Oxonium fluoroborates

Alkylation of pyrazinones and quinoxalinones may be carried out under a variety of conditions and it is usually observed that while O-alkylation may occur under conditions of kinetic control, to yield the corresponding alkoxypyrazines or alkoxyquinoxalines, under thermodynamic control the A-alkylated products are formed. Alkylation using trialkyl-oxonium fluoroborate results in exclusive O-alkylation, and silylation under a variety of conditions (75MI21400) yields specifically the O-silylated products. Alkylation with methyl iodide or dimethyl sulfate invariably leads to A-methylation. 

Dioxopiperazines are amongst the most ubiquitous of natural products (75FOR(32)57) and they are formally derived by the cyclodimerization of a-amino acids (69CCC4000) or their esters. A number of methods are available for their oxidation to the corresponding pyrazines. Treatment of 2,5-dioxopiperazines with triethyl- or trimethyl-oxonium fluorobor-ate followed by oxidation with DDQ, chloranil or iodine results in pyrazine formation, usually in high yields (Scheme 63) (72JCS(P1)2494). 

Quaternization is difficult benzofuroxan is unaffected by triethyl-oxonium fluoroborate. With methyl trifluoromethanesulfonate, an interesting rearrangement occurs, and l-hydroxybenzimidazole-3-oxide (39, R = H) is formed, probably via the N-quaternized derivative (38). Compound 39 (R = Ceils) has been prepared similarly. 

Between sulfur dioxide radical anions, dithionite, and sulfoxylate/sulfite there exists a pH-dependent equilibrium465 (equation 86). Therefore, dithionite has been used as a source of sulfoxylate in order to prepare sulfinate and hence sulfones. Alkylation with triethyl oxonium fluoroborate leads to ethyl ethanesulfinate, alkyl iodides lead to symmetrical sulfones466 (equation 87). 

Carboxyls are converted to methyl esters by reacting with trimethyl-oxonium fluoroborate (Figure 9b). Pepsin was modified with [14C]tri-methyloxonium fluoroborate at pH 5.0 with loss in catalytic activity (48). The relationship between the number of methyl groups that are incorporated and the remaining catalytic activity suggested, that at least two carboxyl groups are essential for activity. 

Cyclic lactams do not normally form hydrazones or other derivatives typical for ketone carbonyl groups. However, 2-methyl-3(2//)-pyridazinone when treated with formylhydrazine and triethyl-oxonium fluoroborate afforded the hydrazone (93) which undergoes oxidative coupling to form azo dyes. Oximes of 4(lH)-pyridazinones are also claimed to exist. ... 

Oxidmion of ketone acetals and ethers. Ketones can be regenerated from the ethylene acetal derivatives by treatment with trityl fluoroborate in dry dichloro-methanc (Nj) at room temperature. Thus the reaction of trityl fluoroborate with cyclohexanone ethylene acetal results in cyclohexanone (80%) and triphenylmethane. The reaction thus involves hydride transfer to the triphenyl carbonium ions. Triethyl-oxonium fluoroborate can also be used but is somewhat less effective than trityl fluoroborate. 

Tyrosine Tryptophan Serine Aspartic Acid Glutamic Acid T etranitromethane Chloramine T iV-Bromosuccinimide 2-Hydroxy-5-nitrobenzyl bromide Diisopropylfluorophosphate Halomethyl ketones Carbodiimides Trimethyl oxonium fluoroborate Isoxazolium salts Chloramine T also modifies histidines and methionines... 

Reaction with ketones (1, 369-370). Mock and Hartman3 have improved the original procedure for synthesis of /3-keto esters from ketones by using triethyl-oxonium fluoroborate as the Lewis acid catalyst rather than boron trifluoride. Thus cyclohexanone (1) is converted into 2-carboethoxycycloheptanone (2) in 90% yield under the new conditions the yield using BF3 was 38%. The reaction is... 

ALKYLATION Benzyltriethylammonium chloride. 9-Borabicyclo [ 3.3.1 ] nonane. Dimethylcopperlithium. 1,3-Dithiane. Hexamethyl-phosphoric triamide. Lithiumdi-isopropylamide. Lithium diisopropylamide Methyl fluorosulfonate. Naphthalene -Lithium. Phenacylsulfonyl chloride. Polyphosphate ester Silver oxide. Triethylaluminum. Triethyl-oxonium fluoroborate. 

Alkylation of perhydro-l,4-dioxopyrrolo[l,2-a]pyrazines with triethyl-oxonium fluoroborate gives l-ethoxy-4-oxo compounds such as 129. These have then been allowed to react with anthranilic acid to give fused quinazolines such as 130. ... 

Camphor diethyl acetal, obtained in only 25-30% yield from camphor and orthoformic ester,215 is obtained in 87% yield from O-ethylcamphor oxonium fluoroborate and sodium ethoxide.18... 

Oxonium fluoroborate Iminoester fluoroborates from carboxylic acid amides... 

Nitro compounds, ar. Nitrobenzene, m-Dinitrobenzene o-Nitrophenol N-Nitramines Oxonium fluoroborate Trialky loxonium salt Trialky loxonium fluoroborate Pyrylium fluoroborate Carboxylic acid esters HCOOC Hs, p-Nitro-phenyl formate Isopropenyl acetate Salicylic acid esters Ethyl cyanoacetate Chloroformic acid esters Ethyl chloroformate, ClCOOC Hg-i Methyl trichloroacetate Diethyl oxalate Ethyl malonate... 

Di(enolethers). Cyclohexane-1,3-dione treated at 5° with 3 equivalents triethyl-oxonium fluoroborate and 3.2 equivalents ethyldiisopropylamine in methylene diloride -> 1,3-diethoxycyclohexa-l,3-diene. Y ca. 100%. F. e., also partial conversion, s. E. Rizzardo, Chem. Commun. 1975, 644. 

Thietanium Ions.—Methylation of m-2,4-dimethylthietan with trimethyl-oxonium fluoroborate gave the c/r-5 -methyIthietanium ion (49) stereo-specifically, but 2,2,4-trimethylthietan gave a mixture of the sulphonium salts (50) and (51) isomeric at sulphur. In the ion (49) and that (52) derived from /m y-2,4-dimethylthietan, the S—Me group adopted the... 

The deep-orange 3-methyl-2,4-diphenylthiazolium 5-thiolate (154) forms yellow solutions in acids, presumably owing to protonation of the exocyclic sulphur. Its oxidation by hydrogen peroxide in acetic-formic acids produces the 5-sulphonate (155) (91%) subsequent treatment with concentrated hydrochloric acid yields 2,4-diphenylthiazole (156). Anhydro-5-hydroxy-3-methyl-2-phenylthiazolium hydroxide (157) resists alkylation by conventional reagents, but is readily 0-alkylated by triethyl-oxonium fluoroborate, yielding 5-ethoxy-3-methyl-2-phenylthiazolium fluoroborate (158). In this respect, it resembles related mesionic systems based on 1,3,4- and 1,2,3-thiadiazoles (c/. Chapter 15). 

Nitro compounds, ar. Nitrobenzene N-Nitramines Oxonium fluoroborate Trialkyloxonium salt Trialkyloxonium fluoroborate Carboxylic acid esters Isopropenyl acetate Chloroformic acid esters Ethyl chloroformate Methyl trichloroacetate... 

Nitro compounds, ar. Nitrobenzene, m-Dinitrobenzene o-Nitrophenol N-Nitramines Oxonium fluoroborate Trialky loxonium salt T rialky loxonium fluoroborate Pyrylium fluoroborate Carboxylic acid esters... 

N,N-Diisopropylthiobenzamide treated overnight at room temp, with trimethyl-oxonium fluoroborate in dry benzene -> N,N-diisopropylbenzamide. Y 95%. -Similarly N -Benzoyl-N -methyl-N -thiobenzoylbydrazine N jN -dibenzoyl-... 

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