By Gary M. Coppola
Cognizance man made chemist! Chiral alpha-hydroxy acids to be had from nature's chiral pool function beginning fabrics in a large choice of enantioselective conversions resulting in commercially very important items. This monograph, a stimulating resource of principles and a necessary reference paintings for examine chemists, specializes in the well known lactic, mandelic, malic, and tartaric acids. Well-chosen examples exhibit how chiral facilities inherent in those basic compounds can be utilized to manage the advent of additional stereogenic facilities. Readers can without delay follow new variations of their personal paintings for the reason that response stipulations are given in convenient tables.
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Extra info for Alpha-Hydroxy Acids in Enantioselective Syntheses
IBu Ar AI""O=S=O'"AIEt 3 oui on3 Alkyl Grignard reagents as well as organolithium reagents add to lib with equal facility. The utility of this process is exemplified in the total synthesis of protomycinolide IV (335), a biogenetic precursor of macrolide antibiotics of the mycinamicin family . The framework of the macrolide can be assembled from the two segments 342 and 348, each of which is derived from ethyl L-lactate via lactamide lib (Schemes 46 and 47). The salient feature of the synthesis is the organoaluminum-promoted stereospecific pinacoltype 1,2-rearrangement common to both segments.
A [3,3] rearrangement of trifluoroacetimidate 293 produces allylic amine 294. Elaboration of the olefin to an ester furnishes the D-valine derivative 296 with 85% ee . A thiazole heterocycle is an effective synthetic equivalent of an aldehyde. Thus, it has been used in a concise synthesis of protected L-( — )-rhodinose (302), a subunit of the antibiotic streptolydigin (Scheme 41). Addition of 2-lithiothiazole to the ester group of 282 affords acylthiazole derivative 297. Reduction of 297 to 298 with L-Selectride proceeds with high syn selectivity (93% ds) according to the non-chelation Felkin—Anh model for diastereoselection.
3 O-(Trifluoromethanesulfonyl) L-Lactic Acid Derivatives OS02CF3 (S)-2-[[(Trifluoromethyl)sulfonyl]oxy]propanoic Acid The enhanced leaving ability of the triflate group allows SN2 substitution reactions to occur at lower temperatures and over shorter periods of time relative to other leaving groups. When 26 1 Lactic Acid O O J^COOB ChV ^^ ^— 44% O A A ChV >^ OTHP + ° 'CH3 184 185 OTHP 187 188 conditions: (a) 147, NaOCH3 , 2 weeks; (b) NaH, THF/ HMPA; (c) n-BuLi; (d) KOH, H2O; (e) 2N HCI, 0-5°C considering inversion reactions of lactates this seems highly desirable due to the tendency toward racemization over prolonged reaction times.
Alpha-Hydroxy Acids in Enantioselective Syntheses by Gary M. Coppola