The Partial de Novo Approach

In general, the de novo approach applied by the Kishi group for the synthesis of C-oligosaccharides relies on acyclic stereocontrol for the construction of the contiguous chiral centers of at least one of the carbohydrate moieties. The first examples of this approach were reported by this group in 1987 and involved the synthesis of four C-disaccharides, including the C-analogs of methyl maltoside and cellobio-side, from a common synthetic intermediate [70]. 

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In summary, the de novo isolation of the cDNAs encoding enzymes of alkaloid biosynthesis is still achieved by using a variety of classical techniques, such as protein purification followed by partial amino acid sequence determination, and by newer techniques such as proteomics coupled to functional heterologous expression. The current status of cloned cDNAs specifically related to isoquinoline alkaloid biosynthesis is schematically presented in Figure 10.8. New additions to this list will certainly be made in the future as a result of a combination of approaches both new and old. 

Synthetic Schemes. There are basically two main routes to preparation of mixed acid (enantiomeric) diacylphosphatidylcholines. The first involves a total de novo synthesis and the second centers on a partial or semisynthesis using a highly purified naturally occurring phosphatidylcholine as starting material. Both approaches have merits and drawbacks, which are noted as follows. 

Conclusions Both Right and Wrong. Basically these results support a specific attack of the enzyme on the sn-2 ester bond, and certainly the release of a specific fatty acid would support such a conclusion. Earlier proof that the phospholipase A2 prefers the sn-3 configuration and will not attack the sn-1 form allows one to be quite comfortable in a decision on its stereospecificity. Yet, no where here has it been proven unequivocally that the partial synthesis outlined in Figure 4-11 above does give the fatty acids substituted in the described positions. The only route to such proof is through the use of de novo synthesis as discussed above. This approach will allow one to make claims as to the position of fatty acids on phosphoglycerides. 

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