Synthesis and biochemical studies of analogs of platelet-activating factor bearing a methyl group at C2 of the glycerol backbone

Synthesis of ether lipids: natural compounds and analogues

Ether lipids are compounds present in many living organisms including humans that feature an ether bond linkage at the sn-1 position of the glycerol. This class of lipids features singular structural roles and biological functions. Alkyl ether lipids and alkenyl ether lipids (also identified as plasmalogens) correspond to the two sub-classes of naturally occurring ether lipids. In 1979 the discovery of the structure of the platelet-activating factor (PAF) that belongs to the alkyl ether class of lipids increased the interest in these bioactive lipids and further promoted the synthesis of non-natural ether lipids that was initiated in the late 60's with the development of edelfosine (an anticancer drug). More recently, ohmline, a glyco glycero ether lipid that modulates selectively SK3 ion channels and reduces in vivo the occurrence of bone metastases, and other glyco glycero ether also identified as GAEL (glycosylated antitumor ether lipids) that exhibit promising anticancer properties renew the interest in this class of compounds. Indeed, ether lipid represent a new and promising class of compounds featuring the capacity to modulate selectively the activity of some membrane proteins or, for other compounds, feature antiproliferative properties via an original mechanism of action. The increasing interest in studying ether lipids for fundamental and applied researches invited to review the methodologies developed to prepare ether lipids. In this review we focus on the synthetic method used for the preparation of alkyl ether lipids either naturally occurring ether lipids (e.g., PAF) or synthetic derivatives that were developed to study their biological properties. The synthesis of neutral or charged ether lipids are reported with the aim to assemble in this review the most frequently used methodologies to prepare this specific class of compounds.

Chemo-enzymatic synthesis of rac 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholine and its analogues

The synthesis of rac 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholines (6a-c), blood platelet activating ether lipid analogues has been achieved in a four-step sequence from epichlorohydrin (1). Etherification of epichlorohydrin with different alcohols namely tetradecyl (2a), hexadecyl (2b) and octadecyl (2c) alcohols gave glycidyl ethers (3a-c) with 78-80% yields. The second step involved opening of the epoxide by acetic anhydride to give acetylated products (4a-c, 76-78% yield), which were subsequently hydrolyzed selectively, a key step of the method employing a 1,3 specific lipase to obtain rac 1-O-alkyl-2- acetylglycerol (5a-c) with 45-50% yields. The hydrolyzed products (5a-c) were phosphorylated to obtain rac 1-O-alkyl-2-acetyl-sn-glycero-3-phosphocholines (6a-c) in 80-85% yields.

Chemical synthesis and physiological activity of sulfonium analogues of platelet activating factor

Phosphatidylsulfocholine (PSC), the sulfonium analogue of phosphatidylcholine (PC), occurs naturally in some diatoms. The replacement of the [formula; see text] group by a [formula; see text] results in an increase in the polar head group size in PSC relative to that of PC, consistent with the observed increase in permeability of PSC bilayers towards urea. It was of interest to see whether replacement of the [formula; see text] group in platelet activating factor (PAF) by an [formula; see text] group leads to any change in platelet aggregation or other physiological activity. Synthesis of the sulfonium analogue of PAF was carried out by suitable modifications of known procedures. The PAF-sulfonium analogue was found to have almost the same platelet aggregating activity as PAF itself, in the concentration range 1-20 microM, but a much lower activity in the range 0.01-1 microM. The analogue had little or no effect on the platelet aggregation activity of PAF when added in the concentration range 0.01-1 microM and had about half the hypotensive activity of PAF towards hypertensive CDF male rats. The sulfonium analogue, however, was much more cytotoxic to HL-60 cells than PAF itself, in the concentration range 0-15 microM; replacement of the acetate group by a benzyl group increased the cytotoxicity to the level of that of the methoxy analogue of PAF. Thus, replacement of the [formula; see text] group by a [formula; see text] group in the polar head group region of PAF results in a relatively small change in its platelet aggregation activity and a decrease in its hypotensive activity, but greatly increases its antitumor activity.

Conversion of methylallyl alcohol into C2-methyl-1,2-di-O-hexadecylglycerophosphocholine, a conformationally restricted phospholipid

The synthesis of C2-methyl-1,2-di-O-hexadecylglycerophosphocholine from methylallyl alcohol is described and the 13C-NMR assignments of all glycerol backbone and headgroup resonances are provided. Bond rotations in the glycerol backbone of this diether phospholipid derivative are impeded by steric hindrance exerted by the methyl group. We discuss the utility of this conformationally restricted C2-alkyl phospholipid analog for study of the influence of conformational changes at the lipid-water interface of bilayers on the interactions between lipids and other membrane components.