Synthetic routes to β-hydroxy β-methylbutyric acid
A cycloaddition reaction between acetone and ketene depending on the conditions produce β-isovalerolactone or 4,4-dimethyloxetan-2-one.[1][2] Hydrolysis of either of these lactones under basic conditions yields HMB. HMB can also be prepared through the exhaustive halogenation of the methyl ketone of diacetone alcohol (the haloform reaction) with sodium hypobromite or sodium hypochlorite.[3][4][5] Diacetone alcohol in turn may be prepared through the aldol condensation of acetone. The carboxylation of tert-butyl alcohol with carbon monoxide and Fenton's reagent (hydrogen peroxide and ferrous iron) has also been used to synthesize HMB.[3][6] Alternatively, HMB can be prepared through microbial oxidation of β-methylbutyric acid by the fungus Galactomyces reessii.[7]
References
↑  (January 1954). "β-Propiolactone. XIV. β-Isovalerolactone". Journal of the American Chemical Society 76 (2): 486–488. DOI:10.1021/ja01631a045.

↑ WO application 2015094925, White TO, "Stable liquid filled hard capsule comprising beta-hydroxy-beta-methylbutyric acid", published 25 June 2015, assigned to Capsugel Belgium Nv

↑ a b  (June 1958). "Syntheses by Free-radical Reactions. V. A New Synthesis of Carboxylic Acids". Journal of the American Chemical Society 80 (11): 2882–2887. DOI:10.1021/ja01544a072.

↑  (September 1903). "Zur Kenntnis des Diacetonalkohols und des Mesityloxyds". Monatshefte für Chemie und verwandte Teile anderer Wissenschaften 24 (9): 765–772. DOI:10.1007/BF01526057.

↑ "Example 5.2" in   (February 2001)  Organic Synthesis Engineering, Category:New York:  Oxford University Press, pp. 102–124  ISBN:  978-0-19-509689-7. 

↑ "Homolytic Mechanism in Metal Catalysis" in   (December 2012)  Organometallic Mechanisms and Catalysis: The Role of Reactive Intermediates in Organic Processes, Category:New York:  Elsevier, p. 67  ISBN:  978-0-323-14410-0. 

↑  (November 1997). "Conversion of beta-methylbutyric acid to beta-hydroxy-beta-methylbutyric acid by Galactomyces reessii". Applied and Environmental Microbiology 63 (11): 4191–4195. PMID 9361403. PMC: 168736.
Own work
Boghog