Benzopyrylium salt

Benzopyrylium Salts. - Dichloromethoxybutane has been used in the cycliz-ation of the benzyl phenyl ketone (134) to give (after addition of perchloric acid) the pyrylium salt (135). The latter reacted normally with methylamine to form the isoquinolinium salt (136). A complex series of transformations has been suggested to account for the formation of the pyrylium salt (138) by the reaction of the chromone (137) with malononitrile-acetic anhydride.  

The azide ion added on to 2-benzopyrylium salts to give the azidopyran, e.g. (139) on thermolysis, these gave a mixture of products in various proportions, depending on the substituents in the salt.  

Chromanones. - One of the recently discovered minor constituents of hashish is a 5-hydroxychromanone called cannabichromanone (140), which was first identified by g.l.c.-m.s. The suggested structure has been confirmed by a synthesis of the compound from 2,2-dimethyl-5-hydroxy-7-pentylchromanone and methyl vinyl ether, a process which also produced varying amounts of the spiran (141), depending on the conditions. The chromanone (140) has no effect on the nervous system. 

The acyl azide (142 R= CON3) has been photolysed in the presence of several aromatic compounds, e.g. benzene, anisole, or aniline. The nitrenes that were produced were in the triplet state, and yielded 3-substituted dihydronaphtho[2,l-Z)]pyrans (142 R=CONHPh, C0NHCeH40Me, or CONHNHPh respectively). Coumarins behaved similarly. Spectral examination has shown that the equilibrium between the butanedione and the cyclic hemiacetal structure (143 R = H, Me, or MeO) lies entirely on the latter s 

Chromanones are known to condense at C-3 with aldehydes under the influence of a variety of condensing agents. In the presence of orthophos-phoric acid, the 3-benzylidene derivative was obtained, but condensation in piperidine gave the benzylchromone. The chromanone (144) has been cyclized under Dieckmann conditions to give (after further treatment) the 6-oxatetracycline derivative (145 = CSPh, =H). A further three steps 

---

Benzopyrylium perchlorate, 3-ethyI-reduction, 3, 662 Benzopyrylium salts C NMR, 3, 590 chromene synthesis from, 3, 756 chromone synthesis from, 3, 829 electrophilic substitution, 2, 49 mass spectra, 3, 618 reactions... 

When 2-phenyl-l-benzopyrylium salts are treated with sodium azide, 2-azido-l-benzopyrans... 

Whereas benzopyrylium salts are resistant to electrophilic substitution, even in the benzene moiety, the benzopyrones [coumarins (81), isocoumar-ins (82), and chromones (83)] are readily halogenated in either ring, with... 

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>. 

Bipyrans can be prepared by quantitative cathodic reductions of the corresponding pyrylium and benzopyrylium salts. According to the nature of the substituents, dimerizations obtained from pyrylium salts take place at the C-2 or C-4 positions (Scheme 156) [274]. 

Benzopyrylium Salts, Coumarins, Chromones, Flavonoids and Related Compounds... 

There are numerous examples of benzopyrylium salts, benzopyransand benzopyranones, and frequently they have trivial names that reflect their long history (see Box 5.1). Many are natural products, and frequently these compounds contain hydroxy or alkoxy groups (sometimes in the form of a sugar residue). Polyhydroxylated natural products based upon 2-phenylbenzopyrylium (flavylium) salts and with ether linkages to sugars are called anthocyanins, whereas without their sugars they are known as anthocyanidins. 

Electrophilic aromatic substitution of other benzo-fused v-deficient systems generally follows predictable pathways. Thus, benzopyrylium salts are in general resistant to electrophilic substitution even in the benzo-fused ring. Chromones behave somewhat similarly, although substitution can be effected under forcing conditions. Coumarins, on the other hand, undergo nitration readily in the 6-position while bromination results in substitution at the 3-position as a consequence of addition-elimination. 

Another type of [5 +1] reaction leading to the synthesis of an (in effect) quinolizin-4-one (198, Scheme 105) involves the concurrent replacement of the oxygen atom of a 2-benzopyrylium salt (197) by nitrogen (from ammonia), and cyclization (77CHE1183). 

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). 

Condensed heteroaromatic cations are reactive in [2 + 4] cycloaddition reactions with inverse electron demand. For instance, 2-benzopyrylium salts (389) react with vinyl ethyl ether to afford... 

Homophthalaldehyde (188) gives isoquinoline, isoquinoline 2-oxide, 3,4-benzopyrylium salts, and 2-alkyl- and 2-aryl-isoquinolinium salts (189) by reaction with NH3, NH2OH, H+ or RNH2, respectively. 

Nucleophilic addition to benzopyrylium salts is another route to chromenes, but the attack may occur at the positions 2 or 4, thus giving rise to chrom-3-enes (30) or chrom-2-enes (31). Thus NaBH4 reduces simple flavylium salts to flav-2-ene121 (which, in its turn, behaves as a nucleophile and gives rise to dimeric products110) whereas 3-substituted... 

Hydrogenation to chromanes, oxidative degradation, and oxidation to pyrylium salts in acid medium have been known for a long time.3 A series of substituted benzopyrylium salts has been recently prepared by oxidation of chromenes with tritylium perchlorate.97 Oxidation of 4-phenylchromenes to coumarins with Cr03-pyridine complex or with Se02 in dioxan has been reported.186... 

Benzopyrylium salts are reduced by sodium borohydride to the 4/T-chromene but LAH yields the 2//-isomer. The reduction products of flavylium salts vary with the conditions used even the method of isolating the product has to be carefully controlled since, for example, 4/f-flavenes may be isomerized by contact with acid to 2//-flavenes. Flavylium... 

Acid-catalyzed addition of water and alcohols to 4/f-chromenes gives the expected products as predicted by Markovnikov s rule (56JCS4785) an anti-Markovnikov addition of methanol followed by the reintroduction of a double bond in the alternative position gives an overall effect of substitution of hydrogen by methoxy and this is effected by treating methyl 2if-chromene-3-carboxylate (166) with triphenylmethyl perchlorate and addition of methanol to the resulting benzopyrylium salt (167) (72CR(C)(274)650). 

---