Components of various functionality

Oxidation of 5 with Mn02 in acetone led to the acetylenic Cio-dialdehyde 6. Partial hydrogenation over Lindlar catalyst [3,4] provided (2 ,4Z,6 )-2,7-dimethylocta-2,4,6-triene-1,8-dialdehyde (7), which was isomerized to the (all- )-Cio-dialdehyde 8. This Cio-building block 8 (Cio-dialdehyde) is by far the most important unit in carotenoid synthesis. 

The acetylenic diol 1 has been used for the preparation of the phosphonium salts 9 (route 7 —and 10 (route 7 7772 70) which have been applied to the synthesis of p,p-carotene (3) [6]. The phosphonium salts 9 and 70 also proved their utility in the syntheses of 7,8-didehydroastaxanthin (402) and 7,8,7 ,8 -tetradehydroastaxanthin (400) [7] and of optically active carotenoids with 3,5,6-trihydroxy-5,6-dihydro-p-end groups [8]. Despite these interesting examples it is noteworthy that, in general, the diphosphonates are much better reagents for double olefination than the corresponding diphosphonium salts [9]. 

The acetylenic diphosphonate 73 is easily obtained by reaction of dibromide 4 with two equivalents of triethylphosphite under Arbusov conditions [5]. The specific advantages of the diphosphonate 13 have been demonstrated in the field of the synthesis of allenic carotenoids [10]. 

An alternative route starts from furan (19). Reaction with bromine in methanol leads to 2,5-dimethoxy-2,5-dihydrofuran (20) which is transformed to but-2-ene-1,4-dial bisdimethyl-acetal (21). Double enol ether condensation with 1-propenyl methyl ether (22), followed by acetal hydrolysis and elimination, provides crystalline (all- -Cio-dialdehyde 8 in an overall yield 50% [13]. 

Wittig condensation of the acetals 23 and 24, followed by hydrolysis of the resulting bis-acetal, constitutes another industrially attractive route to the Cio-dialdehyde 5 [14], 

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