Head-to-tail connectivity

Figure 2.9 The chronically isolated intestinal loop model. Key (B) blood catheter (C) head-to-tail connection of the intestine (IL) internal loop with intact blood supply and innervation and (P) perfusion tubes (Adapted from Poelma and Tukker [130]).

A Ci-symmetric cis-1 bis-adduct with an additional bridge between the addends was obtained in the reaction between two molecules of ethyl propio-late and C6o in the presence of triphenylphosphane.318 Its structure (( )-162, Figure 1.37) was established with the help of the HMBC- (heteronuclear multiple bond correlation) NMR technique and includes stereogenic centers in the addend moiety as well as a noninherently chiral addition pattern. The latter can be related to the head-to-tail connectivity of the two propiolate units. 

Figure 5 Self-assembly of asymmetric 60° ligands 18,19, and 20 using Zn". While 18 and 20 gave mixtures, 19 gave quantitative head-to-tail connectivity. Adapted with permission from Ref 15DT5139. Copyright 2015 Royal Society of Chemistry.

In contrast to the head-to-tail connected polyisoprenoids, squalene (C30) and phytoene (C40) arise from a head-to-head condensation of two molecules FPP or GGPP, respectively. The squalene synthase requires divalent metal ions Mn " as well as an NADPH cofactor. Superimposition of the crystal structure of human squalene synthase with avian FPP synthase reveals a close structural homology, although the sequence identity in the superimposed parts is low [164]. Due to their distinct functionality, the highly a-helical squalene synthases (Fig. 87.11) have been designated an own type of head-to-head prenyltransferases termed E-class. The highly conserved DDXXD motifs in a-prenyltransferases are altered to DTLED and DYLED motifs in the human squalene synthase. 

Isoprene units can he characterized as having a head and a tail as shown in Figure 12.75. Notice that hoth geranyl and farnesyl pyrophosphate are formed by head-to-tail connections of isoprenes. 

The limit of head-to-tail combining is polyisoprene, natural rubber (p. 554). Your drawing of the polymer of isoprene called gutta percha (Problem 12.32) should also have head-to-tail connections. 

Two units of isoprene combine to form terpenes, Cjo compounds such as geraniol. Three isoprene units form sesquiterpenes, C15 compounds such as famesol. Diterpenes are C20 compotmds, triterpenes are C30 compounds, and so on. Myriad terpenoid compounds are known, and most can be traced to derivatives of isoprene as starting material. Most terpenes follow the isoprene rule, which dictates the head-to-t ul formation described earlier. AH of the terpenes shown in Figure 12.76 do not stricdy follow the pattern, but each has at least one head-to-tail connection. 

The selective telomerization exclusively to natural type terpenoids with a head-to-tail connection has not been achieved so far, but some interesting initial steps have been done. In 1975 Hidai et al. had investigated the synthesis of the terpenoid citronellol starting from isoprene [30]. They reacted isoprene and methanol, yielding the telomer 1-methoxy-... 

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