22S-Hydroxylation

CHOE, S., DILKES, B.P., FUJIOKA, S., TAKATSUTO, S., SAKURAI, A., FELDMANN, K.A., The DWF4 gene of Arabidopsis encodes a cytochrome P450 that mediates multiple 22-hydroxylation steps in brassinosteroid biosynthesis, Plant Cell, 1998,10, 231-243. 

Figure 5.22 Hydroxyl population with temperature of silica gel Kieselgel 60. aOH + free OH bridged OH (Gillis-D Hamers model).

Under oxidative stress testing, hydroxyls are oxidized to the corresponding ketone derivative [22]. Hydroxyl groups can also undergo elimination reactions to form the corresponding olefin. 

The sum of the two semi-reactions (2.23a) and (2.23b) gives the overall Eq. 2.22. Hydroxyl-ions represent the chemical species that close the electric circuit through the electrolyte. The alkaline solution contains about 30 wt% of potassium hydroxide and operates at about 80°C. Today, this technology gives... 

It should be noted that catalytic hydrogenation of 22R,23R butenolides from the aldol reactions gave products which had mainly the 24S stereochemistry. Initially, we had reasoned that if hydrogenation occurred at the face of the butenolide ring nearest the C-22 hydroxyl, the first butenolide would yield the 24S product while the second would yield the 24R isomer. The only explanation we can give for the fact that both yield mainly the 24S isomer is that the favored conformation of the sidechain is different in the two compounds so that the more accessible face for hydrogenation for 3 is opposite to that for 13. 

Comparison of the structure of jaborosalactone 2 (85) with that of trechonolide A (64) indicated that both compounds may have closely related biosynthetic precursors. In the former withanolide, ring closure on C-12 has occurred with a C-22 hydroxyl as shown in Fig. (4) pathway a. Oxidation of this intermediate or a related compound to the 22-ketone would allow cyclization between C-23 and the C-12 ketone to give a spiranoid withanolide (Fig. (4) pathway b). The co-occurrence of trechonolide A (64) and jaborosalactone 2 (85) in J. araucana supports this hypothesis. 

Scheme 10-49 6-Endo cyclization allows the stereocontrolled synthesis of 22-hydroxylated steroids.

The abusolute configuration of the C-22 hydroxyl group was confirmed by the advanced Moscher s method. 

Fig. 4. 22 Hydroxylation of the nitrogen of 4-substituted aeetanilides, ineluding phenaeetin (X=OCH3), is generally thought to occur by direct hydrogen abstraction from the nitrogen (a). However, computational results suggest...

Dehydrocholesterol is also a substrate for P450 llAl [1943], forming five 5,7-dienal products, with mono- and dihydroxy substitution [1949]. These include the 22-hydroxyl and 20,22-dihydroxy 7-dehydrocholesterol products. 

Hydroxyphenyl-pyruvate dioxygenase (D 22) Hydroxylation, oxidative decarboxylation Fe +, ascorbic acid, requires a-ketoglutarate... 

Fig. 7.32 Withanolide congener without lactone cyclization prevented by an acylation of the C-22 hydroxyl group and an ester gjyco-sidation of the C-26 carboxyl group (highlighted in grey)...

FIGURE 25.22 Hydroxylation and glycosylation of 6-deoxyerythronolide Bto give3-0-mycarosyl-erythronolide B. 

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