Dibutylboron triflate

Dibutylboron triflate promotes both enol borinate formation and addition.70... 

Boron enolates are often used for aldol reactions. Boron enolates are usually prepared from the corresponding carbonyl compounds, tertiary amine, and a boron source (e.g., dibutylboron triflates). The aldol reactions proceed via a six-membered transition state to give high diastereo-selectivity which depends upon the geometry of the boron enolates. 

Studying the influence of the activator and using the MPEG-DOX-OH support (Scheme 4.18),12 the same group reported the synthesis of trisaccharide 79 using donors 76 and 78 both glycosylations were performed by action of 3 equiv of donor, in the presence of 0.1 equiv of dibutylboron triflate (DBBOTf) as activator and 4-A molecular sieves at -45°C for 30 min. 

Dibutylboron triflate Methanesulfonic acid, trifluoro-, anhydride with dibutylborinic acid (9) (60669-69-4)... 

Oppolzer et al. used (2M)-bornanc-10, 2-sultam as an effective chiral auxiliary to achieve a highly enantioselective syn-aldol reaction17 (Scheme 2.1p). Treatment of A-propionylsultam (46) with dibutylboron triflate and Hunig s base at -5°C in CH2CI2 followed by addition of aldehydes at -78° C provided, after a simple crystallization, the pure vyn-aldols 47a. It is noteworthy that no anti-aldol product was observed in the aldol reactions with any of the aldehydes. From the1 nuclear magnetic resonance (NMR) study, it was confirmed that the boron... 

The Masamune aldol condensation, in common with the Evans aldol condensation, involves a boron enolate of an ester containing a norephedrine derived chiral auxiliary however, unlike the latter, the Masamune aldol delivers a 3-hydroxy-2-methyl carbonyl moiety with the an/z-stereochemistry. Crucial to the success of this reaction is the use of dicyclohexylboron triflate to generate the boron enolate. Note in the Evans aldol condensation, dibutylboron triflate is utilized. 

A highly versatile auxiliary is the Evans oxazolidinone imide (Figure 5.4c, see also Scheme 3.16), available by condensation of amino alcohols [86,87] with diethyl carbonate [86]. Deprotonation by either LDA or dibutylboron triflate and a tertiary amine affords only the Z(0)-enolate. Scheme 5.12 illustrates open and closed transition structures that have been postulated for these Zf0)-enoIates under various conditions, and Table 5.4 lists typical selectivities for the various protocols. The first to be reported (and by far the most selective) was the dibutylboron enolate (Table 5.4, entry 1), which cannot activate the aldehyde and simultaneously chelate the oxazolidinone oxygen [75]. Dipolar alignment of the auxiliary and approach of the aldehyde from the Re face of the enolate affords syn adduct with outstanding diastereoselection, presumably via the closed transition structure illustrated in Scheme 5.12a [75]. The other syn isomer can be formed under two different types of conditions. In one, a titanium enolate is postulated to chelate the oxazolidinone... 

Cyclic enediynes. The realization that a number of antitumor antibiotics contain an enediyne core which is essential for their activity has led to extensive methods for synthesis of this ring system. One approach involves an aldol reaction with an acetylenic aldehyde protected as the Co2(CO)6 adduct. Thus the keto aldehyde 1, prepared in five conventional steps from cyclopcntenc-l,3-dione, on treatment with dibutylboron triflate and N(C2H5)y cydizes to 2 in 69% yield. Attempted dccomplexation with N-methyl-... 

Dibutylboron triflate Methanesulfonic acid, trifluoro-, anhydride with... 

In the pilot plant, on a reaction scale of 24 kg of 11, we were unable to reproduce this yield. The major problem with this chemistry is the quality of the dibutylboron triflate. Fresh samples behaved well, giving in the lab around 70% yield. If the triflate is older the yield drops drastically to 50% or less. Just what older means has not been defined. Certainly after about 3-4 weeks the yield starts to drop. Triflate from different sources (quality ) also had an effect, between no reaction and similar yields to our standard one. The process takes some 6 days of pilot plant time and as it is very dilute, due to the triflate being delivered as a 1. OM solution, we were unable to significantly increase the throughput. The triflate is pyrophoric at higher concentrations or in other solvents. Consequently, we did not want to prepare it nor distil it ourselves. These reagent quality problems along with an analytical method to measure the quality still need to be solved. 

The formation of both of these side products illustrates the lability of the para-methoxyphenyl acetal-protecting group. In the case of 31, the excess dibutylboron triflate present catalyses the ring opening of the acetal and the resulting stabilized carbocation is trapped by the excess enolate. The tactic here is to establish conditions so that 31 can be eliminated. After some experimentation, we found that simply working up... 

High selectivity for either the anti or the syn aldol product can be obtained by using an appropriate thioester and a boron-mediated aldol reaction. The trans-enolate and hence the anti aldol product can be obtained from the 5-tert-butyl ester and dibutylboron triflate (1.68). The trans-enolute (note the convention that the O-metal substituent takes a higher priority than the SR substituent) is favomed owing to the steric interaction of the bulky tert-butyl group with the ligands on the boron... 

This aldol reaction is carried out using a base (triethylamine) and dibutylboron triflate plus benzaldehyde. The aldol product is formed with outstandingly good selectivity and in high yield, and all that remains is to remove the auxiliary with base and isolate the hydroxy-acid as a single diastereoisomer and a single enantiomer. 

The aldol reaction of 282 with 288 was accomplished by use of dibutylboron triflate to give three diastereomeric adducts. One of them was de-O-protected to give an aglycone of elaiophylin, ll,ir-di-0-methylelaiophylidene (289), which was identical with a sample prepared by methanolysis of elaiophylin (284). 

Alain Commer9on synthesised the protected 3-phenylisoserine via a diastereo-selective, dibutylboron triflate-mediated aldol condensation. By a sequence of standard methods, the intended, configurationally stable carboxylic acid was obtained. 

Boron Enolates of AT-Propionyloxazolidinethiones. Boron enolates of fV propionyloxazolidinethiones can be generated under standard enolization conditions with dibutylboron triflate and diisopropylethylamine. The boron enolates react with aldehydes to provide the Evans s// aldol products with excellent diastereoselectivity (eq 7). No oxidative work-up was necessary in the examples reported. ... 

The diastereofacial selectivity of an asymmetric aldol reaction can also be controlled on the enolate side, and this is the basis of the second-generation methods of Evanst l and Masamune.l24] The complementary Evans auxiliaries (66) and (67) are synthesised from (5)-valine and (15,2/ )-norephedrine respectively. TheZ-enolate (68) is formed exclusively on reaction with dibutylboron triflate, and this reacts with aldehydes to give essentially only one aldol product (69). The diastereofacial selectivity derives from the bulky groups on the auxiliaries which force attack from the opposite face. 

---