Scandium-catalyzed allylboration of aldehydes as a practical method for highly diastereo- and enantioselective construction of homoallylic alcohols

A general approach for the allylation of aldehydes using stable, air-tolerant camphor-based chiral allylboronates under Sc(OTf)3 catalysis is described. This practical methodology provides both syn and anti propionate units and other homoallylic alcohols with very high levels of diastereo- and enantioselectivity for several substrates, including functionalized aliphatic aldehydes useful toward the elaboration of complex natural products.     

Determination of total dietary fiber in selected foods containing resistant maltodextrin by enzymatic-gravimetric method and liquid chromatography: collaborative study

A method was developed for determination of total dietary fiber (TDF) in foods containing resistant maltodextrin (RMD) which includes nondigestible carbohydrates that are not fully recovered as dietary fiber by conventional TDF methods such as AOAC 985.29 or 991.43. Because the average molecular weight (MW) of RMD is 2000 daltons, lower MW soluble dietary fiber components do not precipitate in 78% ethanol; therefore, RMD is not completely quantitated as dietary fiber by current AOAC methods. The accuracy and precision of the method was evaluated through an AOAC collaborative study. Ten laboratories participated and assayed 12 test portions (6 blind duplicates) containing RMD. The 6 test pairs ranged from 1.5 to 100% RMD. The method consisted of the following steps: (1) The insoluble dietary fiber (IDF) and high MW soluble dietary fiber (HMWSDF) were determined by AOAC 985.29. (2) Ion exchange resins were used to remove salts and proteins contained in the AOAC 985.29 filtrates (including ethanol and acetone). (3) The amount of low MWRMD (LMWRMD) in the filtrates were determined by liquid chromatography. (4) The TDF was calculated by summation of the IDF, HMWSDF, and LMWRMD fractions having nondigestible carbohydrates with a degree of polymerization of 3 and higher. Repeatability standard deviations (RSDr) were 1.33-7.46%, calculated by including outliers, and 1.33-6.10%, calculated by not including outliers. Reproducibility standard deviations (RSDR) were 2.48-9.39%, calculated by including outliers, and 1.79-9.39%, calculated by not including outliers. This method is recommended for adoption as Official First Action.     

Synthetic application of a substituent controlled claisen rearrangement. Preparation of advanced chiral intermediates for the synthesis of pseudomonic acids

A substituent controlled Claisen rearrangement is employed as the key reaction for a facile construction of chiral intermediates for the synthesis of pseudomonic acids from diacetyl-L-arabinal.     

Compounding Selectivity in Reactions of Diastereoisomeric Radical Intermediates. An experimental demonstration that the yield of a product from a diastereotopic-group-selective reaction can significantly exceed the level of group selectivity

Reduction of a bis-radical precursor, 6-phenyl-1,1-bis[3-(phenylselanyl)propyl]-3a,4-dihydro-1H,3H-cyclopenta[c]furan-5-one ( 6 ), with 3 equiv. of Ph₃SnH provides mixtures of cis,cis- or cis,trans-angular triquinane products (3aα,5aβ,8β,8aR*)- and (3aα,5aα,8β,8aR*)-hexahydro-3a-propyl-8-phenyl-5H-dicyclopenta[b,c]furan-7(8H)-one (cis,cis- 12 /cis,trans- 12 ), in yields that vary from 50%/50% to 91%/6% depending on the reaction concentration. A mechanistic model for this process is proposed that involves a non-selective phenylselenium-group abstraction step followed by successive kinetic resolutions of diastereoisomeric radical intermediates. This reaction shows how yields in group-selective reactions can be compounded to levels above that ostensibly permitted by the level of the group-selective step.     

Polymers for catalytic cation radical chemistry

Cation radical polymers which have cation radical functions at up to 5% of the poly(styrene) monomer sites have been prepared, and their effectiveness in catalyzing the cation radical Diels-Alder reaction is demonstrated.     

Fluorous mixture synthesis of fused-tricyclic hydantoins. Use of a redundant tagging strategy on fluorinated substrates

Simple HPLC experiments were used to identify a redundant tagging scheme wherein six different amino acids were tagged with only four fluorous tags. The tagged amino acids were converted to regiosiomeric mixtures of tricyclic hydantoins. Despite the lack of selectivity, the mixtures were demixed and detagged to give 11 individual pure products in just 25 steps.     

Recent advances in the activation of boron and silicon reagents for stereocontrolled allylation reactions

Despite the popularity of boron and silicon allylation reagents in stereocontrolled synthesis, they suffer from a number of inherent limitations that have slowed down their development as synthetic tools for nucleophilic additions to carbonyl compounds and imine derivatives. These limitations are the low reactivity and diastereoselectivity of allyl trialkylsilane reagents, and the lack of catalytic systems for the activation and substoichiometric control of enantioselectivity in the additions of allyl boron reagents. To develop more efficient and general methods for the control of absolute stereochemistry in the resulting homoallylic alcohols, new approaches aimed at solving the problem of activation of allylic boron and silicon reagents are needed. This Minireview describes a number of recent approaches that have been devised to address this problem.     

Correlation of the rates of solvolysis of benzoyl fluoride and a consideration of leaving-group effects

The specific rates of solvolysis of benzoyl fluoride have been determined at 25.0 degrees C in 37 pure and binary solvents. Together with seven values from the literature, these give a satisfactory correlation over the full range of solvents when the extended Grunwald-Winstein equation is applied. The sensitivities to changes in solvent nucleophilicity and solvent ionizing power are very similar to those for octyl fluoroformate, suggesting that the addition step of an addition-elimination mechanism is rate determining. In the solvent-composition region where benzoyl chloride also shows bimolecular solvolysis, the appreciable k(Cl)/k(F) values are proposed as being primarily due to a more efficient ground-state stabilization for the fluoride.     

A new class of chiral organogermanes derived from C2-symmetric dithiols: synthesis,characterization and stereoselective free radical reactions

A new class of dithiostannanes and dithiogermanes have been prepared from 1,1'-binaphthyl-2,2'-dithiol and 3,3'-bis(trimethylsilyl)-1,1'-binaphtho-2,2'-dithiol. While reduction of 4-butyl-4-chloro-3,5-dithia-4-stanna-cyclohepta[2,1-a;3,4-a']dinaphthalene to the corresponding tin hydride was unsuccessful, 4-tert-butyl-3,5-dithia-4-germa-cyclohepta[2,1-a;3,4-a']dinaphthalene and 4-tert-butyl-2,6-bis(trimethylsilyl)-3,5-dithia-4-germa-cyclohepta[2,1-a;3,4-a']dinaphthalene were obtained by reduction of the parent germanium chlorides with NaBH(4) and LiBH(4), respectively. Kinetic constants for hydrogen transfer to a primary alkyl radical were measured for both germanium hydrides. Reduction of alpha-halo carbonyl compounds by these germanium hydrides occurs with moderate ee values (up to 42%), while hydrogermylation of methyl methacrylate occurs with low selectivity (<3/1) for the former hydride but high selectivity (>10/1) for the latter.     

Comparative Studies of Cathodically-Promoted and Base-Catalyzed Michael Addition Reactions of Levoglucosenone

Regioselective Michael addition of nitro and heterocyclic compounds to levoglucosenone, 1, is effectively catalyzed by amines and also by cathodic electrolysis. In comparison to the base-catalyzed reaction, it was found that under electrochemical conditions the reaction proceeds under milder conditions and with higher yields. Cathodically-initiated Michael addition of thiols to levoglucosenone using small currents produces the previously unknown threo addition product in several instances. The normal erythro isomer, identified as the kinetic product, tends to be formed when large currents are used. In contrast, slow, low current electrolyses promote equilibration of the two forms so that erythro can be converted to threo by the retro reaction and readdition. Addition of 2-naphthalenethiol to (R)-(+)-apoverbenone is also reported.