Benzo pyrylium perchlorate

The two reactants and 70% perchloric acid gave dimethylaminocoumarino[4,3-b]benzo pyrylium perchlorate... 

Vajda and Ruff [64ACH(40)225] were the first to determine the pKa of 2-benzopyrylium ions, namely different hydroxy-substituted 1-aryl-benzo[c]pyrylium perchlorates of type 10. 

At the same time, dimerization of one-electron reduction product was indicated by oscillopolarographic studies in acid for l-R-3,4-diphenyl-6,7-dimethoxybenzo[c]pyrylium perchlorates (R = Me, Et, Ph) (76MI1). As was noted in Section III,F, 1, the benzo[c]pyrylium cation 261, unsubstituted in position 1, undergoes l, 1 -dimerization via the postulated intermediate radical, and the bisisochromene 263 thus formed is readily converted into the initial monomeric salt, but not into the corresponding biscation on treatment with chemical electron acceptors (76KGS999). 

Unexpected results were obtained when chromenone-based cyclophanes was treated with DBU to achieve HNO elimination, followed by DDQ oxidation to give benzo[fo]furan-based cyclophanes as shown below <05TL8789>. DDQ oxidative conversion of (Q-p-[2-hydroxyphenylethylene]benzeneethanol into 2-phenylbenzofuran was also reported . In addition, an unusual rearrangement of substituted 2-phenylbenzo[d]pyrrolo[3,2-fo]pyrylium perchlorate to 2-phenylfuro[23-c]isoquinoline was observed <05SL1036>. 

Condensation of benzo[fr]furan-3(2H)-one with 1 in acetic acid in the presence of triethylamine afforded 2-(3-benzo[fr]furyl)dimedone 481, which upon acylation with acid anhydrides in the presence of 70% perchloric acid led to the corresponding tetrahydrobenzo[t>]furo[2,3-cJ-pyrylium perchlorate 482 (94CHE283). Similarly, benzo[fr]thieno[2,3-c]-pyrylium perchlorate 484 (94CHE283) and benzo[t>]seleno[2,3-cJpyrylium salt 485 were prepared (94CHE283). 

Pyrylium salts,especially perchlorates, tetrafluoroborates, and hexachloroantimo-nates(V), are stable but reactive compounds. Perchlorates have been used extensively, since pyrylium perchlorates tend to be sparingly soluble, however all perchlorates should be treated with CAUTION perchlorates, particularly dry perchlorates can decompose explosively. No pyrylium salts have been identified in living organisms, though the benzo[ ]pyrylium system plays an important role in the flower pigments (section 9.1.6). 

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 %. 

AH attempts to convert dimer 263 into a dimeric 2-benzopyryIium salt, on treatment with triphenylmethyl or acetyl perchlorate, lead only to the rupture of the newly formed C—C bond and to the regeneration of the initial monomeric salt 261, unlike the behavior of dimers of monocyclic pyrylium cations [73DOK(212)370]. Dimerization may be considered a typical reaction for benzo[c]pyrylium-4-oxides of type 19, which react in dimerizations as 1,3-dipoles by analogy with their behavior in cycloadditions (Section III,E,2). 

The reaction of 4-1 dimerization is similar to the primary step of recycli-zation of 2-benzopyrylium salts in acidic nucleophilic media (cf. Section III,C,4,b,i), but the reactive electrophile is the initial cation in this case, and not a proton. Probably for this reason, the 4-1 dimerization of 2-benzopyrylium perchlorates is not observed in acidic nucleophilic media, in contrast to a-1 dimerization (cf. Section III,F,2,b). At the same time, the scope of 4-1 dimerizations is less restricted in terms of structural requirements for 2-benzopyrylium salts in comparison with a a-1 dimerization. Thus, in the latter case, the presence of a methyl group in position 1 of benzo[c]pyrylium cation is compulsory, whereas for 4-1 dimerizations, the nature of the substituent in this position may be different, leading to a variety of 4-1 dimers and, as a consequence, to a wide variety of their transformations. 

Thiophen Analogues of Isoquinoline.—4,7-Dimethylthieno[2,3-c]pyridine has been synthesized by hydrolysing the acetal function of (252). This compound was obtained by the reaction of 2-(3-lithio-2-thienyl)-l,3-dioxolan with diacetyl followed by oximation and reduction. 6,7-Dimethylthieno[3,2-c]pyridine was obtained in an analogous manner, (2,5-Dimethyl-3-thienyl)acetone gave (253) on acylation. Treatment with perchloric acid gave the pyrylium salt (254), which upon reaction with NH3 gave (255). The same reaction sequence with (3-benzo-... 

Russian workers have worked out another approach to benzo-[bjselenophens starting from selenophen derivatives. Acylation of (514) gave (515), which upon treatment with perchloric acid gave the pyrylium salts (516). Upon treatment with secondary amines, (516) gave amino-substituted benzo[b]selenophens, e.g. (517). From the pyrylium salt (518), only (519) was obtained. On treatment of (518) with aqueous hydroxide, (520) was obtained. Other benzo[b]selenophens hydroxylated in the benzene ring have been prepared similarly. Through the condensation of... 

---