Selectride reduction with

The first asymmetric synthesis of (-l-)-abresoline was achieved from the chiral piperidine derivative 153, which upon treatment of its hydroxy side-chain substituent with carbon tetrabromide, triphenylphosphine, and triethyl-amine cyclized to the frarcr-quinazolidine 154. Deketalization and silyl protection of the phenolic group, followed by stereoselective reduction with lithium tri-t -butylborohydride (L-Selectride ), gave an alcohol, which after acylation and deprotection furnished (-l-)-abresoline 155 (Scheme 25) <2005TL2669>. 

E) configuration. The dipolar cycloaddition of 141 with a silyl nitronate shows a slight increase of facial selectivity over 132 (Eq. 2.9). Because the cycloadducts are converted directly to the corresponding isoxazolines, only the facial selectivity can be determined. It is believed that the cycloaddition proceeds on the Re face of the dipolarophile due to shielding of the Si face by the auxihary. Both chiral auxiliaries can be liberated from the cycloadduct upon reduction with L-Selectride. 

The phenyl 1-thioparatoside 145 was activated with TV-iodosuccinimide and silver triflate and reacted with a convenient derivative of the disaccharide (1-d-GalpNAc-(l ->4)-p-D-GlcpNAc. After removal of the pivaloyl-protecting group with sodium methoxide, isomerization of paratose to tyvelose was performed in a one-pot reaction by oxidation with dimethyl sulfoxide and acetic anhydride, followed by reduction with L-Selectride. Selectivity of the reduction was better... 

Reduction with NaBH4 or DIBAL-H furnished a product mixture in which the equatorial allylic alcohol predominated. However, reduction using K-Selectride produced the axial alcohol as the major product (88% yield, 9.8 1 mixture of diastereomers). 

Fig. 13.20. Generation of enolates from a,/3-unsaturated ketones via Birch reduction (top line) or by reduction with L-Selectride (bottom line).

In order to achieve chemodifferentiation of the two ketone groups, carbonyl reduction may be carried out prior to NBS-mediated hydrolysis. Reduction with L-Selectride was found to be highly efficient and stereoselective, producing only one diastereoisomer of the product alcohol (Scheme 12). 

Epoxidation of oxonine 93 with dimethyldioxirane, followed by reduction with diisobutylaluminium hydride (DIBAL-H), resulted in a separable mixture of alcohols 95 and 96, and the side product 94 (Scheme 16). Each of the isomers was submitted to Swern oxidation and sequential stereoselective reduction with L-selectride to achieve desired stereochemistry of the products 97 and 98. Formation of the side product 94 was explained by Lewis acidity of DIBAL-H and confirmed by treatment of oxirane derived from 93 with another Lewis acid, AlMe3, to produce oxocine aldehyde 99 in 35% isolated yield <1997CL665>. Similar oxidative synthetic sequence was utilized for the synthesis of functionalized oxonines as precursors of (-l-)-obtusenyne <1999JOG2616>. 

Of the large number of reducing agents, the most useful are DIBAH and lithium tri-i -butylborohydride. Regardless of the nature of R, DIBAH reduces 3 mainly to the alcohol 4 (the tzn/i-Cram product) in 60-80% de. Reduction with L-Selectride usually proceeds in the opposite sense and in aeeordanee with Cram s chelate rule, but high selectivity is observed only when R is a primary or tertiary alkyl group. 

Analogous alkylation with different electrophiles allows for easy variation of the shielding moiety. The corresponding 3-substituted derivatives have been prepared from either 3-hydroxyisobomeol after separation of the regioisomeric mixture or from 3-eAro-hydroxycamphor regioselectively after reduction with Lithium Tri-s-butylborohydride (L-Selectride). ... 

One means of stereoselective cleavage of biaryl lactones [53] is activation of the carbonyl group with a Lewis acid and subsequent attack with a chiral nucleophile. Conversely, activation can be effected with a chiral Lewis acid followed by attack of an achiral nucleophile. Complexation of a biaryl lactone to the chiral fragment [CpRe (NO)(PPh3)j then reduction with K(s-Bu)3BH (K-selectride) and ring opening of the intermediate rhenium lactolate gives the metalated aldehyde (dr = 75 25) which is converted to the alcohol without essential loss of optical purity (Sch. 6) [54]. 

The unsaturated linkage in enantiomerically pure a-methyl-. -y-unsaturated ketones (45) exerted a powerful stereochemical influence on their reduction with L-selectride, particularly when R is a tri-methylsilyl group. The anti homoallylic alcohols (46) were produced with uniformly excellent stereoselectivity (>93 7) via a Felkin transition state in which the double bond occupied the perpendicular position (equation 12). This Felkin selectivity was sufficient to overcome any chelation-mediated contribution in the reduction of a-vinyl-p-hydroxy ketones (47) to the, 2-syn diols (48) with LiEtsBH in THF at -78 °C (equation 13).58... 

Pyrimidin-2(l//)-ones (85 X = O) and -thiones (85 X = S) on reduction with sodium borohydride afford mixtures of lH-3,4- (86) and l//-3,6-dihydropyrimidinones (87), the product ratios depending on the acidity of the medium, and some further reduction to the fully reduced pyrimidinones may also occur. Similar results are obtained with LAH as the reductant, although l-arylpyrimidine-2-thiones mainly form the corresponding 3,6-dihydropyridine-2-thiones (88), especially if a hydroxy or methoxy group occupies an ortho position in the aryl substituent. This suggests that the group may complex with the reagent and facilitate an intramolecular hydride transfer reaction. L-Selectride reduces N-benzyl-... 

Carretero and co-workers found that dqjrotection and cyclization of the vinyl-sulfone 294 produced a 4 1 mixture of 2,3-cu-disubstituted pyrrolidine 295 and its tram isomer (Scheme 40). A/-Alkylation of the nuxture. with 3-chloro-2-chloro-methylprop-l-ene followed by chromatogttqjhy led to isolation of the pure 2,3-c/s product 296, silylation and base-initiated cyclization of which gave indolizidine 297. Ozonolysis and elimination of the sulfone group yieldied another pivotal intermediate, the bicyclic enone 298. Reduction with L-Selectride afforded an inseparable nuxture of two diastereomeric alcohols 299 (9 1). Separation was accomplished only after dihydroxylation with osmium tetroxide and peracetylation of the resdting tetrols. The synthesis of ( )-241 was completed by hyc lysis of the m or tetraacetate 300. 

The same two ketones featured again in a synthesis of ( )-lasubine II (910) by Pilli et al. (Scheme 119) (368,369). In this case, reaction of A -Boc-2-ethoxypiperidine (925) with enone 926 in the presence of trimethylsilyl triflate sparked off a remarkably efficient (90%) one-pot synthesis involving condensation (via an A -acyliminium ion to give the intermediate 927), deprotection, and intramolecular conjugate addition. The familiar products 921 and 922 were obtained as a 3 2 mixture. Base-induced epimerization of the mixture enriched the latter component, which was converted into ( )-910 by reduction with LS-Selectride (lithium trisiamylborohydride) according to the procedure developed by Comins (vide infra). 

Reactions on the three-carbon bridge of oxabicyclo [3.2.1] compounds have been reported but less systematically studied. Because the majority of these compounds are derived from oxyallyl cation cycloadditions, most experiments on the three-carbon bridge involve addition to the bicyclic ketone. The parent oxabicyclo[3.2.1] ketone 90 undergoes reduction with bulky hydride sources such as L-selectride to generate the endo alcohol, Eq. 68 [112]. Presumably, the selectivity is due to equatorial attack of the hydride at the ketone of the pyran-one in a pseudo chair conformation. The exo alcohol is prepared from the endo alcohol by a Mitsunobu inversion-hydrolysis sequence [113]. 

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