Boronates etherification

Methyl Ethers. Methylation of sucrose is generally conducted under basic conditions. Etherification occurs initially at the most acidic hydroxyl groups, HO-2, HO-T, and HO-3f, followed by the least hindered groups, HO-6 and HO-6. Several reagents have found use in the methylation of sucrose, including dimethyl sulfate—sodium hydroxide (18,19), methyl iodide—silver oxide—acetone, methyl iodide—sodium hydride in N, N- dimethyl form amide (DMF), and diazomethane—boron trifluoride etherate (20). The last reagent is particularly useful for compounds where mild conditions are necessary to prevent acyl migration (20). 

The addition of alcohols to alkenes is also catalyzed by boron trifluoride-hydrogen fluoride, but the reaction proceeds only under vigorous conditions and the yields are generally low.394 Recently, the etherification of alkenes has also been achieved industrially by using acidic montmorillonite395 and macroporous sulfuric acid resins (ion-exchange resins).396... 

C-M bond addition, for C-C bond formation, 10, 403-491 iridium additions, 10, 456 nickel additions, 10, 463 niobium additions, 10, 427 osmium additions, 10, 445 palladium additions, 10, 468 rhodium additions, 10, 455 ruthenium additions, 10, 444 Sc and Y additions, 10, 405 tantalum additions, 10, 429 titanium additions, 10, 421 vanadium additions, 10, 426 zirconium additions, 10, 424 Carbon-oxygen bond formation via alkyne hydration, 10, 678 for aryl and alkenyl ethers, 10, 650 via cobalt-mediated propargylic etherification, 10, 665 Cu-mediated, with borons, 9, 219 cycloetherification, 10, 673 etherification, 10, 669, 10, 685 via hydro- and alkylative alkoxylation, 10, 683 via inter- andd intramolecular hydroalkoxylation, 10, 672 via metal vinylidenes, 10, 676 via SnI and S Z processes, 10, 684 via transition metal rc-arene complexes, 10, 685 via transition metal-mediated etherification, overview,... 

The reaction requires a dehydrating catalyst, which, for reactions in the liquid phase, is an acid substance this is usually a concentrated mineral acid, but may also be an organic sulfonic acid, acid salt, or halogen compound such as boron trifluoride, zinc chloride, or aluminum chloride. For etherification in the gaseous phase the alcohol vapor is passed over a solid catalyst such as A1203, Ti02, or dehydrated alum.656... 

Removal of an aromatic methylenedioxy grotgf. This reaction can be carried out in three steps. Treatment with boron trichloride is known to cleave methylenedioxy groups preferentially (4, 43). The catechol group is then removed by etherification with S-chloro-l-phenyl-l-Zf-tetrazole and hydrogenolysis of the ether, An example is formulated. ... 

Reductive Etherifications and Acetal Reductions. Additional applications of triethylsilane in the reduction of C-0 bonds also continue to surface. The Kusanov-Pames dehydrative reduction of hemiacetals and acetals with trifluorosulfonic acid/EtsSiH has proven especially valuable. Under such conditions, 4,6-O-benzyli-dene acetal glucose derivatives can be asymmetrically deprotected to 6-0-benzyl-4-hydroxy derivatives (eq 28) and thioketone derivatives can be converted to syn-2,3-bisaryl (or heteroaryl) di-hydrobenzoxanthins with excellent stereo- and chemoselectivity (eq 29). Triethylsilane is also useful in a number of related acetal reductions, including those used for the formation of C-glycosides. For example, EtsSiH reductively opens 1,3-dioxolan-4-ones to 2-alkoxy carboxylic acids when catalyzed by HCU. Furthermore, functionalized tetrahydrofurans are generated in good yield from 1,2-0-isopropylidenefuranose derivatives with boron trifluoride etherate and EtsSiH (eq 30). These same conditions lead to 1,4- or 1,5-anhydroalditols when applied to methyl furanosides or pyranosides. ... 

As expected, intramolecular etherification of (47a) took place smoothly by treatment with a catalytic amount of boron trifluoride etherate and in a completely stereospecific manner giving the exomethylene (48) in 90% yield. 

Yanagisawa and Ando [38] reported an elegant [2,3] sigmatropic rearrangement of 7a-benzoylamino-3-methylenecepham sulfoxide (99) into the sulfenate intermediate (100), which could be trapped with mercaptans to give the disulfides (101). Formation of the epi-oxazoline derivative (47a) by treatment with chlorine was followed by intramolecular etherification with boron trifluoride to obtain 3-methylene-1-oxacepham (48) with 10% overall yield from (99). 

Etherification catalysts include mineral acids such as sulfuric and perchloric acid, but these tend to provide double the amounts of dehydration products compared with other catalysts. Lewis acids such as boron trifluoride and iron (Ill)-exchanged montmorillonite clays are also effective at catalyzing these reactions. Boron trifluoride provides the most consistent yields increases in catalyst concentration generally increase ether yields and/or reduce reaction times. Iron (III) clays provided high... 

The 1,5-anti-aldol reaction was performed with chiral boron enolate of 325 and aldehyde 327, prepared by Evans asymmetric alkylation, cross metathesis, and Wittig homologation (Scheme 72), to afford 324 with a 96 4 diastereoselectivity. Stereoselective reduction of C9-ketone provided the 5y -l,3-diol, which was exposed to catalytic f-BuOK to give 2,6-cis-tetrahyderopyran 333 via an intramolecular Michael reaction. Finally, methyl etherification, deprotection, hydrolysis of ester, and Yamaguchi macrolac-tonization yielded the leucascandrolide macrolide 201 (Scheme 73). 

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