Isoprenoids stereoisomerism

S. N., Correlation of Stereoisomerism in Present Day and Geologically Ancient Isoprenoid Fatty Acids, Nature (1968) 218 (5146), 1019-1024. 

Stereoisomeric alcohols (93) and (94) yielded identical ring-expansion products [e.g. (97)] on formation of carbocations.168 This is evidence of a stepwise reaction in sterol biosynthesis, whereby a tertiary cation [e.g. the model (95)] rearranges to a secondary cation (96)-an anti-Markovnikov rearrangement . The synthetic aspects of biomimetic cyclizations of isoprenoid polyenes were reviewed.169 Included was a detailed discussion of carbenium ion-initiated cyclizations, with a discussion of the different mechanisms that have been proposed. A novel biomimetic carbocation polyene cyclization of a daurichromenic ester was reported an unusual 2 + 2-carbocation cyclization occurred as a side reaction.170... 

Natural cytokinins are. / /-substituted adenine derivatives. The N6 side chain is either isoprenoid or aromatic (Figure 10), with the former occurring in greater abundance than the latter. The prenyl side chains vary in the presence or absence of a hydroxyl group and the stereoisomeric position. Common active isoprenoid cytokinins are isopentenyladenine (iP /V6-(A2-isopentenyl)adenine), trans-zeatin (tZ / / -(4-hydro.xyisopentenyl)adenine (A)-2-methyl-4-(1 //-purin-6-ylamino)but-2-en-l -ol), ax-zeatin (cZ (Z)-2-methyl-4-(l//-purin-6-ylamino)but-... 

Structurally, cytokinins are classified into two groups, namely, adenine-type cytokinins represented by zeatin, kinetin, and 6-benzylaminopurine, and phenylurea-type cytokinins which include diphenylurea and thidiazuron. The naturally occurring cytokinins A -(A -isopentenyl)adenine 13, frans-zeatin 14, and c s-zeatin 15 differ in the stereoisomeric hydroxylation at the isoprenoid side chain. frans-Zeatin and A -(A -isopentenyl)adenine are physiologically the most important cytokinins in plants [27]. 

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