Dimethoxyethylene

Racemic pyrone sulfoxide 52 undergoes a diastereoselective inverse-electron-demand 2 + 4-cycloaddition with 1,1-dimethoxyethylene to afford adduct 53 in > 95% yield (equation 49)100 this is the first example of an asymmetric Diels-Alder cycloaddition using a sulfinyldiene as an electron-deficient enophile101. 

The reaction is most effective with azides bearing electron-withdrawing groups, such as nitrophenyl, toluene-p-sulfonyl, and ethoxy carbonyl. Sulfonyl azide adducts give iV-unsubstituted triazoles, the substituent being lost on aromatization. The triazole which is isolated as a minor product of the reaction of 1,1-dimethoxyethylene with ethoxycarbonyl azide has been assigned a 2/7-triazole structure it is likely that other ethoxycarbonyltriazoles formed by this route also have 2H-structures. 

From the fact that formation of the two cycloadducts from 4,4-dimethyl-2-cyclohexenone (55) and 1,1-dimethoxyethylene was quenched with different efficiencies by di-f-butylnitroxide, Chapman241 concluded that two triplet states were involved. This paramagnetic quencher is known, however, to deactivate both singlets and triplets. 

A second major mode of photocydoaddition involves 1.2-addition to the aromatic ring, and this predominates if there is a large difference in electron-donor/acceptor capacity between the aromatic compound and the alkene. It is therefore the major reaction pathway when benzene reacts with an electron-rich alkene such as 1,1-dimethoxyethylene (3.43) or with an electron-deficient alkene such as acrylonitrile (3.441. When substituted benzenes are involved, such as anisole with acrylonitrile (3.45), or benzonitrile with vinyl acetate (3.46), reaction can be quite efficient and regioselective to give products in which the two substituents are on adjacent carbon atoms. 

The reaction between vinyl ethers and unsaturated carbonyl compounds, which provides a powerful synthetic route to dihydropyrans, has been adapted to the synthesis of pyran-2-ones (72CC863). 2-Chloro-1,1 -dimethoxyethylene, which is a protected form of chloroketene, undergoes cycloaddition with a number of enones to give the cis or trans isomers of 3-chloro-3,4-dihydro-2,2-dimethoxypyrans (338) and (339) or a mixture of both. Although... 

Photochemical addition reactions are exhibited by oxazinones. Thus, irradiation of a mixture of the oxazin-4-one (54) and 1,1-dimethoxyethylene yields the [2 + 2] adduct (55) which fragments to the azetine (56) on heating (77TL431). 

Salicylate annelatiun.1 A new method involves conversion of a cyclohexanone llllo the annelated 3-carbomethoxy-2-pyrone by reaction of the enolate with dimethyl iliclhoxyincthylcnemalonate2 to form a 3-carbomethoxy-2-pyrone. This product undergoes a facile Diels-Alder reaction with 1,1-dimethoxyethylene with loss of C02 anil ( II, OII to give a salicylate... 

Caution The reaction should be run behind a shield in a fume hood for protection in ease of explosion. Pressure will build up in the tube einoe 1,1-dimethoxyethylene boils at 89°C and aarbon dioxide is formed. 

Table II. Diels-Alder Reaction ot 3-Carbomethoxy-2-pyrones (2) with 1,1-Dimethoxyethylene Salicylate Formation... 

In the preparative application of [2 + 2]-photocycloadditions of cyclic enones to (substituted) alkenes, two factors concerning product formation are of decisive relevance, namely the regioselectivity and the (overall) rate of conversion. Regarding the regioselectivity in the addition to mono- and 1,1-disubstituted alkenes, Corey had shown that the preferred addition mode of cyclohex-2-enone to isobutene or 1,1-dimethoxyethylene was the one leading to—both cis- and trans-fused—bicyclo[4.2.0]octan-2-ones with the substituents on C(7) [8]. In contrast, in the reaction with acrylonitrile, the alternate orientation was observed to occur preferentially. Similar results were also reported by Cantrell for the photocycloaddition of 3-methyl-cyclohex-2-enone to differently substituted alkenes [14]. No significant differences in the overall rates of product formation for the different alkenes were observed in these studies. In order to explain these observed... 

Subsequently, Posner published the completely regioselective and highly stereoselective cyclo additions of racemic 3-(p-tolylsulfinyl)-2-pyrone (141) (Scheme 70) with 1,1-dimethoxyethylene [133],vinylether,and vinylthioethers [134]. With the first dienophile, the best diastereoselectivity (an 88 12 ratio of the two endo-adducts) was achieved at room temperature in toluene or hexane as the solvent (48 h). A 10 1 endo/exo mixture of cycloadducts was obtained with vinyl-ether in the presence of ZnBr2 as the catalyst, whereas a total endo selectivity was observed in reactions of 141 with vinylthioethers [134] conducted under high pressures. The bridged bicyclic lactone cycloadducts 142 have been used as versatile synthons in the synthesis of shikimic acid derivatives. Although enantio-merically pure samples of compound 141 could be obtained [134] it was not used as a starting material for asymmetric Diels-Alder reactions (the low yield of (S)-141 precluded this). 

B. 6-Methoxy-7-methoxy arbcmyl-l, g, 3,4-tetr ahydromphthalene, 1,1-Dimethoxyethylene (1.1 g, 12.5 mmol, Note 13) is added to a solution of methyl 2-oxo-5,6,7,8-tetrahydro-2H-l-benzopyran-3-carboxylate (507 mg, 2.44 mmol) in dry toluene (2,5 mL, Note 14) in a dry, 11 x 13 mm, resealable glass tube... 

Much larger effects of this type are observed in cycloadditions of enol ethers to tetrazines (Fig. 26), a reaction shown by Sauer and co-workers to be an example of a Diels-Alder reaction with inverse electron demand 75. The rates of 3,6-di-(2 -pyridyl)-s-tetrazine to various enol ethers and styrenes are summarized in Fig. 27. These were obtained by measuring the disappearance of the 540 nm band in the absorption spectra of the tetrazine76. These results are of particular interest, since there is little or no correspondence between the electron-donor ability of the enol ether, as measured by the ir ionization potentials (Table 5), and the rate of reaction of the enol ether. For example, although the conversion of methyl vinyl ether to 1,1-dimethoxyethylene results in a 4.3 times increase in rate, in line with the 0.2 — 0.3 eV decrease in IP, the 1,2-dimethoxyethylenes are 13 to 25 times less reactive than methyl vinyl ether, even though the IPs of these molecules are much lower... 

These pyrones undergo Diels-Alder reactions with electron-deficient alkenes with loss of COj and formation of an aromatic ring. Thus reaction with 1,1-dimethoxyethylene (11, 279-281) provides a regiospecific route to methyl salicylates (e.g., 3), which are convertible in several steps into catechol derivatives (4) or by decarbomethoxylation into phenol ethers (5). Similar reactions with vinylene carbonate or 1, 1,2-trimethoxyethylene provide regiospecific routes to phenols, differentially protected derivatives of catechol, resorcinol, or pyrogallol (equation II). 

Thermal cycloaddition of 346 (Scheme 66) with 1,1-dimethoxyethylene gives the bicyclic lactone 347 (87JOC1889). The reaction is applicable to the synthesis of aklavinone 348. 

Irradiation of a solution of 51 in benzene causes an interesting isomerization to 4-phenyl-4-oxazolin-2-one (56), via an epoxy isocyanate141 lEq. (33)1. In the presence of 1,1-dimethoxyethylene, a 2 + 2 cycloaddition occurs in competition with a-cleavage to give 57. Compound 51 is also reported to react with dimethylacetylene dicarboxylate... 

The single adduct (67a) is obtained by photoaddition of 1,1-dimethoxyethylene to the ester (66a). Adducts (67b) and (68) were also obtained by the photoaddition of the same olefin to the enone (66b). The molecules obtained by this process were chemically elaborated to yield compounds with the proto-illudane skeleton (69). ... 

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