2.3.5.6- Tetrasubstituted pyrylium salts

The regioselectivity in the Grignard reactions of tetrasubstituted pyrylium salts is the consequence of simultaneous effects of substitution patterns in both the substrate and the reagent. Thus the reaction of MeMgl with 2,3,4,6-or 2,3,5,6-tetramethylpyrylium ions provide mixtures of isomeric 2//-pyrans 187 and 188 in the ratio 91 9 or of 2//-pyran 189 and 4i/-pyran 190 in the ratio 48 52, respectively.237... 

Reaction of hydroxylamine with tri- and tetrasubstituted pyrylium salts yields pyridine iV-oxides and/or 2-isoxazo-lines via the intermediacy of keto-ketoximes <1998T9747>. The regiochemistry of 2-isoxazoline formation from unsymmetrical pyryliums, and the Beckman rearrangement of the intermediate keto ketoximes have also been explored C1998T9747, 1999T15011>. 

Tetrasubstituted 4//-pyrans 153 and 431 were found to disproportionate with trifluoroacetic acid359 and with boron trifluoride-hydrogen bromide reagent360 to mixtures of pyrylium salts 395a,b and tetrahydro-pyrans 432359 or 433,360 respectively. 

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