Hydroxycarboxylic acids, protected

Because primary hydroxy groups are somewhat more reactive toward CDI than a carboxy group, the method was modified for chain elongations of co-hydroxycarboxylic acids with protection of the OH-group [41]... 

The data presented in this section illustrate that, with the exception of those accelerating water-reducing admixtures containing calcium chloride, there is an abundance of evidence to support the conclusion that water-reducing admixtures of lignosulfonate chemical form certainly will not accelerate any kind of corrosion with reinforcement and, when used to reduce the water-cement ratio, will form a more permeable and durable protective cover for the reinforcement. In view of the chemical nature of the other types of materials such as the hydroxycarboxylic acids and hydroxylated polymers, it seems most likely that these materials too would have no deleterious effect in this respect. 

This procedure was improved 123 for the synthesis of building units based on amino acids other than Gly, but with nonfunctionalized side chains (Table 6). To suppress the 3-elim-ination and racemization side reactions, triflates of a-hydroxycarboxylic acid esters 124 34 (L = OTf, Scheme 19) were used as substrates for the nucleophilic substitution. In order to prevent polyalkylation, the nucleophilic amine of to-BocNH- or co-tBu02C-alkylamines 33 were temporarily protected with the benzyl group. 115116 This protection also improved the yields and purity of Gly-based building units. In this case commercially available benzyl bromoacetate 34 (L=Br) was used as the substrate. In both cases the nucleophilic sub-... 

Dioxinones Obtained by Resolution or Prepared with a Chiral Auxiliary. 2-Phenyl-4//-1,3-dioxin-4-ones (15) derived from formylacetate or acetoacetate can be readily prepared in enantiopure form by preparative resolution " on cellulose triacetate. These have been used for Michael additions and hydrolysis to long-chain p-hydroxycarboxylic acids, for example the tride-canoic acid (16) from (/ )-(15a). The cuprate adducts formed with the methylphenyldioxinone (i f-flSb) can be hydrogenolyti-cally cleaved directly to p-branched p-hydroxy acids with benzyl protection of the hydroxy functional group see (17) in eq 9. ... 

The regioselective process is suitable for the synthesis of protected (3-amino-a-hydroxycarboxylic acid derivatives (racemic, no chiral ligand added) from the corresponding acrylic acid substrates. On the other hand, 1,4-bis(dihydroquininoxy)-anthraquinone mediates enantioselective formation of A-benzyloxycarbonylphenylserine. ... 

Optically active a-hydroxycarboxylic acids are valuable synthons for synthetic purposes [188-192] and for optical resolutions [193]. Since they are not widely distributed in Nature, several methods of preparation have been developed [2, 161]. However, all these methods cannot compete with the acid catalysed hydrolysis of chiral cyanohydrins [149] (Scheme 6). Since (R)- and (S)-cyanohydrins are readily available by the employment of oxynitrilases, a broad range of chiral a-hydroxycarboxylic acids is accessible. O-Protected or even free cyanohydrins have been used as starting materials and, interestingly, could be hydrolysed in good yield without racemisation upon treatment with concentrated aqueous hydrochloric acid [131,138,149]. 

Optically active 3-amino-2-hydroxycarboxylic acid derivatives are often key components of medicinally important compounds. The synthesis of isopropyl (2i ,35)-3-amino-4-cyclo-hexyl-2-hydroxybutyrate (126) (Scheme 28) takes advantage of a [2 + 2]-cycloaddition reaction of the chiral imines 123, prepared from 63, to assemble the important diastereomeric azetidinone 124 as the crucial precursor for completion of this novel synthesis. Protection of the hydroxy group of 63 as either the TBS ether 119 or the tert-buty ether 120, followed by a DIBAL reduction at —78 °C, produces smoothly one of the aldehydes 121 or 122. Condensation of these aldehydes with either di-p-anisylmethylamine or benzylamine in the presence of anhydrous magnesium sulfate affords the four possible chiral imines 123a—d (Scheme 26). 

A suspension of Pd-C in ethyl acetate vigorously stirred under H2 until uptake of gas ceased, a soln. of /r 5-2-azidocyclohexanol and di- ert-butyl dicarbonate in the same solvent added, and the mixture stirred at room temp, under H2 for 19 h - product. Y 71%. Pre-saturation of the catalyst is important to prevent nitrile formation. F.e. incl. N-protected a-amino-p-hydroxycarboxylic acid esters and 2-acoxyamines, s. S. Saito et al.. Tetrahedron Letters 30, 837-8 (1989). 

Alkoxycarboxylic acids from hydroxycarboxylic acid esters via alkoxy-carboxylic acid esters—a,hydroxyl groups of aminophenols s. 11, 41... 

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