Cyclization Through Dielsalder Reactions

A number of cyclic structures, typically containing cyclohexane rings, are known to be formed via the acetate pathway, but experimental evidence supports cyclization processes different from the aldol and Claisen reactions seen in the biosynthesis of aromatic compounds. They can, however, be rationalized in terms of an enzymic Diels - Alder reaction, represented as the electrocyclic sequence shown in Figure 3.81. Thus, lovastatin can be formulated as arising from two polyketide chains with C-methylation as outlined in Figure 3.82, with relatively few of the oxygen functions being retained in the final product. Accordingly, it is possible that lovastatin is formed by cycliza-tion of the trienoic acid (Figure 3.82), which is likely to arise by a variant of the macrolide

diene dienophile

Diels-Alder reaction

SEnz it

SEnz O

SEnz O

O

lovastatin

O CO2H

SCoA

O O SAM

Diels-Alder

SEnz

Diels-Alder

dihydromonacolin L Figure 3.82

CO2H ,OH

SEnz

CO2H OH

dihydromonacolin L Figure 3.82

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