Boron triacetate

Interesting products may also be produced by introducing boron atoms into the chain. The amount of boron used is usualy small (B Si 1 500 to 1 200) but its presence increases the self-adhesive tack of the rubber, which is desirable where hand-building operations are involved. The products may be obtained by condensing dialkylpolysiloxanes end-blocked with silanol groups with boric acid, or by reacting ethoxyl end-blocked polymers with boron triacetate. 

Boron triacetate, B(OCOCH3)3. Mol. wt. 187.95. Supplier Alfa. Boron triacetate can be prepared by heating boric acid with excess acetic anhydride. It is generally less destructive than BF3 etherate. 

Diel -AldeT catalyst. Kelly and Montury have examined the effect of three Lewis acids on the Diels-Alder reaction of perf-hydroxylated naphthoquinones with l-methoxy-3-methyl-l,3-butadiene magnesium iodide, boron trifluoride etherate, and boron triacetate. All three increase regioselectivity. BF, etherate and B(OAc)3... 

The catalytic effect of boron triacetate was used in a stereocontrolled synthesis of altersolanol B (6). ... 

Unexpectedly boron triacetate induces a marked regioselectivity on the cyclo-addition of isoprene to the anthracene analog (7) of juglone. The uncatalyzed... 

DIELS-ALDER CA I ALYS I S Aluminum chloride. Boron triacetate Florisil Iron. Menthoxylaluminum dichloride. Nafion-H. 

Guingant and Barreto [53] published the pioneering paper describing the synthesis of ochromycinone (35) by a Diels-Alder reaction. The dienone (46) was prepared from 3-ethoxy-5-methyl-cyclohex-2-enone (45) in two steps (alternatively, a diene with SPh instead of OMe could be used). The Diels-Alder reaction with juglone (47) was catalyzed with boron triacetate to overcome the somewhat poor reactivity of the electron-deficient diene 46. The primary adduct 48 could not be isolated but directly eliminated and oxidized to ochromycinone (35) (Scheme 13). 

The procedure described is essentially that given in a recent patent. -Butyl borate can also be prepared by the action of -butyl alcohol on boron triacetate or boric anhydride. 

Reactions of dienes containing optically active auxiliary groups have not been so widely studied as those of chiral dienophiles. There are, however, examples of the use of various chiral auxiliaries attached to either C-1 or C-2 of the diene. The 1-substituted diene 130, derived from mandelic acid, undergoes cycloaddition with dienophiles in the presence of boron trifluoride or boron triacetate (3.94). With the dienophile juglone, the adduct 131 was formed with virtually complete asymmetric induction. The absolute configuration of the product corresponds to reaction of the diene in the conformation in which one face of the diene is shielded by the phenyl substituent. 

The course of acid-catalyzed acetylations with acetic anhydride may depend markedly on the concentration of the acid and the type of acid. Reaction of 7-(3-deoxy-3-nitro-/3-D-galactopyrano-syl)theophylline with acetic anhydride in the presence of one molar equivalent of perchloric acid gave the 2, 4, 6 -triacetate in 89% yield, but, in the presence of only a trace of the acid, 82% of the 4, 6 -diacetate was obtained.93 Treatment of the manno isomer of the nucleoside with acetic anhydride-boron trifluoride gave the 4, 6 -diacetate as a crystalline product (24%), whereas phosphoric acid as the catalyst yielded the 2, 4, 6 -triester (45%). 

Aryllead(IV) triacetates upon treatment with boron trifluoride-diethyl ether complex at room temperature are converted into the corresponding aryl fluorides I in good yield with the only impurity being the arenes 2 resulting from replacement of the lead substituent by hydrogen. This reaction has been extended to in situ generated aryllead(IV) triacetates, which are formed from arylsilanes or arylboranes upon treatment with lead(IV) acetate. 

Some examples of fluorination of substrates bearing group IV-A elements (Ge, Pb) have been reported.The best results are obtained in the reaction of aryllead(IV) triacetates 20 with boron trifluoride-diethyl ether complex. The yields of fluoroaromatics 16 are 50-80%. Triarylboroximes and aryltrimethylsilanes are precursors of the corresponding aryllead(IV) triacetates 20. ... 

Compound (80), prepared from 2-methylcyclopentane-l,3-dione and methyl 2-chloroacrylate followed by the sequence (76)-> (77)-> (78)- (79) (resolved)— (80), combined with (71) (prepared from m-methoxyphenylstyrene and diborane) to form the seco-steroid (82)/ Acetic anhydride-toluene-p-sulphonic acid then cyclized this stereoselectively to furnish the triacetate (83) which on saponification gave the triol (84). Interestingly, this compound reacted with toluene-p-sulphonic acid in alcohol to produce, by dehydration and change of configuration at C-14, the compound (85), which served as a source of various 8a-oestrone compounds. Compound (84) on treatment with boron trifluoride etherate underwent pinacol transformation in preference to dehydration to yield the ketone (86) this ketone was correlated with the known compound (87). 

Two mechanisms have been suggested. A radical mechanism was first proposed and its involvement is supported by the presence of dimeric products. However, the ligand coupling mechanism is now generally accepted. An enol-lead (IV) triacetate intermediate (7) is first formed by reaction of lead tetraacetate with the enol form. Its formation is accelerated by catalysis by boron trifluoride. 14,33 Treatment of the preformed enolate with lead tetraacetate performs a-acetoxylation at lower temperature and more rapidly than in the reaction with the corresponding enol. 4 Ligand coupling then takes place on this intermediate to lead to the a-acetoxycarbonyl derivative. 

Phenylene bis(lead triacetate) reagents, generated in situ from the corresponding bis(boronic acid) derivatives and lead tetraacetate, react with the a-methyl Meldrum s acid derivative to afford the meta-or para-phenylene bis(Meldrum s acid) derivatives in ca 45% yield. 1 Similarly to malonic acid compounds, the unsubstituted Meldrum s acid was very slow to react and the only observed product was the a,a-diarylated product in 7-17%. 

Such a mechanism was unlikely as addition of an external trap, 1,1-diphenylethylene, had no effect on the course of the arylation of p-ketoesters.i l A second approach involved the use of an internal trapping system which had been successfully used in the study of the radical reactions of arenediazonium salts.The internal trap containing reagent, ( rf/io-allyloxyphenyl)lead triacetate (94), can be easily prepared from the corresponding boronic acid. 2 Reaction with various types of nucleophiles, such as ethyl 2-oxocyclopentanecarboxylate (86), mesitol (36), the sodium salt of nitropropane, iodide and azide always afforded the C-arylation products in high yield. No trace of the 3-substituted dihydrobenzofurans, expected in a mechanism involving the intermediacy of free radicals, could be detected. 

As is the case for cross-coupling reactions, arylstannanes and aryltriaUcoxysilanes can be substituted for boronic acids in this method, but would appear to offer few advantages. A number of other, usually Cu(II)-catalysed, reagents can be used to arylate azoles and indoles, of which diaryliodonium salts are the most useful. Aryllead triacetates and triarylbismuth diacetates may find very occasional use, but A-cyclopropylation using tricyclopropylbismuth with cupric acetate is possibly more interesting. ... 

This is analogous to the extraction of sugars with boronic acids described earlier. The recovery of phenylalanine from a fermentation broth has been simplified by using a microporous poly(tetrafluoroethylene) membrane with tri-/j-octylmethylammonium chloride in toluene in the pores.133 Phenylalanine can also be separated using the quaternary ammonium salt with 2-nitrophenyl octyl ether in a cellulose triacetate membrane.134 Kerosene flowing in hollow fiber membranes can remove 99.9% of organic pollutants, such as benzene, p dichlorobenzenc, chloroform, and carbon tetrachloride, from wastewater outside the fibers.135... 

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