Triisopropoxy titanium

The addition of 2-butenyl(triisopropoxy)titanium was applied in the total synthesis of the pyridone fragment of the antibiotics aurodox and efrotomycin57. 2-Alkenyltitanium(IV) reagents exhibit outstanding simple diastereoselectivity in reactions with methyl ketones52,58 (see Table 2). 

A chiral dinuclear Ti(IV) oxide 20 has been successfully designed by Maruoka and coworkers and can be used for the strong activation of aldehydes, thereby allowing a new catalytic enantioselective allylation of aldehydes with allyltributyltin (Scheme 12.18). ° The chiral catalyst 20 can be readily prepared either by treatment of bis(triisopropoxy)titanium oxide [(/-Pr0)3Ti-0-Ti(0/-Pr)3] with (S)-BINOL or by the reaction of ((5)-binaphthoxy)isopropoxytitanium chloride with silver(I) oxide. The reaction of 3-phenylpropanal with allyltributyltin (1.1 equiv) under the influence of 20 (10 mol%) gives l-phenyl-5-hexen-3-ol... 

N1tro-l-phenylethanol by Addition of Methyl-triisopropoxy-titanium... 

In a 250-mL, three-necked flask attached to a short-path distillation apparatus, under argon, 40 mL of abs. benzene is added dropwise to ca. 3 mmol of the chiral diol (and, if necessary, 3 mmol of an alcohol . Then, 10 mL of benzene is distilled off. After addition of an equimolar amount of chloro(triisopropoxy)titanium in pentane, first pentane, then j-PrOH/benzene and finally (except for a small residue) benzene are distilled off, The yellowish residue is partially dried under high vacuum. 

The best stereochemical results, however, were obtained with the new and bulky methylmetallic reagent, methyl triisopropoxy-titanium, (12) and with methylmagnesium chloride (eq. 13). Presumably, the more electrophilic chloromagnesium species formed a stronger complex with the bidendate enone sulfoxide than did the bromo or the lodomagnesium species (13) and thus forced the 3-addltlon to proceed entirely through the chelated and therefore locked conformation shown in model 9. 

In contrast to the results obtained with the jS-alkoxy-a-alkyl-y-lactol 16 (vide supra), a chelation-directed, anti-Cram selective nucleophilic addition to the a-methyl-y-lactol 1 was not only observed with methyllithium and methylmagnesium bromide but also with (triisopropoxy)methyl-titanium72. In fact, the highest diastereoselectivity (> 98 % de) was observed with the titanium reagent in dichloromethane as reaction solvent. A seven-membered chelate 3 with the a-methyl substituent in a pscudoequatorial position has been postulated in order to explain the stereochemical outcome. 

On the other hand, addition of methylmagnesium bromide, as well as (triisopropoxy)methyl-titanium, to the O-benzyl-protected analog 4 proceeds with the facial stereoselection predicted by Cram s open-chain model, although the selectivities are very low72. 

The enantiomeric purity of the ( + )-(/J)-adduct was increased by using lithium tetramethylalu-minate (90% ee), triisopropoxy(methyl)titanium (91 % ee) or methylmagnesium chloride (98-100 % ee, 93 % yield). 

Dimethoxy [ 1 -trimethy lsilyl-1,2-buta-dienyl]borane, 218 Triisopropoxy[l-(trimethylsilyl)-l,2-butadienyl]titanium, 218 Vinyl groups... 

Allyltributyltin, 10 Vinyltrimethylsilane, 343 Homoallylic ethers Allyltrimethylsilane, 11 Crotyltrimethylsilane, 86 Trityl perchlorate, 339 Homopropargylic alcohols Allenylboronic acid, 36 D imethoxy [ 1 -trimethylsilyl-1,2-buta-dienyljborane, 218 Lithium acetylide, 44 Triisopropoxy[l-(trimethylsilyl)-l,2-buta-dienyl]titanium, 218 Hydroperoxides Oxygen, singlet, 228 Hydroxy acids a-Hydroxy acids... 

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