Remote hydroxylation

Remote hydroxyl groups are compatible with BINAP-Rh(l)-catalyzed allylic amine isomerizations... 

Quinkert and coworkers have also described a clever synthesis of the lichen macrolide (+)-aspicilin (17) using this ring opening/trapping strategy in which the trapping is done intramolecularly by a remote hydroxyl functionality, affording a macrolide product... 

In entry 15 of Table 21.10, it is noted that even a remote hydroxyl group directed hydrogenation by the cationic [Rh(diphos-4)(nbd)]+ catalyst to afford a moderate diastereoselectivity (80 20) [23]. This is an interesting example of long-range 1,5-asymmetric induction. 

Katritzky et al. reported that alkyl halides convert to thiols in one pot when treated with a thiopyridine, which initially transforms to the pyridi-nium salt (34). Intramolecular ipsosubstitution takes place in the s t (34) by the remote hydroxyl group to afford the corresponding thiols shown in reaction (49) (85TL469). Thus, pyridinium sulfides readily undergo both... 

Remote hydroxylation. Dehalogenation of ot-bromo ketosteroids by AgSbF6 generates by a series of hydride shifts cyclic oxonium salts, which are converted on hydrolysis into hydroxylated steroids. 

We used these conditions (low concentration and high temperature) in order to obtain a remote hydroxylation of a steroid D-ring (cf. Scheme 18). Thus the synthesis of 16-hydroxy norcholane in 35% yield is the result of a regioselective intramolecular 1,5-H shift [23],... 

Transannular reactions represent an interesting family of hydrogen transfer processes that can be applied for highly selective and efficient remote functionalization. For instance, Zard reported -functionalization in the longifolene series via Barton decarboxylation of isolongifolic acid (Scheme 21, Eq. 21.1) [97]. A spectacular regio-and stereoselective remote hydroxylation of bicyclic ketone is reported by Winkler... 

Conventional chemical syntheses of metabolites are often difficult and can be time consuming e.g. remote hydroxylation. Microbial systems are particularly attractive where stereo or regiospecific transformations are required. 

The use of molar equivalents of methylboronic add and steroid diol affords only the cyclic ester. Acyclic boronate esters will be formed at remote hydroxyl groups if excess reagent is used. While these latter derivatives display undesirable GC properties, the acyclic boronate groups are readily displaced by trimethylsilyiation the cyclic methaneboronate will be unaffected under the mild silylation conditions described above. n-Butyl boronate—methoxime—TMS derivatization has been... 

By contrast, peptide 124 was found to populate a canonical type 11 p-tum in its lowest energy ensemble (Fig. 13b). That it was discovered from a random combinatorial library speaks to the possibly privileged nature of this secondary structure element [188]. Furthermore [183], the structure validated a number of hypotheses based on sequence variations and truncation studies that support a remote hydroxyl-directed mechanism of action for the oxidation of the 6,7-position of famesol by 124. Importantly, a sidechain ether is implicated as a key acceptor of a hydrogen bond from famesol. 

At this point, a Stork-Crabtree directed hydrogenation was pursued, yet we obtained the (—)-2-cpi-cylindricine C 404 in 54% yield and 403 was recovered in 35% yield (entry i in Scheme 12.100). Alternatively, we found that a remote hydroxyl-directed reduction of the vinylogous amide with Na (OAc)BH3 gave (-)-cylindricine 384 in 83% yield (entry ii). 

Salomon, R.G., Sachinvala, N.D., Raychaudhuri, S.R., and Miller, D.B. (1984) Stereocontrol of Michael hydride reducfion by a remote hydroxyl group a strategy for stereorational total synthesis of spatane diterpenes. J. Am. Chem. Soc., 106,2211-2213. 

Myers has documented a related C=N addition, in which the diastereose-lection depends on the presence of a remote hydroxyl substituent [57, 58]. The addition of phenyl acetylide to TBS ether 35 afforded products with little diastereoselectivity (1 1.5 dr, Equation 5). However, when the addition was conducted with the substrate 36 incorporating a free hydroxy group, high diastereoselectivity was noted (11 1 dr). It was hypothesized that this latter result arose through the intervention of the Mg chelate 41. Formation of this chelated magnesium alcoholate 41 enforces a conformation that preferentially exposes one of the quinolinium diastereofaces. 

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