By Gary M. Coppola
Consciousness man made chemist! Chiral alpha-hydroxy acids on hand from nature's chiral pool function beginning fabrics in a wide selection of enantioselective conversions resulting in commercially vital items. This monograph, a stimulating resource of rules and an important reference paintings for learn chemists, makes a speciality of the well known lactic, mandelic, malic, and tartaric acids. Well-chosen examples express how chiral facilities inherent in those easy compounds can be utilized to manage the advent of extra stereogenic facilities. Readers can at once practice new changes of their personal paintings considering response stipulations are given in convenient tables.
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Extra info for Alpha-Hydroxy Acids in Enantioselective Syntheses
1. 1 Acetyl o O CH3 (S)-2-(Acetyloxy)propanoic Acid L-2-Acetoxypropionic acid (123) is readily prepared by the acylation of L-lactic acid with either acetic acid/sulfuric acid in benzene (50% yield)  or with an excess of acetyl chloride (80% yield) . One is not limited exclusively to the use of lactic acid as the starting material. Diazotization of L-alanine in acetic acid furnishes 123 in 47% yield with at least 96% retention of configuration . Esterification of 123 with diazoethane furnishes ethyl (S)-2-acetoxypropionate (222).
Ethyl or methyl lactate is easily protected with this group by treatment with ter£-butyldimethylsilyl chloride in the presence of a suitable base. The use of imidazole in DMF furnishes 401 or 402 in 90-99% yield [126,127,128,130], whereas triethylamine/DMAP in tetrahydrofuran affords 401 in 90% yield . Vinylation of 401 is accomplished by initial saponification with lithium hydroxide to yield the lithium salt 403, followed by addition of one equivalent of vinyllithium to give 404. After OH = CH3^ OTBS = TBS-CI COOR base ChV COOR 401 R = CH3 402 R = Et deprotection, the optical purity of the enone 405 was determined to be >99% ee .
Even protected sugar derivatives displace the triflate group to afford muramic acid analogs . )-lactates (200) and mercaptans give (^)-thiolactates (201) . Magnesiocuprates react with lactate derivative 189 to give the corresponding (*S)-2methylalkanoic acids (202) in moderate yield, however, optical yields are quite impressive, with enantiomeric excesses ranging from 95 to 99% . This is superior to reactions with corresponding tosylates or mesylates where highly competitive reduction or elimination processes predominate.
Alpha-Hydroxy Acids in Enantioselective Syntheses by Gary M. Coppola