Synthesis of unlabelled synthons

In this Section, the preparation of some unlabelled synthons for use in the synthesis of labelled carotenoids is discussed. 

To obtain the highest yields in the synthesis of carotenoids, it is important to keep the synthetic routes as short as possible, especially when working with valuable labelled substances. It is therefore advantageous to use Cs-synthons rather than C2- and C3-synthons. 

For the synthesis of carotenoids labelled in the central part, the C15 + C10 + C15 scheme is used. The syntheses of the labelled Cio-central parts are described in Section B.2. The C15-parts are used in the form of phosphonium salts. For nearly all the end groups of carotenoids, the syntheses of the corresponding Cis-phosphonium salts have been reported [58-61]. Many of these Ci5-phosphonium salts can be prepared in labelled form by application of the reactions discussed in Section B.l. In this Section the preparation of the Cis-phosphonium salts that are used in the synthesis of labelled p,P-carotene (3), astaxanthin (403) and spheroidene (97) is now described. 

The phosphonium salt 69, which provides the end group of astaxanthin (403), can also be synthesized starting from P-ionone (64), in ten steps [60]. Because of the availability of labelled P-ionone (64), this route gives access to the end group of astaxanthin (403) in labelled form. 

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