Phosphorus pentoxide s. a. under

Phosphorus pentoxide (s. a. under MeSO H) 2,3-Dihydro-l,4-dithiin from 1,3-dithiolane ring... 

S g. (0.84 g. atom) of zinc dust, and 52.5 g. (0.46 mole) of 3-methylpentane-2,4-dione (Note 1). The contents of the flask are stirred vigorously (Note 2), and a solution of 42 g. (0.415 mole) of diacetyl monoxime in 150 ml. of glacial acetic acid is added from a separatory funnel at a rate to maintain the temperature of the mixture at 65-70°. The addition takes 1 hour. When the addition is complete, the mixture is refluxed with stirring for an additional 30 minutes. The flask is then fitted for distillation with steam under nitrogen 500 ml. of water is added and steam is introduced. Steam distillation (Note 3) is continued until no more tetramethylpyrrole comes over. This takes 1-2 hours and the distillate amounts to 1-2 1. The tetramethylpyrrole crystallizes from the steam distillate and is collected by filtration, washed with water, and dried over phosphorus pentoxide in a vacuum desiccator. There is obtained 15-18 g. of white plates, m.p. 110-111° (lit., m.p. 112°). 

Since Hausmanrfs and Kipping s initial studies, the synthesis of truxene from 1-indanone has been carried out many times under a wide variety of conditions. Reagents that have been reported to promote this trimerization are generally either strong protic acids or Lewis acids and include hydrochloric acid [15], hydroiodic acid [15], phosphorus pentoxide [15], zinc dust [15], sulfuric acid [17], phosphorus pentachloride [17], zinc chloride in acetic acid [18],... 

Lactoncs arc conveniently prepared by lactonization of a )S-hydroxy acid with acetic acid or phosphorus pentoxide. In the presence of benzcncsulfonyl chloride, cyclization of acid 3 to lactone 4 occurs under basic conditions using pyridine. ... 

Conventional condensation of 1,2-diaminobenzene 16 with 6-fluoro-3,4-dihydro-2H-chroman-2-carboxylic acid 17 under Phillips conditions or using Eaton s reagent (1 10 mixture of phosphorus pentoxide/methanesulfonic acid) yielded 2-(6-fluorochroman-2-yl)-lff-benzimidazole 18 (Scheme 4) [30]. However, irradiating the reaction mixture containing polyphosphoric acid as a catalyst with microwaves afforded the compoimd 18 in comparable yields in a matter of three minutes [30]. 

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