Polyphosphoric acid in cyclization

Polyphosphoric acid in cyclization of ethyl a-acetyl-/3-(2,3-dimethoxy-phenyl)propionate to 6,7-dimeth-oxy-3-methylindene-2-carboxyl-ate, 40, 43... 

In an analogous manner, the same authors have synthesized the dihydro tricyclic purine derivative 199 via a polyphosphoric acid-mediated cyclization of 198 (Equation 51) <1995CC2041>. 

The cyclization of the 2-methyithio derivative (964, R = MeS, R1 = H) was attempted unsuccessfully by heating in polyphosphoric acid, in polyphosphate, or in a mixture of phosphoryl chloride and polyphosphoric acid (76CPB130). 

The cyclization of /V-(6-methyl-2-pyridyl)aminomethylenemalonate (1001, R = Me, R1 = H), labeled on one of the carboxyl groups with l4C, gave labeled 1,8-naphthyridine (1003, R = Me, R1 = H) in boiling diphenyl ether and labeled pyrido[l,2-a]pyrimidine (1002, R = Me, R1 = H) in a mixture of phosphoryl chloride and polyphosphoric acid in 77% and 85% yields, respectively (73MI2 75MI2). 

Indolylthio)phenylacetic acid 168 in 50% polyphosphate ester in methylene chloride at room temperature affords 169 as a major product in 65% yield. In hot polyphosphoric acid (PPA) cyclized 169 and 170 were formed in 15 and 21% yields, respectively, due to the partial isomerization. Heating in PPA for a prolonged period gives a 90% yield of 170 as a sole product of... 

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... 

Pyrrolo-thiazepine 236 was obtained by connecting the a-carbon of the pyrrole moiety with the carbonyl group of 237 using phosphorus pentachloride in anhydrous dichloromethane (Equation 20) <2003MOL678>. A similar procedure was followed by Campiani et al. <1997EJM241>. Petrova et al. reported the synthesis of the related imidazothiazepine by polyphosphoric acid-mediated cyclization <2003SC4355>. 

As mentioned above, the very stable dihydro systems of triazolopyrimidines and pyrazolopyrimidines proceed in an unusual direction upon treatment with HN02. Under the conditions of the well-known heteroaromatization method, i.e., the action of NaN02 in the presence of an acid, compounds 381 and 382 are transformed into their corresponding nitroso derivatives 383 and 384 [174, 297, 323]. Heating compound 383 in the presence of polyphosphoric acid causes cyclization involving aromatic rings at positions 2 and 4 with the formation of a pyrrole ring (compounds 385 and 386) [323] (Scheme 3.101). 

Further acid-catalyzed reactions include the use of />-toluene sulfonic acid-DMF in a cyclization of the protected amino acid 17 (DMF = dimethylformamide Scheme 15) <20040L4941>. This was the key step in the stereoselective synthesis of 5-hydroxypipecolic acid. A similar acid-catalyzed ring closure of a hemiacetal yielded the fused piperidine 18 <2004JOC1872> (Equation 32). The indolizidine alkaloid can be accessed by a Barton-Ester method utilizing a polyphosphoric acid (PPA) cyclization (Scheme 16) <1994T19157>. 

Treatment of the 2-pyrrolyl allyl thioether (498) with acetic anhydride and quinoline at 170 °C (or in A jV-dimethylaniline at ca. 100 °C) results in a thio-Claisen rearrangement to give the 5-(3-allyl-2-pyrrolyl) thioacetate (499), whilst peracid oxidation of (498) produces the non-rearranged sulfone in low yield and Raney nickel reduction of (498) yields 3-propylpyrrole (78CJC221). The polyphosphoric acid-catalyzed cyclization of (2-pyrrolylthio) acetic acid (501 R = R = H) somewhat unexpectedly yields (502) via the Spiro intermediate, instead of forming the expected oxothiolane (500), which can be obtained by a Dieckmann cyclization of ethyl (3-ethoxycarbonyl-2-pyrrolylthio) acetate (501 R = Et, R = C02Et) (B-77MI30506). 

In contrast to 1-hydrazinoisoquinolines (229), boiling 3-hydrazinoiso-quinoline (251) with acetic anhydride gave (78JHC463) the triacetyl derivative 252 and not the triazolo[4,3-( ]isoquinoline (254). Nevertheless, 254 was obtained (78JHC463) by the dehydrative cyclization of monoacetyl derivative 253 with polyphosphoric acid. In contrast to the stable 1,2,4-triazolo[3,4-a]isoquinolines (208), l,2,4-triazolo[4,3-I>]isoquinolines (209) are unstable because of their quinonoid structure. 

Cyclization of the malonates (70) and acrylates (73) has been carried out by the action of heat, in DowthermA, " " - - - - in diethylbenzene" " and in trichlorobenzene. Cyclization has also occurred under acidic conditions in polyphosphoric acid, - " in phosphoryl chloride-polyphosphoric acid, " " in sulfuric acid-acetic anhydride and acetic acid-acetic anhydride mixtures, and by the use of a basic reagent such as sodium ethoxide in ethanolic solution. ... 

Badger and Sasse have described the preparation of 2-, 3-, and 8-bromophenanthridine by the cyclization of the appropriate bromo-formamidobiphenyl with polyphosphoric acid. 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. 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, several other examples have been described, notably the synthesis of the related phenanthridine (5b),the l,2-(6) and 9,10-benzophenanthridines (7), ° and the 1,2 9,10-dibenzophenanthridine (8).2 ... 

Cyclization, of o-acetoacetochloroanilide to 3-acetyloxindole, 1 polyphosphoric acid in, 43 Cyclohexanone in preparation of methylene cyclohexane, 66 2-Cyclohexenone, 14... 

Hydrolysis methods for mono[ C]nitriles and the spectrum of chemical transformations of the resulting [l- C]carboxylic acids differ little from those for their unlabeled counterparts, so that the synthesis of [l- C]tetralones 42 and [l- " C]indanones 46 from w-arylalkyl [ C]nitriles will suffice as examples (Figure 7.11). Acid-catalyzed hydrolysis of 4-phenylbutyro[ C]nitrile followed by polyphosphoric acid-mediated cyclization of the resulting phenyl[l- C]butanoic acid converted it into 42 in 89% radiochemical yield. Sequential bromination and oxime formation provided the a-bromooxime 43, which upon treatment with polyphosphoric acid underwent a Beckman rearrangement to provide the ring-expanded tetrahydro-2H-l-[2- C]benzazepine-2-one derivative 44- This was a key intermediate for the preparation of a series of labeled ACE inhibitors" . The acid-catalyzed cyclization of a mixture of 3-(2-hydroxyphenyl[l- " C]propionic acid and its ethyl ester, prepared by treatment of the propio[ " C]nitrile with ethanolic HCI proved to be a two-step sequence. In the first step, 2- [2- " C ]chromanone I4SI was formed and had to be submitted to... 

Pomeranz-Fntsch Synthesis, Isoquinolines aie available fiom the cycUzation of benzalamiaoacetals undei acidic conditions (165). The cyclization is preceded by the formation of the Schiff base (33). Although the yields ate modest, polyphosphoric acid produces product in all cases, and is especially useful for 8-substituted isoquinolines (166). 

An interesting case of ipso intramolecular alkylation has been observed in the case of the acid-promoted cyclization of the amino alcohols (61). With trifluoroacetic acid the major product was the rearranged thienopyridine (62), whereas with polyphosphoric acid the product formed exclusively was the non-rearranged thienopyridine (63) (82CC793). 

The Frasca method for obtaining 1-arylindazoles also involves a C(3)—C(3a) ring closure (67CJC697). It consists in the cyclization of p-nitrophenylhydrazones of ketones and aldehydes with polyphosphoric acid. The Barone computer-assisted synthetic design program has found several new methods for preparing indazoles (79MI40409). The selected method involves the transformation of jV, jV -diphenylhydrazides (596) into 1-phenylindazoles (597) by means of trifluoromethanesulfonic anhydride. The yields vary from 2% (R = H) to 50% (R = Ph). 

In 1909, Robinson demonstrated the utility of acylamidoketones as intermediates to aryl-and benzyl-substituted 1,3-oxazoles through cyclization with sulfuric acid. Extension of sulfuric acid cyclization conditions to alkyl-substituted oxazoles can give low yields, for example 10-15% for 2,5-dimethyl-l,3-oxazole. Wiegand and Rathbum found that polyphosphoric acid can provide alkyl-substituted oxazoles 4 in yields equal to or greater than those obtained with sulfuric acid. Significantly better yields are seen in the preparation of aryl- and heteroaryl-substituted oxazoles. For example, reaction of ketoamides 5 with 98% phosphoric acid in acetic anhydride gives oxazoles 6 in 90-95% yield. ... 

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