Sentaro Okamoto

The Divalent Titanium Complex Ti(O-i-Pr)4/2i-PrMgX as an Efficient and Practical Reagent for Fine Chemical Synthesis

This account describes synthetic transformations of unsaturated hydrocarbons, such as alkynes, alkenes, and dienes, mediated by the divalent titanium reagent Ti(O-i-Pr)4/2 i-PrMgX, which proceed via (ν 2-alkene)- or (ν 2-alkyne)-titanium intermediates. Many of these transformations are otherwise difficult or require multi-step reaction sequences. Since Ti(O-i-Pr)4 and i-PrMgX are non-toxic and available in bulk at low price, the reagent satisfies the qualifications for use in large-scale synthesis. 1 Introduction 2 A General View of the Ti(O-i-Pr)4/2 i-PrMgX Reagent 3 Generation of an (ν 2-Alkyne)Ti(O-i-Pr)2 and its Utilization as a Vicinal Vinylic Dianion 4 Preparation of Allyl- and Allenyltitanium Reagents and their Synthetic Utility 4.1 Allylic Titanium Reagents 4.2 Allenylic Titanium Reagents 5 Intramolecular Nucleophilic Acyl Substitution (INAS) Reactions Mediated by Ti(O-i-Pr)4/2 i-PrMgX 5.1 Scope of the Reaction 5.2 Synthetic Applications of the INAS Reaction 6 Intra- and Intermolecular Coupling of Olefins and Acetylenes 6.1 Intramolecular Reactions 6.2 Intermolecular Reactions 6.3 Tandem Inter- and Intramolecular Reactions 7 Summary and Outlook

A highly efficient synthesis of optically pure γ-iodo allylic alcohols and their conversion into various optically active allylic alcohols

Abstract Kinetic resolution of γ-iodo allylic alcohols 1 by the Sharpless asymmetric epoxidation reaction proceeds with very large rate differences for the two enantiomers, thus providing a highly efficient method for preparation of optically pure 1 . The alcohols 1 thus prepared can be readily converted into various secondary allylic alcohols through the coupling reaction with nucleophiles.

Ti(O-i-Pr)4/Me3SiCl/Mg-Mediated Reductive Cleavage of Sulfonamides and Sulfonates to Amines and Alcohols

A low-valent titanium generated in situ from Ti(O-i-Pr)(4), Me(3)SiCl, and Mg powder in THF reacted with aryl- and alkyl-sulfonamides of aryl and alkyl amines in a reductive N-S/S-O/S-C bond cleaving pathway to provide the corresponding amines and hydrocarbons (and thiols) derived from the sulfonyl moiety. The reagent could also cleave sulfonates to the corresponding alcohols.

A highly efficient synthesis of γ-halo allylic alcohols and propargylic alcohols with high optical purity: practical method for synthesis of the prostaglandin ω-chain

Abstract Kinetic resolution of γ-trimethylsilyl allylic alcohols 5 by the sharpless process combined with the reactivity of epoxysilyl or vinylsilyl compounds affords a highly efficient method for preparation of optically pure γ-halo secondary allylic alcohols of type 1 – 4 and propargylic alcohols 14 , thus providing a practical route to the prostaglandin ω-chain.

Synthesis and physiological activities of both enantiomers of coriolic acid and their geometric isomers

Abstract Both enantiomers of coriolic acid and their geometric isomers are synthesized highly selectively. These acids are found to exhibit similar inhibition activity toward spore germination of rice blast fungus.

Synthesis of 1-hydroxybicyclo[n.1.0]alkanes (n = 3 and 4) and their silyl ethers from olefinic esters via tandem intramolecular nucleophilic acyl substitution and intramolecular carbonyl addition reactions mediated by Ti(OPr-i)42 i-PrMgCl reagent

Abstract Treatment of olefinic esters with Ti(OPr-i)4/2 i-PrMgCl reagent resulted in intramolecular nucleophilic acyl substitution and successive intramolecular carbonyl addition reactions providing 1-hydroxybicyclo[n.1.0]alkanes (n = 3 and 4) in high yields.

Bicyclization of dienes, enynes, and diynes with Ti(ii) reagent. New developments towards asymmetric synthesis

The title bis-unsaturated compounds undergo bicyclization with (h 2 -propene)Ti(O-i- Pr)2 to give dialkoxytitanacycles, the characteristic reactions of which are highlighted. Substrate- controlled asymmetric cyclizations starting from optically active allenes, enynes having a chiral acetal as a leaving group, and optically active enynoates were found promising to give cyclic compounds with high enantiopurity.

Synthesis of optically active α-trimethylsilyl δα,β-butenolides and their conversion into various butenolides and saturated γ-lactones

Highly efficient method for synthesis of enantiomerically pure α,β-unsaturated butenolides having trimethylsilyl group at α-position (1) and their conversion into various butenolides and saturated γ-lactones are described.

Stereoselective synthesis of optically active substituted piperidines and pyrrolidines from amino acid derivatives by titanium(II)-mediated intramolecular cyclization reaction

Abstract A versatile and high-yielding synthesis of optically active substituted piperidines and pyrrolidines from amino acid derivatives employing titanium(II)-mediated intramolecular cyclization of bis-unsaturated carbonates was developed.

Synthesis and catalytic properties of 4-aryl-2,3-dihydro-4H-pyrimido[2,3-b]benzothiazoles for asymmetric acyl or carboxyl group transfer reactions.

4-Aryl-2,3-dihydro-4H-pyrimido[2,3-b]benzothiazoles (4-Ar-DHPBs) were synthesized and their catalytic activity and selectivity in kinetic resolution of a secondary alcohol as well as in the Steglich rearrangement and related reactions were evaluated. 4-Aryl-DHPBs showed low enantioselectivity in the acylative kinetic resolution of 1-phenylethanol. Conversely, they catalyzed the Steglich rearrangement with moderate to excellent enantioselectivity, demonstrating the possibility for remote stereocontrol by introduction of a substituent at the 4-position of DHPB.