Vinyl cation equivalents

Metalated epoxides can react with organometallics to give olefins after elimination of dimetal oxide, a process often referred to as reductive alkylation (Path B, Scheme 5.2). Crandall and Lin first described this reaction in their seminal paper in 1967 treatment of tert-butyloxirane 106 with 3 equiv. of tert-butyllithium, for example, gave trans-di-tert-butylethylene 110 in 64% yield (Scheme 5.23), Stating that this reaction should have some synthetic potential , [36] they proposed a reaction pathway in which tert-butyllithium reacted with a-lithiooxycarbene 108 to generate dianion 109 and thence olefin 110 upon elimination of dilithium oxide. The epoxide has, in effect, acted as a vinyl cation equivalent. 

Volume 75 concludes with six procedures for the preparation of valuable building blocks. The first, 6,7-DIHYDROCYCLOPENTA-l,3-DIOXIN-5(4H)-ONE, serves as an effective /3-keto vinyl cation equivalent when subjected to reductive and alkylative 1,3-carbonyl transpositions. 3-CYCLOPENTENE-l-CARBOXYLIC ACID, the second procedure in this series, is prepared via the reaction of dimethyl malonate and cis-l,4-dichloro-2-butene, followed by hydrolysis and decarboxylation. The use of tetrahaloarenes as diaryne equivalents for the potential construction of molecular belts, collars, and strips is demonstrated with the preparation of anti- and syn-l,4,5,8-TETRAHYDROANTHRACENE 1,4 5,8-DIEPOXIDES. Also of potential interest to the organic materials community is 8,8-DICYANOHEPTAFULVENE, prepared by the condensation of cycloheptatrienylium tetrafluoroborate with bromomalononitrile. The preparation of 2-PHENYL-l-PYRROLINE, an important heterocycle for the synthesis of a variety of alkaloids and pyrroloisoquinoline antidepressants, illustrates the utility of the inexpensive N-vinylpyrrolidin-2-one as an effective 3-aminopropyl carbanion equivalent. The final preparation in Volume 75, cis-4a(S), 8a(R)-PERHYDRO-6(2H)-ISOQUINOLINONES, il lustrates the conversion of quinine via oxidative degradation to meroquinene esters that are subsequently cyclized to N-acylated cis-perhydroisoquinolones and as such represent attractive building blocks now readily available in the pool of chiral substrates. 

The synthetic usefulness of dioxin vinylogous esters as p-keto vinyl cation equivalents was demonstrated by a variety of reductive and alkylative 1,3-carbonyl transpositions.5 7 Regioselective alkylation and hydroxylation at the a -position (and, in some cases, at the y-position)8 9 further extend the usefulness of 1,3-dioxin vinylogous ester templates in organic synthesis. 

VINYLATION OF EmLATES WITH VINYL CATION EQUIVALENTS... 

VINYLATION OF ENOLATES WITH A VINYL CATION EQUIVALENT trans-3-METHYL-2-VINYLCYCLOHEXANONE... 

Vinyl acetate- Acetic acid vinyl ester Acetic acid ethenyl ester, (108-05-4), 65, 135 VINYL CATION EQUIVALENT, 66, 95 Vinyl cation synthons, 66, 102 Vinyl radical cyclization, 66, 81, 82 VINYLAT10N OF EN0LATES. 66, 95, 104 Vinyl lithium, 66, 53, 55... 

A trimethylsilylethynyl moiety can serve as a vinyl cation equivalent, as shown by an intramolecular reaction with a dienamine [125], The silver ion catalyst acts as an accentuating agent. 

The a-silyl aldehydes can serve as vinyl cation equivalents in reactions with (irignard or alkyllithium reagents (equation II). 

The ring closure of 1,5-keto-acids and their derivatives is a widely used strategy for the synthesis of 2A-pyran-2-ones <1984CHEC, 1996CHEC-II>. The 1,5-keto acid derivatives are typically formed in situ with concomitant ring closure to form 2//-pyran-2-ones. There are three main approaches to access the prerequisite 1,5-keto acid derivatives, namely (i) reaction of an ester enolate with a 2-acyl vinyl cation equivalent, (ii) the reaction of a ketone enolate... 

The exceptional nucleofugality of the phenyliodonio group has been determined in an alkenyl salt and it is about 106 times greater than that of triflate [30]. This remarkable property makes alkenyl iodonium salts excellent vinyl cation equivalents in nucleophilic substitutions. The chemistry of alkenyl iodonium salts is dominated by the transfer of their aliphatic moiety to a variety of nucleophiles other important reactions involve Michael-type addition and alkylidenecarbene generation, along with elimination to alkynes which is actually an undesirable side-reaction. 

Nucleophilic attack on ( -alkene)Fp+ cations may be effected by heteroatom nucleophiles including amines, azide ion, cyanate ion (through N), alcohols, and thiols (Scheme 39). Carbon-based nucleophiles, such as the anions of active methylene compounds (malonic esters, /3-keto esters, cyanoac-etate), enamines, cyanide, cuprates, Grignard reagents, and ( l -allyl)Fe(Cp)(CO)2 complexes react similarly. In addition, several hydride sources, most notably NaBHsCN, deliver hydride ion to Fp(jj -alkene)+ complexes. Subjecting complexes of type (79) to Nal or NaBr in acetone, however, does not give nncleophilic attack, but instead results rehably in the displacement of the alkene from the iron residue. Cyclohexanone enolates or silyl enol ethers also may be added, and the iron alkyl complexes thus produced can give Robinson annulation-type products (Scheme 40). Vinyl ether-cationic Fp complexes as the electrophiles are nseful as vinyl cation equivalents. ... 

Stereospecificity in the addition-elimination mechanism Towards Carbon Nucleophiles and Vinyl Cation Equivalents Vinyl cation equivalents Conjugate Substitution... 

Towards Carbon Nucleophiles and Vinyl Cation Equivalents... 

The corresponding ketones 76 can be made by aliphatic Friedel-Crafts reactions between an acid chloride and acetylene using A1C13 as catalyst. Under these conditions chloride adds to the acetylenic ketone to give the P-chloroenone 77 directly. These compounds are converted into the sulfides 78 by conjugate substitution13 and are used as vinyl cation equivalents in the next section. 

A detailed preparation of 6,7-dihydrocyclopenta-l,2-dioxin-5(4//)-one, useful as a p-keto vinyl cation equivalent, has been described <970S(75)189>, whilst l-ethynyl-4-methyl-2,6,7-trioxabicyclo[2.2.2]octane serves as a propiolate anion equivalent <79SYN404>. 

Hudrlik and Kulkarni have shown that a-t-butyldimethylsilyl aldehydes serve as vinyl cation equivalents for the synthesis of /3,y-unsaturated ketones (and esters). Addition of lithium enolates to the a-silyl aldehydes is highly erythro-selective, enabling products of either E- or Z-geometry to be obtained (Scheme 58). In a related process, phenylselenoacetaldehyde has been used to transform ketones into the corresponding a-vinylketones (Scheme 59) phenyl-selenoacetone enables a- isopropenylation of ketones in an analogous fashion. ... 

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