Crotyl ether

The crotyl ether has been introduced using similar methodology. ... 

The intramolecular Heck cyclization of 4-iodopyridine 175 with a pendant crotyl ether led to a mixture of bicyclic ethers 176 and 177 in which the endocyclic isomer 176 was predominant [138],... 

Furoquinolines have been obtained by ring closure (H2S04) of o-hydroxylated Bz-allylquinolines (180) (to give, e.g., 2-methyl-2,3-dihydrofuro[2,3-A]quinoline, 181).223 Crotyl ethers give the corresponding 2,3-dimethyl derivatives.233 This method has provided the... 

The crotyl ether of the same phenol also rearranges without inversion.8 The only known example of para rearrangement accompanied by inversion is the reaction of a-ethylallyl 2-carbomethoxy-6rmethylphenyl ether (V), which yields the p-(y-ethylallyl) derivative (VI).8... 

A further resemblance of this rearrangement to the Claisen type is to be observed in the occurrence of inversion when the crotyl ether rearranges (IX - X). 

The mechanism is in agreement with the observation46 that crotyl ethers rearrange more rapidly than allyl ethers, because the 7-methyl group would promote the electronic shifts indicated. The cyclic mechanism as written does not explain the abnormal rearrangement, which involves the shift of two hydrogens, but this may involve a cyclic intermediate in which the /3-carbon becomes attached to the ortho carbon atom. 

Substitution in the a- or 7-position of the allyl group increases the rate of rearrangement the crotyl ether of 2,4-dichlorophenol rearranges more rapidly than the allyl ether.46 a-Ethylcrotyl phenyl ether rearranges to the extent of 10% in twenty-four hours at 120°." a-Ethyl-... 

Allyl and crotyl ethers were prepared from 3-propyl-4-quinolones by two different approaches. Displacement of 4-chloro-3-propyl-quinoline by the sodium salt of the two alcohols gave the ethers (1) as clean products, whereas reacting 3-propyl-4-hydroxyquinoline with allyl bromide in the presence of sodium ethoxide afforded not only the allyl ether but also the 1-allylquinolone (2). The products were well-characterized by their ultraviolet spectra and infrared bands. Rearrangement of the ethers at 200° without solvent gave quantitative yields of the corresponding l-allyl-3-propyl-4-quinolones (2). 

For the synthesis of Olivomycin A. the Roush group used a crotyl ether to protect a phenolic hydroxyl group,42 In the final lap of the synthesis, deprotection of the phenolic crotyl ether in 226.1 was achieved with Pd(0) and tributylstan-nane [Scheme 4.226]. No harm befell the sensitive cyclopentylidene acetal and chloroacetate groups. 

Anionic Claisen rearrangement. Acceleration of the Claisen rearrangement of allyl vinyl ethers was originally observed with potassium hydride in HMPT, but even milder conditions are possible using sodium or lithium dimsylate. The catalyzed rearrangement is as stereoselective as the thermal counterpart. Thus vicinal asymmetric centers are formed selectively on rearrangement of (E)- and (Z)-crotyl ethers (equations I and II). 

Hauser noted that diallyl ether (8) also undergoes Wittig rearrangement upon base treatment and suggested that product formation could involve either a 1,2-shift or a cyclic mechanism (equation 3). Later studies by Schdllkopf and Makisumi with substituted allylic ethers (10,11 and 14-16 equations 4 and 5) pointed to a cyclic (SnO mechanism a process allowed by the Woodward-Hoffmann rules. The diastereoselectivity of the reaction was not determined in these cases, but Schollkopf subsequently found that benzyl rrans-crotyl ether (20 equation 6) affords mainly the anti products upon rearrangement of ether (20) with BuLi in THF. Rautenstrauch observed a 1 1 mixture of syn and anti products upon rearrangement of ether (20) in the presence of TMEDA, whereas the cis isomer (23) gave only the syn product (22 equation 7). ... 

These findings have been extended to nonracemic oxazolines (221 Table 23). The methoxy-methyl-substituted derivatives (entries 1-3) rearrange with moderate enantioselectivity and high syn dia-stereoselectivity (entries 2 and 3). The hydroxymethyl analogs, on the other hand, rearrange with excellent enantioselectivity when KH is used as the base (entries 4, 6 and 8-10). However, the crotyl ethers show poor diastereoselectivity (entries 6, 9 and 10). 

Bu2SnO, toluene, THF CH2 CHCH2Br, Bu4NBr, 96% yield. The crotyl ether... 

Pd° cat., BusSnH, AcOH, P-NO2-phenol. The crotyl ether has been cleaved by a similar method. In the following case, isomerization methods failed presumably because of the MTM group, which may poison the catalysts. ... 

Wittig rearrangements of alkynyl ethers follow the stereochemical trends demonstrated by other w-donor-stabilized substrates, and simple diastereoselection is typically excellent for both E- and Z-substrates 37 " 39. Propargyl-stabilized anions of E- and Z-crotyl ethers 27 exhibit high levels of simple diastereoselection. 

Bankston et al. [103] studied the synergistic effect of rhodium(I) addition during the intramolecular Mizoroki-Heck reaction of some crotyl ethers (Scheme 6.38). 

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