Titanium reagent application

Application of this catalytic process was extended to asymmetric intramolecular Diels-Alder reactions. Synthetically useful intermediates with octalin and decalin skeletons were obtained in high optical purity by use of a catalytic amount of the chiral titanium reagent [45] (Scheme 1.57, Table 1.25). The core part of the mevi-nic acids was enantioselectively synthesized by use of this asymmetric intramolecular reaction [46] (Scheme 1.58). 

Allyltrialkoxy- or -tris(dialkylamino)titanium reagents are not capable of chelation-controlled addition reactions with oxy- or amino-substituted carbonyl compounds due to their low Lewis acidity87. To attain chelation control, the application of allylsilanes (Section 1.3.3.3.5.2.2.) and allylstannanes (Section I.3.3.3.6.I.3.2.) in the presence of bidentate Lewis acids like titanium(IV) chloride, tin(lV) chloride or magnesium bromide are the better options. 

This article will outline firstly the application of RCM technology to the preparation of the epothilones with particular emphasis on the generality and mild nature of the process. The second section will describe the use of cyclopentadi-enyl titanium reagents in metathesis processes and, in particular, their application to the preparation of polyether segments of marine neurotoxins. 

Lenoir, D. 1989. The application of low-valent titanium reagents in organic synthesis. Synthesis. 883-97. 

The present titanium reagent was noted to exhibit a wide applicability to the asymmetric Diels-Alder reaction of various prochiral dienes and dienophiles. 

On the other hand, the reaction of the oxazolidone derivative of fumaric acid and butadiene was found to proceed in poor enantioselectivity as compared with the reaction carried out in the presence of excess amounts of the chiral titanium reagent, and the cyclohexenedicarboxylic acid derivative was obtained in 32-45% ee. In order to achieve wide applicability for the catalytic procedure, the reaction conditions were... 

The chiral titanium reagent preparerd in situ from the chiral 1,4-diol and dichlorodiisopropoxytitanium is expected to be applicable to various reactions catalyzed by Lewis acids. We, therefore, investigated the asymmetric synthesis of cyanohydrins from aldehydes and cyanotrimethylsilane employing the chiral titanium reagent.(20)... 

Apart from the Takai method and titanium reagents such as 15, silyl reagents 16 and 17 frequently find application in the synthesis of vinylic silanes from carbonyl compounds. Reagent 16 can be utilized with aldehydes and non-enolizable ketones in a reaction analogous to the Peterson olefination Reagent 17 also reacts successfully with enolizable ketones.6... 

Compound 8 is a secondary allylic alcohol Systems of this type are well suited to the application of Sha/pless epoxidation 4 for kinetic racemate resolution,5 because the chiral titanium reagent bonds directly with the stcreogenic center (C-9) in the transition state (21). The undesiicd enantiomer epoxidi/es more rapidly than 9, so 9 can be separated in enantioinerically pure form (see Chapter b). 

Palominol. Corey has reported the application of an intramolecular titanium-mediated pinacol coupling reaction to the synthesis of a 15-membered ring, en route to palominol, a marine diterpenoid that displays cytotoxicity toward the human colon cell line (Eq. 3.10) [28]. Slow addition of the keto aldehyde (32 h) to the titanium reagent furnished the cyclized product in 53% yield as a mixture of diastereomers (2.1 1). 

E S S CONTENTS Preface, Tomas Hudlicky. Modern Synthetic Design Symmetry, Simplicity, Efficiency and Art, Tomas Hudlicky and Michael Natchus. Toward the Ideal Synthesis Connectivity Analysis, Paul Wender and Benjamin L. Miller. Application of Graph Theory to Synthesis Planning Complexity, Reflexivity and Vulnerability, Steven H. Bertz and Toby J. Sommer. Asymmetric Reactions Promoted by Titanium Reagents, Koichi Narasaka and Nobuharu Iwasawa. The Use of Arene Cis-diols in Synthesis, Stephen M. Brown... 

Some thermally forbidden [2 + 2]-cycloaddition reactions can be promoted by Lewis acids1-6. With chirally modified Lewis acids, the opportunity for application in asymmetric synthesis of chiral cyclobutanes arises (for a detailed description of these methods see Sections D.l. 6.1.3.. D.l. 61.4. and references 7, 28-30). Thus, a chiral titanium reagent, generated in situ from dichloro(diisopropoxy)titanium and a chiral diol 3, derived from tartaric acid, catalyzes the [2 + 2]-cycloaddition reaction of 2-oxazolidinone derivatives of a,/ -unsalurated acids 1 and the ketene thioacetal 2 in the presence of molecular sieves 4 A with up to 96 % yield and 98% ee. Fumaric acid substrates give higher yields and enantiomeric excesses than acrylic acid derivatives8. Michael additions are almost completely suppressed under these reaction... 

The Applications of Low-Valent Titanium Reagents in Organic Synthesis" Lenior. D. Synthesis. 1989, 883. 

Numerous applications of titanium enolate aldol reactions have been reported for the synthesis of natural products and bioactive compounds of pharmaceutical interest. These syntheses were performed by utilizing the titanium enolate methods described above as key steps. There are many transformations that simply cannot be achieved by use of conventional aldol chemistry. Furthermore, the use of inexpensive titanium reagents, operational simplicity, and functional group tolerance make this chemistry very attractive in synthesis. The following applications in synthesis further exemplify the increasing importance of titanium aldol chemistry. 

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