Mitsunobu etherification

Phenols attached to insoluble supports can be etherified either by treatment with alkyl halides and a base (Williamson ether synthesis) or by treatment with primary or secondary aliphatic alcohols, a phosphine, and an oxidant (typically DEAD Mitsu-nobu reaction). The second methodology is generally preferred, because more alcohols than alkyl halides are commercially available, and because Mitsunobu etherifications proceed quickly at room temperature with high chemoselectivity, as illustrated by Entry 3 in Table 7.11. Thus, neither amines nor C,H-acidic compounds are usually alkylated under Mitsunobu conditions as efficiently as phenols. The reaction proceeds smoothly with both electron-rich and electron-poor phenols. Both primary and secondary aliphatic alcohols can be used to O-alkylate phenols, but variable results have been reported with 2-(Boc-amino)ethanols [146,147]. 

The Mitsunobu etherification of polystyrene-bound phenols is usually conducted in THF or NMP, simply by adding the alcohol, the phosphine, and DEAD. Some authors claim that the addition of tertiary amines is beneficial [148], but this seems not always to be the case [146], It is, of course, important that neither the support nor any of the... 

Alternatively, alkyl aryl ethers can be prepared from support-bound aliphatic alcohols by Mitsunobu etherification with phenols (Table 7.13). In this variant of the Mit-sunobu reaction, the presence of residual methanol or ethanol is less critical than in the etherification of support-bound phenols, because no dialkyl ethers can be generated by the Mitsunobu reaction. For this reason, good results will also be obtained if the reaction mixture is allowed to warm upon mixing DEAD and the phosphine. Both triphenyl- and tributylphosphine can be used as the phosphine component. Tributyl-phosphine is a liquid and generally does not give rise to insoluble precipitates. This reagent must, however, be handled with care because it readily ignites in air when absorbed on paper. 

The most common resin-bound substrates for Mitsunobu etherification are primary benzylic alcohols, but a few non-benzylic alcohols have also been converted into aryl ethers (Table 7.13). Support-bound secondary alcohols are less suitable alkylating agents because elimination often predominates. 

Figure 2.34 shows the mechanism of this reaction. A key intermediate is the alkylated phosphine oxide A, with which the carboxylate ion reacts to displace the leaving group 0=PPh3. Figure 2.34 also shows that this carboxylate ion results from the deprotonation of the carboxylic acid used by the intermediate carbamate anion B. Nucleophiles that can be deproto-nated by B analogously, i.e., quantitatively, are also alkylated under Mitsunobu-like conditions (see Figure 2.36). In contrast, nucleophiles that are too weakly acidic cannot undergo Mitsunobu alkylation. Thus, for example, there are Mitsunobu etherifications of phenols, but not of alcohols.

This linker was employed in the synthesis of a library of N-alkylated 5- and 6-alkyloxy-l,2,3,4-tetrahydroisoquinolines 77 involving the following steps Michael addition, acid-catalyzed removal of a THP group, Mitsunobu etherification, quaternization of the nitrogen, and Huenig s base-catalyzed elimination [86] (Scheme 36). 

The first step consisted of the Mitsunobu etherification of an allyl alcohol with the resorcinol monoester, which afforded 102. Cleavage of the benzoyl protecting group released the phenol, which was then attached to the solid support. One-step cleavage of the THP group and bromination was achieved with PPhs/C Br4 to furnish 103. Nucleophilic substitution of the bromide with benzylamine was followed by acylation of the secondary amine wtith N-Boc-allylglycine 104, which resulted in the precursor 105, ready for the metathesis reaction this was performed with catalyst 101 to yield the final product 106. Either 1-octene or ethene was employed to generate 101. 

A Hbrary of aryl ethers has been prepared by Mitsunobu etherification by stirring a mixture of polymer-bound triphenylphosphine (1.5 equiv.), DEAD (1.5 equiv.), the appropriate alcohol (1.5 equiv.), and a phenol (1 equiv.) in dichloromethane at room temperature for 4-12 h ]37]. As described, the resin was filtered off, the solvent was evaporated, and the DEAD-derived side product was removed by short-path silica gel column chromatography. 

The pme enantiomers can then be used in the construction of a final chiral liquid crystal or dopant. The chiral acid 141b can simply be esterified (Scheme 35) onto a simple mesogenic-like core (146) to provide a final chiral material (147). The ethyl ester can be reduced to the alcohol (145a) which can then either be esterified onto a carboxylic acid mesogenic core or, as shown in Scheme 36, be employed in a Mitsunobu etherification (DEAD reaction) to give aryl bromide 148, which is then involved in a conventional arylboronic acid coupling reaction to provide a final chiral liquid crystal (150) which has a particularly wide S range. 

Scheme 21 shows the synthesis of a dihydrofuran derivative 86. Synthesis of this compound was described by Nam et al. [68] utilizing a furanone compound 87 synthesized by Kim et al. [61] via a similar synthetic approach as described in Scheme 17. The lactone was reduced using lithium aluminum hydride to give the diol 88 and intramolecular etherification using the Mitsunobu reaction afforded the dihydrofuran 86 in moderate yield (47%).

Both intermediates 43a and 43b were converted to the final molecule duloxetine (3), as described in Scheme 14.11. Therefore, route A involved direct transformation of the (5)-chloroalcohol 43a into the corresponding iodide, followed by amination and etherification. In contrast, route B consisted of Mitsunobu inversion of (R)-chloroalcohol 43b... 

In the asymmetric synthesis of axially chiral biaryls, the formation of two C-O bonds is the key step in the etherification of 2,2, 6,6 -tetrahydroxybiphenyl 187 (Scheme 21). Sequential etherification of the biaryl 187 with 1,4-di-O-benzyl-L-threitol 188 under the Mitsunobu conditions afforded the monoether 189. After deprotection of the /-butyldimethylsilyl (TBDMS) group with Bu4NF, the intermediate alcohol was again subjected to the Mitsunobu reaction in situ. The intramolecular cyclization proceeded smoothly to give 190 in high yield (for R= Bn, m.p. 138-139°C) <2000JOC1335>. 

An interesting alternative in using diols, but starting from 3,4-dihydroxythiophene-2,5-dicarboxylic acid diethyl ester (the same intermediate as used in the Gogte pathway above), was developed independently by the groups of Reynolds [25] and Bauerle [26], who utilized the Mitsunobu reaction with azodicarboxylic acid ester-phosphane as the etherification agent. 

Chiral alkanols have also been linked as ethers. Of special interest for etherification is the Mitsunobu reaction [160], which can be used for inversion of configuration at the reaction centre of the chiral alcohol component (Scheme 9). 

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