Base-catalyzed fragmentation

Base-catalyzed fragmentation also occurs on treatment of 5,6j9-epoxy-19-aldehyde (hemiacetal, hemiacetal-acetate or 3)5,6/ -acetal) accessible from nitrous acid-acetic acid treatment of 5a-bromo-6jS-hydroxy-19-oximes followed by mild base hydrolysis (yield not reported)... 

FIGURE 6.34 Regeneration of a-tocopherol by base-catalyzed fragmentation of 5a-toco-pheryl ascorbate (47) followed by a redox process. 

The base-catalyzed fragmentation of 4-alkyl-l,2,3-thiadiazoles is a useful method for the preparation of alkyne-1-thiolates <1996T3171>. These alkyne-l-thiolates can then react with carbon disulfide to afford l,3-dithiole-2-thiones. This strategy has been developed to give a synthesis of some novel tetrathiafulvene derivatives <1996T3171>. 

Concerning the mechanism of the rearrangement, it is proposed to involve the addition of the nucleophilic nitrogen of the 1,3-ambident nucleophiles to C-6 (see Section III,A,2,a). This covalent adduct is in equilibrium with the open-chain diamidino compound 83 (R = OCH3, SCH3). Cycliza-tion into 84, followed by a base-catalyzed fragmentation of the nitrogen-carbon bond at N-3 and expulsion of the thiomethyl or methoxy anion, yields the 2-amino-4(5)-phenylpyrimidines (Scheme III.47). 

In conclusion, we demonstrate by this preliminary review of our work that the base-catalyzed fragmentation of B-hydroxynitro-samines is a general transformation. The rate of the transformation is a significant function of structure as well as the catalyst and the conditions. Several other base-catalyzed transformations of nitrosamines have been observed and all of these reactions are expected to be of significance in the area of nitrosamine carcinogenesis. 

Primary, secondary and tertiary amines behave towards 1,2,4-trioxolanes bearing at least one ring hydrogen atom first of all like a proton acceptor, initiating the base-catalyzed fragmentation already discussed in Section 4.33.3.3.1. 

In particular, Schanze proposed that the mechanism of base-catalyzed fragmentation in 2-(4-X-phenylamino)-l,2-diphenylethanol radical cations (Scheme 61) changes with the oxidation potential of the substrate, going from an ElcB-like mechanism (as in Scheme 59, paths a and d) with the more stable radical cations (X = OMe and Me), to an E2-hke mechanism (as in Scheme 59, path e) when X = H, Cl, CN. 

The structure of crepidine (57), a most intriguing alkaloid isolated from Dendrobium crepidatum, has been deduced by X-ray crystallographic analysis of its methiodide. An unusual but readily explained base-catalyzed fragmentation of crepidine methiodide [(64) — (65)] is given in the same report. " Apparently... 

Seright and Henrici [26] mentioned that it is possible for xanthan to undergo the following base-catalyzed fragmentation ... 

The effect of sodium hydroxide on the xanthan flow curves is more than that expected from the charge shielding mechanism observed with sodium chloride. One possible explanation of this effect is base-catalyzed fragmentation reactions [26,32]. Fragmentation reactions break the biopolymer backbone (cellulose-like structure) to smaller saccharide units. Consequently, the hydrodynamic radius of the biopolymer would decrease and the viscosity of the polymer solution would diminish. 

Bromocalixarenes 28b and 28c react with highly enolizable compounds such as 1,3-pentanedione, affording calixarenes incorporating acetylacetonyl groups at the bridges. These groups can be further modified via reaction with hydrazine or via base catalyzed fragmentation as exemplified in Eq. 4.25 for 28b [27, 28]. 

SCHEME 16.41 Base catalyzed fragmentations of nitroso acetals. 

Vasilevsky, S.F., Mikhailovskaya, T.F., Mamatyuk, V.I. et al. (2009) Competition between 5-exo and 6-endo-dig anionic cyclizations of hydrazides of o-acetylenyl benzoic acids and based-catalyzed fragmentation/recyclization of the initial 5-exodig products.of Organic Chemistry, 74, 8106-8117. 

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