Organotitanium reagents in organic synthesis a simple means to adjust reactivity and selectivity of carbanions

Evolution of a Short and Stereocontrolled Synthesis of (+)-7,20-Diisocyanoadociane

A full account of the development of a concise and highly stereoselective synthesis of (+)-7,20-diisocyanoadociane (DICA)─a structurally complex isocyanoditerpene with potent antiplasmodial activity─is described. The strategy that evolved relies on the rapid construction of unsaturated tricyclic precursors designed to undergo stereocontrolled Birch reductions and a subsequent "bay ring" formation to generate the isocycloamphilectane core. This report is divided into three sections: (1) a description of the initial strategy and the results that focused our efforts on a single route to the DICA core, (2) a discussion of the precise choreography needed to enable a first-generation formal synthesis of (±)-DICA, and (3) the execution of a 13-step second-generation synthesis of (+)-DICA that builds on important lessons learned from the first-generation effort.

Unusual room temperature activation of 1,2-dialkoxyalkanes by niobium and tantalum pentachlorides

The outcome of the reactions of MCl(5) (M = Nb, 1a; M = Ta, 1b) with 1,2-dialkoxyalkanes [i.e. MeO(CH(2))(2)OMe (dme), EtO(CH(2))(2)OEt, MeOCH(2)CH(Me)OMe, MeO(CH(2))(2)OCH(2)Cl, MeO(CH(2))(2)O(CH(2))(2)OMe (diglyme)] depends strictly on the stoichiometry. In the 1 : 1 molar ratio reactions, single C-O bond cleavage occurs, resulting in formation of alkyl chloride and of the complexes [upper bond 1 start]MCl(4)[O(R)CH(R')CH(2)O][upper bond 1 end] [M = Nb, R = Me, R' = H, 3a; M = Ta, R = Me, R' = H, 3b; M = Nb, R = Et, R' = H, 5; M = Nb, R = R' = Me, 6; M = Nb, R = CH(2)Cl, R' = H, 7; M = Nb, R = (CH(2))(2)O(Me)NbCl(5), R' = H, 8], which have been characterized spectroscopically. Moreover, minor amounts of the oxo-bridged adducts MOCl(3)(dme)MCl(5) (M = Nb, 4a; M = Ta, 4b) have been isolated in the reactions involving dme. On the other hand, compounds react with two (or more) equivalents of dme mainly via a multiple C-O bond cleavage process, affording MOCl(3)(dme) (M = Nb, 2a; M = Ta, 2b), 1,4-dioxane and methyl chloride. The oxychloride compounds MOCl(3) (M = Nb, 11a; M = Ta, 11b) have been efficiently obtained by addition of TiCl(4) to 2. Compound 2a is reduced in high yield to the Nb(III) species NbCl(3)(dme), 12, upon treatment with SnBu(3)H. The oxychloride NbOCl(3)(diglyme), 9, 1,4-dioxane, CH(3)Cl and the hexachloroniobate salt [MeOCH(2)CH(2)OCH(2)CH(2)O(H)Me][NbCl(6)], 10, have been identified as products of the reaction of 1a with two equivalents of diglyme. The 1 : 2 molar ratio reaction of 1a with MeOCH(2)CH(Me)OMe gives 2,5-dimethyl-1,4-dioxane. Compound 1a reacts with two equivalents of EtO(CH(2))(2)OEt or MeO(CH(2))(2)OCH(2)Cl yielding Cl(CH(2))(2)OCH(2)CH(3) or O(CH(2)Cl)(2) and diglyme, respectively, but not dioxane, suggesting that fragmentation pathways different from that found for dme are operating. The X-ray molecular structures of 4a, 4b and 10 have been determined.

Reactivity of niobium(v) and tantalum(v) halides with carbonyl compounds: synthesis of simple coordination adducts, C-H bond activation, C=O protonation, and halide transfer

The metal halides of Group 5 MX(5) (M = Nb, Ta; X = F, Cl, Br) react with ketones and acetylacetones affording the octahedral complexes [MX(5)(ketone)] () and [TaX(4){kappa(2)(O)-OC(Me)C(R)C(Me)O}] (R = H, Me, ), respectively. The adducts [MX(5)(acetone)] are still reactive towards acetone, acetophenone or benzophenone, giving the aldolate species [MX(4){kappa(2)(O)-OC(Me)CH(2)C(R)(R')O}] (). The syntheses of (M = Ta, X = F, R = R' = Ph) and (M = Ta, X = Cl, R = Me, R' = Ph) take place with concomitant formation of [(Ph(2)CO)(2)-H][TaF(6)], and [(MePhCO)(2)-H][TaCl(6)], respectively. The compounds [acacH(2)][TaF(6)], and [TaF{OC(Me)C(Me)C(Me)O}(3)][TaF(6)], have been isolated as by-products in the reactions of TaF(5) with acacH and 3-methyl-2,4-pentanedione, respectively. The molecular structures of, and have been ascertained by single crystal X-ray diffraction studies.

Intramolecular titanium-mediated aminocyclopropanation of terminal alkenes: easy access to various substituted azabicyclo[n.1.0]alkanes

[reaction: see text] A variety of substituted azabicyclo[n.1.0]alkanes were synthesized by intramolecular titanium-mediated cyclopropanation of N-benzyl-N-(2-alkylalk-3-enyl)formamides and N-benzyl-N-alkadienylformamides. N-Benzylpyrroline upon treatment with Grignard reagents undergoes a titanium-mediated carbomagnesiation to yield N-benzyl-N-(2-alkylbut-3-enyl)amines.