Alkenes continued hydroxylation

Yates and coworkers have examined the mechanism for photohydration of o-OH-8. The addition of strong acid causes an increase in the rate of quenching of the photochemically excited state of o-OH-8, and in the rate of hydration of o-OH-8 to form l-(o-hydroxyphenyl)ethanol. This provides evidence that quenching by acid is due to protonation of the singlet excited state o-OH-8 to form the quinone methide 9, which then undergoes rapid addition of water.22 Fig. 1 shows that the quantum yields for the photochemical hydration of p-hydroxystyrene (closed circles) and o-hydroxystyrene (open circles) are similar for reactions in acidic solution, but the quantum yield for hydration of o-hydroxystyrene levels off to a pH-independent value at around pH 3, where the yield for hydration of p-hydroxystyrene continues to decrease.25 The quantum yield for the photochemical reaction of o-hydroxystyrene remains pH-independent until pH pAa of 10 for the phenol oxygen, and the photochemical efficiency of the reaction then decreases, as the concentration of the phenol decreases at pH > pAa = 10.25 These data provide strong evidence that the o-hydroxyl substituent of substrate participates directly in the protonation of the alkene double bond of o-OH-8 (kiso, Scheme 7), in a process that has been named excited state intramolecular proton transfer (ESIPT).26... 

Osmium tetroxide (0s04, sometimes called osmic acid) reacts with alkenes in a concerted step to form a cyclic osmate ester. Oxidizing agents such as hydrogen peroxide (H202) or tertiary amine oxides (R3N+—O-) are used to hydrolyze the osmate ester and reoxidize osmium to osmium tetroxide. The regenerated osmium tetroxide catalyst continues to hydroxylate more molecules of the alkene. 

Dihydrooxazoles continue to occupy an important place in organic synthesis and medicinal chemistry as they have found use as versatile synthetic intermediates, protecting groups/pro-drugs for carboxylic acids, and chiral auxiliaries in asymmetric synthesis. There are several protocols in the literature for the transformations of functional groups such as acids, esters, nitriles, hydroxyl amides, aldehydes, and alkenes to 2-oxazolines. Newer additions to these methods feature greater ease of synthesis and milder conditions. 

TSl has also been used in commercial epoxidations of small alkenes. A major limitation with this catalyst is its small pore size, typical of many zeolite materials. This makes it unsuitable for larger substrates and products. Again like many zeolites, it is also less active than some homogeneous metal catalysts and this prevents it from being used in what would be a highly desirable example of a green chemistry process - the direct hydroxylation of benzene to phenol. At the time of writing, commercial routes to this continue to be based on atom-inefficient and wasteful processes such as decomposition of cumene hydroperoxide, or via sulfonation (Scheme 1.1-3). 

After the addition and decarboxylation, the chain can continue to grow through the addition of another malonyl fragment (and decarboxylation) forming keto groups interrupted by methylenes until told to stop as dictated by the particular synthase enzyme. Alternatively, reduction at each stage of carbonyl to hydroxyl at each stage of malonyl addition, elimination of water to an ( )- or (Z)-alkene, and... 

The synthesis of chiral hydroxylated pyrrolidines, piperidines, and indolizidines continues to attract attention. The synthon (73) can be prepared stereospecifically from the E-alkene (74) as outlined in Scheme 20 the Z-alkene, produced together with (74) by... 

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