Ring-opening groups

The last group of reactions uses ring opening of carbonyl or 1-hydroxyalkyl substituted cyclopropanes, which operate as a -synthons. d -Synthons, e.g. hydroxide or halides, yield 1,4-disubstituted products (E. Wenkert, 1970 A). (1-Hydroxyalkyl)- and (1-haloalkyl)-cyclopropanes are rearranged to homoallylic halides, e.g. in Julia s method of terpene synthesis (M. Julia, 1961, 1974 S.F. Brady, I968 J.P. McCormick, 1975). 

The keto tautomer (211a) is involved in the high electrophilic reactivin-of the C-5 carbonyl group. Thus ring opening has been reported u ith various amino nucleophilic reagents. 

The reaction of amines with the 4-phenylazo derivative (228) results in their rearrangement into triazolines. Depending on the basicity of the amines and the size of the alkoxy group, three different triazolines (229. 230, and 231) are obtained (Scheme 117) (454. 459, 472). In all cases, the first step involves nucleophilic addition of the amine to the carbonyl group followed by ring opening and further ring closure. 

In the next group of chapters we shall discuss condensation or step-growth polymers and polymerizations in Chap. 5, addition or chain-growth polymers and polymerizations in Chap. 6, and copolymers and stereoregular polymers in Chap. 7. It should not be inferred from this that these are the only classes of polymers and polymerization reactions. Topics such as ring-opening polymeri-... 

Under acidic conditions, furfuryl alcohol polymerizes to black polymers, which eventually become crosslinked and insoluble in the reaction medium. The reaction can be very violent and extreme care must be taken when furfuryl alcohol is mixed with any strong Lewis acid or Brn nstad acid. Copolymer resins are formed with phenoHc compounds, formaldehyde and/or other aldehydes. In dilute aqueous acid, the predominant reaction is a ring opening hydrolysis to form levulinic acid [123-76-2] (52). In acidic alcohoHc media, levulinic esters are formed. The mechanism for this unusual reaction in which the hydroxymethyl group of furfuryl alcohol is converted to the terminal methyl group of levulinic acid has recendy been elucidated (53). 

Rea.ctlons, Butyrolactone undergoes the reactions typical of y-lactones. Particularly characteristic are ring openings and reactions in which ring oxygen is replaced by another heteroatom. There is also marked reactivity of the hydrogen atoms alpha to the carbonyl group. 

Nucleophilic Ring Opening. Opening of the ethyleneimine ring with acid catalysis can generally be accompHshed by the formation of an iatermediate ayiridinium salt, with subsequent nucleophilic substitution on the carbon atom which loses the amino group. In the foUowiag, R represents a Lewis acid, usually A = the nucleophile. 

Vitamins are classified by their solubiUty characteristics iato fat-soluble and water-soluble groups. The fat-soluble vitamins A, E, and K result from the isoprenoid biosynthetic pathway. Vitamin A is derived by enzymic cleavage of the symmetrical C q beta-carotene, also known as pro-vitamin A. Vitamins E and K result from condensations of phytyldiphosphate (C2q) with aromatic components derived from shikimic acid. Vitamin D results from photochemical ring opening of 7-dehydrocholesterol, itself derived from squalene (C q). 

Hydrogen bromide adds to acetylene to form vinyl bromide or ethyHdene bromide, depending on stoichiometry. The acid cleaves acycHc and cycHc ethers. It adds to the cyclopropane group by ring-opening. Additions to quinones afford bromohydroquinones. Hydrobromic acid and aldehydes can be used to introduce bromoalkyl groups into various molecules. For example, reaction with formaldehyde and an alcohol produces a bromomethyl ether. Bromomethylation of aromatic nuclei can be carried out with formaldehyde and hydrobromic acid (6). 

Other classes of yellow couplers reported ia the Hterature include the iadazolones (71) and the benzisoxazolones (72). Neither of these stmctures contains an active methylene group dye formation is beUeved to occur through a ring-opening process. 

Alkylthio derivatives have also been converted into oxo groups, sometimes via the carboxymethylthio derivative in cases, e.g. (98), where ring opening (Section 2.15.4.1) does not occur. 

Removal of AT-oxide groups by PCI3 follows normal behaviour (63JCS6073), but with acetic anhydride the AAoxides (332) underwent a complex ring-opening reaction leading to (333), and an isomeric 8-alkoxy-6-oxide behaved similarly (75H(3)38l). 

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