Benzopyrylium derivatives

Azonia substitution at a naphthalene bridgehead position gives the quinolizinium ion (16). Oxonia substitution, elsewhere, forms the 1- and 2-benzopyrylium ions (17) and (18). The two most well-known monoaza systems with three aromatie fused rings are aeridine (19), derived structurally from anthraeene, and phenanthridine (20), an azaphenanthrene. The better-known diaza systems inelude phenazine (21) and 1,10-phenanthroline (22), while systems with three linearly fused pyridine rings are ealled anthyridines, e.g. the 1,9,10-isomer (23). 

Method B involves the preparation of precursor of 2-alkyl-l-benzo-pyrylium salts, as shown in Scheme ll.50 2-Alkylbenzopyrylium salts have been prepared by condensation of salicylaldehyde with appropriate ketone in acetic acid or by alkylation or reduction of coumarin or chromone derivatives. Reaction of 2-alkylbenzopyrylium salts with salicylaldehyde gives directly a spirodibenzopyran or 2-vinynologue benzopyrylium salt 17 which then can be converted into the spirodibenzopyran by piperidine or pyridine. 

Chromene acetals 39 are accessible from 2-vinyl-substituted phenols via the allylic acetals 38 through oxypalladation of benzyloxypropa- 1,2-diene and a subsequent Ru-catalysed RCM. 2-Substituted chromenes can be derived from the acetals 39 by conversion into the 1-benzopyrylium salts which are then trapped by nucleophiles (Scheme 26) <00TL5979>. In a like manner, 2-aIkoxychromans have been converted into various 2-substituted chromans by sequential treatment with SnCl4 and a silyl enol ether <00TL7203>. 

In the presence of copper(I) salts in acidic media o-ethynyl-benzaldehyde derivatives were found to cycloisomerise to 2-benzopyrylium salts (4.26.), The reaction, although working in the absence of catalyst too, was accelerated by the addition of different metal salts. The reaction was applied in the preparation of azaphilones and related molecules.30... 

In preparing 6-alkoxy and 6,7-dialkoxy derivatives, it is convenient to react esters 12 with an acid anhydride in the presence of a Lewis acid or, preferably, perchloric acid to produce a benzopyrylium salt (14), presumably via 13 (70KGS200) (Scheme 2). Reaction of 14 with ammonia gives then the isoquinolinols 15 in high yields, where X = alkoxy, R1 = H, Me, Pr, /-Pr, and R2 = H or alkyl group (88JMC1363). 

All of the compounds discussed are based on three molecules 2/f-pyran (1), 4//-pyran (2) and the pyrylium cation (3). Names which have been used for the benzologue (4) of 2//-pyran include 2H- 1-benzopyran, benzo-a-pyran, chrom-3-ene and 2//-chromene. A similar situation exists for the corresponding derivative (5) of 4/f-pyran. The unambiguous and simplest name chromene is used in the present work. The benzologue (6) of pyrylium is known both as benzopyrylium and chromylium the former name is preferred here. Higher benzologues are referred to as naphthopyrans, such as 2H-naphtho[ 1,2-6 jpyran (7), but the names xanthene and xanthylium are used for (8) and (9). 

The trivial names flavene (10), isoflavene (11) and flavylium (12) are in widespread use and are retained here. However, these compounds will usually be considered as the phenyl derivatives of 2H-chromene or benzopyrylium rather than in separate sections. 

In this section the formation of benzopyrylium (632), isobenzopyrylium (633) and xanthy-lium (634) salts is covered in addition to the parent system. There is no separate treatment of flavylium salts, which are considered as phenyl derivatives of benzopyryliums. 

The properties of 1,3-oxazinium salts are similar to those of pyrylium salts.4194,219,255-259 1,3-Oxazinium salts are intermediates for the synthesis of derivatives of pyrazole, isoxazole, pyridine, pyrimidine, quinoline, isoquinoline, chromene, benzopyrylium salts, butadiene, etc.4... 

Depending on the initial carbonyl derivatives 38, several types of 2-benzopyrylium perchlorates were synthesized they are shown in Scheme 3. 

This polycyclic 2-benzopyrylium salt was also obtained in good yield from the 2-benzopyrylium salt 56, through intramolecular acylation of the intermediate isochromene derivative 60. The same approach was used later by Elliott and co-workers (84CJC2435) in the synthesis of some benzofc]-phenanthridine analogs. 

The condensation of the 1-methyl group in 2-benzopyrylium salts 92 with carbonyl derivatives such as aromatic aldehydes (70KGS1308) in acetic acid gives rise to 1-styryl-substituted salts 94. The reaction proceeds similarly with orthoesters (74KGS37), isocoumarin derivatives, and di-methylformamide (DMF) (80KGS193) or azomethines (82KGS465). The... 

Oxoniachrysenes 67 are simultaneously 2-benzopyrylium and 1-benzopyrylium salts, but maintain a distinct chemical behavior. 5-Oxoniachrysene salts 67 are interesting mainly as precursors of ben-zo[c]phenantridine derivatives 132, as mentioned in Section II,C,6. However, in contrast to 2-benzopyrylium salts (cf. Section III,C,4,a), compounds 67 react with nitrogen nucleophiles to form stable adducts 131 or products of anomalous ring-opening, namely a-naphthols 66 and esters 70 (77KGS1176). 

Probably, the experimental conditions (temperature and solvent) determine the conversions of 2-benzopyrylium salts 266 into the chrysene derivatives 270 via a-1 dimerization or [4 + 2] cycloaddition (Scheme 14) (89TH2). 

Contrary to a-1 dimers 269, the 4-1 dimers 276 are more susceptible toward regenerating the starting 2-benzopyrylium salts in acidic media. However, under special conditions, one can carry out some transformations of dimers 276 determined by their interfragment interactions. Thus, thermolysis of dimer 281 with catalytic amounts of acetic acid results in the naphthalene derivative 287. This compound, together with another naphthalene derivative 285, is also formed from semiacetal 284 on heating in acetic acid [90KGS(ip3)]. 

The 2-allyl-277-chromene derivative 73 is synthesized from 477-chromen l-one by in situ formation of the 4-silyloxy-benzopyrylium salt 74 followed by reaction with allyltri- -butyltin (Scheme 25) <2001T1005>. 

Stabilization of an organic cation occurs whenever the electron-deficient carbon atom is conjugated with aryl groups or with atoms containing unshared pairs of electrons such as oxygen, nitrogen, or sulfur (7). Examples of cations which are useful in polymerization include triphenylmethyl (I), cycloheptatrienyl (tropylium) (II), and various alkyl (III), aryl (IV), and benzopyrylium (V, VI) derivatives. 

Heterocyclic systems with one heteroatom. Condensed heteroaromatic cations are reactive in [2 + 4] cycloaddition reactions with inverse electron demand. For instance, 2-benzopyrylium salts 403 react with vinyl ethyl ether to afford naphthalene derivatives 405 in good yields via initial adducts 404 <1990KGS315>. Similar transformations (Bradsher reaction) are also known for isoquinolinium salts <1984CC761>. 

Reaction of 2-benzopyrylium perchlorate 430 with hydrazine gives l-aryl-57/-2,3-benzodiazepines 431 (Scheme 223) . 1H-Z,3-Benzodiazepines 433 <1994J(P1)3149, CHEC-III(13.04.9.2)150> and analogous thienodiazepines <1980J(P1)1718> are obtained from electrocyclic reaction of diazo derivatives 432 (Scheme 224). 

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