1,3,2-Dioxaborolane derivative

A 25 mL round-bottom flask equipped with a magnetic stir bar and fitted with a mbber septum was charged with anhydrous THF (4.0 mL) followed by pinacolborane (2 0.57 g, 4.5 mmol). Alkyl/aryl bromide (1 4.5 mmol, 1 M/THF) was added drop-wise over 5 min at room temper-amre (25 °C) with constant stirring for 1 h. Uprui completion of the reaction (monitored by NMR), the reaction mixture was cooled to 0 °C (ice bath) and acidified with 3 M aqueous HCl (3 mL). After 10 min of stirring, the reaction mixmre was warmed to room temperature (25 °C) and stirred for an additional 30 min. The reaction mixture was then transferred to a separatory funnel and extracted with diethyl ether (3x15 mL). The combined organic layers were dried over anhydrous MgS04, filtered, and dried in vacuo to 1,3,2-dioxaborolane derivative 3, identified by means of spectral studies. 

For example, it has been used to elaborate the chiral cyclopropanes subunits of Curacin A[60], and of the structurally fascinating FR-900848 [61] and U-106305 [62]. The chiral dioxaborolane-derived ligand was also effective to synthesize 1,2,3-substituted cyclopropanes [63]. Excellent to outstanding diastere-oselectivities and enantioselectivities were observed when a variety of allylic alcohols were treated with the reagent formed by mixing 1,1 -diiodoethane and di-ethylzinc. It was also shown that functionalized 1,1-diiodoalkanes could also be used in this reaction. 

More recently, dioxaborolane derived from (RR)- +)-N,N,N, W -tetramethyltartaric acid diamide has been used as an efficient chiral controller in Simmons Smith cyclopropanation reaction of allylic alcohols to produce substituted cyclopropyl methanols in high... 

Dioxaborolane derived from dimethyl tartrate has found application in enantioselective epoxidation of unfunctionalized alkenes using TBHP as co-oxidant. [35]... 

Scheme 10.12 gives some examples of enantioselective cyclopropanations. Entry 1 uses the W.s-/-butyloxazoline (BOX) catalyst. The catalytic cyclopropanation in Entry 2 achieves both stereo- and enantioselectivity. The electronic effect of the catalysts (see p. 926) directs the alkoxy-substituted ring trans to the ester substituent (87 13 ratio), and very high enantioselectivity was observed. Entry 3 also used the /-butyl -BOX catalyst. The product was used in an enantioselective synthesis of the alkaloid quebrachamine. Entry 4 is an example of enantioselective methylene transfer using the tartrate-derived dioxaborolane catalyst (see p. 920). Entry 5 used the Rh2[5(X)-MePY]4... 

Hydrozirconations of both vinyl and acetylenic boranes by Srebnik et al. led to 1,1-dime-tallo reagents, which offer the benefits as coupling partners of alkyl- and vinylboranes, respectively [59—62], Initial trials were conducted with B-alkenylborabicyclo[3.3.1]nonanes, but these led to unstable dimetallics. Replacement of the 9-BBN fragment with the pina-colborane-derived analogue produced stable dioxaborolanes 61 (Scheme 4.34). 

The influence of the size and configuration of various cyclic vinyl carriers (Scheme 6.26) on the stereoselectivity of cycloaddition was studied, and included epoxides, (3-lactams, dioxaborolanes, and dioxans (22). Although the anti preference was maintained in all cases, the conformation of the carrier ring must also be taken into account in order to rationalize the stereoselections observed. The highest diastereomeric ratio was observed with the vinyl-tetrahydrofuran derived from glucose, where the conformational mobility of the carrier ring is substantially locked by an acetonide clamp and one face of the C=C bond is effectively shielded (22,165,215). 

The tartaric acid scaffold also led to the design of one of the most effective and general methods to generate enantiomerically enriched substituted cyclopropyhnethanol derivatives. Indeed, the chiral dioxaborolane ligand 19, prepared from tetramethyltartramide and butylboronic acid, is a superb chiral additive in allylic alcohol-directed cyclopropanation reactions (equation 83) . The best procedure requires the use of the soluble bis(iodomethyl)zinc DME complex . The reaction affords high yields and enantiomeric... 

The enantioselective cyclopropanation leading to 1,2,3-snbstitnted cyclopropane derivatives proceeds with high diastereocontrol (equation 86) . It is quite interesting to observe that the same reaction, when run in the absence of the dioxaborolane ligand, led to lower diastereoselectivity. Other functionalized 1,1-diiodoaLkanes can be used as the zinc carbenoid precursor, but it should be noted that up to 2 equivalents of the reagent (4 equivalents of RCHI2) are needed in this process. This reaction has been applied in the synthesis of ambruticin. ... 

The boron atom dominates the reactivity of the boracyclic compounds because of its inherent Lewis acidity. Consequently, there have been very few reports on the reactivity of substituents attached to the ring carbon atoms in the five-membered boronated cyclic systems. Singaram and co-workers developed a novel catalyst 31 based on dicarboxylic acid derivative of 1,3,2-dioxaborolane for the asymmetric reduction of prochiral ketones 32. This catalyst reduces a wide variety of ketones enantioselectively in the presence of a co-reductant such as LiBH4. The mechanism involves the coordination of ketone 32 with the chiral boronate 31 and the conjugation of borohydride with carboxylic acid to furnish the chiral borohydride complex 34. Subsequent transfer of hydride from the least hindered face of the ketone yields the corresponding alcohol 35 in high ee (Scheme 3) <20060PD949>. 

Naming of carbohydrate boronates may be based on three principles (a), the central use of the ester heterocyclic rings 1,3,2-dioxaborolane (five-membered), 1,3,2-dioxaborinane (six-membered), and 1,3,5,2,4-trioxadiborepane (seven-membered, for example, 8 and 9) (b), the use of radical prefixes borylene (Chem. Abstr.) or boranediyl (I.U.P.A.C.) for /BH or (c), ester terminology. Thus, for example, according to these respective procedures, the glycerol derivative 10... 

Photolysis of Cp M(CO)3, where M = MnorRe, Cp = Cp or Cp ", in the presence of pentane and 4, 4, A, A, 5, 5, 5, 5 -octamethyl-2, 2 = bi-1, 3, 2-dioxaborolane (B2PUI2) resulted in formation of primary pentylboronate ester in yields as high as 95% for the Cp " derivatives. Photolysis of trans-Cp "Re(CO)2(Bpin)2 in pentane produced pentylboronate ester in 100% yields suggesting that Cp M(CO)2(Bpin)2 species are likely intermediates in these syntheses. The analogous oxidative addition of a B-B bond has been observed upon photolysis of CP2WH2 and CatBBCat (Cat = 4-tert-butylcatechol) in benzene yielding Cp2W(Bcat)2. ... 

Substituted derivatives of 2-phenyl-1,3,2-dioxaborolans undergo electron-impact-induced rearrangements, giving hydrocarbon ions (detected mass-spectrometrically).315... 

Trimethylsiloxy)-l,3,2-dioxaborolans and -borinans are formed by cleavage of corresponding trialkyl-tin or -germanium derivatives by... 

The Simmons-Smith cyclopropanation of the same 3-(2-phenylcydopropyl)prop-2-enol (110) in its racemic form afforded an inseparable mixture of and a t/-bicyclopropanes 111 in a 1.3 1 ratio. Therefore, the Charette protocol (p 286) was used first to prepare the required allylic alcohol in its optically pure form and the cyclopropanation was carried out in the presence of the tartrate derived dioxaborolane 93. By using (+)- and (— )-tartrate derived dioxaborolane 93, both the syn- and antf-bicyclopropyls 111 were obtained. The diastereoselectivities observed in their formation were consistently greater than 12 1. ... 

Several reports by Gennari et a/. describe the generation of enolates directly from carbonyl compounds by using ethylenedioxychloroborane (B-chloro-2,5-dioxaborolane 118) in the presence of diiso-propylethylamine. Reactions of these enolates, derived from ketones and thiol esters, with aldehydes provide good yields (60-85%) of aldol adducts, whereas esters give unsatisfactory yields (-30%). V en ethyl ketones or propanethioates are employed, Z )-enolates (119) can be formed exclusively using specific reaction conditions, and excellent syn selectivities (92 8 to 99 1) ate observed (Scheme 48). 

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