S optically active

PolyCy-benzyl L-glutamate) (PBLG), 15 109 Poly(y-ketosulfide)s, optically active, 23 711 Poly(P-alanine), 1 292 Poly-P-hydroxybutyrate (PHB), 12 482 Polybetaines, 20 479-482 applications of, 20 482 preparation of, 20 480-481 solution properties of, 20 481-482 synthesis of, 20 479-481 Polyborates, 4 256-258 Polyborosiloxanes, in silicon carbide manufacture and processing, 22 533 Polybrominated diphenyl ethers (PBDEs), 13 142-143 20 56... 

Whereas racemization is the complete loss of optical activity with time, epimerization is the reversible interconversion of diastereoisomers to an equilibrium mixture which is not necessarily optically inactive. Diastereoisomers arise from the combination of the two chiral centers in 9, namely the metal centered, R and S, and the resolved (S) optically active ligand center. The diastereoisomers (RS) and (SS) differ in their properties. 

Enantiomers are stereoisomers that are non-superimposable mirror images of each other. EnrirTm mers have identical physical and chemical (except towards optically aQtJvp reagents) properties except for the direction in which plane-polarized light is rotated. Enantiomers account for a compound s optical activity. 

The bark of the pomegranate tree (Punica granatum L.) was first investigated by C. Tanret who isolated from it four alkaloids which he named pelletierine (204), isopelletierine, methylpelletierine, and pseudo-pelletierine (205, 206). Methylpelletierine and pelletierine were observed to be optically active while isopelletierine and pseudopelletierine were inactive. Later, Piccinini (207) found a fifth alkaloid, isomethylpelletierine in the mother liquors obtained from the preparation of pseudopelletierine. Eventually, however, Hess (208) while reinvestigating the bark obtained pseudopelletierine, Piccinini s isomethylpelletierine and a third, optically inactive base, but found no trace of Tanret s optically active methylpelletierine. He could not find any optically active alkaloids and, considering his third base identical with Tanret s isopelletierine (185) renamed the latter base pelletierine. Further, Hess and Eichel (209) considered Tanret s methylpelletierine and Piccinini s isomethylpelletierine as identical but different from methylated pelletierine and named it methyliso-pelletierine (185). 

Bou J.J., Rodriguez-Galdn A., Munoz-Guerra S., Optically active polyamides derived from L-tartaric add. Macromolecules, 26, 1993, 5664-5670. 

Marshall s optically active stannanes are readily prepared by the addition of tributylstannyllithium to crotonaldehyde (158), oxidation of the ensuing alcohol to the acyl stannane (159), and enantioselective reduction with Noyori s BINAL-H [110] (Scheme 5.27) [107]. Protection of the secondary alcohol furnishes a-alkoxy-substituted allylstannane 160 that partakes in the stereo-specific rearrangement to afford 161 in 80% yield. 

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