The synthesis of the high potency sweetener candidate NC-00637 (1) required large quantities of (S)-2-methylhexanoic acid (2). This acid was first prepared in small quantities by the use of chiral auxiliaries. For large quantities, resolution by classical means and an enzymatic method were investigated. Asymmetric hydrogenation provided a workable solution.
The synthesis of the high-potency sweetener, NC-00637 (1), required selective preparation of the γ-protected glutamic acid. Coupling of the three components could be performed in any order, but the final route involved N-acylation of the protected L-glutamic acid with the acid chloride derived from (S)-2methylhexanoic acid. Activation of the r-carboxyl group allowed condensation with 5-amino-2-cyanopyridine (4). Saponification of the γ-ester 19 then provided the sweetener 1.
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