2-Ambo-a-tocopherol

While there is no universally accepted scientific terminology for representation there is general agreement on a certain working nomenclature (refs. 97,98). Thus a-tocopherol, the natural stereoisomer is as stated earlier (RRR)-a-tocopherol, the 2-epimer is 2-epi-a-tocopherol and a mixture of (RRR) and (SRR)-a-tocopherols is 2-ambo-a-tocopherol. Synthetic a-tocopherol prepared from synthetic phytol or isophytol is called all-rac-a-tocopherol, since it is theoretically a mixture of (RRR) and (SSS). The mixture of reduction products of 5,7,8-trimethyltocotrienol is called 4 -ambo-8 -ambo-a-tocopherol since as a natural product, although it has a 2R centre, the stereochemistry from hydrogen addition at the 4- and 8- positions is probably substantially RS. 

In the original synthesis of a-tocopherol, due to Karrer, dl-tocopherol (present day, 2-ambo-a-tocopherol) was obtained by the reaction of phytyl Ixomide (from natural phytoi) and trimethyihydroquinone In the presence of zinc chloride. 

Synthetic a-tocopherol exhibits, unKke natural a-tocopherol, lower biological activity, because it consists of the C-2 enantiomers, (2J )-a-tocopherol and (2S)-a-tocopherol. In short, synthetic a-tocopherol is (2J S)-a-tocopherol (D,L-a-tocopherol or 2-ambo-a-tocopherol). Synthetic a-tocopherol can also be a racemic mixture of all possible stereoisomers and it then consists of (2RS)-, (A RS)-and (8 J S)-a-tocopherol (or 2-ambo-, A -ambo and 8 -ambo-a-tocopherol), which is called all-rac-a-tocopherol. 

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