The total syntheses of the pyrrolidine alkaloid (+)-197B (1) and pyrrolizidine alkaloid (+)-xenovenine (2) are described. The strategy involves enantioselective syntheses of the aminoallene, (5S,8S)-5-amino-trideca-8,9-diene (3), and the aminoallene-alkene, (5S)-5-amino-pentadeca-1,8,9-triene (4), which then undergo regio- and stereoselective cyclohydroamination catalyzed by the organolanthanide precatalysts Cp‘2LnCH(TMS)2 and Me2SiCp‘ ‘(tBuN)LnN(TMS)2 (Cp‘ = η5-Me5C5; Cp‘ ‘ = η5-Me4C5; Ln = lanthanide; TMS = Me3Si). These reactive organolanthanide complexes efficiently mediate highly diastereoselective intramolecular hydroamination/cyclization (IHC) reactions under mild conditions. The turnover-limiting step in these catalytic cycles is proposed to be intramolecular insertion into the Ln−N bond of the proximal allenic CC linkage, followed by rapid protonolytic cleavage of the resulting Ln−C bond. The rate and selectivity of the insertion process is highly sensitive to the steric demands of the substrate.

No abstract

Photolysis of 0.25 equiv of W(CO)6 in the presence of tertiary amines (triethylamine or DABCO) and highly functionalized terminal alkynyl alcohols catalyzes single-step, high-yield cycloisomerization to endocyclic enol ethers. This transformation is general for each diastereomeric 3,4-bissilyl ether of 5-hydroxy-1-hexyne, leading to enantio- and diastereoselective syntheses of each isomer of 6-deoxy-1,2-glycals. Stereoselective glycosylations have also been demonstrated for each glycal diastereomer, and have been applied in the preparation of d-digitoxose-β-4-d-digitoxose glycal.

Boron trifluoride-etherate promotes the endo-selective oxacyclization of polyepoxides derived from various acyclic terpenoid polyalkenes, including geraniol, farnesol, and geranylgeraniol, providing an efficient and stereoselective synthesis of substituted oxepanes and fused polyoxepanes. The mechanism of the oxacyclization reaction probably involves intramolecular nucleophilic addition of epoxide oxygen to open another epoxide that is activated as an electrophile by the Lewis acid. These oxacyclizations proceed stereospecifically with inversion of configuration upon opening of each epoxide to provide trans-fused polycyclic products. The oxacyclization cascade is terminated by a tethered nucleophile, which may be the carbonyl oxygen of a ketone, ester, or carbonate, or a trisubstituted alkene. The best oxacyclization yields are generally observed with tert-butyl carbonate as the terminating nucleophile, although in some cases the oxacyclization products include formation of tert-butyl ethers as a minor product. The oxacyclization transformations described herein may mimic ring-forming steps in the biosynthesis of trans-syn-trans-fused polycyclic ether marine natural products.

This paper reports the organolanthanide-catalyzed intramolecular hydroamination/cyclization of amine-tethered unactivated 1,2-disubstituted alkenes to afford the corresponding mono- and disubstituted pyrrolidines and piperidines using coordinatively unsaturated complexes of the type (eta(5)-Me(5)C(5))(2)LnCH(TMS)(2) (Ln = La, Sm), [Me(2)Si(eta(5)-Me(4)C(5))(2)]SmCH(TMS)(2), and [Me(2)Si(eta(5)-Me(4)C(5))((t)BuN)]LnE(TMS)(2) (Ln = Sm, Y, Yb, Lu; E = N, CH) as precatalysts. [Me(2)Si(eta(5)-Me(4)C(5))((t)BuN)]LnE(TMS)(2) mediates intramolecular hydroamination/cyclization of sterically demanding amino-olefins to afford disubstituted pyrrolidines in high diastereoselectivity (trans/cis = 16/1) and good to excellent yield. In addition, chiral C(1)-symmetric organolanthanide catalysts of the type [Me(2)Si(OHF)(CpR*)]LnN(TMS)(2) (OHF = eta(5)-octahydrofluorenyl; Cp = eta(5)-C(5)H(3); R* = (-)-menthyl; Ln = Sm, Y), and [Me(2)Si(eta(5)-Me(4)C(5))(CpR*)]SmN(TMS)(2) (Cp = eta(5)-H(3)C(5); R* = (-)-menthyl) mediate asymmetric intramolecular hydroamination/cyclization of amines bearing internal olefins and afford chiral 2-substituted piperidine and pyrrolidine in enantioselectivities as high as 84:16 er at 60 degrees C. The substrate of the structure NH(2)CH(2)CMe(2)CH(2)CH=CH(CH(2))(2)CH=CH(2) is regiospecifically bicyclized by [Me(2)Si(eta(5)-Me(4)C(5))((t)BuN)]LnE(TMS)(2) to the corresponding indolizidine skeleton in good yield and high diastereoselectivity. Thermolysis of (eta(5)-Me(5)C(5))(2)LaCH(TMS)(2) in cyclohexane-d(12) at 120 degrees C rapidly releases CH(2)(SiMe(3))(2) and leads to possible formation of fulvene (eta(6)-Me(4)C(5)CH(2)-) species. The thermolysis product readily reverts to active catalysts upon protonolysis by substrate and exhibits the same catalytic activity as the (eta(5),eta(1)-Me(5)C(5))(2)LaCH(TMS)(2) precatalyst at 120 degrees C in the cyclization of cis-2,2-dimethylhept-5-enylamine. Catalytically-active lanthanide-amido complexes (eta(5)-Me(5)C(5))(2)La(NHR)(NH(2)R)(n) and [Me(2)Si(eta(5)-Me(4)C(5))((t)BuN)]Sm(NHR)(NH(2)R)(n) are shown to be thermally robust species.