Triisopropoxy

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 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). 

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). 

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... 

When the reaction between a trialkylborane and carbon monoxide (8-24) is carried out in the presence of a reducing agent such as lithium borohydride or potassium triisopropoxy-borohydride, the reduction agent intercepts the intermediate 73, so that only one boron-to-carbon migration takes place, and the product is hydrolyzed to a primary alcohol or oxidized to an aldehyde.333 This procedure wastes two of the three R groups, but this problem can be avoided by the use of B-alkyl-9-BBN derivatives (p. 785). Since only the 9-alkyl group... 

MET MY I.ATION /u-Chlorohis(ij-2,4-cyclo-pentadien-l-yl)(dimethylaluminum)-(i-inethylcnelitanium. Triisopropoxy-methyllitanium. 

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... 

Triisopropoxy[l-(trimethylsilyl)-l,2-butadienyljtitanium, 218 (Triisopropylsilyl)allyllithium, 11 Triisopropylsilyl trifluoromethanesul-fonate, 11... 

Trichloroisopropoxytitanium, 300 Trichloromethyltitanium, 216 Triisopropoxymethyltitanium, 213 Triisopropoxy[ l-(trimethylsilyl)-l, 2-bu-tadienyljtitanium, 218 Tungsten Compounds /rans-Bromotetracarbonyl(methyl-methylidyne)tungsten, 49 Hydrogen peroxide-Sodium tungstate, 145... 

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

Aldol reactions. The Mukaiyama aldol reaction employing 1-triisopropoxy-l-r-butylthioethene as donor displays exceptional Cram-type selectivity, thus the bulk of the silyl group has a crucial effect on the level of 1,2-asymmetric induction. Bicyclic lactones are formed by treatment of a hydroxyalkylalkyne substituted with tungsten and also carrying an acetal side chain. ... 

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