Hydroxylation procedures

The reagent Is expensive and poisonous, consequently the hydroxylation procedure is employed only for the conversion of rare or expensive alkenes (e.g., in the steroid field) into the glycols. Another method for hydroxylation utilises catalytic amounts of osmium tetroxide rather than the stoichiometric quantity the reagent is hydrogen peroxide in tert.-butyl alcohol This reagent converts, for example, cyc/ohexene into cis 1 2- t/ohexanedlol. 

An efficient and selective Cu-assisted ortho-hydroxylation procedure for the conversion of benzoate to salicylate has been described, involving trimethylamine N-oxide (TMAO) as the oxidant [191,192]. The reaction was assumed to proceed via oxidation of a Cu carboxylate complex by TMAO to produce the active species (postulated to be a Cu hydroxo complex, but with only circumstantial evidence), followed by oxygen transfer to the benzoate group (Scheme 14). Using a set of different amide derivatives of benzoic acid, Comba and co-workers gained additional mechanistic hints in support of a reactive Cu-oxo or Cu-hydroxo intermediate that is stabilized by a five-membered chelate [193]. A pre-equilibrium involving Cu the ligand, and TMAO was proposed, but details of the reaction are far from clear. 

Other variations in this hydroxylation procedure include the use of t-butyl hydroperoxide72 in place of hydrogen peroxide, and the use of PTC conditions.73... 

Practical aspects are much as discussed earlier, and a good laboratory steroid hydroxylation procedure has been published. Numerous additives (e.g. surfactants, antibiotics and fungicides) have been used to improve microbial hydroxylation of steroids, and some of these have been discussed previously... 

Hydroxylation of the allylic methylene group in a- and 5,6-epoxides 56 and 57 was very effectively realized by a combination of selenium dioxide and pyridine N-oxide in dioxane [22 a]. Starting from 56 the Ip-hj-droxy isomer of 58 was obtained as a single product (Eq. 8) while, under the same conditions, the acetate 57 was converted to a 78 22 mixture of the la- and 1 5-hydroxy isomers 59a and 59b, respectively (Eq. 9). In this context it is interesting to note that a similar hydroxylation procedure with... 

Rubottom, G. M., Vazquez, M. A., Pelegrina, D. R. Peracid oxidation of trimethylsilyl enol ethers. Facile a-hydroxylation procedure. Tetrahedron Lett. 1974, 4319-4322. 

This methodology, in association with a highly diastereoselective hydroxylation procedure, was exploited to great effeet by Solladi6 in the asymmetric synthesis of protected L-arabinitol (17), (Scheme 4.14) [21]. 

In summary, it is certainly feasible to achieve the (bio)hydroxylation of hydrocarbon compounds by using one of the many microorganisms used to date, but it is difficult to predict the likely site of oxidation for any novel substrate using monooxygenases. However, there are three strategies which can be employed to improve regio- and/or stereoselectivity in biocatal3ttic hydroxylation procedures ... 

Calcium ion activity in blood is strictly regulated by a hormonal feedback mechanism. When the activity tends to decrease, the parathyroids secrete parathyroid hormone (PTH), which stimulates renal tubular reabsorption of Ca , K , and Na and decreases the reabsorption of phosphate. The net effect is a decreased excretion of Ca , Na" ", and K and an increased excretion of phosphate. PTH stimulates the hydroxylase activity in the kidney, and there is a negative feedback system which means that an increased amount of 1,25(OH)2D3 inhibits this hydroxylation procedure. Other hormones such as prolactin, estrogen, and growth hormone also stimulate hydroxylase activity, as does as a low serum phosphate concentration [6]. 

Scheme 9). The acetoxymethyl analogue (33) in an alternative hydroxylation procedure using MCPBA only gave lyxo-product, epimerization being discouraged by hemiacetal formation between the C-3 formyl and C-5-hydroxy groups. 

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