Ethers cyclic vinyl

Recently, Ley found that a highly diastereoselective, microwave-induced Claisen rearrangement of an appropriately substituted propargyUc enol ether allows the formation of the sterically congested C8-C14 bond of azadirachtin [55]. 

Thermal rearrangement of the simple allyl ether was successful in preliminary studies (Eq. 3.1.43) [56]. Unfortunately, repeated attempts to induce the Claisen rearrangement of 66 resulted in nearly complete recovery of the starting material (Eq. 3.1.44, Table 3.1.5, entry 1). With these results at hand, the microwave irradiation was attempted to induce the desired rearrangement. As a result, a small increase in yield was observed when 66 was reacted under the conditions given in Table 3.1.5, entry 2. The microwave irradiation of a solution of 68 after only 15 min resulted in a 71% yield of the desired allene 69 as a single diastereoisomer. After extensive optimization studies, it was shown that microwave irradiation with 15 consecutive 60-s pulses allowed the formation of the 69 in 88% yield. 

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The addition of nucleophiles to cyclic fluoroolefins has been reviewed by Park et al. [2 ]. The reaction with alcohols proceeds by addition-elimination to yield the cyclic vinylic ether, as illustrated by tlie reaction of l,2-dichloro-3,3-di-fluorocyclopropene Further reaction results in cyclopropane ring opening at the bond opposite the difluoromethylene carbon to give preferentially the methyl and ortho esters of (Z)-3-chloro-2-fluoroacrylic acid and a small amount of dimethyl malonate [29] (equation 8). 

A comparison of the cationic polymerization of 2,3-dihydrofurans with that of furan and 2-alkylfurans shows that the complications of the latters two, arising from the dienic character of the monomers, obviously vanish when the monomer is a simple cyclic vinyl ether with just one reactive site, viz. the carbon-carbon double bond. However, it also points out that ring opening in the polymerization of furans by acidic catalysts in the absence of water is unlikely, because otherwise it would also occur to some degree in the polymerization of dihydrofurans. 

As these acetals could be converted into the 4,6-O-ethylidene derivatives on treatment with acid, it was reasoned that use of a cyclic vinyl ether, namely, 3,4-dihydro-2H-pyran, might prevent this second process, thus leading to a more useful method of selective acetalation.338 An equimolar reaction with methyl a-D-glu-copyranoside for 4 days in N,N-dimethylformamide led to utilization of 88% of the glycoside, and the 6-(tetrahydropyran-2-yl) ether constituted —85% of the crude reaction-product. In contrast to the steric control apparent in this instance, reaction of 3,4-dihydro-2H-pyran with the axial and equatorial hydroxyl groups in dl-1,4,5,6-tetra-O-acetyl-mi/o-inositol was completely unselective,339 a fact that has been rationalized310 in terms of the probable mechanism of these reactions. 

Tandem carbonyl olefmation—olefm metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefmic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefm metathesis between titano-cene methylidene and the cis-1,2 -disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cyclic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

An ab initio study of the energetics of deprotonation of cyclic vinyl ethers by organolithium reagents has clarified the ring-size-dependent competition between vinylic and allylic deprotonation.The respective transition states involve preequilibrium complexation of lithium to the electron-rich vinyl ether oxygen, prior to deprotonation via a multi-centre process free ions are not formed during the lithiation. 

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

Akiyama applied Im in the inverse-demand aza-Diels-Alder reaction of various acyclic and cyclic vinyl ethers with N-aryl imines derived from o-hydroxyaniline to provide ophcally active tetrahydroquinoline derivatives (Scheme 5.16) [29]. Since aldimines derived from p-methoxyaniline gave no cycloaddition product, a nine-membered cyclic TS (akin to that proposed for the author s Mannich reachon) was invoked to rationalize the high levels of enantio-control. 

Triazines are generally more reactive in [2 + 4] cycloaddition in comparison with 1,2,3-tria-zines. The wide variety of dienophiles can be employed enamines, enaminones, vinyl silyl ethers, vinyl thioethers, cyclic ketene jV,O-acetals, /V-phenylmaleimide, 6-dimethylaminopentafulvene, 2-alkylidene-imidazolidines (cychc ketene aminals), cyclic vinyl ethers, arynes, benzocyclopropene, acetylenes, and alkenes like ethylene, (Z)-but-2-ene, cyclopentene, cyclooctene and bicyclo[2.2.1]hept-2-ene, hexa-1,5-diene, cycloocta-1,5-diene, diallyl ether, cyclododeca-l,5,9-triene,... 

Cyclic vinyl ethers have been deprotonated using f-butyllithium in THF (77TL4187). The carbanions formed, which react readily with a variety of electrophiles, constitute a special type of protected acyl anion. Compound (658), available from the metallated pyran (657) and methallyl bromide, gave the /8-vinyl enone (659) on hydrolysis and cyclization (Scheme 152). 

Besides acyclic unsaturated carbonyl compounds, their alicyclic analogues may be used, leading to fused dihydropyran derivatives. Cyclic vinyl ethers have also been utilized in the process. 

Acylation of the keto acid (637) leads to the isobenzopyrylium salt (638) (77CHE1183). However, the isobenzopyrylium salt (639), a potential intermediate for the synthesis of analogues of berberine alkaloids, results from the formylation of the substituted ketone or the isochromanone (640) using dichloromethyl butyl ether (Scheme 251) (81CHE221). A second product, the 5-oxoniachrysene (641), is formed and this compound may also be obtained by reaction of the isobenzopyrylium salt with phosphorus pentachloride and then with triethylamine. The intermediacy of a cyclic vinyl ether is proposed. 

In addition to 99 and ethyl vinyl ether, other para-substituted thiobenzal-dehyde complexes [W(CO)5 S=C(H)C6H4R-p ] (R = H, Me, OMe, Cl, Br) and other open-chain vinyl ethers R (R2)C=C(R3)OR4 (R R2 = H, Me R3 = H, Me, Ph, C6H4Me-p, C6H4OMe-p R4 = Me, Et, Bu1) as well as cyclic vinyl ethers such as 2,3-dihydrofuran, 4,5-dihydro-2-methylfuran, and 3,4-dihydro-2//-pyran have also been employed successfully.224,251 Thio-and selenoketone complexes did not react with vinyl ethers. 

The photochemical reaction of t-1 with the cyclic vinyl ether 2,3-dihydropyran was initially reported by Rosenberg and co-workers (103) to yield two isomeric cycloadducts 42 and 43. 

Ionic initiators are much more specific toward the type of monomer and can initiate polymerization in some cases where radical initiation is ineffective (cyclic, vinyl ether, or allylic monomers). Monomers suitable for anionic initiation are those with an electron-withdrawing substituent attached to the double bond (phenyl, carbonyl, etc.). The reaction consists... 

Cyclic vinyl ethers have different reactivity patterns. Reaction of cyclobutene and cyclopentene derivatives with S03 leads to the formation of cycloalkenones [182] ... 

E. Kozlowska-Gramsz and G. Descotes, Addition of nitrenes to cyclic vinyl ethers. Application to the synthesis of aminosugars, Tetrahedron Lett., 22 (1981) 563-566. 

One of the most facile methods for construction of the (i-lactam skeleton would be [2 + 2] cyclo additions of isocyanates to alkenes. However, the [2 + 2] cycloadditions of alkyl or aryl isocyanides with either 2,3-dihydrofuran or vinyl ethers often fails under normal conditions. High pressure can surmount this difficulty in certain cases. For instance, alkyl isocyanates 125 underwent [2 + 2] cycloadditions with such cyclic vinyl ethers as 126 to produce the P-lactams 127 (Scheme 34) [80]. 

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]. 

Ozonolysis of the cyclic vinyl ethers 71 (n = 2, 3 or 4) in the presence of M-benzylidene-benzylamine affords the dioxazoles 73 by dipolar cycloaddition to transient carbonyl oxides 72. The dioxazoles fragment to mixtures of phenylcycloalkenes, benzaldehyde and N-formyl-benzylamine by the action of silica gel (94JCS2449). Carbonyl oxides 75 (E = C02Me), generated by thermolysis of the peroxides 74, add to phenyl isocyanate to yield 1,2,4-dioxa-zolidin-3-ones 76 (94JCS3295). 

The presence of a heterosubstituent facilitates deprotonation. 4-Methyl-4H-pyran and 1,4-dimethyldihydropyr-idine are regioselectively metallated at the position next to the heteroatom by LIC-KOR or trimethylsilylmethylpotassium.47 The metallation of cyclic vinyl ethers with Schlosser s base has been successfully used in the synthesis of C-glycosides48 50 via metallation of the glucal followed by a reaction with tributyltin chloride to afford the corresponding tin derivative that could be submitted to coupling reactions with various electrophiles.48... 

The cyclic vinyl ether (182),170 postulated last year as an intermediate in the 21-acetoxylation of the hemiacetal (183), can be prepared in good yield by heating the hemiacetal with aluminium isopropoxide in toluene under argon, followed by direct chromatography on basic alumina.171 The vinyl ether was converted into the 20,21-diol (184) either by reaction with osmium tetroxide or by acetoxylation with lead tetra-acetate, followed by hydrolysis. The hypoiodite reaction, followed by oxidation, allows direct conversion of a 21-acetoxypregnan-20/3-ol (185) into the 18-iodo-20-ketone (186), which is solvolysed in the presence of silver acetate to provide a new route to the 21-acetate of 18-hydroxydeoxycorticosterone hemiacetal (187).171... 

The a-lithiated cyclic vinyl ethers 599 and 603 have been allowed to react with alkyl iodides881,886 and compounds 599, 600909,910 and 606910 with carbonyl compounds. 

For example, methyl 2-phenyl-7,7-bis(phenylthio)heptanoate (4.98) on treatment with 4.94 at room temperature produces the cyclic vinyl ether 4.99 in 68% yield. 

Gree and co-workers have found that bicyclic epoxy alcohols when treated with diethylamino sulfur trifluoride (DAST) can undergo a ring expansion via C-C bond cleavage of the oxirane ring to provide fluorinated cyclic vinyl ethers (Table 17) <20020L451>. 

Regio- and stereochemistry of Paterno-Buchi photocyclo additions of carbonyl compounds and cyclic vinyl ethers leading to formation of oxetanes 04ACR919. 

In some cases, the insertion of dichlorocarbene into the C-H bond of the cyclic vinyl ether competes with its cycloaddition. Furthermore, the structure of the products formed depends on the mode of carbene generation, e.g. formation of 1-4. ... 

Ring-Opening Polymerization of Cyclic Vinyl Ethers... 

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