Convergence Strategies

Scheme 4 Building blocks used in the synthesis of mucih related Fla ahtigen via two different strategies convergent (A+B, then C) or "cassette" (B + C, then A) approaches.

When the initial and final internal states of the system are not well-separated in energy from other states then the closed-coupling calculation converges very slowly. An effective strategy is to add a series of correlation temis involving powers of the distance r. between internal particles of projectile and target to the tmncated close-coupling expansion which already includes the important states. 

Di- and trinucleotides may be used as units instead of the monomers. This convergent synthetic strategy simplifies the purification of products, since they are differentiated by a much higher jump in molecular mass and functionality from the educls than in monomer additions, and it raises the yield. We can illustrate the latter effect with an imaginary sequence of seven synthetic steps, c.g. nucleotide condensations, where the yield is 80% in each step. In a converging seven-step synthesis an octanucleotide would be obtained in 0.8 x 100 = 51% yield, compared with a 0.8 x 100 = 21% yield in a linear synthesis. 

When topological strategies are used concurrently with other types of strategic guidance several benefits may result including (1) reduction of the time required to find excellent solutions (2) discovery of especially short or convergent synthetic routes (3) effective control of stereochemistry (4) orientational (regiochemical) selectivity (5) minimization of reactivity problems and (6) facilitation of crucial chemical steps. 

The details of Sheehan s convergent penicillin synthesis strategy are presented in Schemes 3-5. 

The retrosynthetic analysis described above outlines a triply convergent strategy towards the synthesis of zaragozic acid A (1). The most pleasing aspect of the retrosynthetic strategy is the rapid and enantioselective manner in which the two-dimensional molecule... 

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