[reaction: see text] Norbornene undergoes Ni-catalyzed (1-2 mol% allylnickel bromide/phosphine/NaBARF or AgSbF(6), 1 bar ethylene, -50 degrees C) hydrovinylation (>97% yield), giving either a 1:1 or a 2:1 (norbornene/ethylene) adduct depending on the size of the phosphine. Use of binaphthol-derived phosphoramidite ligand results in up to 80% ee for the 1:1 adduct. The course of the reaction is highly dependent on the ligand (size and configuration of the appendages) and the counteranion present.

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Catalyzed by Pd(0), trialkylsilyltrialkylstannane (R(3)Si-SnR'(3)) reagents undergo highly selective additions to 1,2-dien-7-ynes and 1,2-dien-8-ynes to give 2-vinylalkylidenecyclopentanes with silicon and tin substituents on the double bonds. Similar additions of distannanes and borostannanes show that the reactions with silylstannanes are superior in terms of ease of handling of the bifunctional reagents and the isolation of the products after the reaction. The chemo- and regioselectivities are controlled by the enhanced reactivity of the allene unit, while the (Z)-geometry of the exocyclic stannylvinylidene is a consequence of the syn-carbometalation and subsequent reductive elimination from Pd with retention of configuration at the vinyl carbon. Synthesis of highly functionalized pyrrolidines and indolizidines and the reluctance of certain kinds of allenynes and silicon-tin reagents to undergo the cyclization illustrate the scope and limitations of the reaction. Based on the isolation of intermediates, a mechanism for the formation of the cyclic compounds is proposed. Model transition states to explain the stereoselectivity in cyclization of substituted allenynes are provided. Further elaboration using the vinyltin and vinylsilane moieties should lead to highly functionalized carbocyclic and heterocyclic compounds. Under similar conditions, addition of silylstannanes to highly functionalized allenes gives E-allylstannanes with high stereoselectivity. Functional groups such as THP- and silyl-ethers, lactones, beta- and gamma-lactams, alpha,beta-unsaturated esters, olefins, and substituted acetylenes are tolerated under the reaction conditions.

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Starting from succinamide and 1,2-heptadiene-4-ol, a racemic allene-aldehyde substrate, 20, suitable for R(3)SiSnR'(3)-mediated cyclization was synthesized in six steps and in 21% yield. Stereoselective cyclization (relative cis configuration at the new stereogenic centers of the homoallyl alcohol generated) proceeded smoothly, giving a mixture of indolizidinols bearing five contiguous stereocenters in a combined yield of 80%. Relative configurations of each of the products were unequivocally established by a combination of 2D NMR experiments and single-crystal X-ray analysis. The major indolizidinol obtained in 32% yield was elaborated into indolizidine 5,8-epi-indolizidine 223A via a five-step reaction sequence in 32% overall yield. The second major component (24%) of the key cyclization yielded, in four steps, indolizidine 6,8-epi-223 in 14% yield. Even though revision of the initially postulated structure foiled our original synthetic plans for the natural product, indolizidine 223A, the new stereoselective cyclization strategy and several selective transformations of the indolizidine derivatives reported here may find further applications for the synthesis of highly alkylated indolizidine and other related alkaloids.

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