Isobenzopyrylium salts

Isobenzofuranone — see Phthalides, 552 Isobenzopyrylium salts isocoumarin synthesis from, 3, 834 synthesis, 3, 862, 863, 867, 868 Isobenzopyrylium salts, l-amino-3-aryl-synthesis, 3, 867 Isobenzopyrylium salts, 1-aryl-NMR, 3, 592... 

Isobenzopyrylium salts, 1,3-diphenyl-synthesis, 3, 872 Isobutyropyrrothine biological activity, 6, 1024 Isocaffeine, 8-carboxymethyl-synthesis, S, 593 Isocarboxazid... 

A. Miilier and co-workers [Ber, deut. Chem. Ges. 76, 856 (1943) Ber. deut. chem. Ges. 75, 891 (1942) Acta Chim. Acad. Sci. Hung., in press observed similar behavior with some isobenzopyrylium salts. If warmed in aqueous solution they are transformed into isobenzop5Tapols and these change on warming in ethyl acetate into the isomeric diketoues by ring opening. 

Most reactions are carried out with perchlorates or fluoroborates methylene dichloride is a convenient aprotic solvent for salts with the former anion. Among the various anions, the strongest nucleophile yet reported to give ionized salts (in the case of an isobenzopyrylium salt) is the formate anion (cf. also Section II,B,2,c). 

A related reaction is the acylation of yA disuhstituted benzyl ketones to isobenzopyrylium salts the unsubstituted compounds yield 4-pyrones as shown in Section II,D,3,a. Another related acylation converts unsaturated esters into 2-p3U ones, e.g., ethyl, jS-dimethyl-acrylate into 4,6-dimethyl-2-pyrone. ... 

Dorofeenko and his coworkers have extended this route to the synthesis of a range of cyclic analogues of pyrylium salts. Thus, cyclohexenylacetophenone affords the reduced isobenzopyrylium salt (635) on treatment with acetic anhydride (67MI22402) and 2-(indol-2-yl)cyclohexanone yields the indolo[2,3-rf]pyrylium salt (636) in the same way (69ZOB716). The yields in these reactions are much improved, a feature which may be attributable to the conformational preference for the structure shown for the enone rather than a conjugated arrangement of double bonds. 

Acylation of the keto acid (637) leads to the isobenzopyrylium salt (638) (77CHE1183). However, the isobenzopyrylium salt (639), a potential intermediate for the synthesis of analogues of berberine alkaloids, results from the formylation of the substituted ketone or the isochromanone (640) using dichloromethyl butyl ether (Scheme 251) (81CHE221). A second product, the 5-oxoniachrysene (641), is formed and this compound may also be obtained by reaction of the isobenzopyrylium salt with phosphorus pentachloride and then with triethylamine. The intermediacy of a cyclic vinyl ether is proposed. 

Isobenzopyrylium salts have been obtained from o-acylbenzonitriles by treatment with hydrobromic acid. The l-amino-3-arylisobenzopyrylium derivatives are produced in good yield according to Scheme 260 (78JOC3817). 

Homophthaldehyde is converted into the isobenzopyrylium salt by this route (33JCS555). However, the yield of the unsubstituted pyrylium salt obtained from pentanedial is low, but much better from the enolate (53CB1327). Several other reagents are effective in bringing about the cyclization and oxidation of pentane-1,5-diones and the diverse substitution patterns which can be achieved increase the value of this method. 

Dimethylfuran reacted with an isobenzopyrylium salt generated from a 2-aIkynyl benzaldehyde in the presence of 3 mol% of AuClj and HjO to give the interesting h -acetal cage compound depicted below <03AG(E)4399>. 

Acylation of indenylindenones leads to indenopyrylium salts. A related reaction is the acylation of 3,4-disubstituted benzyl ketones to isobenzopyrylium salts " the unsubstituted compounds yield... 

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