Cyclizations termination

Scheme 13 Catalytic cyclization terminated by -hydride elimination...

In related studies, Terada described the synthesis of optically pure piperidines via a tandem aza-ene-type reaction/cyclization sequence (Scheme 5.18) [32]. The reaction of a monosubstituted enecarbamate and an N-acyl aldimine affords aza-ene-type intermediate 5, which reacts with a second equivalent of enecarbamate to give aldimine 6. Subsequent intramolecular cyclization terminates the aze-ene-type reaction sequence to furnish trans-piperidine 7 in high enantio- and diasterio-selectivities. 

Further insight into the effect of the ionicity on cyclization, termination and propagation is seen in the date repeated by Wilke 131). This is summarized in Table 11. 

The major problem with using Wilkinson s catalyst is that it also constitutes an excellent hydrogenation catalyst [56]. Thus, alkynes and terminal alkenes are not tolerated under the conditions of the coupled catalytic cycles. This implies that radical cyclizations terminated by a CHAT cannot be carried out under these conditions. 

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... 

Trost and Lee [112] reported a similar approach, using vinylcyclopropanols as cyclization terminators, to construct 8-membered rings. The composite of an olefin and a hydroxyl group mediated through a cyclopropane in the form of a... 

Propargylsilanes show predictable behavior as cyclization terminators. Internal additions of propy-nylsilanes to acyclic -acyliminium ions promoted by Lewis acids or protic acids yield a-allenic amides or carbamates (Scheme 54). ... 

Mannich cyclizations are also successful when the carbonyl participant is masked as an acetal or enol ether. Considering the acetal, if the cyclization is conducted in aqueous acid, there is a good possibility that the cyclization terminator is actually the enol of the parent carbonyl i.e. a classic Mannich cyclization). Under anhydrous acid conditions the reactive terminator is most likely the enol ether. 

A practical synthesis of the unsaturated pyrrolizidine (+ )-heliotridane (101) was achieved via stereoselective acyliminium cyclization of (100). The ketene dithioketal substituent served both as an efficient cationic cyclization terminator and as a means of controlling double-bond migration regioselectively into the correct position <93JA3653>. 

A number of Tc-cyclization terminators have been introduced for biasing cationic 7C-cyclizations toward forming five-membered ring products. For an early discussion of this problem see Johnson, W.S. Biorg. Chem. 1976, 5, 51. 

Like free radical polymerization, cationic polymerizations undergo chain transfer and termination reactions. Chain transfers can take place by proton transfer to monomer, by hydride transfer from a chain, and by a variety of alkylations and cyclizations. Termination by coupling two cationic centers is not possible, but termination can occur when propagating centers react with water or other basic contaminants to give stable products. 

Harding and his co-workers have shown that polyene cyclization termination by y-allenes proceeds exclusively via a six-membered ring vinylic cation rather than the alternative five-membered ring alkylic cation (Scheme 23). Johnson et... 

Chiral amine catalysts have also been used in cascade reactions mediated by SOMO catalysis [143] and Lewis base catalysis [144]. MacMillan s group developed a powerful cascade reaction moderated by SOMO catalysis. The radical cation, generated from an enamine in condensation of imidazolidinone catalyst 208 with aldehyde 207 and subsequent oxidation by Cu oxidant, was expected to engage in a series of 6-endo-trig radical cyclizations terminated by a suitable arene to give a cyclohexadi-enyl radical. After a second oxidation, rearomatization, and liberation of the catalyst, the requisite 209 would be generated (Scheme 1.90). 

An N-acyliminium ion was utilized as the electrophile in a key step of Hiemstra s enantioselective synthesis of the unnatural enantiomer of gelse-dine (148, Scheme 11.24) [123]. In this synthetic endeavor it was discovered that treatment with iodide promotes the participation of the allene as the nucleophile to give the cyclized vinyl iodide 147 in 42 % yield. This finding is related to Overman s previous investigations of halide-promoted iminium ion cyclizations with alkynes as the nucleophilic cyclization terminators (cf. 149 150) [124]. The allene cyclization step (145 146—>147) afforded a rapid route to the skeleton of gelsedine (148), thus enabling the total synthesis of this natural product [123]. 

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