Stereoselektive Synthese von Alkoholen, VI1) Asymmetrische Synthesen von 4-Penten-2-ol über Allylboronsäureester chiraler Glycole

Single-Pot Asymmetric Approach toward Enantioenriched Quaternary Stereocenter-Containing Alkylidenecyclobutanes

Enantioenriched alkylidenecyclobutanes possessing a quaternary stereogenic center, usually difficult to access, have been synthesized by combining a double boron-homologation and an allylboration through a highly efficient and diastereoselective one-pot process. Starting from commercially available substrates, this protocol represents a simple way of accessing chiral unsaturated four-membered ring systems with excellent stereoisomeric ratios.

Allylation reactions of aldehydes with allylboronates in aqueous media: unique reactivity and selectivity that are only observed in the presence of water

Zn(OH)2-catalyzed allylation reactions of aldehydes with allylboronates in aqueous media have been developed. In contrast to conventional allylboration reactions of aldehydes in organic solvents, the α-addition products were obtained exclusively. A catalytic cycle in which the allylzinc species was generated through a B-to-Zn exchange process is proposed and kinetic studies were performed. The key intermediate, an allylzinc species, was detected by HRMS (ESI) analysis and by online continuous MS (ESI) analysis. This analysis revealed that, in aqueous media, the allylzinc species competitively reacted with the aldehydes and water. An investigation of the reactivity and selectivity of the allylzinc species by using several typical allylboronates (6a-6d) clarified several important roles of water in this allylation reaction. The allylation reactions of aldehydes with allylboronic acid 2,2-dimethyl-1,3-propanediol esters proceeded smoothly in the presence of catalytic amounts of Zn(OH)2 and achiral ligand 4d in aqueous media to afford the corresponding syn-adducts in high yields with high diastereoselectivities. In all cases, the α-addition products were obtained and a wide substrate scope was tolerated. Furthermore, this reaction was applied to asymmetric catalysis by using chiral ligand 9. Based on the X-ray structure of the Zn-9 complex, several nonsymmetrical chiral ligands were also found to be effective. This reaction was further applied to catalytic asymmetric alkylallylation, chloroallylation, and alkoxyallylation processes and the synthetic utility of these reactions has been demonstrated.

(2R,3R)-1,4-Dimethoxy-1,1,4,4-tetraphenylbutane-2,3-diol: Valuable reagent in the asymmetric synthesis of organoboronates

Tartrate-derived (2R,3R)-1,4-dimethoxy-1,1,4,4-tetraphenylbutane-2,3-diol has found diverse applications in asymmetric organic synthesis. Among these, its utilization as a protecting group for boronic acids has been investigated extensively. Besides being extraordinarily stable and thus allowing a plethora of transformations, it enables access to various diastereo- and enantiomerically pure organoboron reagents and versatile intermediates in asymmetric synthesis.

Asymmetric synthesis of 2 degrees- and 3 degrees-Carbinols via B-methallyl-10-(TMS and Ph)-9-borabicyclo[3.3.2]decanes

Simple Grignard procedures provide methallylboranes 1a and 1b in enantiomerically pure form from air-stable precursors in 98% and 95% yields, respectively. These reagents add smoothly to aldehydes and methyl ketones, respectively, providing branched 2 degrees- (6, 69-89%, 94-99% ee) and 3 degrees- (10, 71-87%, 74-96% ee) homoallylic alcohols.

Enantio- and Diastereoselective Synthesis of syn-β-Hydroxyallylsilanes via a Chiral (Z)-γ-Silylallylboronate

syn-beta-Hydroxyallylsilanes of general structure 11 and 28 are prepared in 50-86% yield and 91-95% ee (for aliphatic aldehydes; 50% ee for benzaldehyde) via the BF(3).Et(2)O-promoted gamma-silylallylboration reactions, using reagents 14 and 15.

[Development of efficient methods for synthesis of nitrogen-containing compounds using carbamates, acylhydrazines, and ammonia]

For the efficient synthesis of divergent nitrogen-containing compounds of pharmaceutical and agricultural importance, the development of efficient, complementary, and new synthetic methodologies is essential. One of the key subjects is how to introduce nitrogen atoms in to organic molecules. This review summaries our recent efforts on this issue, focusing on the use of carbamates, acylhydrazines, and ammonia as nitrogen sources.

Asymmetric Allyl- and Crotylboration with the Robust, Versatile, and Recyclable 10-TMS-9-borabicyclo[3.3.2]decanes

The remarkable versatility and selectivity of the 10-(trimethylsilyl)-9-borabicyclo[3.3.2]decanes (10-TMS-9-BBDs) in the allyl- and crotylboration of representative aldehydes are reported. The new reagents are prepared through air-stable crystalline pseudoephedrine borinic ester complexes of the 10-TMS-9-BBDs (4), which are available in 63% overall yield from B-MeO-9-BBN through a simple two-step procedure. These complexes 4 are directly converted to the corresponding B-allyl-10-TMS-9-BBDs (1) with allylmagnesium bromide, which either can be isolated (98%) or used in situ for the allylations. The remarkable enantioselectivity (96 to > or =99% ee) of these reagents in the rapid (<3 h), asymmetric allylboration process at -78 degrees C is only slightly diminished when it is conducted at 25 degrees C, a phenomenon attributable to its rigid bicyclic structure. In addition to providing the homoallylic alcohols 6 efficiently (68-80%), the procedure also permits the efficient recovery of 4 (68-84%) for the direct regeneration of 1. Alternatively, an oxidative workup procedure can be used for the preparation of 6. The reagent gives predictable stereochemistry and exhibits an extremely high level of reagent control in the allylboration of d-glyceraldehyde acetonide. A simple and efficient procedure has been developed for the preparation of all four geometric and enantiomeric isomers of the B-crotyl-10-TMS-9-BBDs (10) from optically pure enantiomers of B-MeO-10-TMS-9-BBD (3). These reagents 10 also add rapidly (<3 h) and efficiently to representative aldehydes at -78 degrees C, providing ready access to all four of the possible stereoisomers of the beta-methyl homoallylic alcohols 12-15 (69-92%) in high dr (> or =98:2) and ee (94-99%).

Practical and Efficient Multigram Preparation of a Camphor-Derived Diol for the Enantioselective Lewis Acid Catalyzed Allylboration of Aldehydes

[reaction: see text] Chiral diols are important molecules with widespread use as chiral auxiliaries and ligands in enantioselective synthesis. Therefore, efficient and practical syntheses of highly dissymmetrical nonracemic diols are still a meaningful pursuit. Two new routes to access camphor-derived chiral diol 1 have been developed. One route employs camphorquinone (3) as the starting material, affording in only two steps the desired diol in 55% overall yield. The second route, from camphor (2), leads to the desired diol in an efficient four-step synthesis, with an overall yield of 55%.

Lewis Acid Catalyzed Allylboration: Discovery, Optimization, and Application to the Formation of Stereogenic Quaternary Carbon Centers

A full account of the development of the first catalytic manifold for the additions of allylboronates to aldehydes is described. The thermal additions (both diastereospecific and enantioselective) of 2-carboxyester 3,3-disubstituted allylboronates 1 to both aromatic and aliphatic aldehydes give biologically and synthetically important exo-methylene butyrolactones 2 containing a beta-quaternary carbon center. Although the thermal reaction requires 14 d at room temperature to reach completion, the presence of certain metal salts allows for a 12-16 h reaction while preserving the diastereospecificity observed in the uncatalyzed process. Preliminary mechanistic studies on the origin of the catalytic effect are described as well as stereoselective transformations of lactones 2 into cyclic and acyclic stereotriads with potential usefulness as synthetic intermediates.

α-Aminoallylation of Aldehydes with Ammonia: Stereoselective Synthesis of Homoallylic Primary Amines

Three-component reactions of aldehydes, ammonia, and allylboronates were found to provide homoallylic primary amines in high yields with high chemo- and stereoselectivities. A two-step, one-pot, stereoselective synthesis of an uncommon alpha-amino acid, alloisoleucine, was achieved utilizing this reaction.

Lewis Acids Catalyze the Addition of Allylboronates to Aldehydes by Electrophilic Activation of the Dioxaborolane in a Closed Transition Structure

This study addressed the mechanism of the recently reported Lewis acid-catalyzed manifold for additions of allylboronates to aldehydes. A series of control experiments using various reagents and conditions, and kinetic studies by low-temperature NMR spectroscopy, were performed to investigate the origin of the activation provided by the best catalyst, Sc(OTf)3. The results obtained are strongly supportive of a mechanism involving electrophilic boron activation by coordination of the metal ion to one of the boronate oxygens in a closed bimolecular transition state.

Novel isomerically pure tetrasubstituted allylboronates: stereocontrolled synthesis of alpha-exomethylene gamma-lactones as aldol-like adducts with a stereogenic quaternary carbon center

In spite of their inherent isomerization tendency and low reactivity, 1-alkoxycarbonyl vinylcopper(I) intermediates from the conjugate addition of organocuprates onto acetylenic esters were trapped with very high cis-addition selectivity with iodomethylboronic esters in the presence of HMPA. The resulting isomerically pure 3,3-disubstituted allylboronates react with aldehydes in a highly diastereo- and enantioselective manner, providing alpha-exomethylene gamma-lactones with a stereogenic quaternary beta-carbon center.