Endocyclic ring opening

FIGURE 13.1 Ring expansions throngh (a) o-bond rearrangement to an exocyclic carbonium ion center and (b) endocyclic ring opening. 

Another interesting cationic domino process is the acid-induced ring opening of a-cyclopropyl ketones and subsequent endocyclic trapping of the formed carboca-... 

Evidently, the cleavage of the weak endocyclic N-0 bond is the driving force of the ring opening. The nucleophilicity of the /V-oxide oxygen atom in nitronates facilitates the backward cyclization reaction (in the case of minimization of steric hindrance). With regard to the above mentioned one cannot exclude the tautomerism between cyclic nitronates 100 and open (or chain) isomers 101. 

It is worth noting that 6,7-dihydro-2(3//)-oxepinone is an unusual lactone because it can be polymerized by two distinct mechanisms ROP of the cyclic esters by aluminum alkoxides, and the ring-opening metathesis polymerization (ROMP) of endocyclic olefins by the Schrock s catalyst (Fig. 28) [121]. 

Note that some general considerations of ring-opening polymerisation originate from studies of the coordination polymerisation of tiiranes. Other thiacyclic monomers that contain an endocyclic sulphur atom and an exocyclic oxygen atom, such as monothiocarbonates, are instead analogous to five-membered cyclic carbonates. 

As pointed out in the previous sections ring-opening reactions with preferential endocyclic P-P scission is a most typical feature in the reactivity of diphosphiranes. However, the substitution of one aryl group in diphosphiranes 133-133d by a chlorine atom leading to the trans P-chlorodiphosphiranes 153 via the nj-isomer intermediate has... 

Exo- and endo-cyclic ring closure reactions using 0-nucleophiles transform oxocarbenium ions into cyclic acetals. As an example of an endocyclic cyclization, epoxide ring opening of (96) with lithium dimethyl cuprate, and subsequent treatment of the resulting alcohol with acid, smoothly gives the bicyclic acetal (97), ° a key intermediate in the total synthesis of tirandamycic acid (Scheme 47). ... 

Dimroth rearrangement Isomerization of heterocycles in which endocyclic or oxocyclic heteroatoms and their attached substituents are translocated via a ring-opening-ring-closure sequence. 144... 

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