Cyclization-dimerization

The cyclization reactions discussed here either involve the intramolecular reaction of a donor group D with an acceptor group A or a cyclizing dimerization of two molecules with two terminal acceptors and two donors. A polymerization reaction will always compete with cyclization. For macrolides see p. 146 and p. 319 — 329. 

Scheme 17.12 Cyclization-dimerization of a-allenyl acids and ketones.

Scheme 17.13 Alternative mechanism for the cyclization—dimerization reaction of an a-allenyl acid and a ketone.

Now, consider a thermal [2 4 2] cyclization, dimerization of ethylene. This would involve overlap of the HOMO, tt, of one molecule with the LUMO, tt, of the other. But w and n are of opposite symmetry, and, as Fig. 29.21 shows, lobes of opposite phase would approach each other. Interaction is antibonding and repulsive, and concerted reaction does not occur. 

In the decay of the radical cation of 4-methyldiphenylamine (44), for which an electronic absorption spectrum with A.max = 689 nm was observed, the main reaction route is the formation of a benzidine-type dimer, similarly to that in the case of 39+. The dimerization rate constant is 2.3 x 104 M 1 s 1. However, in the presence of a large excess of parent molecules acting as a base, the formation of cyclized dimers was also suggested. In contrast, the formation of cyclic structures was found to be characteristic of the radical cations of 3 -substituted (45) and 3,3 -disubstituted (46) derivatives of diphenylamine. On the basis of CV measurements, the formation of dihydrodiphenylphenazine derivatives may be anticipated and, consequently, 47 is assumed to be the product of dimerization of 45. In both cases the rate constants were as fast as ca 1 x 107 M 1 s-1 the 3-methyl substituent promotes visibly the reaction between the 6-position of the phenyl ring of... 

A full paper describing the synthesis of 2,4-disubstituted furans in high yields through a novel oxidative cyclization-dimerization reaction between two different allenes was provided <05JOC6291>. 

Regioselective [4-F2] cycloadditon of a-thioxothioamides 334 to the C=N bond of heterocumulenes, followed by S extrusion, typically affords 2-thioxothiazoles. However, attempts at using the phenylene-l,2-diisothiocyanate 335 as the heterocumulene did not give the expected mono- or bis-thioxothiazoles 338. Instead, the reaction afforded primarily the pentacyclic thiadiazine derivative 337, which is formed by the rapid cyclization/dimerization of the diisothiocyanate, and precipitates out of the reaction mixture (Scheme 69) <2001HAC617>. 

Lactide (3,6-dimethyl-l,4-dioxane-2,5-dione) is a cyclized dimer of lactic acid (anhydrous lactic acid), and it is used in the manufacture of PLA resin (see Chapters 1 ) for the plastic industry. The compound exists as a translucent white powder or clear flakes, is odorless at room temperature but emits an acrid plastic odor when heated. LA decomposes when exposed to moisture to form the linearized lactic acid dimer followed by hydrolysis to lactic acid monomer [22]. 

Reduction Abstraction of a Hydrogen Atom Intramolecular Cyclization Dimerization Substitution by Solvents ... 

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