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20.2.12.1.1.2.2 Variation 2: Use of Carbon Dioxide

DOI: 10.1055/sos-SD-020-00556

Beignet, J.Science of Synthesis, (200720572.

The addition of carbon dioxide onto the enolate anion of a methyl ketone produces the corresponding intermediate carboxylate that is hydrolyzed to the β-oxo acid 9 (Scheme 4).[‌3‌‌5‌] This method mimics the fixation of carbon dioxide in biological systems, and it is not surprising to find numerous examples of artificial carbon dioxide carriers in the literature. They include thiazolols,[‌13‌] bromomagnesium ureides,[‌14‌] magnesium N,N-dicyclohexylamidinides,[‌15‌] lithium 1,8-diazabicyclo[5.4.0]undec-7-en-6-ide,[‌16‌] and diphenylcarbodiimide with potassium carbonate.[‌17‌] The artificial photosynthesis of β-oxo acids via an aluminum enolate complex carrying a light-absorbing porphyrin ligand is also documented.[‌18‌] However, these methods are only effective with active methylene compounds such as acetophenones;[‌15‌‌18‌] otherwise, they are low yielding and produce a mixture of regioisomeric carboxylic acids when starting from alkan-2-ones.[‌13‌,‌14‌] An efficient chemical synthesis of β-oxo acids 9 is achieved by deprotonation of the desired methyl ketone with a strong base (sodium triphenylmethanide,[‌4‌] sodium amide,[‌5‌] or lithium diisopropylamide[‌3‌]) to generate an enolate which is added to an excess of solid carbon dioxide. The β-oxo acid 9 is obtained in moderate to good yields as a single isomer on protonation of the resultant carboxylate (Scheme 4). This method has been used to prepare the acid 10 in a simple synthesis of (±)-β-trans-bermagotene.[‌3‌] An excess of the base is required since the product contains more acidic protons than the reacting ketone which might explain the moderate yields of the original procedure.[‌4‌,‌5‌] An interesting alternative employs a combination of magnesium halides and triethylamine to form the enolate, but the carboxylation at the terminal methyl group of alkan-2-ones proves unselective.[‌23‌]

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