Protecting groups manipulations

The commercially available biphei rl ketone and ethyl isocyanoacetate underwent a smooth condensation reaction and following protecting group manipulations the hydrogenation snbstrate was produced in excellent yield. ° Examination of the asymmetric hydrogenation of material produced using this protocol revealed that the... 

There were excessive protecting group manipulations and the catalyst loading at the final de-benzylation step was very high. 

Multiple protecting group manipulations drastically reduced the efficiency of the route. 

Overall, the racemic synthesis was accomplished in a 15-step sequence with only a single protecting group manipulation and marked the second total synthesis of ( )-nominine (1) (Scheme 1.20) [59], the first having been completed by Muratake and Natsume in 2004 [19]. The asymmetric synthesis of nominine was accomplished in a 16-step sequence in a similar fashion [60]. 

Removal of Side Products Using solid-phase protocols, unreacted reagents, consumed reagents, and the products of protecting group manipulations are washed away. Solution-phase strategies will be limited to a single transformation per step with subsequent purification of the product (a multistep route) unless the byproducts either (1) are not produced or (2) do not interfere with subsequent transformations. 

Scheme 11.4 Formation of pentasaccharides in a multistep strategy without protecting group manipulation.

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