Alkene enol ester

In 1959 Carboni and Lindsay first reported the cycloaddition reaction between 1,2,4,5-tetrazines and alkynes or alkenes (59JA4342) and this reaction type has become a useful synthetic approach to pyridazines. In general, the reaction proceeds between 1,2,4,5-tetrazines with strongly electrophilic substituents at positions 3 and 6 (alkoxycarbonyl, carboxamido, trifluoromethyl, aryl, heteroaryl, etc.) and a variety of alkenes and alkynes, enol ethers, ketene acetals, enol esters, enamines (78HC(33)1073) or even with aldehydes and ketones (79JOC629). With alkenes 1,4-dihydropyridazines (172) are first formed, which in most cases are not isolated but are oxidized further to pyridazines (173). These are obtained directly from alkynes which are, however, less reactive in these cycloaddition reactions. In general, the overall reaction which is presented in Scheme 96 is strongly... 

See Section 362 (Ester-Alkene) for the formation of enol esters and Section 367 (Ether-Alkenes) for the formation of enol ethers. Many of the methods in Section 60A (Protection of Aldehydes) are also applicable to ketones. 

Geranyl chloride can be prepared from geraniol by the careful use of triphenylphosphine in carbon tetrachloride. Tris(dimethylamino)phosphine reacts with carbon tetrachloride to form the complex (42) which can be used to form the enol esters (43) from acid anhydrides. Similarly, aldehydes form the alkenes (44), and esters or amides of trichloroacetic acid are converted to glycidic esters. ... 

Again, the exclusive formation of six-membered rings indicates that the cyclization takes place by the electrophilic attack of a cationic center, generated from the enol ester moiety to the olefinic double bond. The eventually conceivable oxidation of the terminal double bond seems to be negligible under the reaction conditions since the halve-wave oxidation potentials E1/2 of enol acetates are + 1.44 to - - 2.09 V vs. SCE in acetonitrile while those of 1-alkenes are + 2.70 to -1- 2.90 V vs. Ag/0.01 N AgC104 in acetonitrile and the cyclization reactions are carried out at anodic potentials of mainly 1.8 to 2.0 V vs. SCE. 

Alkenes react with nitryl chloride to give jS-nitroalkyl chlorides, jS-chloroalkyl nitrites and vic-dichloroalkane products. Nitryl chloride reacts with enol esters to give o -nitroketones. ... 

The vinyl carbamates of type 355, although being enol esters, are very stable under acidic or basic conditions and can be handled, similar to usual alkenes, without problems . Quite a number of synthetically useful transformations have been developed over the years. Five aspects can be addressed selectively by suitable reagents ... 

Coupling with enol esters (7, 93). A new synthesis of an alkyl-substituted alkene involves coupling of a lithium dialkyl cuprate with an enol triflate,1 available from a ketone by reaction with triflic anhydride and 2,6-di-t-butylpyridine.2 A wide variety of organocuprates can be used and the geometry of the enolate is largely retained. Reported yields are in the range 60 100%. 

The addition of carboxylic acids to alkynes affords enol esters which are useful as intermediates in organic synthesis.470 As in the addition to alkenes, a catalyst is usually required for high conversions of alkynes to enol esters. Simple acid catalysis has been employed (equation 279),471 but the more common catalysts are Lewis acids, such as boron trifluoride etherate,472 silver nitrate,473 zinc acetate474 and zinc oxide (equations 280 and 281),47S-476... 

Perhaps the most useful type of alkene substrates for these reactions are enol ethers, enol esters and vinyl sulfides. Silyl enol ethers have excellent electron-donor properties, with an ionization potential of about 8 eV and an oxidation potential in various solvents of approximately 1.0-1.5 V vs SCE161. These compounds are easily synthesized by reaction of an enolate with a chlorosilane. (A very recent report synthesized a variety of silyl enol ethers with extremely high stereochemical yield, using the electrogenerated amidate of 2-pyrolidinone as the base.)162 An interesting point is that the use of oxidative or reductive cyclization reactions allows carbonyl functionalities to be ambivalent, either oxidizable or reducible (Scheme 65)163. 

The yields from aldehyde alkylidenation is somewhat lower due to the reductive dimerization of aldehydes with low-valent Ti. Alkylidenation of esters is possible by the reaction of 1,1 -dibromoalkane. TiCU and Zn in the presence of TMEDA to give (Z) vinyl ethers [60], Cyclic vinyl ethers are prepared from unsaturated esters in two steps. The first step is formation of the acyclic enol ethers using a stoichiometric amount of the Ti reagent, and the second step is ring-closing alkene metathesis catalysed by Mo complex 19. Thus the benzofiiran moiety of sophora compound I (199, R = H) was synthesized by the carbonyl alkenation of ester in 197 with the Ti reagent prepared in situ, and the subsequent catalytic RCM of the resulting enol ether 198 catalysed by 19 [61]. 

Alkenation of esters by this method yields enol ethers, and lactones give cyclic enol ethers. 

Intramolecular alkenation of esters 4.95 with 4.94 produces enol ethers 4.97 via titanium-carbene complexes 4.96 (Scheme 4.48). 

The phthalazide bis(cinchona) derivatives [(DHQD)2-PHAL] are the best ligands for the asymmetric dihydroxyla-tion of trans, 1,1-disubstituted, and trisubstituted alkenes, enol ethers, a,p-unsaturated ketones, and a,p- and p,y-unsaturated esters, whereas the DHQD-IND ligand turns out to be superior for c/j -alkenes (Table 1). The bis(cinchona) alkaloid-substituted pyrimidine ligand was found to be the best for monosubstituted terminal alkenes. The addition of Methanesulfonamide to enhance the rate of osmate(VI) ester hydrolysis is recommended for all nonterminal alkenes. 

As with other intramolecular ene reactions, this reaction is best suited to the preparation of cyclopentanes, but can also be used for the preparation of cyclohexanes. The reaction cannot be used for the formation of cyclopropanes or cyclobutanes since the unsaturated carbonyl compound is more stable than the ene adduct. 8,e-Unsaturated ketones (167) do not give cyclobutanes (171) by enolization to give (170) followed by a type I reaction but instead give cyclohexanones (169) by enolization to give (168) followed by a type II reaction. Alkynes can replace alkenes as the enophile. Enols can be prepared from pyrolysis of enol esters, enol ethers and acetals and from -keto esters and 1,3-dicaibonyl compounds. Tlie reaction is well suited to the preparation of fused or bridged bicyclic and spirocyclic compounds. Tandem ene reactions in which two rings are formed in one pot from dienones have also been described. The examples discussed below 2-i63 restricted to those published since Conia and Le Perchec s 1975... 

SnCLrinduced cyclizations between alkenes and enol acetates result in cycloalkanes or bicycloalkanes in high yield (Eq. 59). It is interesting to note that the MesSiOTf-catalyzed reaction can yield fused rather than bicyelo products. Alkenic carboxylic esters, allylic alcohols, sulfones, and sulfonate esters are also cyclized in the presence of SnCU alkenic oxiranes, however, often cyclize in poor yield [89a]. 

Appropriate quenching of a reductively formed lithium enolate with a carboxylic acid anhydride, chloride, methyl chloroformate or diethyl phosphorochloridate yields the corresponding enol esters, enol carbonates or enol phosphates. These derivatives may be transformed into specific alkenes via reductive cleavage of the vinyl oxygen function, as illustrated by the example in Scheme 8. 

Preparation of 2-acyloxy-l,l-dichlorocyclopropanes via the addition of dichlorocarbene to acyloxyalkenes depends on the structure of the alkene and on the mode of generation of the carbene. Thus, ketone enol esters give cyclopropanes irrespective of the method used for the generation of dichlorocarbene. 

Enol ester formation from crotonaldehyde gives the expected / -selectivity 66. Now the silyl enol ether is formed from this ester, also with the expected double bond geometry. The product 67 has three alkenes each is conjugated with at least one oxygen atom. 

Corey and Wright have utilized reductive conversion of a vinyl diethyl phosphate (73) to an alkene 74 in a synthesis of colneleic ester via the enol esters 72.43... 

This method for bromofluorination of ethylenic compounds has been extended by others to symmetrical alkenes,10 terminal allylic alcohols,11 vinyl oxiranes,12 enol esters,13 and vinyl fluorides.14... 

Coupling reaction. Photoinduced three-component coupling combines an alkene (enol ether, silyl enol ether, 1,3-diene, etc.) and ethyl propiolate to form a carbon chain in which the two PhSe groups from PhSeSePh are separately located. The reaction starts from p-addition of PhSe radical to the ester. 

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