Pyridinium salts Grignard addition

Direct addition of Grignard reagents to Zincke-derived chiral pyridinium salts such as 99, meanwhile, allowed subsequent reduction to 1,2,3,6-tetrahydropyridines (e.g., 100, Scheme 8.4.32). This strategy provided entry to asymmetric syntheses of (-)-lupetidin and (+)-solenopsin. Tetrahydropyridines prepared by reduction of chiral... 

An asymmetric synthesis of 1,2-dihydropyridines has been achieved by the addition of Grignard reagents to the pyridinium salt generated from 3-(triisopropylstannyl)pyridine and the chloroformate of 8-arylmenthyl based chiral auxiliaries (Scheme 22) (91JOC7167). 

The regioselectivity of Grignard addition to (V-alkoxycarbon y I pyridinium salts can often be controlled by changing the conditions. For example, C(4) addition can be enhanced by the presence of catalytic amounts of copper salts reaction of salt 288 (R = Me) with lithium dimethylcuprate gives also almost exclusively the 1,4-dihydro product 290 (R = Me, R Me). 

Two examples of the preparation of dihydropyridinones from pyridines via Grignard nucleophilic addition to acyl pyridinium salts <07TL8536 07BMC1106> were published in the literature this year. 

Silylation at nitrogen with t-butyldimethylsilyl triflate, generates pyridinium salts which, because of the size of the A -substitutent, react with Grignard reagents exclusively at C-4 montmorillonite-catalysed addition of silyl enol ethers to pyridines has a comparable effect in producing l-trimethylsilyl-l,4-dihydropyridines carrying an acylalkyl substituent at C-4. ... 

Surprisingly, 1-acyl-pyridinium salts, with Grignard reagents and with organocadmiums, undergo addition at the 2-(major product) and 6-positions rather than at the 1-acyl carbon centre. Formation of acyl-pyridinium is very... 

A review of the literature prior to 1953 on reactions of pyridinium, quinolinium, and isoquinolinium salts is available (31). The reactions will be described here only briefly. The initial observation was that of Freund et al. (32-37), who found that treatment of various derivatives of hydra-stinine (15) with Grignard reagents yielded addition products, such as 16. 

The addition (150-157) of Grignard reagents, alkoxides, hydroxide, sulfides, cyanide, and enolate anions to pyridinium and isoquinolinium salts again provides a variety of cyclic enamines of potential synthetic use. 

Reactions of highly electron-rich organometalate salts (organocuprates, orga-noborates, Grignard reagents, etc.) and metal hydrides (trialkyltin hydride, triethylsilane, borohydrides, etc.) with cyano-substituted olefins, enones, ketones, carbocations, pyridinium cations, etc. are conventionally formulated as nucleophilic addition reactions. We illustrate the utility of donor/acceptor association and electron-transfer below. 

Amuilations of pyridinium- and quinolinium-salts however have not been reported so far. From the addition reactions of C-nucIeophiles like Grignard, organozinc and organotin reagents to pyridinium- and quinolinium-salts have been reported to be valuable methods for the construction of substituted dihydropyridine [4a-4d] and dihydroquinoline-derivatives [4d-4e] results of our studies in the field of benzothiopyrylium-salts [5] we concluded that the sequential intermolecular 1,2-addition/intramolecular 1,4-addition of 4-silyloxyquinolinium-salts 1 with 2-silyloxy-l,3-butadienes 2 might be useful for the diastereoselective preparation of annulated quinolones 3. 

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