Protection 2- -2-propyl ester

The resolution was then based on the enzymatic propanolysis of this derivative in dioxane as solvent. Lip Novozyme 435 selectively cleaves the L-form of the oxazolone producing an L-enriched (81-87% ee) 2-acetamido-3-(heteroaryl)propionic acid propyl ester, the dynamic aspect of the process being based on the continual racemization of the residual oxazolone. The propyl group was then removed with alkali and a second selective enzymatic step to remove the acetyl protecting group with Fluka Acylase 1 produced the L-amino acid at better than 99% ee (Scheme 13). 

Protection of carboxyl groups as 2-(l-adamantyl)-2-propyl esters COOR COOH... 

The most common method for the preparation of PCPP is the macromolecular substitution of the polydichlorophosphazene precursor [37] (see Section 1.1). This requires the use of protecting group chemistry, as it is well known that free acid groups lead to skeletal breakdown reactions. Propylparaben, the n-propyl ester of p-hydroxybenzoic acid has been most widely used for this, with subsequent deprotection of the propyl ester by hydrolysis under basic conditions. The requirement for complete macromolecular reactions has been addressed and structurally homogeneous, fully deprotected PCPP can be attained with the correct structure on relatively large (2 kg) scales [21, 26, 38, 39]. 

Scheme 9. N-Ras peptide synthesis employing choline esters as C-terminal protecting group. EDC 1-ethyl-3 (dimethylamino)propyl-carbodiimide hydrochloride, HOBt 1-hydroxybenzo-tri azole...

Bis(hydroxymethyl)phosphonic acid esters that incorporated thymine were employed as a backbone to prepare short oligonucleotide chains. This chain was prepared by condensation of the bis(4,4 -dimethoxytrityl) protected phosphonic acid and iV or N -(2-hydroxyethyl)thymine in the presence of l-(2-mesitylenesul-fonyl)-3-nitro-l,2,4-triazole or by an Appel reaction with or N -(2-aminoethyl)thymine (89a-h). Selective removal of one DMT-group and phos-phitylation yielded the building blocks for solid supported synthesis of the short oligomers by the phosphoramidite approach. Holy has reported the synthesis of 8-amino and 8-substituted amino derivatives of acyclic purine nucleotide analogues. The 8-amino, 8-methylamino- and 8-dimethylamino-adenine and -guanine analogues of iV-(2-phosphonomethoxyethyl) and (S)-iV-(3-hydroxy-2-phosphono-methoxy-propyl) derivatives of purines (90a-i), were prepared by... 

The levulinoyl esters are prepared from the free hydroxyl group by treatment of levulinic acid with DCC [255,256] or l-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride (EDAC) [257] in the presence of DMAP (O Scheme 38). Additionally, 3 - and 5 -0-levulinyl protected derivatives of 2 -deoxy nucleosides have been prepared by regioselective enzymatic acylation using a variety of lipases and acetonoxime levulinate as acylating agent [258]. In contrast to other ester substituents, the 0-levulinoyl group is far less prone to migration [259]. 

---