Boranes syntheses with

Air-sodium hydroxide/hydrogen peroxide 2-Ethylenealcohols from boranes Synthesis with addition of 4 C-atoms... 

Highly enantioenriched 4-alken-l-yn-3-ol moieties present in many bioactive acetylenic metabolites from sponges have been efficiently obtained by reduction of the parent 1-trimethylsilyI-4-alken-l-yn-3-one 18 with Alpine-borane or with BH3-SMe2 in the presence of chiral oxazaborolidines, followed by desilylation of the resulting alcohol. This strategy has been applied to the first stereoselective synthesis of petrofuran 19 <99SL429>. 

Further developments are likely as the chemistry of the compounds described above is explored. Moreover, entirely new dimensions may be added. For example, the synthesis of tungsten-alkylidyne complexes with carba-borane ligands with cage structures smaller than the icosahedral C2B9 fragment should result in the isolation of new electronically unsaturated metal cluster and electron-deficient molecules of types as yet unknown. 

An important method for the synthesis of carbaboranes is the reaction of a borane cluster with an alkyne. However, an analogous reaction of a metallaborane cluster with an alkyne is not commonly used to prepare metallacarbaborane clusters. Equation (21) illustrates one example. 

Conjugated enynes are of importance for themselves, as well as for the synthesis of conjugated dienes. The cross-coupling reaction of 1-alkenyl(disiamyl)boranes (3c) with 1-bromo-l-alkynes (Scheme 2-34) provides conjugated enynes in high yields [45]. 

In contrast to the oxidative deavage of the C10-C11-moiety, hydroboration followed by oxidation furnishes Cll-functionalized quinine and quinidine derivatives. As an illustration, only transformations in the quinidine series are shown (cf. Scheme 12.14). Cll-aldehyde 58 was obtained either upon direct oxidation of the borane species with PCC/Si02 or via an improved stepwise procedure including (i) oxidation with Me3NO-2 H20 to yield the terminal alcohol 57 and (ii) subsequent Dess-Martin oxidation [35], Oxidation and esterification of alcohol 57 using Jones reagent and MeOH/HCl gave the Cl 1-ester 59, which is a suitable precursor for the synthesis of cinchona alkaloid macrocycles (Scheme 12.15) [38],... 

As has been the pattern in recent years, there has been considerable interest in the synthesis and characterisation of phosphide reagents derived from metals other than lithium, sodium, and potassium, and also in studies of the structure of metallophosphides in the solid state. A new route to P-chiral phosphine-boranes of high enantiopurity is afforded by treatment of the borane complexes of methyl(phenyl)phosphine with a copper(I) reagent, giving the copper-phosphido intermediate (83), which, on subsequent treatment with an iodoarene in the presence of a palladium(O) catalyst, gives the related chiral t-phosphine-borane (84), with retention of configuration at phosphorus. Organophosphido systems... 

D.S. Matteson (1995) Stereodirected Synthesis with Organo-boranes, Springer, Berlin. 

Borane reacts with electron pair donors to form Lewis acid-base complexes. The most common forms for use in synthesis are the THF and dimethyl sulfide complexes. Stronger bases, particularly amines, form less reactive adducts. 

The addition reaction of carbon-11 labelled cyanide ion to the bisulphite addition adduct of an aldehyde has been extended to prepare carbon-11 labelled amines. Maeda and coworkers prepared both p- and m-octopamine [2-(p-and m-hydroxyphenyl)-2-hydroxyethyl-amine] from the corresponding benzaldehyde by reducing the cyanohydrin formed in the reaction between the appropriate benzaldehyde and cyanide ion both under enzymatic conditions and by the basic modification of the Bucherer-Strecker synthesis, with borane-THF. The synthesis of / -octopamine is presented in equation 64. 

The stereoselective synthesis of the (RJi)- (or (S,S)-ligands) 99 was performed in several steps using the ephedrine methodology with (-t)- or (—)-ephedrine, respectively. The key step of the synthesis is the methano bridge formation by reaction of the carbanion derived methyl phosphine borane 98 with the chloro... 

The chlorophosphine boranes 97 are efficient starting reagents for the synthesis of various classes of P-chiral phosphorus compounds. Reactions of chlorophosphine boranes 97 with nucleophiles, such as carbanions, phenoxides, phenylthiolates, or amides, leads to the formation of corresponding organophos-phorus compounds 100-103 in yields of 53-99% and with up to 99% ee. This method was also used for the preparation of various classes of symmetric and asymmetric P-chiral ligands useful for asymmetric reactions, catalyzed by complexes of transition metals (Scheme 31) [52, 60, 61]. 

Another interesting preparative example was reported by Kurosu and Kishi (Scheme 6.9) [21]. An advanced steroid intermediate 39 was prepared in racemic form by total synthesis. Upon borane reduction with 33 as a catalyst, the reaction afforded 40 and 41 as the exclusive products (each with >99% ee). The authors noted that this approach allows access to either enantiomeric product from the same total synthesis because the diastereomers 40 and 41 may be separated and re-oxidized to give the corresponding enantiomers of the starting ketone 39. This is likely to be an easier approach compared to classical resolution of the advanced intermediate. 

The terminal alkynes react with 2 equiv of 9-BBN and affords, quantitatively, the corresponding 1,1-diboraylalkanes. Soderquist has reported that this trialkyl-borane reacts with 1 equiv of benzaldehyde or 1-NaphCHO in 2 h at 25 °C, and quantitatively form B-ArCH20-9-BBN and B-alkenyl-9-BBN, exclusively with tram configuration [18]. The frans-B-alkenyl-9-BBN undergoes selective oxidation [18] with 1 equiv of anhydrous trimethylamine-N-oxide (TMANO) [19] and affords almost quantitatively the corresponding stable fraws-B-vinyl-9-oxa-10-borabicyclo[3.3.2]decane derivatives (traws-B-vinyl-OBBD). tram-B-Yinyl-OBBD derivatives are inert to atmospheric oxygen and are unreactive toward protonolysis (HOAc, 25 °C, 8 h) or insertion process (PhCHO, neat, 80 C, 6 h). The reaction sequence for the synthesis of fraws-B-vinyl-OBBD is outlined in Scheme 20.6 [18]. 

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