Oxazole, sugar derivatives

The synthesis of oxazole C-nucleosides in moderate yields by Tosmic addition to sugar derived aldehydes and concomitant cyclization has been reported. In particular, aldehyde 130 gave 131 in 48% yield <03SL1619>. 

The fused oxazole derivative 100 was synthesized from 3-azido-3-deoxy-l,2 5,6-di-O-isopropylidene-a-D-allofuranose, the key final step being the reaction of methyl 3-amino-3-deoxy-5,6-0-isopropylidene-p-D-alloside with DMF dimethyl acetal. The annelated pyranoside 101 was obtained by cyclization of a branched chain hexosulose derivative, and the fused triazole-piperidinoses such as 102 were made by a radical cyclization of a 3-pyrazolo-6-iodo-sugar derivative. The spiro-isoxazohne 103 and related isomers have been synthesized by dipolar cycloadditions of mesitonitrile oxide to 2-deoxy-2-C-methyl-ene-pentonic acids, themselves available in five steps from D-mannitol. Intra-... 

Fused oxazoles, particularly tetrahydrofuro[3,2-d]oxazoles, are abundandy present in natural products, modified sugar derivatives, and important bioactive molecules. Ahmed and coworkers have reported a highly efficient, novel, microwave-assisted, metal-free, diastereoselective synthesis of... 

In the second method, which can be applied to compounds with an optically active center near the potentially tautomeric portion of the molecule, the effect of the isomerization on the optical activity is measured. In favorable cases both the rate of racemization and the equilibrium position can be determined. This method has been used extensively to study the isomerization of sugars and their derivatives (cf. reference 75). It has not been used much to study heteroaromatic compounds, although the very fact that certain compounds have been obtained optically active determines their tautomeric form. For example, oxazol-5-ones have thus been shown to exist in the CH form (see Volume 2, Section II,D,1, of article IV by Katritzky and Lagowski). 

Of the many substituted and functionalized alkenes that have been combined with diazo dipoles to give A -pyrazolines or products derived from them (i.e., A -pyrazolines, pyrazoles, cyclopropanes), only a selection will be mentioned. These include ot-alkylidene-cycloalkanones (62), -flavanones, -thioflavanones, -chroma-nones, and thiochromanones (63,64) a-arylidene-indanones and -indolones (65) diarylideneacetones (66) l-benzopyran-2(77)-ones (coumarins) (67,68) 4-nitro-1,2-oxazoles (69) 2-alkylidene-2-cyanoacetates (70) dimethyl 2,3-dicyanofuma-rate (71) tetracyanoethylene (72) tetraethyl ethylenetetracarboxylate (72) 1,4-quinones (35,73-75) 2-X-l,l,l-trifluoro-2-propene [X = Br, (76), SPh, SOPh, S02Ph (77)] nitroalkenes (78) including sugar nitroalkenes (79) 1-diethoxyphos-phoryl-1-alkenyl-sulfoxides (80) methyl 2-(acetylamino)cinnamate and -acrylate... 

Considerable work was done to induce chirality via chiral auxiliaries. Reac tions with aromatic a-ketoesters like phenylglyoxylates 21 and electron-rich al kenes like dioxoles 22 and furan 23 were particularly efficient (Scheme 6). Yield up to 99% and diastereoselectivities higher than 96% have been observed whet 8-phenylmenthol 21a or 2-r-butylcyclohexanol 21b were used as chiral auxiliarie [14-18]. It should be noted that only the exoisomers 24 and 25 were obtained from the reaction of dioxoles 22. Furthermore, the reaction with furan 23 wa regioselective. 24 were suitable intermediates in the synthesis of rare carbohydrate derivatives like branched chain sugars [16], Other heterocyclic compounds liki oxazole 28 [19] and imidazole 29 [20] derivatives as well as acyclic alkenes 3fl 31, and 32 [14,15,21,22] were used as olefinic partners. Numerous cyclohexane derived alcohols [18,21-24] and carbohydrate derivatives [25] were used as chiri... 

The synthesis of all three sugars proceeds through a common intermediate, L-/yxo-1,4-lactone (393). Lactates 377 or 391 are converted to the lithio derivative 389 by alkaline hydrolysis with lithium hydroxide. The carboxyl ate is then activated with DPP A and reacted with the sodium salt of ethyl isocyanoacetate to give oxazole 390 (Scheme 53). 

Various 2-(alditol-l-yl)oxazole derivatives, e.g. 78, have been obtained in modest yields from sugar lactone oximes via their (3-vinyl ethers, e.g. 76- 77 (Scheme 16). ... 

Sugar lactone oxime O-vinyl ethers such as (109) formed oxazole derivatives such as (110) on thermolysis (200°, 2 min) (Scheme 11), and a mechanism involving initial radical cleavage of the N-0 bond was proposed. ... 

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