Substituted vinyl ethers

Recently, a Wittig olefination/Ir atalyzed etherification sequence was developed as a method for selective preparation of substituted vinyl ether (see Section 3.1.2.9). However, no convenient syntheses are available for the selective preparation of simple propenyl ethers. Accordingly, Hiersemann has investigated the re- 

The Ciaisen rearrangement of the (Z, )-21 and E,E)-21 mixture afforded the a-keto esters as an anti and syn mixture. As shown in Section 3.1.3.1, the configuration of the rearrangement products is based on the estabUshed chair-like transition state. Therefore, the thermal rearrangement of (Z,E)- and (E,E)-21 should lead to anti- and syn-a-keto esters, respectively. 

In the synthesis of ( )-fluorobotryodiplodin, the thermal rearrangement of vinyl substituted fluorinated methyl ether with the (Z,Z) geometry proceeded in a highly stereoselective manner, giving a ketone with syn geometry in 92% yield (Eq. 3.1.25) [33]. 

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Intermolecular befera-Diels-Alder reactions of enamino ketones with highly substituted vinyl ethers. Effect of high pressure on the kinetics and diastereoselectivity [77]... 

Microwave-assisted Heck reaction of (hetero)aryl bromides with N,N-dimethyl-2-[(2-phenylvinyl)oxy]ethanamine, using Herrmann s palladacycle as a precatalyst, yielded the corresponding /3-(hetero)arylated Heck products in a good EjZ selectivity (Scheme 79) [90]. The a/yd-regioselectivity can be explained by the chelation control in the insertion step. This selectivity is better than 10/90 when no severe steric hindrance is introduced in the (hetero)aryl bromides. The process does not require an inert atmosphere. There is evidence that a Pd(0)/Pd(II)- and not Pd(II)/Pd(IV)-based catalytic cycle is involved. Similarly, other j6-amino-substituted vinyl ethers such as... 

Extension of this tandem process to create five contiguous stereogenic centers has been accomplished by using 2-substituted vinyl ethers (Eq. 8.114).178 The results for the cycloaddition... 

There have been two main approaches to the development of dipolarophile facial selectivity (1) the use of chiral substrates, templates, and auxiharies and (2) the use of chiral rhodium catalysts [35]. In one of the earhest examples of chiral substrate selectivity, Pirmng and Lee reported a selective hydroxy-directed cycloaddition with chiral hydroxy-substituted vinyl ethers [95]. This effort was followed by a number of chiral template approaches to diastereocontrol, including the use of (R)- or (S)-phenylglycinol to form a cycHc phenyloxazinone for the facially selective cycloaddition of isomtinchnones [96, 97]. Padwa and Prein demonstrated acycHc diastereofacial control in the cycloaddi-... 

In a substituted vinyl ether, the oxygen atom deshields the a-proton by an inductive effect and shields the /3-proton by resonance. 

Very recently, the water-soluble binuclear ruthenium allenybdene complex [ RuCl(ju-Cl)(C=C=CPh2)(Ph2P(2-0S(0)2C6H4))2 2] Na4 was used to perform selective transetherification of substituted vinyl ethers into acetals and aldehydes according to the solvent (Scheme 23) [110]. 

Propenyl Ethers and Unsaturated Cyclic Ethers Propenyl ethers (CH3—CH=CH—OR R = ethyl, isobutyl, etc. cis- and trans-isomers) and 3,4-dihydrofuran are linear and cyclic a,/3-unsaturated ethers, that can be regarded as / -substituted vinyl ether derivatives. For these monomers a few controlled/living cationic polymerizations have been reported. The HI/I2 system is generally effective for both linear and cyclic monomers [181,182,183], whereas a recent study by Nuyken indicates that the IBVE-HI adduct coupled with nBu4NC104 is suited for 3,4-dihydrofuran (see Section V.A.4) [184]. A variety of mono- and bifunctional propenyl ethers can readily be prepared by the ruthenium complex-catalyzed isomerization of corresponding allyl ethers [185]. 

K platinum-on-asbestos catalyst at 290° is used in the conversion of t liethyl acetal to ethyl vinyl ether (42%). Ketals of the type C(OCHj)jCHj are readily split by heating with a small amount of p-I oluenesulfonic acid to yield substituted vinyl ethers of the type t lC(OCHj)=CHj (92%). In the presence of excess of a higher alcohol, fl OH, the olefinic ether corresponding to this alcohol, RC(OR )=CHj, is formed in high yield. Similarly, treatment of alkoxy ketals with acid I atalysts gives alkoxy dienes and dialkoxy olefins. ... 

The pyrolysis of allyl vinyl ethers derived by acid- or mercuric ion-catalyzed ether exchange of ethyl vinyl ether or substituted vinyl ethers with allylic alcohols produces the corresponding y, 8-unsaturated aldehydes or ketones, respectively. 

The procedure for 1-ethoxy-1-phenylethene is an illustration of a general synthetic method for 1-substituted vinyl ethers, which are not accessible via 1-metalla-tion [9] (compare Chap. IV, Exp. 1). The synthesis starts with the reaction of a Grignard compound with a 1,2-dibromoalkyl ether [231]. A halogen atom in the 1-position of an ether group is very readily replacable, even under non-polar conditions. This reaction is a special case of nucleophilic displacement in which the... 

Substituted vinyl ethers may react more sluggishly, but often they can be forced into reaction at high pressure. This strategy was employed to synthesize bicyclic lactone 40, which was prepared as an intermediate in synthesizing a novel 4 -hydroxybutyl vitamin D ana-log. Retention of the olefin stereochemistry in the cycloadduct suggests that this reaction proceeds via a concerted rather than a stepwise mechanism. 

Elevated pressure was also necessary to force the reaction of pyrone 47 with substituted vinyl ethers 48E and 48Z. Interestingly, geometric isomer 48Z reacts considerably faster and with better stereoselectivity than 48E. Thus, 48Z cycloadded to give 38% of cycloadducts 49... and 49 in a ratio of 18 1. By contrast, 48E cycloadded to give a very low yield of cycloadducts 50-.,- and in a ratio of 2 1. In each reaction, the olefin stereochemistry was retained in the reaction products, indicating that these cycloadditions occur in a concerted rather than in a stepwise fashion. 

Mahadevan, V, C.V Viswanathan, and F. Phillips. Conversion of Fatty Acid Aldehyde Dimethyl Acetals to the Corresponding Alk-l-enyl Methyl Ethers (Substituted Vinyl Ethers) During Gas-liquid Chromatography,/. Lipid Res. 8 2-6 (1967). 

The aliphatic Claisen rearrangement of secondary allyhc ethers has been recognized to provide E double bonds (Eq. 3.1.20) [28, 29]. The Claisen rearrangement of C4 and C5 substituted vinyl ether provided a 90 10 ratio of ( )- to (Z)-unsaturated aldehydes (Table 3.1.1, entry 1). An increase in the steric bulk of the C5 substituent produced higher stereoselectivity (entry 3). 

There is an additional steric effect that affects the rate of formation of the intermediate. As the bond to C2 closes, the carbonyl oxygen atom is brought against the substituent cis to the smaller of the two substituents on Ci in our example it is the one trans to the alkoxy group. As a result, when its steric requirements are greater than that of the second substituent on C2 the cis-substituted vinyl ether reacts more rapidly than the trans an effect that increases with increasing bulk of R<. 

A substituted vinyl ether, 1,2-dimethoxyethylene, has been polymerized to high molecular weight in the presence of alkyl aluminium chloride at —50 C in... 

Vinyl ethers are known to react with ruthenium carbene complexes to give the so-called Fischer carbenes which show greatly reduced olefin metathesis activity [13]. Therefore, ethyl vinyl ether is typically use din the nonfunctional termination of most living ROMP reactions using ruthenium initiators. Figure 3.2. When substituted vinyl ethers are employed, functional groups or even complex molecules can be transferred onto the polymer chain end in one step. This type of functional termination reaction works only for ruthenium carbene complexes because titanium, molybdenum, and tungsten carbenes are tolerant toward vinyl ethers. 

Figure 3.2 Reaction with vinyl ethers, (top with substituted vinyl ethers. A functional arrow) Nonfunctional termination (methylene group or fragment R is transferred onto the transfer) of a living ROMP with ethyl vinyl polymer chain end. ether, (bottom arrow) Functional termination...

Figure 3.3 An example of an azide-end-functional polymer via the reaction of chain end with substituted vinyl ether.

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