(<i>R</i>,<i>R</i>)‐2,3‐Butanediol and (<i>s</i>)‐pinanediol allylboronates in chiral synthesis of (2<i>S</i>,3<i>S</i>)‐3‐methyl‐5‐hexen‐2‐ol

Abstract

Abstract ( R , R )‐Butanediol (dichloromethyl)boronate ( 1 ) with 1 equiv. allylmagnesium halide yields ( R , R )‐2,3‐butanediol (1S)‐(1‐chloro‐3‐butenyl)boronate ( 3 ) together with the diallylated product ( R , R )‐2,3‐butanediol (1‐allyl‐3‐butenyl)boronate ( 4 ). The formation of 4 is unprecedented in reactions of α‐chloroboronic esters with Grignard reagents. With methylmagnesium bromide 3 yielded ( R , R )‐2,3‐butanediol (1 S )‐(1‐methyl‐3‐butenyl)boronate ( 5 ), which failed to hydrolyze with water. Hydrolysis of 3 yielded impure α‐chloroboronic acid, which was esterified with pinacol and treated with methylmagnesium bromide to form 6 , which with (dichloromethyl)lithium followed by methylmagnesium bromide yielded diastereomeric boronic esters 7 and 8 . Oxidation by hydrogen peroxide yielded (2 S ,3 S )‐ and (2 R ,3 S )‐3‐methyl‐5‐hexen‐2‐ol ( 9 and 10 , ees unknown). Treatment of ( s )‐pinanediol allylboronate ( 11 ) with (dichloromethyl)lithium at −100°C followed by zinc chloride at up to 25°C has proceeded in the normal way to form ( s )‐pinanediol (1 S )‐(1‐chloro‐3‐butenyl)‐boronate ( 12 ), which has been elaborated via 13 , 14 , and 15 to (2 S ,3 S )‐3‐methyl‐5‐hexen‐2‐ol ( 9 ) in 95% de.