Phenolic oxazolines

In addition, Rowlands has involved chiral sulfoxide-containing ligands for the catalytic addition of McsSiCN to aldehydes. " The ligand structure was based on a phenolic oxazoline scaffold with introduction of the sulfur substituent via cysteine derivatives. The best enantioselectivities of up to 61% ee were obtained with the bulkiest tert-butyl substituted ligand (Scheme 10.42). The effect of the sulfoxide configuration was studied, showing that the use of... 

The synthesis of Ciufolini, published in 2001 [6], is based on the strategy he had previously developed to assemble the spirobicyclic skeleton through an oxidative cyclization of phenolic oxazolines (see 3.2.1). The strategic plan for the synthesis of FR901483 (1) is outlined in Scheme 21 and starts with the same disconnection C(7)-C(8) as used by Snider and Sorensen, involving a retroaldol process which leads to aldehyde 6. This advanced intermediate could be prepared from the spirodienone 49 available from an adequate oxazoline such as 50, which comes from two tyrosine units. 

In summary, the total synthesis of (-)-FR901483 was accomplished by an oxidative cyclization of a phenolic oxazoline to a spirolactam in the key step. The longest linear sequence leading to FR901483 through a Snider-type inversion encompasses 20 steps from commercial L-tyrosine and it proceeds with an overall yield of 1%. The alternative synthesis involving a Sorensen-type Mitsunobu inversion is shorter (17 steps), and it affords identical overall yield (1.3%). 

The oxidative spirocyclization of phenolic substrates containing an internal nitrogen nucleophile provides a useful tool for the construction of nitrogen heterocycles [287, 315-318], For example, the hypervalent iodine-induced cyclization of phenolic oxazolines 251 affords the synthetically useful spirolactam products... 

In turn the oxazoline-containing polymer may then react very rapidly (e.g. at 240°C) with such groups as carboxyls, amines, phenols, anhydrides or epoxides, which may be present in other polymers. This reaction will link the two polymers by a rearrangement reaction similar to that involved in a rearrangement polymerisation without the evolution of water or any gaseous condensation products (Figure 7.14). 

Brunner and Berghofer (48) investigated ligands composed of a combination of salicylaldimines and oxazolines. Intriguing effects of the electronic character of the phenol were noted. The electron poor p-nitrophenol 74b provided the trans cyclopropane in 53% ee, compared to 6% ee using the parent phenol ligand 74a. 

A reaction which is analogous to the orthoester synthesis but which utilises glycosyl oxazolines affords means of synthesising, 2-trans-2 amino-2-deoxy-glycoside derivatives. Treatment of the gluco compomd (17) with phenol in the presence of toluene- -sulphonic acid thus gave the glycoside (18), and it was also utilised in a nucleoside synthesis 48). 

Bis(oxazoline) ligands have also been used in Wacker-type cycUzations. " For example, the phenolic derivative 238 was cyclized in the presence of ligand 13b complexed with palladium(ll) to yield the 2,3-dihydrobenzofuran 239 in 86% yield with an ee of 94% (Fig. 9.70). 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

The more challenging task of direct C-N bond formation has recently been accomplished by utilization of the oxazoline ring as the nitrogen donor [76 - 78]. This approach was demonstrated by the preparation of spirolactams such as 17 from oxazoline derivatives of tyrosine and related phenolic acids by oxidation with BAIB in trifluoroethanol (Scheme 25), and similar conversions of indole-tethered oxazalones to tetracyclic products through spirolactam intermediates. It is noteworthy that phenolic amides, amines, and iminoethers were not useful for this purpose [78]. 

Although diastereoselective intramolecular al-koxypalladations have been investigated intensively and have found application in synthesis (see above), there are few examples of enantioselective alkoxypalladations [2bJ. For instance, Hosokawa et al. were able to cyclize unsaturated phenol derivatives of type 62 in the presence of chiral (// -allyl-PdOAc complexes, i.e. 63), but only with modest enantioselectivities. Under the same conditions the conversion of the phenol 65 to chroman 66 (a compound related to vitamin E) proceeded in acceptable yields, but with only low asymmetric induction. Newer results by Uozumi et al., for instance the Pd-catalyzed cyclization of 67 to 68 in the presence of a chiral bis-oxazolin ligand [15], show that much higher enantioselectivities can be achieved, at least for certain substrates. 

A Pd-catalyzed oxidative cyclization of phenols with oxygen as stoichiometric oxidant in the noncoordinating solvent toluene has been developed for the synthesis of dihydrobenzo[ ]furans (Equation 136). Asymmetric variants of this Wacker-type cyclization have been reported by Hayashi and co-workers employing cationic palladium/2,2 -bis(oxazolin-2-yl)-l,l -binaphthyl (boxax) complexes <1998JOC5071>. Stoltz and co-workers have reported ee s of up to 90% when (—)-sparteine is used as a chiral base instead of pyridine <2003AGE2892, 2005JA17778>. Attempts to effect such a heteroatom cyclization with primary alcohols as substrates, on the other hand, led to product mixtures contaminated with aldehydes and alkene isomers, which is in contrast to the reactions with the Pd(ii)/02 system in DMSO <1995TL7749>. 

Fifty-six years ago the very first siderophore, mycobactin, was isolated by the crystallization of the aluminum complex. Mycobactins from Gram-positive Mycobacteria and the closely related nocobactins from Nocardia embody a series of lipid-soluble siderophores located in the lipid-rich boundary layers of these bacteria (Figure 2(c)). The X-ray structure revealed that iron binding in mycobactins is accomplished by two hydroxamates, a phenolate group, and oxazoline nitrogen. 

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