A-Chloroboronic esters

The heterocyclic scaffolds are prepared from pyroglutamic acid [154, 155]. 1-aminoalkyl boronic acid pinanediol esters are readily available through a diastereoselective homologation with dichloromethyllithium, providing (5)-a-chloroboronic esters. Aminolysis of the chloride yielded... 

Boronic ester homologation. (R,R)-2,3-Butanediol- and (-(- )-pinanediol have been used as the chiral adjuncts in a diastereoselective homologation of dichloromethaneboronic esters (1) to the (aS)-a-chloroboronic esters (2). Reaction of 1 with an alkyllithium produces a borate complex (a), which rearranges diastereoselectively in the presence of ZnCl, to 2 with introduction of a chiral center adjacent to boron. The reaction permits... 

Allylboronates homoallylic alcohols. Vinyllithium reagents react regio- and ster-eospecifically with this a-chloroboronic ester (1) to form an ate complex (a) that rearranges to an allylboronate (2) Allylboronates are known from the work of Schlosser and Hoff-... 

The chiral directing groups are pinanediols derived from osmium tetroxide-cat-alyzed oxidation of either (+)-a-pinene or (-)-a-pinene with trimethylamine oxide or with NMO (N-methyl-morpholine-N-oxide). The (s) and (r) notations shown in the abbreviations refer to the configuration of the chiral center in the a-chloroboronic ester using the appropriate pinanediol. 

A recent report by Chan and Chong describes the enantioselective reduction of acylstannanes to the oi-alkoxy organostannanes by the chiral 2,2 -dihydroxy-l,l -binaphthyl modified lithium aluminum hydrides. Matteson has also described a possible route to such chiral organostannanes utilizing the chiral a-chloroboronic esters. ... 

Olefins. The reaction of tertiary a-chloroboronic esters (preparation, 5, 201) with silver nitrate in aqueous ethanol at 25 results in formation of olefins in good yield. For example, (1) is converted into cyclohexylidenecyclo-hexane (2) in 83% yield, and (3) is converted into (4). 

Boronic esters (3) can be homologated to a-chloroboronic esters (4) (Scheme 2) replacement of the chlorine atom by carbon nucleophiles, such as Grignard reagents, and oxidation of the carbon-boron bond gives a new route to alcohols/ If the sequence is performed with (+)- or (-)-pinanediol boronic esters it can be made into a directed chiral synthesis of alcohols, as illustrated in Scheme 3 from phenyl boronic acid the homologation steps were found to occur with diastereoselectivities in excess of 90%, the chlorine replacement proceeds with inversion, and the oxidative step with retention of configuration. 

Thus, the pinane diol derivative 78 was reacted with dimethyl-4-bromobutylboronate 79 to afford the boronic ester 80 in good yield. A one-carbon homologation was then achieved by first treating the boronic ester 80 with lithium methylene dichloride, which affords an intermediate boronate 81, that undergoes a diastereoselective rearrangement to afford the a-chloroboronic ester 82. 

The boronic acid ester B was synthesized by transesterification of the corresponding pinacolester A with (lR,2R)-l,2-dicyclohexyl-l,2-dihydroxyethane. Stereoselective chlorination of B was carried out with (dichloromethyl) lithium and zinc chloride. Reaction of the obtained chloroboronic ester C with lithio 1-decyne followed by oxidation of the intermediate D with alkaline hydrogen peroxide afforded the propargylic alcohol E. Treatment with acid to saponify the tert-butyl ester moiety and to achieve ring closure, produced lactone F. Finally, Lindlar-hydrogenation provided japonilure 70 in an excellent yield and high enantiomeric purity. 

Stereoselective Synthesis Using a-Chloroboronic Acid Esters... 

By convention the (s) and (r) representations used in Equations B6.1 and B6.2 refer to the configuration of the a chiral centre in the o.-chloroboronic esters that the homochiral pinanediol subsequently generates in the reactions described below.]... 

Pinanediol (lR)-(l-benzyloxypentyl)boronate (48) prepared from chloroboronic ester 47 provided a model for the reaction conditions needed (Scheme 8.11) [29]. Rearrangement of the benzyloxy group from boron to carbon tended to be sluggish, and best results were obtained when about 1 mol of dimethyl sulfoxide was added as a promoter. The reactants are customarily mixed at -78 °C and allowed to warm to room temperature afterward, though there is no evidence that the cold mixing is necessary. 

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