Five-membered ring formation Subject

The preference for closure to the five- rather than the six-membered ring is opposite to what would be predicted on the basis of radical stability (pri < sec). This matter has been the subject of considerable study, but a detailed explanation for the preference for five-membered ring formation has yet to be developed. It is known that the cyclization is very rapid (/ 1 x 10 sec ) and irreversible. Generation of the cyclopentylmethyl radical does not lead to products derived from the opened 5-hexenyl radical. 

Various alioyolio epoxides have been subjected to the action oJ oarbanionoid reagents. Cyclopentene oxide (Eq. 887) is unique thus far, in that it gives no lactone.870 Lactone formation is prevented b the high activation energy required to establish irwwt-fusion between two five-membered rings.870... 

Cyclization attendant by coupUng from molecules containing both an amino group and an allene unit has apparent utility in synthesis. Formation of five- or six-membered rings is subject to change of the substituent pattern of the allene moiety. The bond distance between the nitrogen atom and the unsaturation is of necessity important. Diazetidines have been acquired from 4-hydrazinyl-l,2-alkadienes. ... 

A dehydrogenation was also observed when the 3-carboline dienyne 145 was subjected to the phosphine-free catalyst 4 at 80°C in toluene (Scheme 2.51) [28]. Formation of 146 implies initial attack of the ruthenium carbene on the allyl group attached to the nitrogen atom embedded in the five-membered ring. 

The intramolecular version of ester condensation is called the Dieckmann condensation.217 It is an important method for the formation of five- and six-membered rings and has occasionally been used for formation of larger rings. As ester condensation is reversible, product structure is governed by thermodynamic control, and in situations where more than one product can be formed, the product is derived from the most stable enolate. An example of this effect is the cyclization of the diester 25.218 Only 27 is formed, because 26 cannot be converted to a stable enolate. If 26, synthesized by another method, is subjected to the conditions of the cyclization, it is isomerized to 27 by the reversible condensation mechanism. 

The transition metal catalysed formation of five membered heterocycles through the insertion of a triple bond has also been explored. o-Halophenyl-alkynylamines, propargylamines and propargyl-ethers have been subjected to ring closure reactions. These processes, however also require the presence of a second, anionic reagent, which converts the palladium complex formed in the insertion step to the product. 

Due to the existence of five-membered furan rings in low-rank coals [49], the pyrolysis mechanism of furan has become the subject of several experimental studies [50-52]. Recently, some detailed theoretical studies have been carried out to understand the formation processes of CO, C3H4 (propyne and aUene), ketene, acetylene, and so on. 

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