Symmetrical carotenoids 92 Wittig condensation

Carotenoids of symmetrical structure are synthesized by double Wittig condensations that use as central building blocks either crocetindialdehyde (536) as a C2o-component (bond formation between C(7)/C(8) and C(7 )/C(8 )) or with the Cio-dialdehyde 2,7-dimethylocta-2,4,6-trienediaI (34) (bond formation between C(11)/C(12) and C(1 l )/C(12 )). 

The double Wittig condensation between the Cps-phosphonium salt 45, which was prepared from natural (-)-(2/ )-lavandulol (46) in a seven-stage synthesis sequence with retention of configuration, with crocetindialdehyde (536) as a symmetrical C2o-huilding block, gave the Cso-carotenoid (25, 2 5)-tetraanhydrobacterioruberin (47) [66] (Scheme 11). 

The most effective method for the synthesis of symmetrical carotenoids has proved to be the double Wittig condensation of a Cis-phosphonium salt 48 with the symmetrical Cio-di aldehyde 34. 

Formation of the C-C double bond between C(15)/C(15 ) (synthesis strategy 3) has been used in recent studies more rarely than the other strategies for synthesizing the carotenoid skeleton, but the synthesis of a series of symmetrical and unsymmetrical C4o-carotenoids by strategy 3 (synthesis principle C20 + C20) has been summarized [86]. The Wittig condensations of retinal (69) with various phosphonium salts and of retinylphosphonium salt with aldehydes for the preparation of p,p-carotene (3) and 8 -apo-carotenoids is considered in Chapter 3 Part VII. 

---