Improved synthesis of aryltriethoxysilanes via palladium(0)-catalyzed silylation of aryl iodides and bromides with triethoxysilane.

The scope of the palladium-catalyzed silylation of aryl halides with triethoxysilane has been expanded to include aryl bromides. A more general Pd(0) catalyst/ligand system has been developed that activates bromides and iodides: palladium(0) dibenzylideneacetone (Pd(dba)(2)) is activated with 2-(di-tert-butylphosphino)biphenyl (Buchwald's ligand) (1:2 mol ratio of Pd/phosphine). Electron-rich para- and meta-substituted aryl halides (including unprotected aniline and phenol derivatives) undergo silylation to form the corresponding aryltriethoxysilane in fair to excellent yield; however, ortho-substituted aryl halides failed to be silylated.

Palladium-catalyzed arylation of allylic benzoates using hypervalent siloxane derivatives.

Palladium-catalyzed cross-coupling of hypervalent arylsiloxane derivatives proceeded in good to excellent yields with allylic benzoates. Arylation occurred with complete inversion of configuration. The scope and limitations of this reaction, an alternative to the Stille coupling, is summarized.

A highly stereoselective approach to the synthesis of functionalized pyran derivatives by Lewis acid assisted ketal reduction and allylation.

Reduction of bicyclic ketal 1 gave functionalized pyran derivatives 7a or 7b in a highly stereoselective manner, depending upon the reduction conditions utilized. For example, treatment of ketal 1 with TiCl4/Et3SiH produced exclusively diol 7b with the 2,5-syn relationship in good yield. Alternatively, reduction of ketal 1 by DIBALH gave 2,5-anti-diol 7a stereoselectively. Alane reductions of ketal 1 were highly stereoselective also; however, the syn/anti selectivity observed was strongly dependent on the ratio of reagents employed for in situ generation of the alane. Lewis acid catalyzed allylation of ketal 1 gave pyran 10 in a stereospecific alkylation reaction.

Improvements in cross coupling reactions of hypervalent siloxane derivatives.

[formula: see text] The scope of the palladium-catalyzed cross coupling reaction of aryl halides with phenyltrimethoxysilane has been expanded to include aryl bromides, heteroaryl bromides, and aryl chlorides. A more general Pd(0)-catalyst/ligand system has been developed to activate bromides: palladium(II) acetate (Pd(OAc)2) is activated with triphenylphosphine (PPh3) or tri-o-tolylphosphine (P(o-tol)3) (1:2 molar ratio of Pd:phosphine). Coupling of aryl chloride derivatives required addition of 2-(dicyclohexylphosphino)biphenyl (Buchwald's ligand) to Pd2dba3 (tris-(dibenzylideneacetone)dipalladium(0)) (1:1.5 molar ratio of Pd:phosphine).

Development of crosslinked polyamines suitable for synthesizing complex ampholytes for isoelectric focusing.

Commercially available ampholytes used in isoelectric focusing applications vary widely from source to source in their resolving power. This study was initiated to develop alternative ampholyte formulations for high resolution preparative and analytical isoelectric focusing. Initial IR spectroscopy studies showed that divalent acid esters would efficiently crosslink available polyamines with complete consumption of ester. Fast atom bombardment mass spectroscopic analysis of resulting crosslinked polyamines showed extensive structural heterogeneity of the resulting polyamine mixture. Conversion of the polyamine mixture to functional zwitterions using alpha,beta-unsaturated carboxylic acids yielded mixtures giving smooth pH gradients in acrylamide gel isoelectric focusing. Further analysis of these mixtures in immobilized pH gradients showed increases in heterogeneity of available carrier species over similar zwitterion mixtures made using only commercially available polyamine monomers. The mixtures were also more heterogeneous than commercially available ampholytes when analyzed by picric acid precipitation in immobilized pH gradients.

(2S-[2 alpha,3 alpha,3a beta,6 beta(R*),7 alpha, 7a alpha])-6-(3-benzyloxy-2-propyl)-2-hydroxy-2,3,7-trimethylhexahydro-4H- furo [3,2-c]pyran-4-one, a rearrangement product of pyranone derivatives in the tirandamycin A series.

The six- and five-membered heterocyclic rings are cis-fused and adopt twist-boat and half-chair conformations, respectively. The average bond distances are: Csp3-Csp3 1.520 (6), Csp3-Csp2 1.494 (6), C-Cbenz 1.360 (10) and Csp3-O 1.425 (5) A; the C = O and Csp2-O bond lengths are 1.214 (5) and 1.324 (5) A, respectively.

Structure of trans-3,3-dichloro-4-(alpha-chlorobenzyl)-1-methyl-5-phenyl-2-pyrrolidi none.

C18H16Cl3NO, Mr = 368.69, triclinic, P1-, a = 10.360 (1), b = 10.397 (1), c = 10.810 (2) A, alpha = 60.84 (1), beta = 57.22 (1), gamma = 70.97 (1) degrees, V = 852 (1) A3, Z = 2, Dx = 1.437 Mg m-3, Mo K alpha radiation, lambda = 0.71073 A, mu = 0.54 mm-1, F(000) = 380, T = 293 (1) K, R = 0.0287 for 2330 observed reflections with I greater than 3 sigma (I). The five-membered ring has an envelope conformation and the 4-(alpha-chlorobenzyl) and 5-phenyl groups are trans with respect to each other.

Structure of a lactone, 2,4,6-trideoxy-4,6-dimethyl-2-(2-oxopropyl)-7-O- (phenylmethyl)-L-glycero-D-manno-heptono-1,5-lactone 3-(3,5-dinitrobenzoate).

C26H28N2O10, Mr = 528.5, triclinic, P1, a = 9.524 (1), b = 11.8187 (8), c = 12.615 (1) A, alpha = 66.512 (7), beta = 83.321 (9), gamma = 88.758 (8) degrees, V = 1293.0 (4) A3, Dx = 1.357 g cm-3, Z = 2, lambda(Cu K alpha) = 1.54178 A, mu = 8.99 cm-1, F(000) = 556, T = 293 K, final R = 0.039, wR = 0.048 for 2425 reflections with I greater than 3 sigma(I). The tetra-substituted valerolactone ring has a boat conformation and the relative stereochemistries of the three pairs of adjacent substituents are cis-trans-trans.

Structure of a lactone, (3 alpha,4 alpha,6 alpha)-4-(tert- butyldimethylsiloxy)-3,4,5,6-tetrahydro-6-methyl-3-(2-oxopropyl)- 2H- pyran-2-one.

C15H28O4Si, Mr = 300.5, monoclinic, P2(1)/a, a = 11.194 (4), b = 10.944 (3), c = 14.815 (5) A, beta = 95.01 (3) degrees, V = 1808 (2) A3, Dx = 1.104 g cm-3, Z = 4, lambda(Mo K alpha) = 0.71069 A, mu = 1.43 cm-1, F(000) = 656, T = 293 K, final R = 0.052, wR = 0.058 for 1630 reflections with I greater than 3 sigma(I). The valerolactone ring is in a boat conformation and the three ring substituents are cis.

Structure of (2'R*,5S*)-(+/-)-5-(tetrahydro-2'H-pyran-2'-yl)-2(5H)-furanone.

C9H12O3, Mr = 168.2, triclinic, P1, a = 5.5385 (6), b = 8.543 (1), c = 10.373 (1) A, alpha = 113.329 (9), beta = 100.78 (1), gamma = 99.82 (1) degree, V = 426.2 (2) A3, Dx = 1.31 g cm-3, Z = 2, Cu K alpha (lambda = 1.54178 A, graphite monochromator), mu = 8.2 cm-1, F(000) = 180, T = 293 K. Final R = 0.036, wR = 0.081 for 1559 reflections with I greater than 3 sigma(I). The relative configurations at the two chiral centers are (S,R). The dihedral angle between the five- and six-membered rings (O1--C4--C5--O3) is 69.9 (1) degree. The tetrahydropyran is in a chair conformation with the furanone ring equatorial.