Phenylfluorenyl group

The volume concludes with the preparation of four useful starting materials. The highly electrophilic tricarbonyl reagent DIMETHYL MESOXALATE finds application as a two-electron component in various pericyclic processes. 9-BROMO-9-PHENYL-FLUORENE is becoming increasingly used for the protection of primary amines, particularly amino acids and amino esters. Two methods for introducing the 9-phenylfluorenyl group are illustrated in the preparations of (S)-N-(9-PHENYLFLUOREN-9-YL)ALANINE AND (S)-DIMETHYL N-(9-PHENYLFLUOREN-9-YL)ASPARTATE. 

Protection of amines The 9-phenylfluorenyl group is introduced by treatment of an amine with 1 in CH3CN with K3P04 as base and Pb(N03)2 as a bromide scavenger. The 9-PhFl group is much more stable than the trityl group to protonolysis, but can be removed with TFA in aqueous CH3CN. 

Phenylfluorenyl protecting groups were removed by hydrogenolysis on 10% Pd/C in 10 1 EtOH-AcOH at 5 atm hydrogen pressure (Scheme 4.84).331... 

Houk control concerns electrophilic attack on alkenes, enolates and the like. The alkylation of enolate 56 would be an example if it were not held in a ring by chelation. It can in fact be difficult to tell whether chelation is involved or not with many enolates and the outcome of the reaction may tell which. Chamberlin s asymmetric preparation of both pyrrolidine 2,3-dicarboxylic acids 141 and 142 from natural aspartic acid illustrates this perfectly. The key to the stereochemical control is the very large protecting group 9-phenylfluorenyl-143 introduced by Rapoport.22... 

A number of hnker units designed specifically for immobihzation of amines have also been developed. One noticeable example exploits the versatility of the 9-phenylfluorenyl-9-yl group (PHFI). The PHEI group has previously been used as a protecting group for amines and was adapted into a hnker unit by Bleicher (Table 1.1, Entry 12). Cleavage from this... 

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