9-Borabicyclo nonane reactions with

Z-Boron enolates can also be obtained from silyl enol ethers by reaction with the bromoborane derived from 9-BBN (9-borabicyclo[3.3.1]nonane). This method is necessary for ketones such as 2,2-dimethyl-3-pentanone, which give E-boron enolates by other methods. The Z-stereoisomer is formed from either the Z- or E-silyl enol ether.20... 

In the second step, achiral 9-borabicyclo[3.3.1]nonane (9-BBN) adds to the less hindered diastereotopic face of a-pinene to yield the chiral reducing agent Alpine-Borane. Aldehydes are rapidly reduced to alcohols. The reaction with deuterio-Alpine-Borane, which yields (R)-a-d-henzy alcohol in 98% enantiomeric excess ( ) reveals a very high degree of selectivity of the enantiotopic faces of the aldehyde group in a crowded transition state ... 

Organoaluminum reagents, 202 1,1,1-Trifluoroacetone, 323 Trityllithium, 338 Zinc chloride, 349 Stereoselective aldol reactions With boron enolates Boron trichloride, 43 Chlorodimethoxyborane, 73 9-(Phenylseleno)-9-borabicyclo-[3.3.1]nonane, 245 With silyl enol ethers... 

An improved route to 2a-hydroxycholesterol has been devised as part of the preparation of 2a-hydroxy-vitamin D3 (263 R1 = R4 = R5 = R6 = H, R2 = R3 = OH).123 Hydroxylation of the A bond of cholesta-l,5-dien-3/3-ol by means of 9-borabicyclo[3,3,l]nonane followed by reaction with alkaline hydrogen peroxide produced the 2-equatorial 2a,3a-diol in 70—80% yield. The conventional four-step sequence, acetylation, bromination, dehydrobromination, and hydrolysis, gave 2a -hydroxycholesta-5,7-dien-3/3-ol which was converted into 2a-hydroxy-vitamin D3. The isomeric 2/3-hydroxy-vitamin D3 has also been reported.124 Reaction of the 1/6,2/3-oxide obtained by peroxidation of the adduct (265) with lithium aluminium hydride results in a mixture of 2/3,3/3-dihydroxycholest-5,7-diene and its 1/3,3/3-dihydroxy-epimer in the ratio 8 1. Irradiation of the former 5,7-diene furnished the expected previtamin, which on equilibration gave 2/3-hydroxy-vitamin D3 (263 R1 = R4 = R5 = R6 = H, R2 = a-OH, R3 = OH). 

This reaction is general , with simple routes to trialkylhydroborates available. The method is effective for boranes from B(C2Hj)3 to such complex ones as phenyl-9-borabicyclo[3.3.1]nonane . Reaction occurs when a slight xs of (t-C4H,)Li in pentane is added dropwise to borane in THF at — 78°C with vigorous stirring. The lithium trialkylhydroborate is formed quantitatively ... 

Borabicyclo[3.3.1]nonane (9-BBN-H) has received greater study than the others and a separate section is devoted to its reactions, with the other dialkyl- and diaryl-boranes considered as a group. Separate sections are also devoted to dihaloboranes, alkylchloroboranes and catecholborane and its analogs. 

An important feature of the Suzuki reaction is that it allows cr/Z Z-alkenyl or alkyl-aryl coupling of fi-a//cy/-9-borabicyclo[3.3.3]nonane derivatives with haloalkenes and haloarenes without concomitant p-hydride elimination. The required organoboranes are obtained by hydroboration of the appropriate alkenes or functionally substituted alkenes with 9-BBN. The coupling reaction tolerates the presence of functional groups in both reaction partners, thus circumventing a requirement of their prior pro-tection. ... 

Hydroboration of an allylic chloride gave a cyclopropane 14 when the intermediate or-ganoborane was treated with aqueous sodium hydroxide. 9-Borabicyclo[3.3.1]nonane is particularly suitable the geometry present in the alkene is retained in the product. 3-Bromopropyne underwent reaction with two equivalents of 9-borabicyclo[3.3.1]nonane, subsequent treatment with methyllithium and then with hydrogen peroxide gave cyclopropanol. 

The new base is also superior to potassium /-butoxide for the reaction of organoboranes with ethyl bromoacetate (2, 192-193) and with ethyl dibromo-acetate (2, 195).6 Yields are higher and it is not necessary to avoid an excess of the base. In the case of trialkylboranes the usual reaction conditions were used. For an unknown reason the reaction with B-alkyl-9-borabicyclo[3.3.1]nonanes in THF was unsuccessful. However, if the reaction is carried out in a solution roughly 50 50 in THF and /-butanol, satisfactory yields are obtained. 

Homologated products can be prepared by using 9-alkyl-9-borabicyclol3.3.1]nonanes with the appropriate homologated alkyl group. The diastereoselective hydrobora tion of steroidal olefins with 9-borabicyclo[3.3.1]nonane followed by the reaction with chloroacetonitrile has also been utilized in the construction of steroidal side chains23-24. 

Borabicyclo[3.3.1]nonane (9-BBN) has found use in the selective hydro-boration of alkenes in the presence of other reducible functional groups and its reaction with alkynylstannates has been studied. o-Stannyl- and a-silyl-substituted crotyl-9-BBN show promise as reagents for the stereo-regulated synthesis of acyclic systems. A series of papers covers the question of olefin-alkyl exchange in. 5-alkyl-9-BBN s, " the kinetics of reduction of substituted benzaldehydes with 9-BBN, and the kinetics and mechanism of hydroboration of alkynes with 9-BBN dimers. Selective dehalogenation of tertiary alkyl, benzyl, and allyl halides in the presence of secondary or primary alkyl or aryl halides is possible with (165). The... 

The preparations of lithium and sodium (cyclooctane-1,5-diyl)dihydro-borates(l-) in tetrahydrofuran proceed via isolable, stable etherates. These can be made solvent-free simply by heating under vacuum. 9-Borabicyclo[3.3.1 ]-nonane dimer (9-BBN) can easily be prepared from cycloocta-1,5-diene2 by reaction with tetraethyldiborane(6), tetrahydrofuran-borane8,9 or dimethyl sulfide-borane.10 The synthesis of alkali metal (cyclooctane-1,5-diyl)dihydroborates is achieved by addition of 9-BBN to a suspension of the alkali metal hydride in tetrahydrofuran. Lithium hydride reacts more slowly than sodium or potassium hydride. The reactions are brought to completion by heating under reflux. 

It is interesting to note that ate-complexes obtained from a 3-alkyl-7-methylene-3-borabicyclo-[3.3.1]nonane (127) react with acetyl chloride in-a similar way, giving 3,5-dimethylene-cyclohexylmethyl(dialkyl)boranes (128). If compounds (128) are used in the known reaction with aromatic aldehydes <65IZV898,66ZOB62), the complete dehydroboration of 1-boraadamantane (5) is achieved, affording 1,3,5-trimethylenecyclohexane (129) (Scheme 51) <83JOM(246)l29>. 

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