Benzo pyrylium salts

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. 

The present review will not deal with chrom-2-enes, nor with benzo-pyrylium salts, nor with ring-fused pyrano-heterocycles. The reader... 

The aim of this review is to survey the advances in the chemistry of chrom-3-enes (2//-l-benzopyrans, chrom-/l -enes, a-chromenes) in the last decade. This subject was previously summarized by Wawzonek in 1951. A brief but clear treatment of chrom-3-ene chemistry can be found in Dean s book (1963), where a detailed account is given of all the natural products known at that time. A section of a recent book on the synthesis of natural compounds has been dedicated to chromenes. The present review will not deal with chrom-2-enes, nor with benzo-pyrylium salts, nor with ring-fused pyrano-heterocycles. The reader... 

Chromone-coumarin tautomerism 7, 250 Chromone ring (s. a. Benzo-pyrylium salts)... 

Three different fused systems have been reported in the literature thieno[3, 2 4,5]thieno[2,3-c]pyridine (f/r-thiophenes), thieno[2, 3 4,5]thieno[2,3-c]pyridine (tram- thiophenes) and thicno[3,2-g]thicno [3,2-d pyridine derivatives. So far, the parent thienothienopyridines have only been prepared via the corresponding pyrylium salts (see below) all others are benzo or extended carbocyclic derivatives, for example, thienothienoquinolines. 

Pyrylium cations form pyridines with ammonia and pyridinium salts with primary amines (B-82MI 505-02). For example, 2,4,6-triphenylpyrylium cation (261 Z=0) yields 2,4,6-triphenylpyridine with ammonia, the corresponding 1-methylpyridinium salt with methylamine, and pyridine 1-arylimines with phenylhydrazine. Xanthylium ions (210), where ring opening cannot readily occur, form adducts (262) with ammonia, amines, amides, ureas, sulfonamides and imides. Similar adducts (e.g. 263) are formed by benzo[( ]pyrylium ions. 

The goal of the present review is to demonstrate the wide potential for using newly developed methods to synthesize 2-benzopyrylium salts and to discuss in detail the numerous and unusual transformations of this class of oxygen cations in comparison with monocyclic pyrylium salts, i.e., the study of influence of benzo[c]annelation. 

On the basis data known so far, one can conclude confidently that the chemistry of benzo[c]pyrylium salts is very promising, not only because of its practical outcome leading to isoquinolines, but mainly because of a multiplicity of unexpected transformations. 

The same authors used this approach to synthesize pyrylium salts condensed at the c-site with different 7r-excess heterocycles (72MU). However, these systems with heterocyclic rings replacing the benzo ring are outside the scope of this review and will not be discussed. 

The direct interaction of benzyl carbonyl compounds 38 and 48 with aromatic aldehydes in the presence of PPA or AC2O + HC104(86KGS125 87TH1), or with acetals or acylals of aliphatic or aromatic aldehydes in the presence of triphenylmethyl perchlorate (73KGS881 75ZOR1962), gives rise to benzo[c]pyrylium salts 62 in yields of 15-70 %. 

The further behavior of benzo[c]annelated adducts 100 depends on the structure of substituents in the initial cation 30, nature of the nucleophile, thermodynamic parameters of the final products, and conditions of the experiment. The reaction may be stopped at the step of adduct 100 (b, 1 in Scheme 5) or may be continued with the formation of ring-opened intermediate 101 (b,2). However, the latter step has some specific features, in comparison with monocyclic pyrylium salts, as a consequence of the presence of the annelated benzenoid ring in benzo[c]pyrylium cations. 

The interaction between ammonia and l-R1-3-aryl-substituted ben-zo[c]pyrylium salts 30, as well as 3,4-diaryl-substituted indeno[e l,2-]-benzo[c]pyrylium, and other substituted cations having a similar fragment is more complicated, especially in hydroxyl-containing solvents. In these cases, mixtures in different ratios are obtained that contain isoquinolines 138, 3-hydroxy-3,4-dihydroisoquinolines 137, OR-adducts 109, anhydro-bases 119, diketones 29, a-naphthylamines 140, and a-naphthols 141 (88UP2). 

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