Ethers, cyclohexenyl

In general, hydrogenolysis of vinylic compounds is favored by platinum and hydrogenation by ruthenium and rhodium 31,55,59,72,106). In the reduction of 4-methyl-1-cyclohexenyl ether, the order of decreasing hydrogenolysis to give methylcyclohexane was established as Pt Ir > Rh > Os Ru = Pd (52). 

A quantitative comparison of metals in the hydrogenation of vinyl ethers has been made, The extent of hydrogenolysis in hydrogenation of l-ethoxy-3-methylcyclohexene decreased in the order Pt Os > Rh Ir > Pd > Ru U24e)-, in the case of ethyl 4-methyl-1-cyclohexenyl ether, the order was Pt Ir > Rh > Os Ru Pd (124d). In ethanol, ketal formation is a... 

Diels-Alder reaction, of acrolein with B-butyl cyclohexenyl ether, n-butyl vinyl ether, and ethyl isopropenyl ether, 34, 30... 

The following reactions of propargyl 3-vinyl-2-cyclohexenyl ethers 160 also involved initial isomerization to an alkoxyallene. The terminal allenic C=C double bond participated in an intramolecular [4+ 2]-cycloaddition [133],... 

Under comparable conditions the submitters found that the corresponding dihydropyran derivatives were similarly obtained by the condensation of acrolein with methyl vinyl ether in 80-81% yield, with ethyl vinyl ether (77-85% yield), with w-butyl vinyl ether (82% yield), with ethyl isopropenyl ether (50% yield), and with w-butyl cyclohexenyl ether (40% yield). Other <, /3-un-saturated carbonyl compounds that have thus been condensed with ethyl vinyl ether are crotonaldehyde (87% yield), meth-acrolein (40% yield), a-ethyh/3-n-propylacrolein (54% yield), cinnamaldehyde (60% yield), /3-furylacrolein (85% yield), methyl vinyl ketone (50% yield), benzalacetone (75% yield), and benzal-acetophenone (74% yield). 

The carbon skeleton of natural product galanthamine was prepared by the ring closure of an enantiopure aryl-cyclohexenyl ether. The transformation, run in the presence of a palladium-dppp catalyst and silver carbonate, led to the diastereoselective formation of the tricyclic product (3.25.). It is worth mentioning, that the chiral information in the ether intermediate was introduced in the palladium catalysed Tsuji-Trost reaction (N.B. the synthetic approach depicted in 3.13. and 3.25. are much alike).31... 

Asymmetric ethylidene transfer has been achieved in the reactions of 1-cyclohexenyl ethers carrying a chiral auxiliary with 1,1-diodoethane/diethylzinc 39. Asymmetric induction in the reaction of diazofluorene with fumaric esters bearing chiral alcohol moieties has been investigated (equation 84)140,141. Kinetics of intramolecular cyclopropanation in... 

A process related to those shown in Scheme 14 involves the asymmetric Mo-catalyzed conversion of tertiary carbocyclic cyclopentenyl ethers to the corre-sponsing cyclohexenyl ethers with enantioselectivity (e.g., 67—>69, Scheme 15) [25]. A remarkable and unusual attribute of this class of transformations is that significantly higher levels of enantioselectivity are observed when ten substrate equivalents of THF are used as an additive. As an example, 69 (Scheme 15) is formed in only 58% ee in the absence of THF (<5% conversion is observed when THF is used as solvent). As also shown in Scheme 15 (70—471), enantioenriched spirocycles may be accessed easily by a similar approach in this case, no additive effect is observed. 

As is readily noted from the results summarized in Table 8E.13, enantioselectivity is very sensitive to a variety of factors such as the nucleophile and the nature of the allylic system as well as the ligand used. As expected, the enantioselectivity varied greatly with the structure of the nucleophile. Higher enantioselectivities were consistently obtained from the reactions of 2-cyclopentenyl phenyl ether than from the corresponding reactions of 2-cyclohexenyl ether. The biphenyl-derived DiPHEMP (7a) proved to be more effective than closely related BINAP (4) for this reaction. 

Povarov has reported that the addition of ethyl- 1-cyclohexenyl ether to benzylideneaniline and a number of its derivatives in the presence of boron trifluoride etherate provides the adducts (171a-d). 171a can be converted into 6-phenyl-7,8,9,I0-tetrahydrophenanthri-dine either by treatment with /j-toluenesulfonic acid or oxidation with potassium permanganate.200... 

Very few pericyclic reactions of carbene complexes have been studied that are not in the cycloaddition class. The two examples that are known involve ene reactions and Claisen rearrangements. Both of these reactions have been recently studied and thus future developments in this area are anticipated. Ene reactions have been observed in the the reactions of alkynyl carbene complexes and enol ethers, where a competition can exist with [2 + 2] cycloadditions. Ene products are the major components firom the reaction of silyl enol ethers and [2 + 2] cycloadducts are normally the exclusive products with alkyl enol ethers (Section 9.2.2.1). As indicated in equation (7), methyl cyclohexenyl ether gives the [2 -t- 2] adduct (84a) as the major product along with a minor amount of the ene product (83a). The t-butyldimethylsilyl enol ether of cyclohexanone gives the ene product 9 1 over the [2 + 2] cycloadduct. The reason for this effect of silicon is not known at this time but if the reaction is stepwise, this result is one that would be expected on the basis of the silicon-stabilizing effect on the P-oxonium ion. 

The cyclohexenyl ether is prepared from the bromide and K2CO3 in acetone. It is cleaved with HCl in ether (92-98% yield) and with 10% Pd/C, 10% KOH, MeOH. ... 

As an example, treatment of allyl as-5-methyl-2-cyclohexenyl ether (5) with butyllithium in tetrahydrofuran at —85 JC affords the [2,3] Wittig product 6. which is then transformed to the oxy-Cope product 7 by reaction with potassium hydride in the presence of 18-crown-6. However, this two-step sequence is accompanied by the tandem process 5 -> 6 -> 7. since reaction of 5 gives a 62 14 mixture of 6 and 7. The overall transformation proceeds with exclusive cis selectivity1178. 

Dicyclopen tadiene, 32,41 36,33 37,65 Dicyclopropyl ketone, 38,19 Diels acid, 35, 38 Diels-Alder reaction, of acrolein with n-butyl cyclohexenyl ether, n-butyl vinyl ether, and ethyl iso-propenyl ether, 34, 30 of butadiene with maleic anhydride, 30, 93... 

Fig. 4.7. Br0nsted relation for the hydrolysis of methyl cyclohexenyl ether. (Reproduced from reference 21 by permission of the American Chemical Society.)...

Svennebring, A., Nilsson, R and Larhed, M. (2007) Microwave-accelerated sprro-cyclizations of o-halobenzyl cyclohexenyl ethers by palladium(O) catalysis. J. Org. Chem., 72, 5851-4. 

Br0nsted relation for hydrolysis of methyl cyclohexenyl ether... 

Claisen rearrangement of allyl vinyl ethers is catalyzed by palladium(II). Treatment of E-2-buten-l-yl 1-cyclohexenyl ether (R = CH3, = H) with 5% PdCl2(MeCN)2 at... 

In 2007, Larhed and co-workers reported a palladium-catalyzed cyclization of o-halobenzyl cyclohexenyl ethers. " A number of new spiro[cyclohexane-l,l -isobenzofuran]-based compounds were synthesized by palladium(O)-catalyzed S-exo cyclization of a series of cyclohexenyl o-halobenzyl ethers. Controlled microwave heating was found to promote both product yield and reaction rate without compromising the selectivity. Heck cyclization of... 

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