Pyrans from pyrylium salts

The facile nucleophilic displacement of a 4-methoxy group from pyrylium salts provides syntheses of 4-substituted pyrans. In the presence of triethylamine, 4-methoxypyrylium salts react with aromatic nitro compounds to give 4-benzylidene-4//-pyrans (73JOC2834). 

Like the 2-amino-2//-pyrans, analogous 2-azido-2//-pyrans have been observed to be unstable intermediates in the known synthesis of oxa-zine derivatives from pyrylium salts and NaN3 see Section V.G.3. Only some sterically hindered pyrylium cations can generate the intermediates, and attempts have been made to rationalize these observations by quantum-chemical approaches (84T3539 84T3549). Exceptionally, 4-azido-4//-pyran 124 was isolated after the reaction of 4-methyl-2,3,5,6-tetraphenylpyrylium perchlorate with NaN3 (84T3539). 

Finally, a novel synthetic route involves formation of the pyridine ring from a fused pyran intermediate, e.g. (139) - (140) (70CB1250, 80JOC1918, 73JCS(P1)823). If a pyrylium salt is used, a quaternary pyridopyrimidinium salt such as (141) is formed 77KGS14S4). 

K-pyran synthesis from, 3, 761-762 pyridazine synthesis from, 3, 54 pyrylium salt synthesis from, 3, 872 reduced, 2, 45... 

As in the synthesis of other bipyridines, several routes to 4,4 -bipyridine have been devised where one of the pyridine rings is built up from simpler components. For example, a dimer of acrolein reacts with ammonia and methanol in the presence of boron phosphate catalyst at 350°C to give a mixture of products including 4,4 -bipyridine (3.4% yield), and in a reaction akin to ones referred to with other bipyridines, 4-vinylpyridine reacts with substituted oxazoles in the presence of acid to give substituted 4,4 -bipyridines. ° ° Condensation of isonicotinaldehyde with acetaldehyde and ammonia at high temperatures in the presence of a catalyst also affords some 4,4 -bipyridine, and related processes give similar results,whereas pyran derivatives can be converted to 4,4 -bipyridine (56% conversion), for example, by reaction with ammonia and air at 350°C with a nickel-alumina catalyst. Likewise, 2,6-diphenyl-4-(4-pyridyl)pyrylium salts afford 2,6-... 

The addition of ammonia or primary and secondary amines to pyrylium salts is usually accompanied by a rapid ring opening without detection of the expected 2//-pyran intermediates. The exceptional behavior of the salt 160a was observed in its reaction with Af./V-dimethylaniline, affording 85% of 4H-pyran 210, whereas analogous addition products from toluene and methoxy-benzene were too unstable to be isolated.214... 

Analogously, 2//-pyrans 212a were prepared from sodium azide in acetonitrile and the corresponding pentasubstituted pyrylium salts at -35°C.254... 

In contrast to 4//-pyrans, the 2//-pyrans have been rarely reported to be aromatized to pyrylium salts. 2,6-Dimethyl-4-methoxy-2//-pyran (323) was easily converted to the corresponding perchlorate 373 with perchloric acid.318,319 A similar oxidation was reported for 2,4,6-triphenyl-2//-pyran.217 The formation of pyrylium salts 375 and 377 from 374 and hydrogen chloride261 or from 376 and perchloric acid181 are not oxidations. 

The same approach allows preparation of various pyrylium carboranes from the corresponding 4/f-pyran carboranes 174a,b and 175b by the action of acetyl perchlorate,245 perchloric acid,244 and triarylamine radical cation salts,244,245 as well as electrochemically.243 The oxidation of condensed 4H-pyran 345 with trityl perchlorate, 2,3,5,6-tetra-substituted 4f/-pyrans 431 and 153 with tropylium tetrafluoroborate or 153 with heterocyclic salt 393 led to useful preparations of pyrylium salts 394,330 395a,359 and 395b,360 respectively. 

Bis-4,4 -pyrylium salts 214 (R = r-Bu, Ph) were obtained from trityl perchlorate with 163b,c221 or perchloric acid with spirocyclic 4//-pyran 18.52... 

Pyrans and thiins are also easily aromatized, e.g. (483) + S2Cl2 — 1-benzothiinium ion. 2H-Thiins are aromatized by hydride acceptors such as triphenylmethyl cations to give thiinium salts, and similar conversions produce pyrylium salts from pyrans. 

A ring transformation similar to the oxidative conversion of the pyrylium salts to the acylfurans is the formation of 5-alkyl-3,4-dichlorofuran-2-carbonyl chlorides from tetra-chloro-2//-pyrans by oxygen at room temperature. A hyperoxide is proposed as an inter-... 

The distribution of charge in the resonance forms (28)-(31) suggests that nucleophiles may attack at C-2, C-4 or C-6 (or at C-2 or C-4 in 1-benzopyrylium cations, and at C-l, C-3 or C-4a in 2-benzopyrylium ions) but they most commonly add at C-2 for example, attack by cyanide ion gives a 2//-pyran (37) which exists partly or wholly as the acyclic isomer (38). Steric and electronic effects in the reactants probably have a role in determining the course of the reaction of trisubstituted pyrylium salts with nucleophiles. A mixture of both 2H- and 4H-pyrans is sometimes produced, for example, from methoxide ion and 2,4,6-triphenylpyrylium perchlorate (39) no acyclic product was detected in this reaction... 

Irradiation of 2,6-dimethylpyran-4-one (357) in ethanol or benzene or in the solid state yields the cage-like molecule (358) from which the pyran-4-one is regenerated by hydrochloric acid (63JA1208). When the photodimerization is conducted in water but in the absence of air, a small amount of 4,5-dimethylfuran-2-aldehyde is also formed (63JA2956). Pyran-4-one is photoisomerized to pyran-2-one (359) the intermediates postulated resemble those suggested in the irradiation of pyrylium salts (Section 2.23.2.1) (79JA7521). 

The reaction of pyrylium salts with Grignard reagents or organolithium compounds leads to the pseudobase by attack at C-2 (59CB2042). A similar reaction occurs with the anions derived from malononitrile and diethyl malonate, although the pyran derivatives were not always isolated (59CB2046). 

The anions derived from various active methylene compounds react with 2,6-disubstituted pyrylium salts to form 4-substituted 4//-pyrans (57AG720, 59CB46). Grignard reagents react to give similar compounds (60AG777). 

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