With pyrylium ions

Cycloaddition reactions in pyrone derivatives A and D and activated pyrones B, C, and E (Scheme 46) with pyrylium-ion character are important in synthetic methodology because the initial bicycloadducts F-J, which can have up to four contiguous stereocenters as well as variously membered rings (3, 4, 5, 6, 7, etc.), can be formed in one procedure. These bicycload-... 

However, secondary amines yield aniline derivatives 50 with pyrylium ions in analogy to the example 43 44 ... 

This is an Sn2 process, since inversion is found at R. Another good leaving group is NTS2 ditosylamines react quite well with acetate ion in dipolar aprotic solvents RNTs2 4- OAc — ROAc. Ordinary primary amines have been converted to acetates and benzoates by the Katritzky pyrylium-pyridinium method (p. 447). Quaternary ammonium salts can be cleaved by heating with AcO in an aprotic solvent. Oxonium ions can also be used as substrates RsO -f R COO —> R COOR R2O. 

Pyrylium ions react with ammonia or primary amines to give pyridinium ions (see p. 447). ... 

This is manifest in the reactivity of 180/180-Z1 which was generated from 3-bromo-41-f-pyran (283) by /3-elimination of hydrogen bromide with KOtBu (Scheme 6.61). Whether or not this reaction was conducted in the presence of styrene or furan, the only product identified was tert-butyl 4H-pyran-4-yl ether (284). This is in line with the relationship of the intermediate to a pyrylium ion. Thus, the addition of the tert-butoxide ion to 180/180-Zj has to be expected at the 4-position with formation of the vinyl anion 285, which is then protonated to give 284. Likewise, the attack of the nucleophile is predicted at C2and C6 leading to the vinyl anions 286, which... 

The amount of a given isomer in reaction mixtures is affected by substitution patterns of the starting pyrylium ions and by the structure of a reagent, in accordance with quantum chemical interpretations using various LCAO-MO methods.201-2028 Almost all additions to 2,6-disubstituted pyrylium ions occur at position 4 to give 4//-pyrans. Unsubstituted or 2,4,6-trisubstituted substrates may be attacked, on the other hand, at a or y positions providing 2H- and/or 4//-pyrans. Limited information on tetrasubstituted and penta-substituted pyrylium ions leads to the conclusion that the former afford AH-pyrans, whereas the latter tend to be attacked by nucleophiles only at positions 2 or 6. 

Two isomeric 2//-pyrans 167 and 169 together with a 4//-pyran 168 may be formed from substituted pyrylium ions 166 and organometallic reagent RM (Eq. 9). 

The regioselectivity of the addition of organometallics to 2,4,6-trisub-stituted pyrylium ions 166 (R2 = R4 = H) is mainly determined by the structure of the entering group R. Thus 2,4,6-trimethylpyrylium perchlorate 177 was found to react with MeMgl, MeLi, or MeNa exclusively to give 2,2,4,6-tetramethyl-2Z/-pyran (176),212,220 237 238,246 whereas i-PrMgX, /-PrLi,... 

However, if a tetrasubstituted pyrylium ion possessed one or two condensed six-membered carbocyclic rings, then only 4//-pyrans 191, 192, 27, and 193 are isolable (35 to 92%) after reaction with PhCH2MgCl236 or with other Grignard reagents RMgX.210... 

Equilibrium aromatizations of 4//-pyran (5), 4//-thiopyran (7), and 4H-selenopyran (8) with appropriate pyrylium ions were explored for the estimation of relative stabilities of the latter.351... 

Disproportionation has been observed frequently with thiopyrans and rarely with 4//-pyrans, and all cases involve a tetragonal carbon center (position 2 or 4) bearing at least one C—H bond. Some molecules of the substrate are aromatized to corresponding thiopyrylium or pyrylium ions and others reduced to dihydro or tetrahydro products. The relative abilities of pyrans and thiopyrans to disproportionate were interpreted within a proposed hydride transfer mechanism by a CNDO/2 method.45... 

Fused benzene rings aid nucleophilic attack on pyridines, pyridinium and pyrylium ions, and pyrones the loss of aromaticity involved in the formation of the initial adduct is less in monobenzo derivatives and still less in linear dibenzo derivatives than in monocyclic compounds. For the same reason, the tendency for this initial adduct to re-aromatize is less for benzopyridines. Fused benzene rings also influence the point of attack by nucleophilic reagents attack rarely occurs on a carbon atom shared with a benzene ring. Thus, in linear dibenzo derivatives, nucleophilic attack is at the y-position (157). 

Pyrylium salts react with hydroxide ions in a complex series of equilibria involving the pseudo-base (207) and ring-opened forms (208) and (209). 

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 aromatic character of pyrylium salts is also suggested by their UV spectra. A comparison can be drawn with the pyridinium ion, since this system is iso-ir-electronic with pyrylium and belongs to the same symmetry point group (C2v) (7lPMH(3)67). Thus, the energies of their respective molecular orbitals will be comparable and this is reflected in the similarity of their UV maxima. 

The presence of a 4-(4-dimethylaminophenyl) group enhances the visible absorption differences between the pyrylium and thiopyrylium salt analogues. Thus, on reaction of 625 with hydrosulfide ion a change in hue from magenta (540 nm) to blue (580 nm) is observed this color change serves as the basis for a new chemodosimeter selective for hydrosulfide (Scheme 240) <2003JA9000>. 

The first step involves the formation of a pyridinium ion by reaction of a pyrylium ion with a primary amine the second step (dequaternization) has been studied more extensively than the first (amine + pyrylium). This important work on dequaternization deserves special mention because, besides the value for synthesis and understanding of steric acceleration, it sheds new light on the mechanism of aliphatic nucleophilic substitution (84CSR47). 

Monocyclic 1,3-oxazepines 419 can be prepared by reaction of aliphatic diazo compounds with 1,3-oxazinium perchlorates 418 (Scheme 194) <1974S187>. Tetra- and penta-phenyl-1,3-oxazepines 421 (R = H or Ph) have been obtained by the reaction of azide ion with pyrylium salts 420 (Scheme 195) <1978H(11)331>. 

An attempt to prepare 2,4,6-triphenylselenopyrylium cation (10) by reaction of the corresponding pyrylium ion 8 with Na2Se was unsuccessful (78AP170). 

The reaction has been successfully carried out also with 2,6-di-tert-butylthiopyrylium (26) and selenopyrylium (27) (87JOC2123), but not with the corresponding pyrylium ion (25), which apparently does not form a phosphonium salt with triphenylphosphine (79JOC4456). 

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