Esters coupling

Now, it is widely known that proline at the N-terminal position causes problems of steric hindrance by using active ester couplings in the polycondensation step as well as in the synthesis of the tri- or hexapeptides. This is often a stringent restriction also if proline or glycine are intended to be in the C-terminal position. 

In the early solution phase syntheses of oligonucleotides, coupling of phosphate diesters was used. A mixed 3 -ester with one aryl substituent, usually o-chlorophenyl, was coupled with a deprotected 5 -OH nucleotide. The coupling reagents were sulfonyl halides, particularly 2,4,6-tri-i-propylbenzenesulfonyl chloride,53 and the reactions proceeded by formation of reactive sulfonate esters. Coupling conditions... 

Scheme 11.8 Synthesis of the triflate and boronic ester coupling partners...

Scheme 3 Amino Acid Thioesters by Active Ester Coupling or from Thiocarboxylic Acids113-161 rl — ...------- R1...

Fig. 14.50. Crossed McMurry reactions ketone/ester coupling (top) and ketone/amide coupling (bottom).

Schem 13.17 Kinetic resolution of amino acid esters coupled with pyridoxalphosphate mediated in situ racemization.

Corey et al reported that the complex formed by chromium(VI) oxide and 2,4-dimethylpentane-2,4-diol can be used as a catalytic oxidant with peroxyacetic acid as a cooxidant. When used stoichiome-trically, secondary alcohols are oxidized quickly even at -20 C, but the oxidation of primary alcohols is slow and large amounts of ester coupling are observed. 

Wu X, Ling C-C, Bundle DR. A new homobifunctional p-nitro 42. phenyl ester coupling reagent for the preparation of neoglycoproteins. Organic Lett. 2004 6 4407-4410. 

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