Unnatural products synthons

Synthesis of Natural and Unnatural Products from Sugar Synthons... 

This methodology was later applied to the desymmetrization of the pendant dione moiety 235 to give products 236 embodying four contiguous stereocenters with complete stereochemical control resulting in high enantiomeric excesses (Scheme 106) (186). The resulting products can be used as synthons to set chiral centers in the total synthesis of both natural and unnatural products. 

D-Proline (16) is an unnatural amino acid and an important chiral synthon for the synthesis of a variety of biologically active compounds. There are few chemical methods for asymmetric synthesis of this compound. Almost all processes for the production of D-proline at scale are based on resolution of d/-prolinc, and most of them involve the racemization of L-proline (17). 

R) -1 sopropylideneglycerol is a useful C3-synthon in the synthesis of (S)-P-blockers, e. g. (S)-metoprolol. Also, (R)-isopropylideneglyceric acid may be used as the starting material for the synthesis of biologically active products. The resolution is carried out by selective microbial oxidation of the (S)-enantiomer (Fig. 19-9). The chemical synthesis of (R)-isopropylideneglycerol starts either from unnatural L-mannitol or from L-ascorbic acid (Fig. 19-10). In comparison to the biotransformation, here stoichiometric quantities of lead tetra acetate are needed. 

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