Boric acid dehydrating agent

Boric acid is a mild dehydrating agent suitable for removal of water from some primary, secondary, or tertiary alcohols. Since the acid and the alcohol form first a trimeric metaboric ester, which then regenerates the boric acid when it decomposes to the olefin,36 the reaction is somewhat similar to pyrolysis of carboxylic esters but the boric acid dehydration occurs at appreciably lower temperatures (250-300°). Olefins are readily obtained by heating approximately molar equivalents of boric acid and 1-octanol, 1-heptanol, 1-hexanol, (—)-menthol, cyclohexanol, or 5cyclohexane-methanol, and cyclobutanemethanol.38... 

Kozikowski and Stein (281) used the INOC strategy to prepare the 2-methyle-necyclopentanone derivative 172, which in turn was converted to sarkomycin (173), an antitumor agent (Scheme 6.81). The key step involved the treatment of nitroalkene 169 (obtained from bromide 168) with p-chlorophenyl isocyanate-triethylamine, which furnished a single diastereomeric isoxazoline 170 in 55% yield. This compound was transformed to the aldol product 171 by Raney nickel hydrogenation using wet acetic or boric acid, followed by dehydration to the a,p-enone 172 (281), a precursor of 173. 

Mono- or diacyl derivatives that undergo cyclization to benzoxazoles on heating or under the influence of dehydrating agents are formed as intermediates in this reaction [134, 452, 453, 459, 461 173], Phosphorus oxychloride [453, 457], boric anhydride [455, 461, 462], or polyphosphoric acid [134, 471] are used as condensing agents. In particular, 2-hydroxy-5-nitrobenzoxazole, used for the synthesis of antivirus medicines, has been obtained by the reaction of condensation of 4-nitro-2-aminophenol with (NH2)2CO in pyridine [474],... 

The following may be employed as dehydrating agents phosphoric acid, boric acid, potassium bisulphate, sodium sulphate, etc. However, the preparation of acrolein is not very satisfactory when these substances are used, the yield being not above 30-40% of the theoretical. It was only as a result of the... 

The reactants were heated at reflux in m-cresol (200 °C), with boric acid as a dehydrating agent, to obtain aromatic polymers which showed a 10% weight loss at 460-500 C in nitrogen. It has been suggested that the exocyclic carbon-carbon double bond enhances the reactivity of the lactone with diamines. This is supported by the use of relatively low reaction temperatures, and by the high yields and molecular weights obtained. 

A stirred mixture of heptanal, boric acid, and m-xylene refluxed 18 hrs. using a Dean-Stark trap to remove water 2-pentyl-2-nonenal. Y almost 100%. F. e., also with 10-hydroxy-10,9-boroxarophenanthrene, or with porous boron oxide as catalyst and dehydrating agent, s. R. D. Offenhauer and S. F. Nelsen, J. Org. Ghem. 33, 775 (1968). 

---