Epimerization coupling reactions

Despite the rapidity of the coupling reactions epimerization may be extensive. It may be substantially reduced if the condensations are carried out using high concentrations of AA OR. This is a consequence of the differing dependencies of epimerization (first-order) versus coupling (second-order) on AA OR concentration. Both processes are sensitive to the stereochemistry of the reactants, and the balance of factors is such that coupling of (S)-AA OMe with A-... 

The aldehyde function at C-85 in 25 is unmasked by oxidative hydrolysis of the thioacetal group (I2, NaHCOs) (98 % yield), and the resulting aldehyde 26 is coupled to Z-iodoolefin 10 by a NiCh/CrCH-mediated process to afford a ca. 3 2 mixture of diaste-reoisomeric allylic alcohols 27, epimeric at C-85 (90 % yield). The low stereoselectivity of this coupling reaction is, of course, inconsequential, since the next operation involves oxidation [pyridinium dichromate (PDC)] to the corresponding enone and. olefination with methylene triphenylphosphorane to furnish the desired diene system (70-75% overall yield from dithioacetal 9). Deprotection of the C-77 primary hydroxyl group by mild acid hydrolysis (PPTS, MeOH-ClHhCh), followed by Swem oxidation, then leads to the C77-C115 aldehyde 28 in excellent overall yield. 

The first synthesis, by method a, of amylostatin (XG) was reported by Kuzuhara and Sakairi. The synthon for the cyclohexene moiety was the benzylated allyl bromide 382, derived from D-glucose by the sequence 378 — 382 of the Perrier reaction. The coupling reaction of 382 using an excess of 4-amino-T,6 -anhydro-4,6-dideoxymaltose tetrabenzyl ether (383), and sodium iodide in DMF for 3 days produced a mixture of the epimeric monocarba-trisaccharide derivatives, separation of which gave the protected derivatives in 15% yield. 

A method by Berger[12] utilized the stereoselective hydrolysis of diastereomeric peptides by leucine aminopeptidase. Z-L-Ala-D-Ala-OH was coupled to an all L-Ala peptide such as l-Ala-L-Ala-ONbz. Epimerization during coupling resulted in the formation of a small amount of L-Ala-L-Ala-L-Ala-L-Ala after deprotection, and since the peptidase has an absolute specificity for the all-L peptide, only the epimerized product was hydrolyzed. Quantification of the degradation products gave the extent of epimerization. These classical procedures, however, are specific to the particular coupling reaction under consideration and the results may not be fully applicable to all systems. Furthermore, they give no direct information about the rate of racemization. 

There are numerous studies of control of racemization for specific subsets of amino acids. For example, Benoiton has carried out extensive studies on racemization of Al-methyl amino acids. In particular, McDermott and Benoiton1261 demonstrated that the presence of tertiary amine salts in coupling reactions had a profound effect on activated TV-methyl amino acids in contrast to the nonmethylated form. In one example, when Z-Ala-MeLeu-OH was coupled to the tosylate salt of Gly-OBzl with ethyl 2-ethoxy-l,2-dihydroquinoline-l-carboxylate (EEDQ) in the presence of TEA, 15% of the l-d dipeptide was formed with a yield of 68%, compared to 0.5% for Z-Ala-Leu-OH with a yield of 78%. When the free base of Gly-OBzl was utilized in a DCC/HOSu coupling in the absence of tertiary amine, no l-d products were detected. The authors attributed this increased susceptibility to epimerization to an ox-azolonium intermediate, which can epimerize by proton abstraction or merely by tauto-merization (Scheme 11). 

A small amount (<5%) of a second product is routinely observed during the coupling reaction. Isolation and preliminary spectroscopic characterization suggests this product may be a diastereomer that could, presumably, arise from epimerization of the L-Ala a-stereocenter during the peptide-coupling event. 

A step-by-step peptide synthesis from the N- to the C-terminus is not possible with chemical methods as it risks partial epimerization due to the repeated carboxy activation procedures, In constrast, the stereo- and regiospecificity of serine and cysteine proteases ensures integrity of the stereogenic center and allows ecological reaction conditions without side-chain protection. Scheme 4 shows the synthesis scheme using clostripain and chymo-trypsin as catalysts.The second coupling reaction was carried out by enzyme catalysis in a frozen aqueous system (see Section 4.2.3.1). 

As the half time of the epimerization was about fifty times longer than that of the coupling reaction, the epimeric ratio formed under kinetic control was only slightly influenced by it. Therefore, the stereoselectivity at C-l of the coupling reaction in the non-benzylated series was... 

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