Pyrylium salts cycloaddition

The vinylogous 3,5-hexadien-2-one (16) adds in a 1,4 cycloaddition with zl -dehydroquinolizidine (17) to form compound 18 (26). A similar 1,4-cycloaddition reaction takes place between pyrylium salts and the pyrrolidine or morpholine enamines of cycloalkanones (26a). 

In this section, some recent uses of pyrans in synthesis are described. Much of the chemistry of this class of compounds involves Diels-Alder cycloadditions, metal- and thermal-assisted reorganization reactions, and the preparation and reactions of organometallic derivatives. The chemistry of the pyrylium salts, useful precursors to substituted aromatics (B-64MI11600), will not be discussed here. 

Cycloaddition reactions of pyrylium salts with alkenes continue to be explored in the context of natural products synthesis, as shown in Schemes 12-14. In the example shown in Scheme 12, a pyrylium-ylide [5+2] cycloaddition was performed (128—> 129), allowing access to intermediates reminiscent of the core framework of the diterpene antibiotic, guanacastepene <2001TL4947>. 

Benzo[c]pyrylium salts (e.g., 135) also undergo cycloadditions with azomethines via the intermediacy of 136 and 137 to give dihydroisoquinolinium salts such as 138 (Scheme 15) <1997MC204>. 

Schmidt has demonstrated that selected /J-acyl vinyl carbocations, generated from /J-chlorovinyl ketones226 or from the in situ acylation of acetylenes, react with acetylenes to provide pyrylium salts presumably with 4ir participation of the cationic oxabutadienes in [4+ + 2] cycloadditions [Eq. (55)].226... 

IS+2JCycloaddition. A new route to complex seven-membered rings involves activation of an unsaturated pyronc such as 1 with CH3OTf (2 equiv., 20°) to form a pyrylium salt (a), which undergoes [5+2]cycloaddition at 25° when exposed to CsF to give 2 in 84% yield. 

Electron-transfer-mediated cycloadditions of 2-(l-cyanovinyl)indoles can be carried out with cyclohexadienes and electron-rich styrenes the reactions are catalysed by photoexcited pyrylium salts or triarylaminium radical cations (Equation (91)) <93AG(E)90i>. 

In order to avoid the requirement of high temperatures, Wender et al. have developed an alternative method and milder activation process to perform this type of cycloaddition, even at room temperature. This involved an initial 0-4 alkylation of the P-hydroxy-y-pyranone, to produce a highly reactive 4-aLkoxypyrylium salt, followed by 0-3 desilylation to give the desired oxidopyrylium intermediate. As shown in Scheme 20.34, alkylation of 78 with methyl triflate at 20 °C generated a pyrylium salt. Upon exposure of this salt to anhydrous cesium fluoride, the cycloaddition proceeded smoothly at room temperature to give the... 

Other Acid-catalyzed Cycloadditions. Isobutene has been used to form pyrylium salts (eqs 30 and 31), oxetanes by Boron Trifluoride-catalyzed [2 + 2] cycloaddition (eq 32), and 3,4-dihydropyrans by Diels-Alder reactions (eq 33). ... 

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). 

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