9 -Z-neoxanthin

The occurrence of xanthophyll dioxygenase, which releases xanthoxin from xanthophyll, was found using the ABA-deficient mutant vpl4oi Z. mays.650 Xanthophyll dioxygenase cleaves the double bond between C-ll (C-11 ) and C-12 (C-12 ) of 9-Z-violaxanthin (43) and 9 -Z-neoxanthin to give xanthoxin (5).651 The mRNA of xanthoxin dioxygenase in the leaves of P. vulgaris increased upon water stress and decreased upon... 

Luteochrome was hardly effective at all as a reversing agent, when compared to (8 R)- and (S Sj-luteoxanthin, (13Z)-zeaxanlhin, (13Z)-lutein, (9Z)-zeaxanthin, (9Z)-violaxanthin and (9/Z)-neoxanthin and their high fluorescence activity ratios on the mouse lymphoma cells (Table 4). 

The (9 Z)-isomer of neoxanthin (234) has been obtained by direct synthesis, [see Chapter 3 Part HI for the synthesis of the optically pure (all-E)-neoxanthin (234)]. The (9 Z) double bond was introduced via the (9Z)-epoxyaldehyde 40 (Scheme 16). Horner-Emmons reaction with the acetylenic Cio-diphosphonate 41, used in the synthesis of (all-E)-neoxanthin (234), furnished, after purification by chromatography, the (9Z)-15,15 -didehydrophosphonate 42 (yield 45%). Subsequent Horner-Emmons reaction of 42 with the allenic Ci5-aldehyde 43 gave (9 Z)-15,15 -didehydroneoxanthin 44 in 43% yield. Partial hydrogenation of the triple bond led to the (15Z,9 Z) isomer of 234 (yield 75%), and thermal isomerization furnished [(9 Z)-neoxanthin (9 Z)-234, m.p. 134-136°C] [38]. 

The most likely precursors of abscisic acid (44) are 9 -Z-violaxanthin (45) and 9-Z-neoxanthin (46). -Violaxan-thin (3) is converted to 9 -Z-neoxanthin in fluridone-treated etiolated plants of Lycopersicon and Phaseolus seedlings following exposure to light (Parry and Morgan, 1991). Cleavage of these xanthophylls across the 11,12- (11, 12 -) double bond may produce xanthoxin, which is readily converted to abscisic acid by plant tissues. Xanthoxin (47) serves as a source of abscisic acid in tomato and wheat. The epoxide oxygen atom becomes the tertiary hydroxyl oxygen atom of abscisic acid. 

Violaxanthin and neoxanthin (42) have E double bonds at C-9 and C-9, and these bonds should isomerize to Z double bonds to form the Z double bond at C-2 of ABA. The isomerase of the double bond seems to be essential, but has not been found so far. 

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