upon the substitution pattern of the ylid $\text{4}$ and the nature of the electrophile.     

Chemie der lithium-enolat-heptafulvene: regiospezifische aldol-addition

Aldehydes and ketones add to the cycloheptatriene ester anion $\text{2a}$ and to the carboxylic acid dianion $\text{2b}$ at C-7 (α-attack) or C-2 (C-5) (γ-attack), depending on the starting compounds.     

Chemistry of 1,4-dioxene II : A new method for the two-carbon homologation of aldehydes and ketones to α-hydroxymethyl ketones

1,4-Dioxen-2-yl lithium $\text{2}$ reacts with ketones or aldehydes to give alcohols $\text{3}$ which lead after an allylic rearrangement under mild conditions, followed by reduction and hydrolysis to α-hydroxymethyl ketones in fair yields.     

Reduction d'acetals γ-bromo α-acetyleniques par les sels chromeux : Acces aux acetals et aldehydes α-alleniques.

Reduction of γ-bromo α-acetylenic acetals $\text{1}$ with chromous ions (THF + HMPA) in the presence of AcOH affords α-allenic acetals $\text{2}$ whose careful hydrolysis yields the corresponding aldehydes $\text{3}$. A new access to the dried bean beetle pheromone (racemic), via a Witting reaction with $\text{3a}$ is reported.     

A one-step synthesis of γ-pyrones

The potassium enolate of 4-methoxy-3-buten-2-one reacts with acid chlorides anhydrides and acylimidazoles by C-acylation and $\text{in situ}$ cyclization to afford 2-substituted γ-pyrones directly.     

Biomimetic synthesis of bacterial c50 carotenoids decaprenoxanthin and c.p. 450

Alkylation of the distal double bond of pseudoionone $\text{4}$ has been carried out with isoprene epoxide (ZnCl₂ /MeNO₂ ) leading directly to α-$\text{cis 10 a}$, α-$\text{trans}$$\text{10b}$ and γ $\text{10c}$ hydroxyprenylionones. The α-$\text{cis}$ and γ-isomers have been converted in few steps into the C₅₀ carotenoids decaprenoxanthin $\text{1}$ and C.p. 450 $\text{3}$ respectively.     

Reactivity and selectivity in the cyclization of sila-5-hexen-1-yl carbon-centered radicals

A trio of sila-5-hexen-1-yl radicals has been prepared from the corresponding halides by reaction with tri-$\text{n}$-butyltin hydride (deuteride). The radicals possessing a dimethylsilyl function α or β to the carbon radical center demonstrated marked reduction in total (but especially $\text{exo-trig}$) cyclization compared to the all-carbon system. The γ-silyl radical behaved, contrariwise, quite comparably to the all-carbon system. The difference in cyclization found in the α-silyl radical was demonstrated to result from both a pronounced decrease in cyclization rate $\text{via}$ the expected $\text{exo-trig}$ mode and $\text{and from a significantly enhanced rate of hydrogen abstraction from TBTH}$. Both the α- and γ-silyl radicals cyclized $\text{via}$ the $\text{endo-trig}$ mode at rates close to that of the parent 5-hexen-1-yl radical itself. The cyclizations studied were demonstrated to be irreversible. The kinetic control thus shown by the preferred formation of $\text{endo}$ cyclized product from the α- and β-silyl radicals is highly unusual and represents $\text{the first report of carbon-centered 5-hexen-1-yl type radicals violating the Baldwin-Beckwith rule}$$\text{exo-trig}$ cyclization preferred by 5-hexen-1-yl radicals). Rationalization of the cyclization behavior of the α- and γ-silyl radicals involves both steric and electronic factors. The behavior of the most unusual case, the β-silyl radical, which has the lowest cyclization propensity and no $\text{exo}$ mode product, remains largely unexplained because its hydrogen abstraction rate from TBTH is unavailable as yet. Some speculative considerations involving the preferred radical conformation in this system and its relation to cyclization are given.     

Stereoselective synthesis of (e)-2-alkene-1,4-diols via metallated allylic sulphoxides

Reaction of allyl sulphinyl anion with chiral α-methylaldehydes affords α- or γ-adducts in highly regio-controlled fashion, depending on reaction conditions. From the α-adducts $\text{syn}$ (E)-2-alkene-1,4-diols are obtained as major (d.r. 2:1 ~ 28:1) products by thiophile promoted desulphurization.     

3-Bromo-2-(tert-butylsulfonyl)-1-propene. A multi-coupling reagent Part 1.

3-Bromo-2-(tert-butylsulfonyl)-1-propene $\text{4}$ is easily available in two steps from allyl-tert-butylthioether $\text{5}$ (70% overall yield). It reacts with a great variety of carbanions to give functionnalized α, β-unsaturated sulfones of type $\text{7}$ in high yields. A second nucleophile (lithium cuprates) can then be added to compounds $\text{7}$ to furnish functionnalized sulfones of type $\text{3}$ (Y=SO₂-tert-butyl).     

Captodative substituent effects XXI. : Synthesis of selenenylated captodative olefins via selenenyl halide addition to olefins bearing electron-withdrawing substituents

Addition of methane- and benzeneselenenyl bromide or chloride and benzene sulfenyl chloride to carbon-carbon double bonds substituted by electronwithdrawing groups is achieved in solvents of different polarity. Two regioisomeric adducts $\text{6}$ and $\text{7}$ or $\text{8}$ and $\text{9}$ are generally formed, which can be interconverted by equilibration in refluxing acetonitrile. It is of mechanistic interest that the regioisomers may also derive from selenenyltrihalide adducts. In comparison to acrylic esters, the propiolic ester reacts more slowly, producing mainly the α-selenenyl adduct. Dehydrohalogenation of adducts provides a general and valuable method for the preparation of olefins carrying methyl or phenylselenenyl groups in α-position to electron-withdrawing substituents.     

γ-alkylation of α,β-unsaturated ketones: α'-phenylsulfonyl groups as regioselective control elements

The α'-phenylsulfonyl derivative of E-3-penten-2-one ($\text{3}$) undergoes predominant γ-alkylation with a variety of alkyl iodides when first converted into a trilithiated intermediate with excess lithium diisopropylamide in tetrahydrofuran-hexane.     

The palladium-catalysed reductive addition of aryl iodides to propargyl alcohols: a route to /gg,/gg-diaryl allylic alcohols

The reaction of aryl iodides with ethynyl and arylethynyl,dialky $\text{l}$ carbinols in the presence of the tri- or dialkylammonium formate-palladium reagent provides a convenient route to γ,γ-diarylallylic alcohols. In the presence of arylethynyl,alkylcarbinols a lack of regioselectivity was observed and mixtures of ß,γ-, γ,γ-diarylallylic alcohols, and α,ß-unsat$\text{u}$ rated ketones were obtained.     

Stereospecific 2-vinylcyclohexanol formation via internal attack of an allylsilane moiety on an aldehyde group

Acid catalyzed cyclization of ($\text{E}$)-HCO-(CH₂)-₄-CH=C(Me)-CH(SPh)SiMe₃ affords $\text{cis}$-2-substituted cyclohexanol, while ($\text{Z}$) compound affords the $\text{trans}$ isomer. Those cyclized products are transformed to the corresponding α-methyl γ-lactones.     

1-AZA-1,3-dienes. Diels-alder reactions with α, β-unsaturated hydrazones.

The α,β unsaturated hydrazone $\text{3}$ reacts regioselectivity with a wide range of dienophiles to give the corresponding [4+2] adducts. Reductive cleavage of the NN bond in the adducts gives tetrahydropyridines.     

β- and α-lithiaton of methyl β-methoxyacrylate: efficient synthesis of α,γ-substituted methyl tetronates - structure of aspertetronins and gregatins

Methyl β-methoxyacrylate ($\text{1}$) can be lithiated successively in β- and α-positions. Reaction with two electrophiles (at first a carbonyl compound) leads to α, γ-substituted methyl tetronates in a two step synthesis. Application of this method to the synthesis of methyl tetronate whose structure was assigned as that of gregatin B indicates that the gregatins and aspertetronins have the isomeric structures $\text{17}$.     

Reactivite des epoxydes—IV: Mise en evidence de complexes HMPT-lithium lors de l'ouverture des epoxydes par les amidures de lithium—interet sur la plan synthetique

The study of the ratio $\text{|HMPT|}\text{|Amide|}$ effect on the percentage of α-elimination products during the opening of 3,4-epoxycyclooctene 1 with Et₂NLi leads to notice the formation of two complexes: HMPT, Li⁺ and 2HMPT, Li⁺. The α-elimination is entirely suppressed when the second complex is formed. With 5,6-epoxycyclo-octene 2 as a substrate, the study of the ratio $\text{|HMPT|}\text{|Amide|}$ effect on the β-elimination evidences the formation of a third complex: 4HMPT. Li⁺; up to a concentration of |HMPT| = 2|amide| β-elimination is still possible, but for |HMPT|=4|amide| γ-elimination is mainly observed. These conclusions have been applied to 3-allyl epoxy-cylooctane 3 reaction which is able to lead to α, β and γ-elimination.     

Diastereofacial selectivity via aldol reactions using ethyl dithioacetate and ethyl dithiopropionate enolates

The lithium enolate of ethyl dithioacetate reacts with α-methyl aldehydes to yield the aldol products in which the $\text{syn}$ configuration in the positions β and γ to the thiocarbonyl of the product is favored over the $\text{anti}$ configuration. This selectivity is solvent-dependent, and is enhanced at lower temperatures. In most cases, $\text{syn:anti}$ product ratios obtained under these conditions varied from 57:43 to >99:1, depending upon the structure of the α-methyl aldehyde. When the lithium enolate of ethyl dithiopropionate was allowed to react with α-methyl aldehydes, only two out of the four possible diastereomers were detected in the product mixtures.     

Hochsubstituierte tricyclopentanone und cyclopropenyldiazomethane - vorstufen für tetrahedrane?

Reaction of tri-$\text{tert}$-butylcyclopropenylium tetrafluoroborate (=7) with the Li-salt of $\text{tert}$-butyldiazomethane leads to the astonishingly stable diazo compound (=6). The trimethylsilyl derivative (=8) can be prepared via the same route. On photolysis or pyrolysis both cyclopropenyldiazomethanes are fragmented into the corresponding acetylenes.     

Solvent polarity effects on the regioselectivity of photoaddition reactions of arene-ethene systems

Use of polar solvents in the $\text{ortho}$ protocycloaddition of ethenes to anisole greatly increases the regioselectivity of the process and the major adducts reflect 1,2-attack onto the arene.     

Reaction of aldehydes and ketones with t-butyl bromide-dimethyl sulphoxide

Reacting aldehydes and ketones with the “Bu^tBr-Me₂SO” system produces the corresponding α-bromoderivatives $\text{2}$. In the case of ketones, is possible, bromination is obtained exclusively at the more highly substituted α-position. With slight modifications of the reaction conditions (add to obtain “in situ” formation of either dimethyl(2-oxo-2-phenylalkyl)sulphonium salts $\text{3}$ or of α-methylthioderivatives $\text{4}$. Dimethyl(l-m bromide ($\text{3h}$) during crystallization undergoes spontaneous resolution of the two enantiomers, as demonstrated by single crystal X-ray analysis a