Alkenes allyl ethers

The 7, i5-unsaturated alcohol 99 is cyclized to 2-vinyl-5-phenyltetrahydro-furan (100) by exo cyclization in aqueous alcohol[124]. On the other hand, the dihydropyran 101 is formed by endo cyclization from a 7, (5-unsaturated alcohol substituted by two methyl groups at the i5-position. The direction of elimination of /3-hydrogen to give either enol ethers or allylic ethers can be controlled by using DMSO as a solvent and utilized in the synthesis of the tetronomycin precursor 102[125], The oxidation of the optically active 3-alkene-l,2-diol 103 affords the 2,5-dihydrofuran 104 in high ee. It should be noted that /3-OH is eliminated rather than /3-H at the end of the reac-tion[126]. 

There are several reagents that lead to isomerization of a double bond to form a new alkene. Allyl decyl ether (CH2=CHCH20CioH2i) was isomerized to 1-decyloxy-l-propene (CH3CH=CHOCioH2i) by treatment with NaHFe(CO)4. Many of these reactions were discussed in 12-2. 

In a related reaction, pyrolysis of allylic ethers that contain at least one a hydrogen gives alkenes and aldehydes or ketones. The mechanism is also pericyclic"" ... 

An interesting extension of this reaction is shown in the asymmetric kinetic resolution of cyclic allylic ether 44 under alkene coupling conditions. Use of (R)-12 as the catalyst gives (R)-45 in > 99% ee at 58% conversion. The ethylated product 46 is also formed in the reaction in 94% ee (Eq. 7) [25]. The reaction is effective for six- to eight-membered 3-oxacycloalkenes 47 as well as for a wide variety of alkoxycycloalkenes 48 [27], with some resolution dependency on the ring size of 47 (Fig. 2) [26]. 

Nickel-bpy and nickel-pyridine catalytic systems have been applied to numerous electroreductive reactions,202 such as synthesis of ketones by heterocoupling of acyl and benzyl halides,210,213 addition of aryl bromides to activated alkenes,212,214 synthesis of conjugated dienes, unsaturated esters, ketones, and nitriles by homo- and cross-coupling involving alkenyl halides,215 reductive polymerization of aromatic and heteroaromatic dibromides,216-221 or cleavage of the C-0 bond in allyl ethers.222... 

Scheme 40 Zr-catalyzed reaction of allyl ethers with EtMgBr to produce 3-ethyl-1 -alkenes.

Since nucleophilic addition to a metal-coordinated alkene generates a cr-metal species bonded to an -hybridized carbon, facile 3-H elimination may then ensue. An important example of pertinence to this mechanism is the Wacker reaction, in which alkenes are converted into carbonyl compounds by the oxidative addition of water (Equation (108)), typically in the presence of a Pd(n) catalyst and a stoichiometric reoxidant.399 When an alcohol is employed as the nucleophile instead, the reaction produces a vinyl or allylic ether as the product, thus accomplishing an etherification process. 

Figure 6.3. Various modes of addition of cyclic allylic ethers to an (ebthi)Zr—alkene complex.

Ruthenium complexes B also undergo fast reaction with terminal alkenes, but only slow or no reaction with internal alkenes. Sterically demanding olefins such as, e.g., 3,3-dimethyl-l-butene, or conjugated or cumulated dienes cannot be metathesized with complexes B. These catalysts generally have a higher tendency to form cyclic oligomers from dienes than do molybdenum-based catalysts. With enol ethers and enamines irreversible formation of catalytically inactive complexes occurs [582] (see Section 2.1.9). Isomerization of allyl ethers to enol ethers has been observed with complexes B [582]. 

Ru(H20)6], which is a precursor of ROM polymerization of cyclic dienes has also been found to possess good alkene isomerization activity [1], Among others it catalyzed the isomerization of allylphenyl ether to a vinylphenyl ether (Scheme 9.1) at room temperature. Allyl ethers are stable to acids and bases, while vinyl ethers are easily cleaved in acidic solutions. Therefore this isomerization gives a mild method for removal of protecting allyl groups under exceedingly mild conditions. 

The dihydroxylation of alkenes is a useful sttategy for the synthesis of polyols and these can be nitrated to the corresponding nitrate esters. Evans and Gallaghan " synthesized both the mono- (74) and di- (70) allyl ethers of pentaerythritol and used these for the synthesis of some novel nitrate ester explosives. 

Tab. 10.8 summarizes the application of rhodium-catalyzed allylic etherification to a variety of racemic secondary allylic carbonates, using the copper(I) alkoxide derived from 2,4-dimethyl-3-pentanol vide intro). Although the allyhc etherification is tolerant of linear alkyl substituents (entries 1-4), branched derivatives proved more challenging in terms of selectivity and turnover, the y-position being the first point at which branching does not appear to interfere with the substitution (entry 5). The allylic etherification also proved feasible for hydroxymethyl, alkene, and aryl substituents, albeit with lower selectivity (entries 6-9). This transformation is remarkably tolerant, given that the classical alkylation of a hindered metal alkoxide with a secondary alkyl halide would undoubtedly lead to elimination. Hence, regioselective rhodium-catalyzed allylic etherification with a secondary copper(l) alkoxide provides an important method for the synthesis of allylic ethers. 

There are numerous examples of highly syn diastereoselective cyclopropanation of allylic ethers in the literature, and most of them are alkenes prepared from protected glyc-eraldehyde. Some examples are illustrated in Figure 596.140-143 jjj cases, the... 

Copper(I) salts of enamines have been allylated with the 2-allyloxybenzimidazoles to give y,5-unsaturated ketones upon hydrolysis (79CL957). Primary allylic ethers react preferentially at their a-carbon with retention of double bond configuration whereas secondary allylic ethers react mainly at the y-carbon to afford alkenes of predominantly (E)-stereochemistry. 

Cycloaddition to a cyclic allyl ether The key step in a synthesis of lineatin (3), the aggregation pheromone of the bark beetle, is the addition of dichloroketene to the alkene 1. Under usual conditions (POCl3, 8,156) the desired adduct is obtained in 7% yield. Fortunately, substitution of 1,2-dimethoxyethane for POCl3 increases the yield of 2 to 50-60%. 

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