Di-3-pinanylborane

Addition of alane and borane to alkenes affords a host of alkylated alanes and boranes with various reducing properties (and sometimes bizarre names) diisobutylalane (Dibal-H ) [104], 9-borabicyclo[3.3.1]nonane (9-BBN) (prepared from borane and 1,5-cyclooctadiene) [705], mono- [106,107] and diiso-pinocampheylborane (B-di-3-pinanylborane) (both prepared from borane and optically active a-pinene) [108], isopinocampheyl-9-borabicyclo[3.3.1 Jnonane alias B-3-pinanyl-9-borabicyclo[3.3.1]nonane (3-pinanyl-9-BBN) (prepared from 9-borabicyclo [3.3.1]nonane and a-pinene) [709], NB-Enanthrane prepared from 9-borabicyclo[3.3.1]nonane and nopol benzyl ether) [770] and others. ... 

More recently, the same group achieved a simple, highly stereocontrolled total synthesis of (+)-hirsutic acid (Scheme LXXIX) ". This chirally directed effort developed subsequent to reaction of dl-728 with (+)-di-3-pinanylborane, alkaline hydrogen peroxide oxidation, chromatography, PCC oxidation, and hydrogenolysis. The dextrorotatory hydroxy ketone 729 was nicely crafted into keto aldehyde 730 from which 720 was readily obtained. Once again, the Wacker oxidation played an instrumental role in annulation of the third five-membered ring. The remainder of the asymmetric synthesis was completed as before. 

ASYMMETRIC GRIGNARD CROSSCOUPLING Ferrocenylphosphincs. ASYMMETRIC HYDROBORATION Dilongifolylborane. ( + )-Di-3-pinanylborane. Monoisocamphenylboranc. Thexylborane. 

FIG. 1. Examples of asymmetric synthesis, (a) Addition of a Giignard reagent to a chiral a-keto ester, (b) Chiral hydroboration with di-3-pinanylborane. (c) Chiral heterogeneous hydrogenation. 

In a 2-L, three-necked flask fitted with a condenser, mechanical stirrer, and a gas inlet tube Is placed 90.0 g (0.66 mol) of (-)-a-pinene (Note 15), The flask 1s cooled to 0°C and under an inert atmosphere a total of 300 mL (0,30 mol) of 1 M borane in tetrahydrofuran (Note 16) Is added dropwise over a 1-hr period. The solution is stirred for 18 hr at 0°C during which time a white precipitate of (+)-di-3-pinanylborane forms. This, solution is then cooled to -78°C. The ca. 0.30 mol solution of methyl 2,4-cyclopentadiene-l-acetate (Part B) 1s transferred at -78°C to a 500-mL pressure-equalizlng dropping funnel through a U-tube in an Inert atmosphere, and is added rapidly, In one portion, to the stirring solution of di-3-pinanylborane at -78°C. After this mixture is stirred for 6 hr at -78°C, the bath temperature is allowed to rise to 0°C and the mixture is stirred for 16 hr at 0°C to complete the hydroboratlon reaction. 

Reduction191 of the imine (367) with diborane followed by a catalytic hydrogenation produced 20a(20S)- and 20/3(20R)-amines, (370) and (371), in the ratio 55 45. Reduction employing (+ )-di-3-pinanylborane gave a similar mixture. However, when chirality was introduced into the imine as in (368) [prepared from (S)-phenylethylamine] diborane reduction yielded uniquely the 20a -amine (370) (funtuphyllamine A). The imine (369), prepared from ( + )-(R)-phenylethylamine, gave (370) (8%) and (371) (92%). 

An extremely efficient synthesis of lactone [41] is provided (33,34) by asymmetric synthesis (Fig. 10). Alkylation of the anion of cyclopentadiene with methyl bromoacetate gave the unstable diene [59], Immediate asymmetric hydroboiation with (+)-di-3-pinanylborane gave, after oxidative workup, the hydroxy ester [60] in about 95% e.e. Lactonization involved conversion to mesylate [61] and saponification. The crystalline lactone [41] was readily brought to an enantiomerically pure state. This route is apparently the basis for commercial quantities of compound [41], the Corey lactone, and other prostaglandin intermediates offered by the Hungarian firm Chinoin. 

Xylitol has been derived from the product of photo-oxidation of cyclopentadiene [484] which is (Z)-(4i 5)-4,5-epoxypent-2-enal. An elegant total synthesis of acosamine and of daunosamine (13 steps, 15% overall yield) was developed at Roche starting from cyclopentadiene [485], The key-step is the asymmetric monohydroboration of 5-methylcyclopentadiene with (—)-di-3-pinanylborane. 

Reduction of chiral amines. Reduction of the 20-iminopregnane derivative (1) with either diborane or ( )-di-3-pinanylborane gives, after debenzylation, about equal amounts of the 20a- and 20/3-amine (2a and 2b). However, reduction... 

Diphosphopyridine nucleotide, 36 Diphosphorus tetraiodide, 243-244 Di-3-pinanylborane, 161 Di- -propylcopperlithium, 245 2,2 -Dipyridine, 52 2,2 -Dipyridyl disulfide, 246-247 a-Disaccharides, 569 Disiamylborane, 62... 

Table 10.1. Stereoselectivity in Some Hydroboration-Oxidations of Alkenes with Di-3-pinanylborane ...

The desired configuration at the chiral center created on reduction is the one shown. Reduction with zinc borohydride is essentially nonstereoselective, giving a 1 1 mixture of the desired alcohol and its diastereomer. A complex hydride prepared from di-3-pinanylborane and methyllithium (6) increases the ratio in favor of the... 

Di-3-pinanylborane is a versatile chirial reagent for selective hydroboration and exhibits a remarkable asymmetric stereoselectivity when applied to the hydroboration of c/s-olefins, H. C. Brown, N. R. Ayyangar, and G. Zweifel, J. Amer. Chem. Soc., 86, 397 (1964). See also J. D. Morrison and H. S. Mosher in "Asymmetric Organic Reactions, Prentice-Hall, Englewood, 1971, p. 220-241. 

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