Asymmetric Synthesis of Methoxylated Ether Lipids: Total Synthesis of n-3 Polyunsaturated Docosahexaenoic Acid-Like Methoxylated Ether Lipid.

The first total synthesis of a docosahexaenoic acid (DHA)-like methoxylated ether lipid (MEL) is reported. This compound constitutes an all-cis methylene skipped hexaene framework identical to that present in DHA, the well-known omega-3 polyunsaturated fatty acid. The polyene C22 hydrocarbon chain, bearing a methoxyl group in the 2-position and R-configuration at the resulting chiral center, is attached by an ether linkage to the pro-S hydroxymethyl group (sn-1 position) of a glycerol backbone. The asymmetric synthesis is highly convergent and based on the polyacetylene approach involving iterative copper-promoted coupling reactions of propargyl bromides with terminal alkynes and semihydrogenation of the resulting hexayne. Starting from enantiopure R-solketal and racemic epichlorohydrin, the targeted MEL was accomplished in an 8.2% yield over eight steps (longest linear sequence) involving an enantio- and diastereopure glyceryl glycidyl ether key C6-building blocks from which the polyynes were constructed.

Synthesis of enantiopure reversed structured ether lipids of the 1-O-alkyl-sn-2,3-diacylglycerol type.

This report describes the synthesis of reversed structured 1-O-alkyl-2,3-diacyl-sn-glycerols (DAGEs) possessing a pure saturated even number fatty acid (C6:0-C16:0) at the sn-2 position along with a pure EPA or DHA located at the terminal sn-3 position of the glycerol backbone of chimyl, batyl and selachyl alcohols. These adducts were synthesized by a highly efficient two-step chemoenzymatic process involving an immobilized Candida antarctica lipase to introduce pure EPA and DHA activated as oxime esters exclusively to the sn-3 terminal position of enantiopure chimyl, batyl and selachyl alcohols in excellent yields. The saturated fatty acids were subsequently incorporated to the remaining sn-2 position of the resulting 3-monoacylglyceryl ethers (3-MAGEs) using EDAC coupling agent in the presence of DMAP in very high to excellent yields (85%-98%). No losses of enantiomeric composition were observed during these processes. The multiple utilities of the resulting focused library of reversed structured DAGEs are discussed including how such compounds may possibly be utilized within the pharmaceutical area.

Activation of n-3 polyunsaturated fatty acids as oxime esters: a novel approach for their exclusive incorporation into the primary alcoholic positions of the glycerol moiety by lipase.

A novel approach has been developed for activating the highly bioactive long-chain n-3 polyunsaturated fatty acids EPA and DHA as oxime esters and incorporating them exclusively to the end-positions of glycerol and enantiopure 1-O-alkylglycerols. The Candida antarctica lipase B was observed to display a superb regioselectivity when using the acetoxime esters of EPA and DHA as acyldonors under mild condition to keep acyl-migration side-reaction under complete control. Regiopure 1,3-diacylglycerols, 1-O-alkyl-3-acyl-sn-glycerols and their antipodes possessing EPA and DHA were afforded in very high purity and yields.

Ether lipids.

The naturally occurring 1-O-alkyl-sn-glycerols and their methoxylated congeners, 1-O-(2'-methoxyalkyl)-sn-glycerols, are biologically active compounds, ubiquitously found in nature as diacyl glyceryl ether lipids and phosphoether lipids. The chief objective of this article is to provide a comprehensive and up to date review on such ether lipids. The occurrence and distribution of these compounds in nature are extensively reviewed, their chemical structure and molecular variety, their biosynthesis and chemical synthesis and, finally, their various biological effects are described and discussed. An unprecedented biosynthesis of the 2'-methoxylated alkylglycerols is proposed. The first synthesis of enantiopure (Z)-(2'R)-1-O-(2'-methoxyhexadec-4'-enyl)-sn-glycerol, the most prevalent 2'-methoxylated type alkylglycerol present in cartilaginous fish, is described. It was accomplished by a highly convergent five step process.