Aldol consecutive asymmetric

The control of the three consecutive asymmetric centers in a-alkylated y-amino-p-hydroxy acids is achieved by aldol condensation of a chiral aldehyde and a chiral reagent (Scheme 17 and Table 6), e.g. boron enolate 43,150 79 oxazolidinones of Evans type lit80 or 45 [68>69>801 or Brown s or Roush s crotylorganoboron reagents 46[81 and 47J81,82 respectively. 

S,3S,4R)-4-Amino-3-hydroxy-2-methylpentanoic acid (3), having three consecutive asymmetric centers, was prepared by the aldol condensation of (R)-2-aminopropionaldehyde derivative (15) and -boron enolate (16) (Fig. 11) [42]. The aldol product (17) was obtained in 35 1 stereoselectivity and used directly as an active ester for the subsequent peptide synthesis, i.e., the coupling with tripeptide S (18) facilitated by cupric acetate afforded tetrapeptide S (19) [43]. These studies were included in the first total synthesis of deglyco-BLM and BLM by Umezawa and his co-workers which included our group in 1982 [44, 45] and the synthesis by Hecht s group was completed in the same year [46, 47], proving the entire structure of BLM. 

Stevastelins are depsipeptides exhibiting immunosuppressant activity. The first total synthesis of stevastelin B was described by Y. Yamamoto and co-workers. To construct four consecutive stereocenters, the Evans aldol reaction and the Roush asymmetric allylation were utilized. In the allylation step, the authors used (S,S)-diisopropyltartrate-derived ( )-crotyl boronate. The anti homoallylic alcohol product formed as the only diastereomer. 

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