Di-isobutylaluminum Hydride

With the co side chain at C-12 in place, we are now in a position to address the elaboration of the side chain appended to C-8 and the completion of the syntheses. Treatment of lactone 19 with di-isobutylaluminum hydride (Dibal-H) accomplishes partial reduction of the C-6 lactone carbonyl and provides lactol 4. Wittig condensation8 of 4 with nonstabilized phosphorous ylide 5 proceeds smoothly and stereoselectively to give intermediate 20, the bistetra-hydropyranyl ether of ( )-1, in a yield of -80% from 18. The convergent coupling of compounds 4 and 5 is attended by the completely selective formation of the desired cis C5-C6 olefin. 

The H—D exchange between triethyltin deuteride and di-isobutylaluminum hydride has been studied by Neumann 84). A four-center transition state has been proposed for this reaction... 

In order to replace the chlorine atom by the desired hydroxy functionality, di-isobutylaluminum hydride (DIBAL-H) reduction followed by an acid-catalyzed acetalization with isopropanol in refluxing benzene had to be performed to protect the lactone moiety as an acetal and provided the thermodynamically preferred a-anomer 38 together with 5-8% of the other epimer. The conversion of the... 

DI-ISOBUTYLALUMINUM HYDRIDE (and ALKYLALUMINUMS generally) [(CH3)2CHCH2]2A1H... 

Numerous other metal hydride reagents have been developed to accomplish a variety of specialized reductions. The only other one that will be discussed here is di-isobutylaluminum hydride, i-Bu2A1H or DIBALH ... 

Reduction of 308 with sodium borohydride gives 24% tetraphenylethylene with lithium borohydride, 75% of 2,2-diphenylethanol and 16% of 2,2-diphenylethyl disulfide with di-isobutylaluminum hydride, 39% l,l,3,3-tetraphenyl-2-propanone and 30% of benzophenone. Lithium aluminum hydride gives four products that may be formed by way of cyclopropanone intermediates. ... 

Pentadiene and dialkyl alanes give addition products from which only pentane is split off on hydrolysis (88). Nothing further is known yet about the constitution of these compounds, e.g., if aluminum heterocycles (6- or 5-ring compounds) are formed. Tris(3,3-dimethyl-pentamethylene) dialane is said to be formed from 3,3-dimethyl-l,4-pentadiene and di-isobutylaluminum hydride in 98% yield, even under forcing conditions at elevated temperatures (160°C). The compound distills at low pressures (b.p.io-t 132°-140°C) (212). 

Double bonds of benzene and related aromatic compounds do not react with diazoalkanes in 1,3-cycloadditions. The corresponding benzo-annellated dihydro-pyrazoles such as (1 a,6a)-6,9,9-trimethyl-7,8-diazabicyclo[4.3.0]nona-2,5,7-triene (6.98, R = H) can be synthesized, however, from methyl cyclohexa-l,4-dienecarboxy-late (6.97, R = H), as shown by Klarner et al. (1990). The reduction of the ester group to a methyl group was carried out with di-isobutylaluminum hydride (DIBAL-H), esterification of the OH group with methanesulfonyl chloride and reduction with lithium triethyl borohydride. The second double bond was introduced by bromina-tion with Br2 on a polymeric carrier after Bongini et al. (1980). Cycloaddition with 2-diazopropane in ether at -- 5 °C and the following steps gave 6.98 in a yield of 58%. 

Deprotection of the tertiary benzyl M,M-diisopropylcarbamates is difficult. Acidic conditions lead to the destruction of the tertiary substrates. Occasionally, treatment with UAIH4 in refluxing THF [57,136], or with a large excess of di-isobutylaluminum hydride, Hberates the carbinols, but ester and carbonyl groups are also reduced. 

The diastereomeric retinaldehyde derivatives (394), (395), (396), and (397) were synthesized from the ketone (390) by the following procedure (Nakanishi et aL, 1976) The allenic ketone (390) was reacted with the C2 phosphonate (167) to give the mixture of ester isomers (391), which was reduced with di-isobutylaluminum hydride (DIBAL) and then oxidized with manganese dioxide. The 9 -cis aldehyde (392) was separated by chromatography and was reacted with the C5 phosphonate (166) to give a mixture of 9Z, 2>E and 9Z 13Z isomers of the ester (393). Treatment of the 9-cis, 13-trans ester with DIBAL and manganese... 

Addition to chiral, bicyclic acetals has been exploited in an approach to the synthesis of the tetrahydropyran subunit of the polyether nigerin. The particular acetal generated by the di-isobutylaluminum hydride reduction of aliphatic esters undergoes aldol addition in good yields (eq 16). ... 

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