By Gary M. Coppola
Recognition artificial chemist! Chiral alpha-hydroxy acids on hand from nature's chiral pool function beginning fabrics in a large choice of enantioselective conversions resulting in commercially vital items. This monograph, a stimulating resource of rules and an important reference paintings for study chemists, specializes in the well known lactic, mandelic, malic, and tartaric acids. Well-chosen examples convey how chiral facilities inherent in those easy compounds can be utilized to regulate the advent of additional stereogenic facilities. Readers can without delay follow new changes of their personal paintings in view that response stipulations are given in convenient tables.
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Extra info for Alpha-Hydroxy Acids in Enantioselective Syntheses
OBOM LiAIH4 OBOM OBOM Ph3P / Br2 Et3N CH3 OH 282 283 Br 284 Construction of the C-7 to C-13 subunit of erythronolide A and B aglycones proceeds through the highly functionalized tetrahydropyran 289. The genesis of the stereochemistry associated with 289 is lactic acid. The anti-a\\y\ic alcohol 285 is formed from 282 in a single reaction using a mixture of 2-propenylmagnesium bromide and lithium borohydride . Under the reaction conditions the lithium borohydride does not compete with the Grignard reagent for the ester, but after addition of the Grignard reagent to the ester the intermediate ketone is reduced preferentially by lithium borohydride rather than reacting further with 40 1 Lactic Acid excess Grignard reagent.
Treatment of 123 with thionyl chloride furnishes acid chloride 227 in high yield. By the route outlined in Scheme 33, a variety of 6-deoxy-L-hexoses can be prepared from common intermediate 230 . ), where the desired a-anomer is separated by column chromatography and recrystallization. Reduction of the olefin and ketone gives optically pure methyl a-L-amicetoside (231). 123 227 228 OAc d, e CH30 OCH3 229 230 231 Scheme 33 conditions: (a) SOCI2; (b) propionaldehyde dimethyl acetal, EtMgBr, CuCI, THF; (c) H2 , Pd / BaSO4 , quinoline, EtOAc; (d) 1N NaOH, dioxane; (e) H3PO4 , CCI4; (f) H2 , 10% Pd / C, MeOH, 1 atm; (g) LiAIH4 , ether Compound 230 can also be converted to the monosaccharides methyl a-L-mycaminoside (232) in three steps and methyl a-L-oleandroside (233) in four steps.
Lactates 377 or 391 are converted to the lithio derivative 389 by alkaline hydrolysis with lithium hydroxide. The carboxylate is then activated with DPPA and reacted with the sodium salt of ethyl isocyanoacetate to give oxazole 390 (Scheme 53). The oxazole ring is readily cleaved with 10% HCI, which also removes the MOM group, and quantitatively gives 3-amino-5-methyltetronic acid. 4 O-Protected Lactic Acid Derivatives CH3 CH3 CH3 •s. /~~° W HO- •/ HO-"/ Vo H2r/ 387 386 We HO"-/ WOH H2N I CH3 H2N 53 388 OMOM OMOM OMOM a CH3"-/ b, c 3 100% COOEt ) 70% 390 389 377 a, b, c OH 70% OMOM d 91% 391 Scheme 53 conditions: (a) LiOH, THF, H2O; (b) DPPA, DMF; (c) CNCH2COOEt, NaH; (d) CICH2OCH3 , /-Pr2NEt, CH2CI2 OMOM CH 3 -"/ COOEt DIBAL 2.
Alpha-Hydroxy Acids in Enantioselective Syntheses by Gary M. Coppola