In summary, as it has been related above, the increasing use of RCM to achieve
the medium-ring oxacycles must be highlighted.
2.2 Cyclization to Form Carbon–Carbon Single Bond
This strategy has been widely utilized for the construction of medium-sized ethers.
This section is focused on metal-catalyzed cyclization, free radical cyclization, and
alkylations.
2.2.1 Free Radical Cyclization
Free radical cyclizations are important synthetic tools due to mild reaction conditions, along with high levels of chemoselectivity and stereoselectivity. However,
there are problems associated with the formation of medium-sized rings. Thus,
there are only few examples in the literature for the construction of eight-membered
ring systems, in general, and ethers, in particular, by radical cyclization. Herein,
recent advances in radical cyclizations that have led to the development of new
methods to efficiently construct eight-membered ethers are reported. To the best of
our knowledge, up to now nine- and ten-membered ring ethers have not been
synthesized by using radicals.
Majumdar [127] reported thiophenol-mediated radical cyclization reactions to
form benzoxocine derivatives. As shown in Scheme 30, the eight-membered ether
148 was formed from enyne 145 by using a regioselective thio-radical addition
Scheme 29 Effect of the catalyst on the olefin geometry in the RCM reported by Fu ¨rstner et al.
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the medium-ring oxacycles must be highlighted.
2.2 Cyclization to Form Carbon–Carbon Single Bond
This strategy has been widely utilized for the construction of medium-sized ethers.
This section is focused on metal-catalyzed cyclization, free radical cyclization, and
alkylations.
2.2.1 Free Radical Cyclization
Free radical cyclizations are important synthetic tools due to mild reaction conditions, along with high levels of chemoselectivity and stereoselectivity. However,
there are problems associated with the formation of medium-sized rings. Thus,
there are only few examples in the literature for the construction of eight-membered
ring systems, in general, and ethers, in particular, by radical cyclization. Herein,
recent advances in radical cyclizations that have led to the development of new
methods to efficiently construct eight-membered ethers are reported. To the best of
our knowledge, up to now nine- and ten-membered ring ethers have not been
synthesized by using radicals.
Majumdar [127] reported thiophenol-mediated radical cyclization reactions to
form benzoxocine derivatives. As shown in Scheme 30, the eight-membered ether
148 was formed from enyne 145 by using a regioselective thio-radical addition
Scheme 29 Effect of the catalyst on the olefin geometry in the RCM reported by Fu ¨rstner et al.
344
E. Soriano and J. Marco-Contelles