Polyunsaturated bridges

Figure 7. Relationship of oxidation and degree of polyunsaturation. Polyunsaturation is measured as the methylene bridge index (MBI), which is a more precise measure of extent of unsaturation and oxidizability than the double bond index. It is the mean number of 6is-allylic methylene bridge positions per fatty acid (or fatty acyl chain) in a lipid ensemble. The rate of lipid radical formation measures formation of an oxidative product, while O2 consumption (% O2 lost per sec) is a measure of utilization of a reactant. (Drawn using our data abstracted from Wagner, B.A., Buettner, G.R., and Bums, C.P. 1994, Biochemistry 33 4449-4453).

Hoye described an RCM-based total synthesis of the 20-membered marine macrolide dactylolide 288 and its subsequent conversion to the natural carbinolamide zampanolide 289 (Scheme 56), both feature a common highly unsaturated macrolide core, bridging a m-2,6-disubstituted 4-methylene tetrahydropyran unit. When the polyunsaturated acyclic lactone 286 (1 1 epimeric mixture around the /i r/-butyldimethylsilyl (TBS)-protected carbinol center) was in situ protected with bis-trimethylsilylacetamide (BSA) and then treated with catalyst G in benzene at 60 °C, each diastereomer smoothly cyclized to the corresponding cycloalkene 287 with exclusive ( )-geometry at the newly formed double bond. 

Bourre JM, Bonneil M. Clement M, Dumont O, Durand G, Lafont H, et al. Function of dietary polyunsaturated fatty acids in the nervous system. Prostaglandins Leukotrienes Essential Fatty Acids 1993 48 5-15. Bridge EM, lob LV. The mechanism of the ketogenic diet in epilepsy. Bull Johns Hopkins Hosp 1931 ... 

Fig. 4. Chain reaction of lipid peroxidation. An oxidant removes an electron from a PUFA (step 1) to form a lipid radical. Molecular rearrangement causes formation of a reactive conjugated diene (step 2). This can react with active singlet molecular oxygen ( 02), which is in an excited state rather than the ground state to form a peroxyl radical (step 3). Also, transition metals can react with oxygen to produce potent metal-containing oxidants that may allow simultaneous binding or bridging of a biomolecule and oxygen (B6, K4, W5). The peroxyl radical can be detoxified by an antioxidant to a lipid peroxide (step 4) or the peroxyl radical can act as an oxidant to remove an electron from another PUFA (step 5), effecting a chain reaction of autooxidation. PUFA, polyunsaturated fatty acid (R5). Dot indicates unpaired electron in radical forms.

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