[Concerning the chemistry of vitamin E. 7. The stereospecific cyclization of (3'R, 7'R, 11'R)-alpha-tocopherolquinone]

Synthesis of Vitamin E

Vitamin E compounds are biologically essential fat-soluble antioxidants derived from 6-chromanol. This chapter covers the representative literature on the preparation of various stereoisomeric forms and homologues of tocopherols and tocotrienols, and of respective starting materials and intermediates. The industrially most relevant (all-rac)-alpha-tocopherol is generally built up by coupling of arenes with aliphatic precursors. For the synthesis of optically active vitamin E components, various strategies are compiled. In approaches to chiral chroman and side chain building blocks, a broad variety of methods from the repertoire of asymmetric synthesis were applied, that is optical resolution and use of chiral pool starting materials and chiral auxiliaries in stoichiometric as well as catalytic amounts, including catalysis by metal complexes, microorganisms, and enzymes. Most efforts were directed to (2R,4'R,8'R)-alpha-tocopherol due to its prominent biological activity. Syntheses of different stereoisomers (and mixtures thereof) and (beta-, gamma-, delta-) homologues of tocopherols and tocotrienols, as well as methods for their interconversion, are also described.

A Stereocontrolled Synthesis of δ-trans-Tocotrienoloic Acid

[reaction: see text] A concise stereoselective total synthesis of a naturally occurring polymerase beta inhibitor, delta-trans-tocotrienoloic acid (2), is described. The key step in the synthesis is an acid-catalyzed cyclodehydration reaction. Additionally, this report corrects a previously reported structural assignment, defines the absolute stereochemistry of 2, and defines key structural requirements for polymerase beta inhibition.

Supernatant protein factor in complex with RRR-alpha-tocopherylquinone: a link between oxidized Vitamin E and cholesterol biosynthesis

The vast majority of monomeric lipid transport in nature is performed by lipid-specific protein carriers. This class of proteins can enclose cognate lipid molecules in a hydrophobic cavity and transport them across the aqueous environment. Supernatant protein factor (SPF) is an enigmatic representative of monomeric lipid transporters belonging to the SEC14 family. SPF stimulates squalene epoxidation, a downstream step of the cholesterol biosynthetic pathway, by an unknown mechanism. Here, we present the three-dimensional crystal structure of human SPF in complex with RRR-alpha-tocopherylquinone, the major physiological oxidation product of RRR-alpha-tocopherol, at a resolution of 1.95A. The structure of the complex reveals how SPF sequesters RRR-alpha-tocopherylquinone (RRR-alpha-TQ) in its protein body and permits a comparison with the recently solved structure of human alpha-tocopherol transfer protein (alpha-TTP) in complex with RRR-alpha-tocopherol. Recent findings have shown that RRR-alpha-TQ is reduced in vivo to RRR-alpha-TQH(2), the latter has been suggested to protect low-density lipoprotein (LDL) particles from oxidation. Hence, the antioxidant function of the redox couple RRR-alpha-TQ/RRR-alpha-TQH(2) in blocking LDL oxidation may reduce cellular cholesterol uptake and thus explain how SPF upregulates cholesterol synthesis.

Tocopheryl acetates from the pupal exocrine secretion of the squash beetle, Epilachna borealis (Coccinellidae)

The oily droplets on the pupal integumental hairs of the squash beetle Epilachna borealis contain a mixture of alpha-, beta-, gamma-, and delta-tocopheryl acetates as major constituents. In addition, the secretion contains a number of minor components that appear to be dehydrocongeners of the major components. This is the first report of the occurrence of acetate esters of any tocopherol in nature.