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Cyclization by Intramolecular Addition Reactions

The most important process (which accounts for most of the uses of radical cychzations in synthesis) is the selective 5-exo-cyclization of the 5-hexenyl radical to give the cyclopentyl methyl radical. This occurs even though the alternative - a 6-endo cyclization to give a more stable, cyclohexyl radical - is thermodynamically more favorable. Thus, the 5-ex o-cycHzation proceeds under kinetic control. The preference for 5-ex o-cyclization is explained by an early transition state with little product character. The transition state is a strain-free chair-like arrangement, which nicely accommodates the stereoelectronically required attack angle on the alkene. This model also nicely explains the stereochemical outcome of the cycHzation reaction. Assuming that substituents prefer to adopt pseudo-equatorial positions in the chair-Uke transition state, we see why  [Pg.137]

The l-methyl-5-hexenyl radical also gives predominantly the ds-product, due to a combination of steric and stereoelectronic factors. 5-exoCydization proceeds with [Pg.137]

1) When the alkene has a substituent at the 5-position G-endo cyclization is then faster than 5-exo. [Pg.138]

2) In the cyclization of vinyl radicals. Here, the 6-ring product is formed by rearrangement of the kinetic 5-ring, via a cyclopropyl methyl radical. The ratio of 5-exo to G-endo products can be controlled to some extent by altering the concentration of BUjSnH. [Pg.138]

Cyclization of the 6-hexenyl radical could conceivably be used to make cyclohexanes via ( -exo ring closure. However, this reaction is far less widely used in synthesis than the 5-hexenyl radical cyclization since  [Pg.139]


See other pages where Cyclization by Intramolecular Addition Reactions is mentioned: [Pg.86]    [Pg.136]   


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Addition, intramolecular cyclization

By cyclization

By intramolecular

Cyclization reactions

Cyclizations intramolecular

Cyclizations intramolecular addition

Intramolecular addition

Intramolecular cyclization

Intramolecular reactions addition

Intramolecular reactions cyclizations

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