Homochiral segment

Fig. 2.37 Turn-like motifs induced by y-pep-tides consisting of unlike-y -amino acid residues. (A) 14-membered H-bonded turn segment formed by y-tetrapeptide 150 with homochiral residues (Cin = cinnamyl). Summary of characteristic NOEs (as arrows) in the turn segment collected in pyridine-c/5 [202]. (B) 14-membered H-bonded turn form...

Scheme 7-16 shows that a similar synthetic route leads to the asymmetric synthesis of optically active 62. The synthesis that began from homochiral aldehyde (—)-52 used this newly discovered asymmetric epoxidation three times, 52 —> 58, 58 —> 68, and 68 —> 61, finishing the conversion from 52 to 61 by following a shortened route. The last chiral center to be built is C-27, and the addition of allyltin to the aldehyde derived from 61 proceeds with high stereoselectivity to give the chiral aliphatic segment 62. 

An ingenious experimental example for the reverse of the socalled la coupe de Roi (i.e. the dividing of finite geometric objects into isometric segments) — namely the assembly of two homochiral compounds into an achiral one — was provided by making use of appropriate [2.2]metacyclophanes. Whereas self-coupling of (+)-4-(bromomethyl)-6-(mercaptomethyl)[2.2]metacyclophane (cf. formulas 55 to 60 for... 

Figure 2-45. Dissection of an equilateral triangle into (a) Achiral (b) Homochiral and (c) Heterochiral segments after Shubnikov and Koptsik [72], Used with permission from Nauka Publishers, Moscow.

The efficient synthesis of racemic triphyophilline (60) paved the way for the first asymmetric total synthesis of the 5,1-linked napthylisoquinoline alkaloid (-)-ancistrocladine (1) (ref. 47). The approach once again relied on the successful intramolecular strategy used previously and required the development of asymmetric synthesis of the tetrahydroisoquinoline portion of 1. The route to isoquinoline segment 79 began with the synthesis of homochiral amphetamine derivative 83 via a reductive methylation protocol (Scheme 10) (ref. 54, 55) Thus, condensation of ketone 80 with (S)-... 

Stereocontrolled polymerization requires elimination of the intermolecular transfer (segmental exchange) between the homochiral chains containing repeating units of opposite configuration. Otherwise, even if initially stereoselective polymerization proceeds giving the homochiral chains. 

---