Regioselective etherification

Synthesis.—Reisch, Mester, and co-workers have made important contributions this year by synthesizing the alkaloids furacridone (34) and ( )-rutacridone (37) for the first time. Regioselective etherification of 1,3-dihydroxy-jV-methylacridone (32 R = H) gave the acetal (33), which furnished furacridone (34) as the major product of acid-catalysed cyclization (Scheme 4). Claisen rearrangements of the 3-allyloxy-acridone (32 R = CH2CH=CH2) and the propargyl derivative (32 R = CH2C=CH) were also studied.18... 

The regioselective etherification of 4,6-di-0-benzyl-l,2-0-isopropylidene-myo-inositol in the 3- or 5-position has been found to be strongly dependent on the nature of the alkylating agent used. ... 

Diastereoselective syntheses of dihydrobenzo[f>]furans have been accomplished by a rhodium-catalyzed regioselective and enantiospecific intermolecular allylic etherification of o-iodophenols as a key step, providing the corresponding aryl ally ether 122, which leads to a dihydrobenzo[b]furan by treatment of the intermediate aryl iodide with tris(trimethylsilyl)silane and triethylborane at room temperature in the presence of air <00JA5012>. 

Although palladium catalysts have played the most prominent role in this area, other metals have also been found to catalyze allylic etherification reactions, often providing complementary stereochemical outcomes. A few ruthenium catalyst systems have been used for the O-allylation of phenols,143,144 including an enantioselective version utilizing [Cp Ru(MeCN)3]PF6 that provides promising ee s, albeit with diminished control of regioselectivity (Equation (25)).145... 

Blocking the C-l OH of D-fructose and L-sorbose (Scheme 25) was effected in excellent yields through regioselective isopropylidene acetalation of the free ketoses, followed by etherification (benzylation or allylation) of the remaining primary alcohol. Acid-catalyzed hydrolysis of the isopropylidene groups and condensation with HSCN efficiently produced a sole fused bicyclic OZT. 

Tab. 10.7 Regioselective rhodium-catalyzed allylic etherification with ortfio-substituted phenols.

Enantiospecijic Rhodium-QjtalYzed Allylic Alkylation 207 Tab. 10.8 The scope of the regioselective rhodium-catalyzed allylic etherification reaction. 

Tab. 10.8 summarizes the application of rhodium-catalyzed allylic etherification to a variety of racemic secondary allylic carbonates, using the copper(I) alkoxide derived from 2,4-dimethyl-3-pentanol vide intro). Although the allyhc etherification is tolerant of linear alkyl substituents (entries 1-4), branched derivatives proved more challenging in terms of selectivity and turnover, the y-position being the first point at which branching does not appear to interfere with the substitution (entry 5). The allylic etherification also proved feasible for hydroxymethyl, alkene, and aryl substituents, albeit with lower selectivity (entries 6-9). This transformation is remarkably tolerant, given that the classical alkylation of a hindered metal alkoxide with a secondary alkyl halide would undoubtedly lead to elimination. Hence, regioselective rhodium-catalyzed allylic etherification with a secondary copper(l) alkoxide provides an important method for the synthesis of allylic ethers. 

The regioselectivity of the reaction was also investigated by means of NMR and HPLC studies. These investigations clearly showed that the etherification reaction... 

The fluoro-lactonization of alk-4-enoic acids and the fluoro-etherification of alk-4-enols with pentachloro-l-fluoropyridinium triflate (lo) proceed smoothly in a regioselective manner when the substrates contain an aryl substituent on the double bond.63 Some stereospecificity is observed in the case of 5-exo ring closure, possibly due to participation of the oxygen atom in the aryl-stabilized cationic intermediate. 

The etherification of calixarene analogues containing methylene and 1,3-propylene bridges leads in good yield and regioselectively to 1,3-di-O-substituted derivatives 90.175 To prevent enantiomerization the ether residues must be at least propyl... 

P-Hydroxy selenides are conveniently prepared from epoxides by treatment with sodium phenylse-lenide (Scheme 32) and by the addition of benzeneselenenic acid and its derivatives to alkenes (Scheme 33), - -" although in some cases these reactions are not regioselective. Useful phenylseleno -etherification and -lactonization reactions have been developed which can be regioselective (equation 42 and Schemes 34 and 35). -" " Selenide- and selenoxide-stabilized carbanions have been used in addition reactions with aldehydes and ketones, - and the reduction of a-seleno ketones also provides a route to P-hydroxy selenides. ... 

Similarly, chelation-assisted palladium-catalyzed oxidative functionalizations of C—H bonds with, for example, hypervalent iodine(III) reagents turned out to be particularly valuable. These protocols allowed for, inter alia, regioselective acetoxyla-tion or etherification of aromatic and aliphatic C— H bonds [17-19], and also halogenations of arenes (Scheme 9.3) [20, 21]. 

Cellulose, Polysaccharide, Activation, Cellulose solvents. Esterification, Etherification, Ionic liquids, NMR spectroscopy. Nucleophile displacement. Protecting groups, Regioselective functionalization... 

In a bromo-etherification process, amino alcohols 147, derived from the chiral pool, cyclized in a 1-endo manner to give rise to a range of chiral polysubstituted 1,4-oxazepanes 148 in good yields with moderate-to-excellent regio- and diastereoselectivities. The regioselectivity observed was... 

One-pot regioselective functionalization of glucopyranosides acetalation /reductive etherification... 

In this context, a one-pot regioselective protection of persilylated monosaccharides, using copper(ii) triflate as a single catalyst was able to promote acetalation-reductive etherification-acylation reactions. In the same way, the easy to handle, cheap and environment-friendly FeCls 6H2O complex was a very efficient catalyst to promote regioselective acetalation and reductive etherification on glucopyranosides. Hence, treatment of per-O-silylated a-methyl-u-glucopyranoside 50 with benzaldehyde (3 equiv) and triethylsilane (1.1 equiv) in the presence of... 

Epoxide 150 was then opened with lithium 2-ethylhexanoate to provide 151. This was not the first choice of nucleophile Lots of experiments were required to overcome problems such as acyl transfer and regioselectivity. There is no doubt that search is an important part of the word research . Williamson etherification of the C17 alcohol gave 152, and catalytic hydrogenation of the olefin provided 153 as the major product. This was converted to the Wender intermediate 154 (133) in a straightforward manner. 

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