Boronic esters chiral

Synthesis of OC- and P-Ghiral Ketones, Esters, and Nitriles. Chiral boronic esters are convenient precursors of a-chiral ketones (R COR ), which can be prepared via the dialkylborinic ester or dialkylthexyl route (524,525). 

The conversion of chiral boronic esters iato optically pure B-aIkyl-9-BBN derivatives followed by reaction with a-bromoketones, a-bromoesters, or a-bromonitriles leads to the homologated P-chiral ketones, esters, and nitriles, respectively (526). 

In three separate papers, the use of chiral boronic esters in 1,3-dipolar cycloadditions with nitrile oxides have been described (316-318). The reaction of 203 with nitrile oxides proceeded with low diastereoselectivities (Scheme 12.58). 

PhCHO) being less effective substrates than their aliphatic counterparts. The relative complexity and low yields (40-60%) of the starting allenylboronic acid coupled with the requirement that the chiral boronic esters have to be freshly prepared immediately before their use in the allenylboration detract somewhat from the overall value of the process as do the long reaction times (20 h, -78 °C), the hydrolysis of the chiral boron ligation upon work-up and the relative inefficiency of chirality transfer for aromatic substrates. 

This iterative procedure has been utilized with great effect in asymmetric synthesis via chiral boronic esters derived from chiral alcohols. ... 

This methodology has been developed by Matteson and coworkers, with particular emphasis on chiral boronic esters derived from pinanediol. Sequential chiral centers can be assembled by an iterative procedure with excellent enantiomeric excess at each step. The two chiral centers in eldanolide (37), for example, were assembled in this manner (Scheme 36). ... 

Chiral boronic esters react with organolithium reagents to form diorganylalkoxyboranes (borinic esters). Subsequent reaction with the anion of dichloromethyl methyl ether then yields chiral ketones by rearrangement of both of the groups on boron (Scheme 42). No racemization is observed in this sequence and alkyl-, aryl- or alkynyl-lithium reagents can be used. 

This homologation method with chiral boronic esters has wide application and its synthetic potential has been reviewed16. 

Hydrolysis of 1,3,2-dioxaborolanes is thermodynamically disfavored, no doubt as a result of the same kinds of entropic factors that favor formation of cyclic acetals in preference to acyclic acetals. Hydrolysis of a 1,3,2-dioxaborolane (1) converts three molecules to two, but hydrolysis of a boronic acid dimethyl ester keeps the total number of molecules at three (Scheme 5). (It m be noted that hydrolysis of a cyclic acetal with one molecule of water keeps the number of reactant and product molecules equal at two, and hydrolysis of an acyclic acetal converts two molecules to three.) Adding base does almost nothing to the equilibrium, since hydroxide ion coordinates to the boronic ester 1 as well as to the boronic acid product. Furthermore, it spears that the trans R° substituents in 1 further stabilize the stmcture. Chiral boronic esters of this series are harder to hydrolyze than pinacol boronic esters, and treatment of pinacol boronic esters with DICHED results in liberation of the pinacol and formation of the DICHED boronic ester. 

Enantioselective Addition Reactions to Clyoxylic Acid-derived Iminium Ions using Chiral Boronic Esters... 

The use of chiral boronic esters in the Petasis borono-Mannich reaction has been reported to result in low levels of enantioselectivity of the adducts at room temperature (6-15% ee) [48]. Auxiliaries used in this study by Scobie and co-workers included pinanediol and tartaric acid derived alkenylboronates. Morpholine was the only secondary amine used, with the primary amine ethyl glycinate failing to react. 

Dialkoxyboronic esters are a priori very weak Lewis acids. Diels-Alder reaction of an ortko-boronoanilide dienophile with cyclopentadiene proceeds faster than both its para isomer and the unsubstituted derivative, thereby confirming that self-activation by internal coordination is operative with ortHo-boronoanilide (Equation 32 vs. 33). However, the level of 1,8-stereoselection observed in the Diels-Alder reaction of chiral boronic ester derivatives is only minimal (up to 18% de) [31],... 

There continues to be considerable effort applied to the use of organoboranes in chiral synthesis. A convenient procedure for the synthesis of various 6-chiral boronic esters of essentially 100% ee which are not available by direct asymmetric hydroboration methods has been developed and their synthetic utility described. 

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