Polyphosphoric acid Phosphorus pentoxide

Cyclodehydration 1,5-Diazabicyclo[4.3.0]nonene-5. 2,3-Dichloro-5,6-dicyano-l, 4-benzo-quinone. l,l-Dichloro-2,2-difluororoethyIene. Dichloroketene. Hydrobromic acid. Phos-phoryl chloride-Phosphoric acid-Phosphorus pentoxide. Polyphosphoric acid. Thionyl chloride. 

Dilution of polyphosphoric acids with water in absence of cooling may lead to a large exotherm. Thus, tetraphosphoric acid diluted from 84 to 54% phosphorus pentoxide content rapidly attains a temperature of 120-140°C. 

A powerful and efficient method for the preparation of poly(ketone)s is the direct polycondensation of dicarboxylic acids with aromatic compounds or of aromatic carboxylic acids using phosphorus pentoxide/methanesulfonic acid (PPMA)16 or polyphosphoric acid (PPA)17 as the condensing agent and solvent. By applying both of these reagents to the synthesis of hexafluoroisopropylidene-unit-containing aromatic poly(ketone)s, various types of poly(ketone)s such as poly(ether ketone) (11), poly(ketone) (12), poly(sulfide ketone) (13), and poly-... 

The cyclization of arylaminomethylenemalonates (701) on the action of phosphorus pentoxide in nitrobenzene gave quinoline-3-carboxylates (702) in 16-85% yields (74JMC137). Polyphosphoric acid proved to be a more effective cyclization agent than the phosphorus pentoxide-nitrobenzene system. For example, a chloro derivative (701, R = Et, R1 = Cl, R2 = R3 = R4 = H) gave quinoline-3-carboxylate (702, R = Et, R1 = Cl, R2 = R3 = R4 = H) in 46% yield on the action of polyphosphoric acid, whereas the yield was only 16% in the phosphorus pentoxide-nitrobenzene system. 

Af-Ethyl-A-(3-halo-2-methylphenyl)aminomethylenemalonates (106, R = Et, R1 = Me, R2 = Hlg, R3 = H) were heated in polyphosphoric acid, prepared from phosphoric acid and phosphorus pentoxide, at 140°C for 40 min. The reaction mixture was then poured into water, and the product was hydrolyzed with 10% aqueous sodium hydroxide to give quinoline-3-carboxylic acids (696, R = Et, R1 = Me,R2 = Hlg) in 68-70% yields (80GEP3007006). 

Aqui et al. investigated the cyclization of diethyl A-(3-substituted phe-nyl)aminomethylenemalonates (251) under different cyclization conditions (75JHC557). For cyclization, they applied a 4 7 mixture of concentrated sulfuric acid and acetic anhydride, polyphosphoric acid, polyphosphate (prepared from phosphorus pentoxide and diethyl ether in chloroform), phosphoryl chloride, and Dowtherm A (see Table VI). They found the most effective cyclization conditions were thermal cyclization (heating in Dowtherm A) and polyphosphate. 

Isopropylidene A[-ethyl-/V-(4-pyrimidinyl)aminomethylenemalonates (1199) were cyclized by heating in a mixture of polyphosphoric acid and phosphorus pentoxide at 100°C for 1 hr to give pyrido[2,3-t/Jpyrimidine-6-carboxylic acids (1200) in 50-65% yields [83JAP(K)23692]. Higher yields (—86%) were achieved when the cyclizations were carried out in polyphosphate [83JAP(K)72583]. 

This synthesis of benzo[6]thiophenes is probably the most widely exploited single method for such syntheses since its introduction about 1950 by a variety of workers. It is applicable to a great number of highly substituted benzo[6]thiophenes, and intermediates are generally readily available. In 1949 Werner (49RTC509) reported the synthesis of several 3-alkyl- and 2,3-dialkyl-benzo[( ]thiophenes by cyclodehydration of arylthioacetones (99) with phosphorus pentoxide or anhydrous zinc chloride. Arylthioacetaldehydes (99 R = H, R1=H, R2 = Me, Et or Ph) have been cyclized in 65-80% yields with phosphorus pentoxide. In 1950 Tilak (50PIA(A)(32)390) introduced the polyphosphoric acid-promoted cyclization of... 

The condensing agent used in earlier studies was phosphorus pentoxide and it was believed that this led predominantly to chromones. Sulfuric acid, on the other hand, was considered to effect cyclization to the coumarin. Later workers have used polyphosphoric acid or phosphoryl chloride, though in general this change only brought about improvements in the yield. 

Polyphosphoric acid can be prepared by the addition of 200 g of phosphorus pentoxide (P2O5) to 100 mL of an 85% solution of phosphoric acid and heating to 170°C with vigorous stirring until all of the P2O5 is dissolved (ca. 6 hr). 

Only resinous products and methyl 6-acetamidopyridine-3-carboxylate were obtained when methyl 6-aminopyridine-3-carboxylate was reacted with ethyl 2-acetoxyacetoacetate by heating in phosphorus pentoxide, methanesulfonic acid, or polyphosphoric acid in the absence or presence of a solvent such as toluene, xylene, or methylene chloride. However the desired methyl 3-acetoxy-2-methyl-4-oxo-4//-pyrido[ 1,2-a]pyrimidine-7-carboxylate 93 was obtained when the above components were reacted in N, A-dimethylacetamide in the presence of polyphosphoric acid at 100°C for 48 hours (84FES837). 

Badger and Sasse have described the preparation of 2-, 3-, and 8-bromophenanthridine by the cyclization of the appropriate bromo-formamidobiphenyl with polyphosphoric acid.25 In the case of 2-bromo-2 -formamidobiphenyl a higher concentration of phosphorus pentoxide in the acid was necessary to effect ring closure, and a simple steric effect was invoked.26 Nevertheless, the Morgan-Walls reaction has been used to obtain several overcrowded compounds in which unfavorable steric factors operate. Following the original report of the preparation of the 1,10-dimethylphenanthridine (5a) by this procedure,27 several other examples have been described, notably the synthesis of the related phenanthridine (5b),28 the l,2-(6)29 and 9,10-benzophenanthridines (7),30 and the 1,2 9,10-dibenzophenanthridine (8).29... 

When amides are dehydrated using phosphorus pentoxide as a reagent (Figure 7.2) the solidification of the reaction mixture caused by the resulting polyphosphoric acid is often... 

The use of polyphosphoric acid as a reagent for the synthesis of 2-substituted imidazo[4,5-h]pyridines (311) has been frequently reported <89JHC289,90JHC1825). For example, condensing p-aminosalicylic acid with an equimolar amount of 2,3-diaminopyridine (310) in polyphosphoric acid (85% phosphorus pentoxide) for 3 h at 200°C gave good yields of the derivative (311) (Equation (25)). 

The ring synthesis of many heteroaromatic compounds frequently involves cyclization of a suitable precursor with loss of either water or a simple alcohol such as methanol or ethanol. This may be brought about by treatment with polyphosphoric acid (PPA), which is prepared by dissolving phosphorus pentoxide in concentrated phosphoric acid. In the example of Protocol 14 this approach is used to prepare 3-phenylbenzofuran 55 from phenoxymethyl phenyl ketone 54. The temperature must be carefully controlled, however, since as shown in Scheme 14, an acid-catalysed rearrangment by way of 56 gives the isomeric 2-phenylbenzofuran 57 at higher temperature. This useful feature allows the convenient preparation of either 55 or 57 uncontaminated by the other isomer. 

One of the advantages of using the acetic anhydride system is that the reaction can be perfoimed at room temperature, which is in contrast to a report that trifluoroacetic anhydride is an unsatisfactory activator at this temperature.Attempts have been made to use acylating activators such as benzoic anhydride, phosphorus pentoxide or polyphosphoric acid, although these tend to give lower yields of carbonyl compounds. ... 

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