Base-promoted Ring-opening

There is no published mechanistic study on the Auwers flavone synthesis. The mechanism may involve the nucleophilic addition of oxonium 7, derived from 1, with hydroxide to give 8. Base-promoted ring opening of 8 could provide the putative intermediate 9, which then could undergo an intramolecular Michael addition to form 10. Expulsion of bromide ion from 10 would then give flavonol 2. 

Azepin-2-ones are prepared by the ring expansion of 2,6-dialkylphenolates with ethereal chloramine at -70 °C (Scheme 28) (B-69MI51600), and by the mechanistically similar base-promoted ring opening and rearrangement of spiroquinol ethers (234) (81JOC4077). 

Na, J. Houk, K. N. HUvert, D. Transition state of the base-promoted ring-opening of isoxazoles. Theoretical prediction of catalytic functionalities and design of haptens for antibody production, 7. Am. Chem. Soc. 1996,118, 6462-6471. 

Hu, Y., Houk, K.N., Kikuchi, K., Hotta, K., Hilvert, D. Nonspecific medium effects versus specific group positioning in the antibody and albumin catalysis of the base-promoted ring-opening reactions of benzisoxazoles. J. Am. Chem. Soc. 2004,126, 8197-205. 

Hurst <93AHC(58)215> has summarized the results of nitration of 1-phenyl-A -pyrazolines for instance, l,5-diphenyl-3-aryl-2-pyrazolines are nitrated by potassium nitrate in sulfuric acid to afford the 1-p-nitrophenyl derivative in quantitative yield. Thermal ring opening of A -pyrazolin-3-carboxylic acids has been used as a method of preparation of )8-aminonitriles <94T7543>, while base-promoted ring opening of 1,1 -disubstituted-3-amino-A -pyrazolinium salts affords either N,N-disubstituted hydrazines or a,)S-unsaturated amidrazones <86SC585>. 

Scheme 6.39 Tandem base-promoted ring opening/Brookrearrangement/allylic alkylation of O-silyl cyanohydrins of [i-Silyl-a.[i-epoxyaldehydes.

Silyl and siloxy substituents can be used to assist both the acid- and base-promoted ring-opening of cyclopropanes, and several variants on this theme are possible. The ester (160), for example, has been converted into the substituted propene (161) by the action of NaCHjSOjMe. This product, and others similarly prepared from the sodium salts of alternative sulphones and sulphoxides, has been further elaborated to silyl-substituted cyclopentanes and ring-opening of (162) to enones(163). ... 

Dioxepanes 63 were hydrolyzed with aqueous hydrochloric acid to the starting diol. A thionyl chloride promoted ring-opening of dioxepane 63 to intermediate 64 has been reported. When treated with base, compound 64 can be transformed into vinylic ether 65 in 58% yield (81ZOR1047) (Scheme 31). 3-Methylfurazan-4-acetic acid was converted to the vinyl derivative 66 via an esterification, reduction, mesylation, and base elimination sequence (81JHC1247) (Scheme 31). 

Grubbs and coworkers studied a series of Ru-based catalysts and identified two lead catalysts for alkene cross metathesis that could provide the desired hetero-coupled product with high Z-selectivity for the resulting alkene moiety (Fig. 39) [91-93]. Comparably, Hoveyda and coworkers developed a number of Mo-based catalysts that enabled the selective synthesis of Z-alkenes [94, 95]. They successfully demonstrated that the novel molybdenum adamantylimido complexes (catalysts 154-156) could promote ring opening/cross metathesis (ROCM) of oxabicycle (157 or 158) in good yield and excellent Z-selectivity (Fig. 40) [96]. These elegant studies paved the way to develop further more effective catalysts for Z-selective alkene metathesis and utilize these methods for the synthesis of complex natural products. 

Our aim in this chapter is to present the most important advances in the domino reactions of carbohydrates based on the metal-promoted ring opening of cyclic sugars. These procedures, promoted by different low-valent organometaUic reagents, start with the addition of the metal to an electrophihc site (i.e., an alkyl hahde, a carbonyl, an alkene, or an alkyne), followed by reductive ehmination to the open-chain sugar. From this open-chain sugar intermediate, the next reaction takes place under the same reaction conditions. 

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