Boronic acid anhydrides boroxines

Boronic. . . (s. a. Mercuride-boronation) 20, 488 Boronic acid anhydrides s. Boroxines ---- esters 16, 156... 

Boronic acids readily dehydrate at moderate temperatures (or over P4O10 at room temperature) to give trimeric cyclic anhydrides known as trialkyl(aryl)boroxines ... 

Cyclotriboroxanes (RBO)3 (also known as boroxines) are formally anhydrides of the corresponding boronic acids RB(OH)2 only mild heating is required to convert these dibasic acids into a wide range of cyclic trimeric anhydrides [eqn (9.18)]. This dehydration process can be adapted for the synthesis of B3O3 rings with different aryl groups on the boron atoms by using two or more arylboronic acids in the appropriate stoichiometric ratio. 

Boroxines result from the condensation of boronic acids. RBfOH). The cyclic Irimeric anhydride of methylboronic acid is (IVfeBO),. Give a balanced equation for the reaction of (MeBO)j with water (See Brown, H. C. Cole, T. E. Organometallics 1985.4. 816.)... 

A second peptide coupling employing 2-pyrazine carboxylic acid (24) and TBTU affords 25. Finally the boronic ester moiety is removed under acidic conditions using isobutyl boronic acid. This regenerates pinanediol boronic ester 16, which can be used in another batch run of the process. Crystallization from ethyl acetate gives bortezomib in its anhydride (boroxine) form 26. The overall yield for the route is 35% with a typical purity of > 99% w/w. 

Boronic acids readily undergo self-condensation to give cyclic anhydrides, and either these trisubstituted boroxins (11), or the acids themselves, react spontaneously, but not necessarily completely, in solution, with suitable diols, to give cyclic boronates 12 (see Scheme 1),... 

The mechanism we have proposed to explain boronic acid-catalyzed amidation is depicted in Sch. 2. Arylboronic acid usually contains different amounts of cyclic tri-meric anhydrides (boroxines). The rate-determining step is the generation of ArB (OCOR)(OH). 

The steroid or mixture of steroids (lO mol) and an appropriate boronic add (11 mol of alkyl- or benzeneboronic acid) are dissolved in 1 ml of ethyl acetate and left at room temperature for 5 min. In obstinate cases a greater excess of the boronic add may be used. The solution may be analysed directly by TLC, GC or GC-MS. If necessary, the solution may be evaporated to dryness for silylation in 0.1 ml of dry pyridine using hexamethyldisilazane (100 /zl) with a trace of trimethyl-chlorosilane for 5 min at room temperature. Unconsumed boronic adds present no problems in GC or GC—MS, but elute as the trimeric anhydrides (boroxines) [50]. 

Despite their Hmited structure determination capabilities, ultraviolet and infrared spectroscopy were determinant characterization techniques in the early days of boronic acid research [332]. Notable IR absorptions are the strong H-bonded OH stretch (3300-3200 cm ), and a very strong band attributed to B-O stretch (1380-1310 cm ). IRis particularly diagnostic of the presence of boronic anhydrides. Upon anhydride (boroxine) formation, the OH stretch disappears and a new strong absorption appears at 680-705 cm [68]. 

Boroxines (3.29), the cyclic anhydrides of boronic acids, are of spedal interest in view of their possibly aromatic character. The parent compound (3.29)... 

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