Branched synthesis

Figure 22.10 illustrates the pathway for de novo pyrimidine biosynthesis. It differs from de novo purine synthesis in that the pyrimidine ring is synthesized separate from the ribose sugar (in purines, the ring is built upon the sugar - see Figure 22.4). In addition, de novo pyrimidine biosynthesis is not branched. Synthesis leads to UMP, from which CTP is ultimately made. By contrast, de novo purine biosynthesis branches after IMP is produced (Figure 22.6).

The "branched synthesis according to the fragment strategy 8) of Scheme 2 corresponds analogously to Pathway 2. 

Scheme 13.20 Twelvefold branching synthesis using two-directional synthesis.

Although there is not much information available in the literature concerning optimal solvents for long chain branch formation, it appears that paraffinic solvents are preferable to aromatics. The use of aliphatic solvents for long chain branch synthesis has been recommended. This choice has also been supported for the catalytic system bis(cyclo-pentadienyl) zirconium dimethyl/perfluorotriphenyl boron. It was found that this catalyst system gave... 

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