Samarium iodide mediated cyclization

Cossy has reported a synthesis of a-kainic acid that establishes the stereogenic centers on a preformed pyrrolidine ring (Eq. 20) [40], Thus, ketone 57 was prepared from L-pyroglutamic acid in 11 steps. Samarium iodide-mediated cyclization of 57 gave 58 as a mixture of stereoisomers at the carbinol carbon. Dehydration gave 59, and a 6-step sequence, starting with oxidative cleavage of the double bond, provided a-kainic acid. One notable aspect of this synthesis is the use of an enamide as a free-radical acceptor in the key cyclization. This process has been used in a number of alkaloid syntheses as will be seen in the next section. 

Samarium(II)-iodide-mediated cyclizations in natural product syntheses 04CR V3371. 

The first total synthesis of ( )-oxerine (8) was performed by Aoyagi and co-workers (187), and is shown in Scheme 6. The route is based on the use of a samarium iodide-mediated intramolecular cyclization as the key... 

Samarium(II) iodide-mediated cyclization of aryl radicals onto alkene and alkyne acceptors provides an excellent route to nitrogen- and oxygen-based heterocycles (eq 29). ... 

There are few reports in which the 5-lactone ring is constmcted through a samarium iodide-mediated reductive cyclization of 1,5-dicarbonyl compounds. In the first example. Fang, Tsai et al. have reported the synthesis of 5-lactones using samarium... 

The occurrence of the indole subunit is well established within the class of natural products and pharmaceutically active compounds. Recently, the Reissig group developed an impressive procedure for the assembly of highly functionalized in-dolizidine derivatives, highlighting again the versatility of domino reactions [8]. The approach is based on a samarium(II) iodide-mediated radical cyclization terminated by a subsequent alkylation which can be carried out in an intermolecular - as well as in an intramolecular - fashion. Reaction of ketone 3-11 with samarium(II) iodide induced a 6-exo-trig cyclization, furnishing a samarium enolate intermediate... 

Tadano, K, Isshiki, Y, Minami, M, Ogawa, S, Samarium(II) iodide-mediated reductive cyclization approach to total synthesis of the insect sex attractant (—)-anastrephin. Tetrahedron Lett, 33, 7899-7902, 1992. 

Scheme 7. Samarium(II) iodide and dimanganese decacarbonyl-mediated cyclizations followed by trapping with TEMPO...

A radical-mediated two-step synthesis of spiroketals has recently been reported the strategy (an example is shown in Scheme 8) involves an intramolecular radical cyclization of a ketal precursor (14), which could be easily prepared from 2-methylene-3,4-epoxyoxolanes (13). As alternative to tin-hydride mediated cyclization of alkynyl halides, samarium(II) iodide has been used to generate the alkyl radical which adds intramolecularly to the triple bond. ... 

An alternative route to 1,2-diselenines involved titanium-mediated cyclizations of terminal alkynes or allenes, followed by trapping with selenocyanogen and then treatment with tetrabutylammonium fluoride or samarium(ll) iodide (Schemes 16 and 17) <2000JA5052>. Mixed alkyne-allene reactions are also successful using this procedure (Scheme 18). Titanacycles 85-87 are key synthetic intermediates in these reactions. 

F. Ketyl-olefin cyclization mediated by samarium(II) iodide 563... 

F. Ketyl-Olefin Cyclization Mediated by Samarium(ll) Iodide [33]... 

Samarium(n) iodide further mediates the cyclization reactions of alkynyl halides (eq 31). When treated with Sml2, the alkynyl halides are converted to the cyclized product in good yield. Addition of DMPU as cosolvent provides slightly higher yields in some instances. 

---