By Vadim A. Soloshonok and Kunisuke Izawa (Eds.)
content material: New thoughts of designing micro- and nano encapsulation platforms --
Micro- and nanoencapsulation for nutrition applications.
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Extra resources for Asymmetric Synthesis and Application of α-Amino Acids
The conditions for enantiodivergent cyclization have been applied to various amino acid derivatives (Table VII). Five-membered cyclization of 27, 31, and 33 with K H M D S in D M F at -60 °C gave 28, 32, and 34 in 98, 97, and 95 % ee, respectively, with retention of configuration (entries 1, 3, and 5). On the other hand, use of L T M P in T H F gave the cyclization products in 81-91% ee with inversion of configuration (entries 2, 4, and 6). Similar phenomena were observed in four-membered cyclization.
Direct H P L C analysis of the reaction mixture showed that ee of 6 was at least 65% in the reaction medium below -20 °C. Thus, the chiral information of 3 appears to be memorized in the enolate intermediate 5 as dynamic axial chirality and then regenerated as central chirality in the product 4 (3,4). = [/2 2 axially chiral enolate \ RI _ I^OMV X I Y J ' 1 M 2 VMQJ 1 _^R / Y ent-\ planar chiral enolate = 1 M ent-2 Figure 1. Dynamic Chirality of Enolate Structure 2. Strategy for Asymmetric a-Alkylation of a-Amino Acids via Memory of Chirality Nonproteinogenic ct,a-disubstituted-a-amino acids have attracted considerable attention because of their utility as conformational modifiers of biologically active peptides and as enzyme inhibitors (5).
4-δ 5) values large and positive for 1,4-regioisomers and small and negative for 1,5-regioisomer, was confirmed by further studies carried out in our laboratories. See: a) Dondoni, A . ; Marra, A . J. Org. Chem. 2006, 71, 7546-7557. ; Dondoni, A . ; Marra, A . Org Biomol. Chem. 2006, 4, 3225-3227. (c) Nuzzi, A . ; Massi, A . ; Dondoni, A . QSAR Comb. Sci. 2007, 26, 1191-1199. (d) Marra, A . ; Vecchi, A . ; Dondoni, A . J. Org. Chem. 2008, 73, in press. Rostovtsev, V . V . ; Green, L. ; Fokin, V .
Asymmetric Synthesis and Application of α-Amino Acids by Vadim A. Soloshonok and Kunisuke Izawa (Eds.)