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3- -4,5-dihydrooxazole aldehyde

Enhanced anti selectivity is observed in reactions of lithiated 4.5-dihydrooxazoles bearing an additional substituent which facilitates the formation of rigid azaenolates by internal chelation of lithium13. Thus, reaction of 2-ethyl-4,5-dihydro-4,4-dimethyloxazole (10) with 2-methylpropanal gives a 56 44 mixture of adducts while (R)-2-ethyl-4,5-dihydro-4-(methoxymethyl)-oxazolc (12) reacts with the same aldehyde to yield a 90 10 mixture of adducts 1313. [Pg.609]

Using the above procedures, allyl a-azido alkyl ethers of type 281 were prepared by employing an unsaturated alcohol such as allyl alcohol [76] (Scheme 32). The reaction of an aldehyde with allyl alcohol and HN3 in a ratio of 1 3 9 carried out in the presence of TiCl4 as catalyst provided azido ethers 281, 283, and 285 in 70-90% yield. The ratio of reagents is critical to ensure a high yield of azido ether and to prevent formation of acetal and diazide side products [75]. Thermolysis of azido alkenes 281, 283, and 285 in benzene (the solvent of choice) for 6-20 h led to 2,5-dihydrooxazoles 282,284, and 286, respectively, in 66-90% yield. [Pg.41]

The alkylation of metalated imines, hydrazones, 4,5-dihydrooxazoles, 4,5-dihydroisoxazoles, 5,6-dihydro-4/7-1,2-oxazines and 2,5-dialkoxy-3,6-dihydropyrazines (i.e., azaenolates) is a commonly used method in asymmetric synthesis of enantiomerically enriched aldehydes, ketones, spiroacetals, amines, /J-oxo esters, carboxylic acids, lactones, 1,3-amino alcohols, /(-hydroxy ketones and amino acids. [Pg.969]

Keywords imidate, dimethyl aminomalonate, aldehyde, 1,3-dipolar cycloaddition, 4,5-dihydrooxazol... [Pg.72]

The tosyl compound reacts with aldehydes in the presence of potassium carbonate to yield 5-alkyl- or 5-aryl-oxazoles, the intermediate dihydrooxazoles (which can be isolated) eliminating toluene-p-sulfinic acid (Scheme 30). Use of acyl chlorides in place of aldehydes leads to 4-tosyloxazoles (288). Furthermore, alkylation of tosylmethyl isocyanide with an alkyl halide RfX, followed by treatment with an aldehyde R2CHO, yields a 4,5-disubstituted oxazole (289). A related reaction is that of A-tosylmethyl-iV -tritylcarbodiimide with aromatic aldehydes under phase-transfer catalysis to yield 2-tritylaminooxazoles which are readily converted into 2-amino-5-aryloxazoles (equation 117) (81JOC2069). [Pg.220]

The preparation of 2,5-dihydrooxazoles from a-hydroxy ketones, aldehydes and gaseous ammonia is illustrated in equation (165). A derivative of 2,5-dihydrooxazole is obtained by the 1,3-dipolar cycloaddition of benzaldehyde to the nitrilium ylide (306), generated from a-chlorobenzylidene-p-nitrobenzylamine (equation 166). [Pg.228]

Oxiranes are converted into 4,5-dihydrooxazoles by treatment with cyanides in the presence of sulfuric acid (equation 171). a-Lithiated isocyanides react with aromatic aldehydes to furnish 2-lithiodihydrooxazoles (equation 172) (79JCS(P1)652). [Pg.229]

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. [Pg.531]

Another efficient process has been described for the silylcyanation of aldehydes in the presence of salen-titanium alkoxide complexes [86]. When a chiral C2 symmetric bis(dihydrooxazole)-Mg complex is employed with aldehydes as substrates, both excellent chemical yields and enantiomeric excesses are obtained [87]. Chiral titanium complexes are also derived from optically active sulfoxi-mines and a titanium alkoxide precursor [88]. [Pg.486]

There are many methods for the cleavage of 4,5-dihydrooxazoles once they have served their purpose. An effective method for hydrolyzing them back to carboxylic acids employs trifluoro-methanesulfonic anhydride <92SC13>. Intermediate ring-opened esters (100) are A(-methylated, then saponified to the acids (Scheme 34). The oxazoles may also be converted into aldehydes or nitriles. In a one-pot, two-step procedure, 4,5-dihydrooxazoles are transformed into alcohols (101) <93TIj4893>. Chloromethyl methyl ether converts the dihydrooxazoles into ring-opened amides in the first step, and these are reduced with diisobutylaluminum hydride (Scheme 35). [Pg.287]

Several routes to 4-alkyl-2,5-dihydrooxazoles, which were not available by previous methods, have been discovered. Thermolysis of allyl a-azidoalkyl ethers (181) gives 2,5-dihydrooxazoles via triazoline intermediates <88JOC27>. The starting materials are obtained from aldehydes, allyl alcohol, and hydrazoic acid (Scheme 89). A hlorination of oxazolidines with r-butyl hypochlorite, followed by dehydrochlorination using potassium superoxide, also provides 4-alkyl-2,5-dihydrooxazoles in 40-93% yield <92TL7751>. [Pg.309]

Transition-metal complexes bearing chiral ligands catalyze an asymmetric aldol condensation of isocyanoacetates with aldehydes to afford a mixture of cis- and /rons-4,5-disubstituted-4,5-dihydrooxazoles (188) in high optical purity (Equation (27)). Both gold and silver ferrocenylphos-phine complexes are effective <94TL2713>. [Pg.311]

A new simple synthetic route to 2,5-dihydrooxazoles 71 by cycloaddition of allyl azido ethers 70 via triazoUnes was shown by Hassner et al. [37]. Earlier, they demonstrated that cr-azido ethers can be easily prepared from aldehydes using an alcohol, hydrazoic acid and titanium tetracliloride as well as the fact that thermolysis of azido ethers in the absence of a double bond forms imi-dates [35,36]. Using the above mentioned facts, the allyl azido ethers 70 were synthesized in good yields employing an aldehyde, an allyl alcohol and HN3 in a 1 3 9 ratio in presence of a Ti catalyst (Scheme 12). Allyl azido ethers 70, on thermolysis in benzene, proved to be ideal substrates for the formation of 2,5-dihydrooxazoles 71 in 66-90% yield. To show that oxazolines are formed via triazolines and not via an independent nitrene pathway, thermolysis of 70 was followed by NMR in hexadeuteriobenzene at 70 °C. [Pg.21]

Acetyl azides like 94 can be cyclized to dihydrooxazoles 95 in high yield (Fig. 35). Furthermore, the protocol can be extended to six-membered rings as shown in Fig. 36 for the synthesis of pipecolic acid 100 from azido-aldehyde 96. [Pg.52]

Primarily, TosMIC is a-deprotonated and added to the aldehyde C=0 to give 42, which cycUzes via O-attack at the isonitrile carbon ( 43) protonation leads to the dihydrooxazole 44, which undergoes base-induced elimination of p-toluenesulfinic acid to the oxazole 41. [Pg.173]


See other pages where 3- -4,5-dihydrooxazole aldehyde is mentioned: [Pg.1031]    [Pg.1031]    [Pg.302]    [Pg.212]    [Pg.160]    [Pg.510]    [Pg.529]    [Pg.302]    [Pg.337]    [Pg.212]    [Pg.1216]    [Pg.206]    [Pg.182]   


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4,5-Dihydrooxazoles

4,5-dihydrooxazol

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