Trifluoroborane-etherate

Due to difficulties in separating (VII) from the IiCl byproduct, it was found to be advantageous to prepare (VII) by the reaction of a tris(imino)borane with trifluoroborane-etherate as illustrated in Eq. (26). 

Trifluoroborane-etherate was used in this reaction. The same route seems to be quite convenient for the preparation of many other (diphenylmethylene-amino)boranes including dihaloborane derivatives. Similarly, B-monohalo derive atives such as [(C6Hs)2ON—B(C6H5)C1] and B-diaryl derivatives were synthesized by the cited method 47)... 

The combination of mercury(II) oxide and trifluoroborane etherate provides a mild method for deprotecting some dithianes139140 but with substrates that are unusually susceptible to acid-catalysed side reactions, further transformations may be observed [Scheme 2 65].141... 

The cycloalkylidene acetals are formed in essentially the same way as their iso-propylidene counterparts. Thus reaction of a diol with cyclopentanone [Scheme 3.42]M or cyclohexanone, usually with provision for the removal of water, accomplishes the transformation in good yield In the double protection of mannitol [Scheme 3.43], triethyl orthoformate served as the dehydrating agent and trifluoroborane etherate was the acid catalyst 65... 

Surprisingly, Lewis acids are seldom applied to the deprotection of TBS ethers even though in certain circumstances they have advantages. A case in point is the transformation depicted in Scheme 4,39.5S The hydrolytic lability of the ox-alimide function in 39.1 precluded desilylation with TBAF or methanolic acid but trifluoroborane etherate in acetonitrile59 at 0 °C rapidly and cleanly accomplished the desired desilylation to give the diol 39,2 in 93% yield. 

A mixture of phosphoric acid and trifluoroborane etherate was used by Hof mann-La Roche workers to prepare a rm-butyl ether and a rm-butyl ester in one pot from the homochiral p-hydroxy add 124.1 as shown in Scheme 4.124.225 The method can be applied to molecules containing an allylic triisopro pylsilyl ether [Scheme 4.125].143... 

The preparation of /m-butyl ethers requires a large excess of isobutene (bp -6.9 °C) and the inconvenience and potential hazard of a pressure vessel or a sealed tube in some cases, ten-Amyl (T m) ethers have the same lability as tert-butyl ethers towards acids, and they are prepared the same way, by reaction of the alcohol with 2-methyl-l-butene (bp 31 °C) in the presence of trifluoroborane etherate, but they require only a slight excess of the reagent [Scheme 4.128].228 Deprotection of rm-butyl and fer/-amyl ethers can be accomplished by treatment with a catalytic amount of TBSOTf. When a stoichiometric... 

Most Lewis acids such as zinc bromid in methanol, dichtoroethylalane in di-chloromethane, trifluoroborane etherate and ethane-1,2-dithiol in methanol,372 or iron(lll) chloride373 will also cleave trity ethers. Scheme 4.202 illustrates the use of zinc bromide in dichloromethane374 to remove a trityl ether in the presence of two a Hylic TBS ethers during a synthesis of ACRL toxin 111b.375... 

Haloboranes will cleave MOM ethers in functionally dense substrates. Trichloro-borane without the nucleophilic assistance of thiol or thioether additives has been used457 and trifluoroborane etherate, in the presence of thiophenol, was used by Corey and co-workers in a synthesis of Ginkolide A45K and by Vedejs and Larsen to cleave a tertiary MOM ether in a synthesis of Fulvine and Cris-patine [Scheme 4.24b]459... 

A mild procedure for the appendage of MOM groups to acid-sensitive substrates is illustrated by the protection of the allylic alcohol in Avermectin derivative 259.1 using [(methoxymethyl)thio]-2-pyridine (259 2) sitver(I) Inflate and sodium acetate in THF at room temperature [Scheme 4.259],479 Primary secondary and tertiary alcohols and phenols are methoxymethylated in good yield though phenols are slower to react. Reagent 259.2 (bp 66 °C/0.088 kPa) is easily prepared in 75% yield by the reaction of pyridine-2-thiol with dimethoxy-methane activated by trifluoroborane etherate. 

We now turn to some rarities. 2-(Trimethylsilyl)ethyl ethers do not belong to the alkoxymethyl acetal clan but they are included here because the conditions used to cleave them resemble the conditions used to cleave SEM ethers. The 2-(trimethylsilyl)ethyl ethers in 300 1 en route to the siderophore Nannochelin A [Scheme 4.300] were impervious to trifluoroacetic acid in dichloromethane (1 1) for five hours at 0aC. thus allowing selective scission of the terr-butyl ester. The two hydroxamic acids were then liberated with trifluoroborane etherate in the final step.549... 

Since its introduction in 1986315, the 2-(trimethylsilyl)ethylsuifonyl group has been widely adopted in alkaloid, carbohydrate and amino acid chemistry, it is every bit as stable as the arylsulfonyl groups discussed above it is impervious to the ravages of trifluoroacetic add, hot 6 M HC1, trifluoroborane etherate, or 40% HF in ethanol. Nevertheless, under very specific non-reductive conditions, it can be cleaved. The link with 2-(trimethylsilyl)ethyl esters, 2-(trimethylsilyl)ethoxycarbon-y t derivatives, and 2-(thmethylsilyl)ethoxymethyl ethers hardly needs comment. 

A diastereoisomeric mixture of dicobalt hexacarbonyl complexes 33 reacted with trifluoroborane etherate at -20 °C to give the reduced product 3.3 (minus a benzyl ether) as a single diastereoisomer after decomplexation of the metal with cerium(IV) ammonium nitrate. Suggest a mechanism for the formation of 33 which accounts for the stereochemistry of the product. 

Early reports of hydroboration reactions often recommended in situ generation of diborane from sodium tetrahydroborate (borohydride) and trifluoroborane etherate or some similar mixture. Indeed, it is still reasonable to use this method for simple hydroborations, provided that nothing more complicated than oxidation of the resultant organoborane is intended. Otherwise, the approach should be avoided, particularly since borane is now commercially available in the form of several Lewis base complexes. 

The use of trifluoroborane etherate and titanium tetrachloride in thioa-cetalisation reactions is common but milder Lewis acids such as zinc chloride will suffice in some circumstances as shown by the large scale transprotection reaction depicted in Scheme 2.93. ... 

Trost first introduced the di-/err-butylsilylene derivative as a means for protecting 12- and 1,3-diols during a synthesis of Pillaromycinone derivatives. Di-rerr-butylsilylene derivatives are not as robust as isopropyiidene or benzylidene acetals and their use is best reserved for systems requiring deprotection under very mild conditions. Di-isopropylsilylene derivatives are occasionally used but they usually only survive in highly crowded environments. Di-ferf-butylsily-lene derivatives survive hydroboration with 9-BBN, mild oxidation (e.g., the Dess-Martin, ozone). Lewis acids such as trifluoroborane etherate and titanium tetrachloride, mild acids (pyridinium p-toluenesulfonate). camphorsulfonic acid, strong bases such as rer/-butyllithium (THF, -50 °C), DDQ, and sodium meth-oxide in methanol at 0 — conditions used to cleave acetate esters. 

EDCI), the nitrogen and hydroxyl groups were protected as the N,0-acetal 1233 by reaction of 1232 with 22-dimethoxylpropane in acetone in the presence of trifluoroborane etherate. Finally, reduction of the N-methoxyamide with lithium aluminium hydride afforded Gamer s aldehyde in 88% overall yield for the 4-step sequence, Epimerisation of the stereogenic centre is negligible under these conditions. 

The 5-trityl ether of cysteine has been prepared by reaction of the amino acid with triphenylmethanol in acetic acid in the presence of trifluoroborane etherate or with triphenylmethanol in neat trifluoroacetic acid, Slightly lower mp and specific rotation were recorded for the product obtained by the more... 

Good yields of dialkylalkynylboranes, RaBC=CR, are obtained by reaction of RaBOMe with R C=CLi followed by trifluoroborane etherate. w-Alkynyl-boranes can be obtained from its isomers in which the triple bond is in a different position by isomerization with potassium 3-aminopropylamide (KAPA). 1-Bora-adamantane has been synthesized by sequences involving allylboronation and hydroboration reactions, and has been converted into adamantan-l-ol by cyanida-tion or carbonylation. Scheme 12 shows the application of anions derived from... 

Brown et ah discovered a simple preparation of organoboranes inaccessible by hydroboration, e.g., aromatic, benzylic, allylic, by sonication of the halides, most of the time bromides, with magnesium and trifluoroborane etherate. The Grignard reagent formed in situ undergoes metal exchange in high yields (Eq. 35). 70... 

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