Convergent block synthesis linear

In spite of the poor selectivity in two of the coupling reactions, the overall yield of the mannohexaose 321 from the block synthesis was higher than that achieved previously in the linear synthesis, thereby clearly highlighting the well-known advantages of convergent syntheses. 

Assembling a target composed of 65 building blocks (n = 64) looks less than appealing if attempted via a linear route, as this route is virtually blocked by the arithmetic demon . At the same time, though, the task does not look prohibitively complicated if a convergent route for its synthesis is feasible. 

To provide an operationally and commercially feasible alternative process for 229, every operation of the discovery route was analyzed systematically and the key issues to be addressed and the problems found were attempted one by one. One of the first problems with this discovery synthesis was that it was linear and all the building blocks were added in a stepwise fashion. To provide a convergent synthesis, we planned the synthesis of the key intermediate (240) using the Stetter reaction of an enone (232) with an advanced cyanoal-dehyde (242) (Scheme 30.42). 

In spite of die very poor selectivity in two of the key coupling reactions, comparison of the two routes to 29 revealed the convergent one to be the more efficient of the two syntheses with 8 steps and 4.4% overall yield, excluding synthesis of die monosaccharide building blocks, as compared to the 11 steps and 1.9% overall yield of the linear route. 

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