Cycloisomerization stereospecific

Marshall and co-workers used the silver-catalyzed version of this cycloisomerization as the final step in the synthesis of (-)-kallolide B from precursor 115 (Scheme 15.33) [51, 54]. Again, the reaction is stereospecific, as has also been demonstrated in the synthesis of kallolide A [55] and other examples [77]. 

Parker has outlined an elegant, enantioselective synthesis of L-vancosamine derivatives commencing from noncarbohydrate precursors (Scheme 17.38) [116]. This approach features a diastereoselective allenylstannane addition and W(CO)5-catalyzed cycloisomerization to construct the pyranose core. Oxidative cyclization of the C4-carba-mate 128 is performed with 10 mol% Rh2(OAc)4 and proceeds stereospecifically to give the crystalline oxazolidinone 129 (86%). All told, synthesis of this useful L-vancosa-mine glycal equivalent covers seven steps from (S)-(-)-ethyl lactate 127 and is accomplished in 44% overall yield. 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

Six-membered dienes with an exocyclic double bond can also be obtained by gold-catalyzed cycloisomerization of 1,6-enynes. This transformation is stereospecific i.e., substrates with defined double-bond geometries afforded the products as single stereoisomers (Scheme 4-23). Low temperatures and electron-withdrawing... 

Dichloro(p-diphenylphosphinopolystyrene)nickel(II) jchromous chloride 2-VinylmethyIenecyclopentanes from 1,2,7-trienes Stereospecific cycloisomerization via catalytic intramolecular carbametalation... 

Without additional reagents Di(ene)cyclobutenes from 1,5-diynes Stereospecific cycloisomerization... 

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