Employing propargylic cations, anions, and radicals

Particularly useful in cyclic dialkyne chemistry is sulfide as a nucleophile. This is due to a discovery made by Regen and co-workers, who found that the yield of dialkyl sulfides is strongly enhanced when the sulfide reagent is first adsorbed onto AI2O3 thus, the substitution reaction takes place on the surface of the aluminum oxide. As the preparation of cyclic sulfides from dibromides is a stepwise process, these conditions were expected to increase the yield of cyclic sulfides dramatically as one end of the chain is immobilized after displacement of the first bromine [14]. Nicolaou first used this principle in the preparation of a series of cyclic thiadiynes [Eq. (6)] [15]. The cyclic thiadiynes prepared in this way are very useful as they can be converted to cyclic enediyne systems via the Ramberg-Backlund ring-contraction reaction (see below). 

However, not only heterocyclic but also carbocyclic systems can be made via nucleophilic attack at propargylic centers. This is usually done intramolecularly as shown in Eq. (7), when a malonic ester anion displaces a propargylic chloride [16]. 

A very interesting and powerful new cyclization method employing intramolecular attack of a propargylic-allylic anion at a propargylic aldehyde is the Nozaki cyclization [Eq. (10)] even the heavily strained nine-membered ring 37 can be obtained by this procedure [18]. 

Propargylic anions, although not yet frequently used, seem to be very promising candidates for new ring-closure reactions. This is illustrated by the reaction shown in Eq. (11) when l,9-bis(bromomagnesium)-2,7-nonadiyne (41) is cyclized with Me2SiCl2 to afford the silacyclodecadiyne 42 [20]. 

Propargylic radicals are generated on reduction of metal-complex-stabilized Nicholas cations with Zn. This principle, which has been found very recently [21], can be used for ring-closure reactions via intramolecular radical recombination [Eq. (12)]. 

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