Vinyl ketene acetal

As an extension of this work, these authors have applied this catalyst system to vinylogous asymmetric Mukaiyama-type aldol reactions, involving silyl vinyl ketene acetals and pyruvate esters. These reactions afforded the corresponding y,5-unsaturated a-hydroxy diesters with quaternary centres in high yields and enantioselectivities of up to 99% ee (Scheme 10.25). It was shown that the presence of CF3CH2OH as an additive facilitated the turnover of the catalyst. 

Chiral dienes have proved to be less popular in asymmetric Diels-Alder reactions than their chiral dienophile counterparts. This is primarily a result of the problem of designing a molecule that incorporates a chiral moiety, such as the formation of a chiral isoprenyl ether or vinyl ketene acetal.187-190 In addition, diastereoselectivities often are not high,54 191-199 as illustrated by the cycloaddition of the chiral butadiene 5 with acrolein (Scheme 26.4). Improved stereoselection is observed through the use of double asymmetric induction, although this is a somewhat wasteful protocol.35,54 177 200... 

Later, the dimethylphenylsilyl adduct 75 d was not only converted to the vinyl-ketene acetal 81 by a Wittig-Horner reaction via the ester 80, but also by reduction of 80 to the aldehyde 84 followed by silylation of the corresponding anion to the silylenol ether 85. Vinylcyclohexenes without terminal substitution (e. g., 83) was prepared via an alcohol derived from 80 or 84 [73,74]. The least substituted diene 83 was alternatively prepared from the triflate 82 in a Stille coupling with trialkylvinyl stannane [75] (Scheme 24), a reaction also used by Toshima etal.[76]. 

Thermolysis of 1,3-dioxane (75) (Equation (14)) gives the eight-membered ring unsaturated lactone (76) in 84% yield, presumably through a vinyl ketene acetal, which undergoes a Claisen rearrangement <86CC325>. 

Martin et developed a biogenetic entry into Ala-methylvellosimine (168) which employed an iminium ion-mediated cyclization similar to van Tamelen s original proposal. As shown in Scheme 4, dihydrocar-boline 169, which was readily obtained from commercially available D-tryptophan, was allowed to react with the vinyl ketene acetal 170 to afford a single product which was directly converted into the i-butyl ester 171. The Nb acylation of amine 171 with diketene furnished an intermediate P-keto amide that underwent facile cyclization via an intramolecular... 

During this early period, a very ingenious free-radical route to polyesters was used to introduce weak linkages into the backbones of hydrocarbon polymers and render them susceptible to bio degradabihty (128—131). Copolymerization of ketene acetals with vinyl monomers incorporates an ester linkage into the polymer backbone by rearrangement of the ketene acetal radical as illustrated in equation 13. The ester is a potential site for biological attack. The chemistry has been demonstrated with ethylene (128—131), acryhc acid (132), and styrene (133). 

Stansbury and Bailey. A review by Colombam on addition-fragmentation processes is also relevant. Monomers used in ring-opening are typically vinyl (e.g. vinylcyclopropane - Scheme 4.20 Section 4.4.2.1) or methylene substituted cyclic compounds (e.g. ketene acetals - Section 4.4.2.2) where addition to the double bond is followed by p-scission. 

Considering the above-mentioned facts, according to which simple diazoketones yield dihydrofurans with ketene acetals but cyclopropanes with enol ethers, one exports an interlink between these clear-cut alternatives to exist, i.e. substrates from which both cyclopropanes and dihydrofurans result. In fact, providing an enol ether with a cation-stabilizing substituent in the a-position creates such a situation The Rh2(OAc)4-catalyzed decomposition of -diazoacetophenone in the presence of ethyl vinyl ether produces mainly cyclopropane 82 (R=H), but a small amount of dihydro-... 

The reactions of nitroalkenes (42) with various enols (43b) (vinyl ethers, silyl, and acyl enolates, ketene acetals) have been studied in most detail (110, 111, 125—154). As a mle, these reactions proceed smoothly to give the corresponding nitronates (35f) in yields from high to moderate. As in the reactions with enamines, the formation of compounds (44b) is attributed to the ambident character of the anionic centers in zwitterionic intermediates analogous to those shown in Scheme 3.43. 

A wide range of olefins can be cyclopropanated with acceptor-substituted carbene complexes. These include acyclic or cyclic alkenes, styrenes [1015], 1,3-dienes [1002], vinyl iodides [1347,1348], arenes [1349], fullerenes [1350], heteroare-nes, enol ethers or esters [1351-1354], ketene acetals, and A-alkoxycarbonyl-[1355,1356] or A-silyl enamines [1357], Electron-rich alkenes are usually cyclopropanated faster than electron-poor alkenes [626,1015],... 

The alkyne hydrozirconation protocol was also applied to acetylenic tellurides furnishing the zirconated vinyl tellnrides in cis fashion and high regioselectivity. Subsequent treatment with tellurenyl halides affords telluro ketene acetals with total retention of configuration. ... 

Treatment with water gives the corresponding Z tellurides. Reaction with BuSeBr, iodine and A -bromosuccinimide (NBS) gives respectively Te-Se ketene acetals, a-iodo- and a-bromo vinyl tellurides as mixtures of Z and E stereoisomers (in contrast with the total retention of configuration of the above-discussed Zr/Te exchange reactions). The acylation was effected with acylchlorides in the presence of Cul. ... 

Alkyl, aryl, allyl, benzyl, vinyl, ethynyl tellurides, tellurobutadienes, divinyltellurides, tellurobutenines, telluro(thio)- and telluro(seleno)ketene acetals and j3-(phosphorovinyl) tellurides are susceptible to such exchange, giving the corresponding lithium compounds trapped in sequence with selected electrophiles. ... 

W. J. Bailey s(52,53) work with ketene acetals deserves mention as potentially a route to biodegradable addition polymers. Its novelty resides in the instability of the vinyl radical and rearrangement to introduce a polyester linkage into a radically produced polymer. As we shall see in the next section, polyesters are biodegradable hence, their Introduction into a polymer with a C-C backbone produces weak links which fracture the polymer into oligomers which we have seen are biodegradable. This chemistry is exemplified schematically, below. 

The first route relies on the ROP of cyclic ketene acetals [1-3]. The electron-rich double bond is prone to react with radicals and electrophiles. Therefore, this class of monomers undergoes cationic and radical polymerization. For example, radical initiators react with the double bond to provide a new tertiary radical (Fig. 2). Two distinct mechanisms of polymerization can then take place direct vinyl polymerization or indirect ring opening of the cycle accompanied by the formation of a new radical, which is the propagating species (Fig. 2). The ester function is formed... 

The reactivity of the produced complexes was also examined [30a,b]. Since the benzopyranylidene complex 106 has an electron-deficient diene moiety due to the strong electron-withdrawing nature of W(CO)5 group, 106 is expected to undergo inverse electron-demand Diels-Alder reaction with electron-rich alkenes. In fact, naphthalenes 116 variously substituted at the 1-, 2-, and 3-positions were prepared by the reaction of benzopyranylidene complexes 106 and typical electron-rich alkenes such as vinyl ethers, ketene acetals, and enamines through the Diels-Alder adducts 115, which simultaneously eliminated W(CO)6 and an alcohol or an amine at rt (Scheme 5.35). 

In order to carry out this reaction by a one-pot procedure, we examined the same complexation reaction in the presence ofaketene acetal, which led us to the discovery of the novel reaction pathways described below [32]. Thus, treatment of o-ethynylphenyl ketone 104 with a catalytic amount of W(CO)5(thf) in the presence qf4equiv of 1,1-diethoxyethylene at rt gave a novel polycyclic compound 120 in good yield as a single stereoisomer. Both vinyl ethers and ketene acetals can be employed as the electron-... 

Whereas 260 does not react with electron-rich dipolarophiles, the more delocalized isomiinchnone 261 does react with both electron-rich and -deficient dipolarophiles (154). A detailed FMO analysis is consistent with these observations and with the regiochemistry exhibited by diethyl ketene acetal and methyl vinyl ketone as shown in Scheme 10.36. The reaction of 261 with the ketene acetal to give 262 is LUMO-dipole HOMO-dipolarophile controlled (so-called lype III process). In contrast, the reaction of 261 with methyl vinyl ketone to give 263 is HOMO-dipole LUMO-dipolarophile controlled (so-called lype I process). In competition experiments using a mixture of A-phenylmaleimide and ketene acetal only a cycloadduct from the former was isolated. This result is consistent with a smaller energy gap for... 

Among the most commonly applied chiral moiety for nitrones (2) is the N-a-methylbenzyl substituent (Scheme 12.6) (18-25). The nitrones 8 with this substituent are available from 1 -phenethylamine, and the substituent has the advantage that it can be removed from the resulting isoxazolidine products 9 by hydrogeno-lysis. This type of 1,3-dipole has been applied in numerous 1,3-dipolar cycloadditions with alkenes such as styrenes (21,23), allyl alcohol (24), vinyl acetate (20), crotonates (22,25), and in a recent report with ketene acetals (26) for the synthesis of natural products. Reviewing these reactions shows that the a-methylbenzyl group... 

Diels-Alder reactions have featured heavily during the period of review. Ar-Vinyl-2-oxazolidinone has been reported as a dienophile for the first time, including the preparation of various tetrahydro-277,77/-pyrano[4,3-. ]pyrans <2002SL952>. The heterodiene cycloaddition reaction of 3-formylchromone with a series of ketene acetals formed from C2-symmetric l,2-diarylethane-l,2-diols is completely diastereoselective (Scheme 40) <1995J(P1)2293>. 

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