Allylboranes, synthesis

The synthesis of chiral a-substituted allylborane 2 via the hydroboration of 1,3-cyclohexa-diene with diisopinocampheylborane has been reported21. 

In 2009, Ritter and coworkers reported a selective Fe-catalyzed hydroboration of 1,3-dienes to produce linear ( )-y-disubstituted allylboranes under mild conditions when a combination of Ll-FeCl2 and magnesium metal as a catalyst was used. Tbe branched ( )-allylboranes were obtained by using L2-FeCl2 instead of Ll-FeCl2 (Scheme 26) [88]. For the synthesis of 2-borylallylsilanes, this method was superior to the previously reported silaboration of allenes [89]. 

In 1987, Vaultier and coworkers [27] developed a combination of a [4+2] cycloaddition of a bora-1,3-diene to provide an allylborane, which then reacts with an aldehyde to give a highly functionalized alcohol. The Lallemand group, as well as Hall and colleagues, has recently used this procedure. In an approach for the synthesis of the antifeedant natural product clerodin (4-83), Lallemand and coworkers performed a three-component domino reaction of 4-80, 4-81 and methyl acrylate to give 4-82 (Scheme 4.18) [28]. 

The nucleophilic attack on an acceptor-substituted allene can also take place at the acceptor itself, especially in the case of carbonyl groups of aldehydes, ketones or esters. Allenic esters are reduced to the corresponding primary alcohols by means of diisobutylaluminum hydride [18] and the synthesis of a vinylallene (allenene) by Peterson olefination of an allenyl ketone has also been reported [172]. The nucleophilic attack of allenylboranes 189 on butadienals 188 was investigated intensively by Wang and co-workers (Scheme 7.31) [184, 203, 248, 249]. The stereochemistry of the obtained secondary alcohol 190 depends on the substitution pattern. Fortunately, the synthesis of the desired Z-configured hepta-l,2,4-trien-6-ynes 191 is possible both by syn-elimination with the help of potassium hydride and by anti-elimination induced by sulfuric acid. Analogous allylboranes instead of the allenes 189 can be reacted also with the aldehydes 188 [250]. 

Chiral addition of allyl metals to imines is one of the useful approaches toward the synthesis of homoallylic amines. These amines can be readily converted to a variety of biologically important molecules such as a-, / -, and y-amino acids. Itsuno and co-workers utilized the allylborane 174 derived from diisopropyl tartrate and cr-pinene for the enantioselective allylboration of imines. The corresponding iV-aluminoimines 173 are readily available from the nitriles via partial reduction using diisobutylaluminium hydride (DIBAL-H) <1999JOM103>. Recently, iV-benzyl-imines 176 have also been utilized for the asymmetric allylboration with allylpinacol boronate 177 in the presence of chiral phosphines as the chiral auxiliaries to obtain homoallylic A -benzylamines 178 in high yield and selectivity (Scheme 29) <2006JA7687>. 

The transmetallation process was extended to the preparation of vinylboranes709,710, and the superior reactivity of organotins over organosilicons was elegantly demonstrated by Williams and coworkers for the preparation of the optically active allylborane 24 from the corresponding allylic stannane in the total synthesis of (—)-Hennoxazole A711 (equation 54). 

The Brown allylboration was used in the enantioselective total synthesis of (-)-calicheamicinone 3318 (Scheme 3.1o). Thus the lactol 34, readily prepared from tetronic acid, was treated with the allylborane d35 to give 36 in a highly stereoselective manner (95% ee, > 98% de). Compound 36 was converted to the aldoxime 37 by standard chemistry. Generation of the nitrile oxide with aqueous sodium hypochlorite was accompanied by spontaneous [3 + 2]-dipolar cycloaddition to afford 38 in 65% yield. 

In the synthesis of a library of (+)-murisolin (39) and its 15 other stereoisomers, Curran and others used Brown allylation strategy to obtain the four diastere-oisomers of the homoallylic alcohol 4019 (Scheme 3.1p). Thus, the homoallylic alcohol (S,S )-40 was prepared in 95% ee from allylborane reagent l35 and corresponding aldehyde. By using the Mitsunobu reaction, (R,S )-40 was obtained. Similarly, (R,R)-40 was synthesized via the enantiomeric borane reagent d35... 

Corey s asymmetric allylation methodology was utilized in the total synthesis of amphidinolide T3 (95), a marine natural product that exhibits significant antitumor properties37 (Scheme 3.1gg). The asymmetric allylation of the aldehyde 96 was carried out successfully with chiral allylborane reagent generated in situ from allyltributylstannane and (R,R)-82 to furnish the homoallylic alcohol desired (97) in 85% yield with excellent diastereoselectivity. Subsequent conversion of the alcohol to the tosylate ester followed by treatment with potassium hydroxide resulted in formation of the trisubstituted tetrahydrofuran 98. 

Murata, M., Watanabe, S., Masuda, Y. Regio- and stereoselective synthesis of allylboranes via platinum(0)-catalyzed borylation of allyl halides with pinacolborane. Tetrahedron Lett. 2000, 41, 5877-5880. 

The synthesis of methyl ketone 281 began with the reaction between the tetra-substituted allylborane 279 and 2,3-0-isopropylidene-D-glyceraldehyde 48. The resulting homoallylic alcohol 280, obtained in 73% yield and excellent selectivity (exact ratio not defined) [231], was converted in two steps to the methyl ketone 281. Aldol condensation between the lithium enolate of 281 and aldehyde 278 (structure shown in Scheme 11-12) gave, after protection of the initial adduct, the Felkin diastereomer 282 as the only reported product in 54% yield. This adduct... 

Stereoselective synthesis of the C28-C46 fragment of phorboxazole A has been achieved. In this approach the correct relative stereochemistry at Csy/Csg was established by use of allylborane which was prepared by transmetalation between allylstannanes and a boron bromide reagent derived from homochiral diamine and BBrj (Scheme 12.24) [62]. 

A particularly useful borane is the chiral reagent (-)-B-allyl(diisopinocampheyl)borane.l 3 Although the chemistry of acyl addition to aldehydes and ketones to give alcohols will not be formally discussed until Section 8.4.C, the addition of this chiral allylborane to carbonyl compounds will be presented here. In a simple example taken from Smith s synthesis of (-)-macrolactin A,n 156 was treated with (-)-B-... 

The reaction proceeds with excellent enantioselectivity as shown, but in some cases the diastereoselectivity is modest, which is true when the allylborane reagent is substituted, as in crotylborane derivatives. In the Omura, Smith and co-workers synthesis of (+)-lactacystin, for example, reaction of the B-crotyl derivative shown with aldehyde 158 gave a 4 1 mixture of 159 and 160 in high yield. Each diastereomer was formed with high enantioselectivity. 

Since allenes are easily obtained from the corresponding olefins [6, 7], the route olefin-allene-allylborane-carbonyl addition is general sequence and is a viable alternative to Grignard-based process for the synthesis of complex structures. 

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