Pyridine 1-oxide, 2-methyl-, reaction with Grignard

The addition of Grignard reagents to aldehydes, ketones, and esters is the basis for the synthesis of a wide variety of alcohols, and several examples are given in Scheme 7.3. Primary alcohols can be made from formaldehyde (Entry 1) or, with addition of two carbons, from ethylene oxide (Entry 2). Secondary alcohols are obtained from aldehydes (Entries 3 to 6) or formate esters (Entry 7). Tertiary alcohols can be made from esters (Entries 8 and 9) or ketones (Entry 10). Lactones give diols (Entry 11). Aldehydes can be prepared from trialkyl orthoformate esters (Entries 12 and 13). Ketones can be made from nitriles (Entries 14 and 15), pyridine-2-thiol esters (Entry 16), N-methoxy-A-methyl carboxamides (Entries 17 and 18), or anhydrides (Entry 19). Carboxylic acids are available by reaction with C02 (Entries 20 to 22). Amines can be prepared from imines (Entry 23). Two-step procedures that involve formation and dehydration of alcohols provide routes to certain alkenes (Entries 24 and 25). 

An acceptable method is to oxidize methyl benzyl carbinol (l-phenyl-2-propanol) to phenylacetone (methyl benzyl ketone) with chrome oxide (CrO3) in pyridine solvent. The problem with this is that methyl benzyl carbinol is not commercially available, and so must be made from benzyl chloride grignard reagent and acetaldehyde. This grignard works well, although there can be a problem getting unreacted benzyl chloride out of the product. Their boiling points are very close, so distillation does not separate them completely. But the real question is Why make the synthesis of phenylacetone a two-step process when it can be done with one reaction ... 

Apart from the technical route described to p-apo-8 -carotenal, readily available vitamin A alcohol (Cjo) has served as an intermediate in the form of the phosphonium salt by reaction with the monodiethyl acetal of a Cio dial (ref. 54). The required Cjo monodiethylacetal was obtained (ref.5, p409) by the reaction of the mono aldehyde-protected derivative, the enol ether of methylmalonaldehyde, (C4) with the acetylenic Grignard reagent from trans 3-methyl-2-penten-4-yn-l-ol (C ) followed by acidic dehydration and partial reduction with Lindlar catalyst to give firstly 8-hydroxy-2,6-dimethylocta-2, 4,6-triene-l-al (Cio). Protection of the hydroxyl group by acetylation in pyridine solution with acetyl chloride and formation of the diethyl acetal with ethyl orthoformate followed by hydrolysis of the acetyl group and oxidation afforded the final CIO aldehyde component (D)shown in Scheme 15a. 

Reactions of pyridine 1-oxide with Grignard reagents have been little studied. The reported cases give very poor yields of 2-alkylpyridines. Only one example of a Grignard reaction with a pyridone seems to have been recorded with benzyl magnesium chloride, l-methyl-2-pyridone gave 2-benzal-1 -methyl-1,2-dihydropyridine5 . 

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