N-terminal alkylation, stereoselective

Table 5.8 Stereoselective N-terminal alkylation of dipeptides by chiral phase-transfer catalysis.

Asymmetric phase-transfer catalysis with (S,S)-lg can be successfully extended to the stereoselective N-terminal alkylation of Gly-Ala-Phe derivative 61 (i.e., the asymmetric synthesis of tripeptides), where (S,S)-lg turned out to be a matched catalyst in the benzylation of DL-61, leading to the almost exclusive formation of DDL-62. This tendency for stereochemical communication was consistent in the phase-transfer alkylation of DDL-63, and the corresponding protected tetrapeptide DDDL-64 was obtained in 90% yield with excellent stereochemical control (94% de) (Scheme 5.30) [31]. 

Chiral PTC with S,S)- [or (R,R)-[ 9g can be successfully extended to the stereoselective N-terminal alkylation of LL-, DL-49 and DDL-50, and the corresponding protected tri- or tetra-peptide was obtained in high yield with excellent... 

Hebach and Kazmaier reported the synthesis of cyclic peptidomimetics containing an alkylated amino acid via Ugi-4CR of N-terminal-protected aloc-amino acids, allyl isocyanoacetate, and chiral amines in trifluoroethanol. Allylic esters of tripeptides 193 were obtained in high yields and good stereoselectivity. Metathesis with 5% of Grubbs first-generation catalyst gave 16-membered cyclic peptides 194 in 30-50% yield (Scheme 2.69) [101]. 

The regio- and stereoselective dimerization of terminal alkynes into disubstituted enynes is efficiently catalyzed by rare-earth metal alkyl and hydride complexes, as reported independently by Bercaw et al. and Teuben et al. in 1987 [211,212]. Takaki and coworkers have shown that complexes Ln[N(SiMe3)2]3 when combined with an amine additive (typically, ArNH2 compounds) afford an active species for the... 

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