Vinyl ethers groups

Three selective methods to remove protective groups are receiving much attention assisted, electrolytic, and photolytic removal. Four examples illustrate assisted removal of a protective group. A stable allyl group can be converted to a labile vinyl ether group (eq. 4) a /3-haloethoxy (eq. 5) or a /3-silylethoxy (eq. 6) derivative is cleaved by attack at the /3-substituent and a stable o-nitro-phenyl derivative can be reduced to the o-amino compound, which undergoes cleavage by nucleophilic displacement (eq. 7) ° ... 

The UV cure system contains an epoxy or a vinyl ether functionalized PDMS polymer and a photo catalyst [36]. This latter, a diaryliodonium salt is photolyti-cally decomposed to form an active acid that polymerizes the epoxy or vinyl ether groups and crosslinks the network. 

A recently developed application of the Ramberg-Backlund reaction is the synthesis of C-glycosides. The required thioethers can be prepared easily by exchange with a thiol. The application of the Ramberg-Backlund conditions then leads to an exocyclic vinyl ether that can be reduced to the C-nucleoside.95 Entries 3 and 4 in Scheme 10.6 are examples. The vinyl ether group can also be transformed in other ways. In the synthesis of partial structures of the antibiotic altromycin, the vinyl ether product was subjected to diastereoselective hydroboration. 

Hydroxy-containing fluorovinyl ether monomers (5,6) were prepared in excellent yields (80-90%) in a single step from the corresponding esters (1,3)12"14 with sodium borohydride in absolute ethanol. Protection of the sensitive vinyl ether groups was not required during the reduction. In contrast, the use of a more powerful reducing agent, such as lithium aluminum hydride, resulted in the reduction of the double bond ... 

Intermolecular coupling of a vinyl ether with styrene at a carbon anode in methanol is successful, giving a mixture of the cross coupled product and the two homocoupled products [49], Intramolecular coupling between an enol ether and an alkene centre, as in 24 and 25, proceeds to give the cyclized product in good yield [50], Five and six membered rings can be constructed in this way. An easily oxidised vinyl ether group is necessary to initiate the reaction and the second alkene... 

VNP, and only homopolymer of polyl2-(cinnamoyloxy)ethyl vinyl ether] (PCEVE) was obtained. It seems that the cationic reactivity of the vinyl ether group in VNP was decreased because of the electron attracting nitro group attached at the 4-position on the phenoxide. 

The chiral anisole derivative 37 has been used in the synthesis of several asymmetric functionalized cyclohexenes (Table 9) [22]. In a reaction sequence similar to that employed with racemic anisole complexes, 37 adds an electrophile and a nucleophile across C4 and C3, respectively, to form the cyclohexadiene complex 38. The vinyl ether group of 38 can then be reduced by the tandem addition of a proton and hydride to C2 and Cl, respectively, affording the alkene complex 39. Direct oxidation of 39 liberates cydohexenes 40 and 41, in which the initial asymmetric auxiliary is still intact. Alternatively, the auxiliary may be cleaved under acidic conditions to afford /y3 -allyl complexes, which can be regioselectively attacked by another nucleophile at Cl. Oxidative decomplexation liberates the cyclohexenes 42-44. HPLC analysis revealed high ee values for the organic products isolated both with and without the initial asymmetric group. 

The plasmalogens represent an unusual type of glycerophosphatide that has been isolated from animal and plant tissues. Acid hydrolysis of these materials yields long-chain aldehydes and mono-acylated a-glycerophosphate. The long-chain aldehydes originate from vinyl ether structures. The vinyl ether group appears usually (if not always) in... 

Basic hydrolysis cleaves the carboxylic acid ester bohds but doesn t affect the ether bond. Acidic hydrolysis cleaves all three groups bonded to glycerol and produces an aldehyde from the vinyl ether group. 

In the development of the tetrafunctional initiator 24, the spatial shapes of initiator molecules turned out to be crucial for obtaining well-defined initiators [140]. As shown in Scheme 11, 24 is prepared from the corresponding tetrafunctional phenol via a reaction with 2-chloroethyl vinyl ether to attach vinyl ether moieties, followed by addition of trifluoro-acetic acid or hydrogen iodide. In this acid addition, the four vinyl ether groups should be well separated spatially. If the vinyl ether groups are located too close to each other, the treatment with the acid leads to intramolecular cyclization and other side reactions. 

A series of similar polymers (V) has been prepared directly from monomers by cationic polymerization of the vinyl ether group of the monomer, the cinnamoyl group being unreactive under these conditions ( ). 

Photo-initiated polymerization of pendant vinyl ether groups attached to a polystyrene backbone has been used as a convenient method of crosslinking. However, the potential commerical importance of this is marred by the fact that a slow thermal initiation involving the acceptor is also possible and occurs, for example, during processing of the carrier polymer. ... 

This compound proved to be the most powerful odorant agent in polyether polyols. Even in trace amounts, this substance confers an unpleasant odour to polyether polyols. The formation of this compound takes place in the purification step, in acidic media, involving the terminal propenyl-ether groups (in fact vinyl ether groups are very sensitive to acidic attack). The probable mechanism is the following cationic cyclisation ... 

The opposite type of reaction has also been reported, viz. one in which the heterocyclic molecule reacts via an electron-rich double bond with electron-poor olefins, in particular with tetracyanoethylene. Tanny and Fowler43 found that 2-azabicyclo[3.1.0]hex-3-enes reacted with tetracyanoethylene via a (2 + 2)-cycloaddition of the enamine double bond to give 13. Other electron-deficient reactants, such as JV-phenyl-maleimide, reacted differently, yielding an 8-azabicyclo[3.2.1]oct-2-ene (16). This type of reaction possibly occurs via a concerted [ 2 +ff2 +n2]-cycloaddition.43 At room temperature tetracyanoethylene also readily formed (2 + 2)-cycloadducts with heterocycles that contained a vinyl ether group for instance, 3,4-dihydro-2ff-pyran, 2,3-dihydrofuran, and 2,2-dimethyi-l,3-dioxole afforded the adducts 17-19 in yields of... 

X3CCH2O are readily converted to alkoxy groups by the action of alcohol <80CLI599>. Also an exocyclic vinyl ether group can be replaced by Br by the action of bromine which in turn may be converted to methoxy by methanol <84JGU486>. The chlorine in (86 R = Cl, X = O) has also been replaced by Mn(CO)s <831CAL139>. 

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