Dinuclear Tricyclic Transition State Model for Carbonyl Addition of Organotitanium Reagents: DFT Study on the Activity and Enantioselectivity of BINOLate Titanium Catalysts.

In the presence of a catalytic amount of chiral BINOL derivatives (or BINOLs), a mixture of various organometallic compounds with Ti(O iPr)4 undergoes enantioselective addition to aldehydes and ketones. Although the catalyst and reacting nucleophile of the reaction have been elucidated to be ( BINOLate)Ti2(O iPr)6 and RTi(O iPr)3, respectively, little is known about the properties of short-lived intermediates and transition structures. In this work, the mechanism of this reaction is investigated with the aid of DFT (M06) calculations. The study provides support for the following mechanistic understandings: (i) The direct racemic reaction proceeds through a pathway involving initial aggregation of RTi(O iPr)3 with Ti(O iPr)4 followed by carbonyl addition of the resulting dinuclear aggregate. (ii) The enantioselective reaction takes place through a pathway involving initial ligand exchange of RTi(O iPr)3 with ( BINOLate)Ti2(O iPr)6 followed by the addition of the resulting chiral dinuclear titanium species via a chiral BINOLate-chelated, tricyclic transition structure. (iii) The enantioselective pathway is favorable not because BINOLate ligands accelerate the carbonyl addition but because the ligands stabilize the chiral dinuclear species against deaggregation through a chelating bridge. (iv) The chiral transition structure serves as a model accounting for the re-face addition generally observed in the reaction of aldehydes with ( R)- BINOLs.

Method for Catalytic Enantioselective Alkylation of Aldehydes Using Grignard Reagents as Alkyl Sources.

Alkyltitanium reagents, generated in situ from Grignard reagents and ClTi(O iPr)3, can be employed without further manipulation in the enantioselective alkylation of aldehyde by the catalysis of a chiral titanium complex derived from DTBP-H8-BINOL. The reaction is performed with good stoichiometry [1.5 equiv each of Grignard reagents and ClTi(O iPr)3] at a low catalyst loading (2 mol %), affording a variety of chiral secondary alcohols in high enantioselectivity and yields and, hence, realizing an asymmetric version of the Grignard reaction in an indirect manner.

Silica-Supported Catalyst for Enantioselective Arylation of Aldehydes under Batch and Continuous-Flow Conditions.

A silica-supported 3-aryl H8-BINOL-derived titanium catalyst exhibited high performance in the enantioselective arylation of aromatic aldehydes using Grignard and organolithium reagents not only under batch conditions but also under continuous-flow conditions. Even with a simple pipet reactor packed with the heterogeneous catalyst, the enantioselective production of chiral diarylmethanols could be achieved through a continuous introduction of aldehydes and mixed titanium reagents generated from the organometallic precursors. The pipet reactor could be used repeatedly in different reactions without appreciable deterioration of the activity.

Application of a Heterogeneous Chiral Titanium Catalyst Derived from Silica-Supported 3-Aryl H8-BINOL to Enantioselective Alkylation and Arylation of Aldehydes.

A 3-aryl H8-BINOL was grafted on the surface of silica gel using a hydrosilane derivative as a precursor, and the resulting silica-supported ligand (6 mol %) was employed in the enantioselective alkylation and arylation of aldehydes in the presence of Ti(OiPr)4. The reactions using Et2Zn, Et3B, and aryl Grignard reagents all afforded the corresponding adducts in high enantioselectivities and yields. The silica-immobilized titanium catalyst could be reused up to 14 times without appreciable deterioration of the activity.

Catalytic Enantioselective Arylation and Heteroarylation of Ketones with Organotitanium Reagents Generated In Situ.

A practical and useful, catalytic enantioselective method has been developed for the synthesis of tertiary diaryl and aryl heteroaryl carbinols starting from commercially available aromatic ketones and aryl or heteroaryl bromides. In this method, organotitanium reagents are generated in situ from the bromides by lithiation with nBuLi followed by transmetallation of the resulting organolithiums with ClTi(OiPr)3 . Treatment of the ketones with the titanium reagents in the presence of (R)-3-(3,5-bistrifluoromehthylphenyl)-1,1'-bi-2-naphthol (BTFP-BINOL) affords the corresponding tertiary alcohols in high enantioselectivities and yields. The reaction can also start with furan and 2-thienyllithium. The method is operationally simple and can be conducted on a 10-mmol scale without any difficulties.

Enhancement of the Catalytic Activity of Chiral H8 -BINOL Titanium Complexes by Introduction of Sterically Demanding Groups at the 3-Position.

The activity of chiral titanium catalysts derived from H8 -BINOL ligands in the enantioselective arylation of an aldehyde with PhTi(OiPr)3 is significantly enhanced by an increase of the size of the substituent at the 3-position. High enantioselectivity (> 90 % ee) can be obtained even at a substrate/catalyst ratio (S/C) of 800 for DTBP-H8 -BINOL (DTBP=3,5-di-tert-butylphenyl) and DAP-H8 -BINOL (DAP=3,5-di(9-anthraceny)phenyl). These titanium catalysts are successfully applied to the enantioselective arylation and heteroarylation of aldehydes at a S/C ratio of 400 by using organotitanium reagents generated in situ from bromide precursors. The remarkable weakening of the intramolecular aggregation of the two -Ti(OiPr)3 units in a DPP-H8 -BINOL (DPP=3,5-diphenylpheny)-derived bis-titanium complex is revealed by X-ray and variable-temperature (VT)-NMR studies. Based on these observations, a catalytic cycle, involving the rate-limiting aryl group transfer followed by aldehyde complexation and enantioselective arylation, is proposed to account for the high activity of the 3-substituted H8 -BINOL catalyst system.

Development of Highly Active Chiral Titanium Catalysts for the Enantioselective Addition of Various Organometallic Reagents to Aldehydes.

The catalytic enantioselective carbonyl addition reaction has long attracted the interest of chemists because of its synthetic importance. Although many highly enantioselective reactions have been developed, with few exceptions the reactions are carried out at relatively high catalyst loadings, making them less practical for scale-up applications. In addition, organometallic reagents employed as carbon nucleophiles have been limited to those with relatively low reactivity, such as diorganozincs and arylboronic acids. In an effort to enhance the practicality, a highly active and enantioselective chiral titanium catalyst system was recently developed in our laboratory, enabling the enantioselective carbonyl addition reaction to aldehydes using various organometallic reagents (RM; M = MgX, Li, BY2 , ZnX, AlMe2 ) at lower catalyst loadings (≤5 mol %).

Enantioselective alkylation of aldehydes using functionalized alkylboron reagents catalyzed by a chiral titanium complex.

A practical method is developed for the synthesis of enantioenriched functionalized secondary alcohols through catalytic enantioselective alkylation of aldehydes. Functionalized alkylboron reagents, [FG-(CH2)n]3B (FG = Br, TIPSO, PhtN, CO2(i)Pr, and CN) prepared from terminal olefin precursors by hydroboration, undergo enantioselective addition to aldehydes in the presence of a catalytic amount (5 mol %) of 3-(3,5-diphenylphenyl)-H8-BINOL and excess titanium tetraisopropoxide to afford the corresponding functionalized alcohols in high enantioselectivities up to 99% ee.

Catalytic enantioselective alkylation of aldehydes by using organozinc halide reagents.

Functionalized alkylzinc halides can be employed in the enantioselective addition to aldehydes by using a titanium(IV) catalyst derived from a H(8)-binaphthol derivative in the presence of [Ti(OiPr)(4)] and MgBr(2). A range of functionalities, including olefin, chlorine atoms, protected alcohols, amides, and cyano groups, are tolerated in the present reaction, providing the corresponding functionalized alcohols in high yields and enantioselectivities (see scheme).

Practical enantioselective arylation and heteroarylation of aldehydes with in situ prepared organotitanium reagents catalyzed by 3-aryl-H8-BINOL-derived titanium complexes.

A highly efficient and practical method for the catalytic enantioselective arylation and heteroarylation of aldehydes with organotitanium reagents, prepared in situ by the reaction of aryl- and heteroaryllithium reagents with ClTi(OiPr)3, is described. Titanium complexes derived from DPP-H8 -BINOL (3d; DPP=3,5-diphenylphenyl) and DTBP-H8 -BINOL (3e; DTBP=3,5-di-tert-butylphenyl) exhibit excellent catalytic activity in terms of enantioselectivity and turnover efficiency for the transformation, providing diaryl-, aryl heteroaryl-, and diheteroarylmethanol derivatives in high enantioselectivity at low catalyst loading (0.2-2 mol%). The reaction begins with a variety of aryl and heteroaryl bromides through their conversion into organolithium intermediates by Br/Li exchange with nBuLi, thus providing straightforward access to a range of enantioenriched alcohols from commercially available starting materials. Various 2-thienylmethanols can be synthesized enantioselectively by using commercially available 2-thienyllithium in THF. The reaction can be carried out on a 10 mmol scale at 0.5 mol% catalyst loading, demonstrating its preparative utility.

Ghrelin reactive autoantibodies in restrictive anorexia nervosa.

OBJECTIVE: Subjects with restrictive anorexia nervosa (AN) display increased basal plasma levels of ghrelin that normalize after refeeding. The mechanism responsible for increased ghrelin levels in AN is unknown. We studied if changes of ghrelin reactive autoantibodies (autoAbs) could explain elevated plasma ghrelin in AN. METHODS: Plasma levels of autoAbs reactive with ghrelin and des-acyl ghrelin were measured by enzyme-linked immunosorbent assay in subjects with AN before and 1 mo after hospitalization (refeeding) and compared with healthy controls and with plasma levels of ghrelin peptides. RESULTS: Decreased levels of immunoglobulin (Ig) G, IgM, and IgA classes of autoAbs reacting with acyl ghrelin were found in patients with AN. Addition of des-acyl ghrelin but not of acyl ghrelin peptides at 10(-8) M to plasma before enzyme-linked immunosorbent assay showed in patients with AN but not in controls high levels of IgG autoAbs reacting with des-acyl ghrelin as a result of dissociation of des-acyl ghrelin autoAbs in immune complexes. Plasma levels of acyl and des-acyl ghrelin peptides correlated negatively with des-acyl ghrelin IgG autoAbs. Body mass index, which improved after refeeding, correlated with an increase of acyl ghrelin IgM autoAbs. CONCLUSION: These results show that in patients with AN, ghrelin IgG autoAbs exist mainly as immune complexes with des-acyl ghrelin accompanied by a decrease of a free fraction of these autoAbs binding acylated and des-acyl ghrelin. This decrease of bioavailable ghrelin autoAbs may underlie a long-term elevation of plasma ghrelin levels and the resulting phenomenon of ghrelin resistance in malnourished patients with AN.

Plasma nesfatin-1 concentrations in restricting-type anorexia nervosa.

Restricting-type anorexia nervosa (AN-R) is characterized by chronic food restriction and severe emaciation due to various cognitive biases such as a distorted self-image. In spite of several treatments, AN-R continues to be a refractory disease because of its unknown pathogenesis. Although previous studies have shown that changes in feeding regulatory peptides such as ghrelin are involved in anorexia, few reports have described the relationship between AN-R and nesfatin-1, a recently identified satiety peptide. Therefore, we examined the plasma nesfatin-1 levels in AN-R patients to determine its role in AN-R. A total of 15 women participated in the study; 7 patients with AN-R and 8 age-matched healthy controls (average BMI, 13.02 ± 0.30 vs. 21.57 ± 0.48, respectively). Our results showed that plasma nesfatin-1 levels were significantly lower in AN-R group than in control group (6.23 ± 0.70 ng/ml vs. 8.91 ± 0.85 ng/ml, respectively, P<0.05). Plasma acyl ghrelin and des-acyl ghrelin levels were significantly higher in AN-R group than in control group (acyl ghrelin: 62.4 ± 10.15 fmol/ml vs. 27.20 ± 5.60 fmol/ml, P<0.01 and des-acyl ghrelin: 300.17 ± 55.95 fmol/ml vs. 107.34 ± 40.63 fmol/ml, P<0.05). Although AN-R is associated with emaciation for a prolonged period, our result suggested that nesfatin-1 levels may be regulated by nutrition status and response to starvation.

Catalytic enantioselective synthesis of secondary allylic alcohols from terminal alkynes and aldehydes via 1-alkenylboron reagents.

A practical one-pot method has been developed for preparing enantioenriched secondary allylic alcohols starting from terminal alkynes and aldehydes. Hydroboration of terminal alkynes with dicyclohexylborane and subsequent reaction of the resulting alkenylboron reagents with aldehydes in the presence of a catalytic amount (5 mol %) of 3-(3,5-diphenylphenyl)-H(8)-BINOL and excess titanium tetraisopropoxide afforded the corresponding allylic alcohols in high enantioselectivities up to 94% ee.

Alkylative carbocyclization of omega-iodoalkynyl tosylates with alkynyllithium compounds through a carbenoid-chain process leading to (1-iodoprop-2-ynylidene)tetrahydrofurans and -cyclopropanes.

Alkylative carbocyclization reactions of omega-iodoalkynyl tosylates with alkynyllithium compounds to give products with incorporated iodine atoms are described. Slow addition of 2-(3-iodoprop-2-ynyloxy)ethyl tosylates to 1-alkynyllithium compounds in tetrahydrofuran at 40 degrees C followed by additional stirring at this temperature gives (Z)-3-(1-iodoprop-2-ynylidene)tetrahydrofurans stereoselectively in good to moderate yields. Under similar conditions at 0 degrees C, 4-iodobut-1-ynyl tosylates react with 1-alkynyllithium compounds to give (1-iodoprop-2-ynylidene)cyclopropanes. The carbocyclization reactions are proposed to proceed through a new carbenoid-chain process involving the exo cyclization of a lithium acetylide intermediate and the vinylic substitution of the resulting TsO,Li-cycloalkylidenecarbenoids (Ts=tosyl) by 1-alkynyllithium compounds.

Oxazaborolidinone-catalyzed enantioselective Friedel-Crafts alkylation of furans and indoles with alpha,beta-unsaturated ketones.

allo-Threonine-derived oxazaborolidinone (10 mol %) catalyzes the Friedel-Crafts alkylation of furans and indoles with simple acyclic alpha,beta-unsaturated ketones to give products with high yield and high enantioselectivity. The use of N,N-dimethylaniline (2.5-10 mol %) as an additive is essential for enantioselectivity.

Asymmetric Mukaiyama aldol reaction of nonactivated ketones catalyzed by allo-threonine-derived oxazaborolidinone.

Asymmetric Mukaiyama aldol reaction of nonactivated ketones is realized for the first time by using an oxazaborolidinone catalyst derived from O-benzoyl-N-tosyl-allo-threonine. By employing a dimethylsilyl ketene S,O-acetal as a nucleophile, a variety of acetophenone derivatives afford the corresponding tertiary beta-hydroxy carbonyl compounds with high enantioselectivity up to 98% ee.

Plasma obestatin concentrations are negatively correlated with body mass index, insulin resistance index, and plasma leptin concentrations in obesity and anorexia nervosa.

BACKGROUND: Obestatin is a recently identified ghrelin gene product that was reported to inhibit appetite and gastric motility in contrast to ghrelin. We investigated fasting obestatin and ghrelin levels in patients with obesity and anorexia nervosa. METHODS: Fasting plasma obestatin, acyl-ghrelin, desacyl-ghrelin, leptin, glucose serum adiponectin, and insulin were measured in 10 obese subjects, 11 restricting-type anorexics, and 11 control subjects. RESULTS: Obese group had significantly lower levels of obestatin (p < .01), while anorexic group had significantly higher levels (p < .01). Obestatin was negatively correlated with body mass index (BMI) (r = -.74), glucose (r = -.56), insulin (r = -.55), leptin (r = -.66), and also with the homeostasis model assessment of insulin resistance (HOMA-R) (r = -.49) and was positively correlated with acyl-ghrelin (r = .65) and desacyl-ghrelin (r = .60). No correlation was seen between obestatin and adiponectin, but the latter was negatively correlated with both acyl-ghrelin and desacyl-ghrelin. Desacyl-ghrelin to acyl-ghrelin ratio was significantly different between anorexic and control groups (p < .05), while no difference was seen between obese and control groups. CONCLUSIONS: Both obestatin and ghrelin are increased in anorexic and decreased in obesity. We suggest that obestatin is a nutritional marker reflecting body adiposity and insulin resistance.