A HIGHLY STEREOSELECTIVE SYNTHESIS OF TRANS-TRISUBSTITUTED OLEFINIC BONDS

The Julia olefin synthesis has been modified in such a way that homoallylic bromides containing trans-trisubstituted olefinic bonds are formed with high stereoselectivity. These substances are useful as intermediates in the preparation of polyolefinic isoprenoid types. The new scheme is illustrated by the synthesis of trans-1-bromo-3,7-dimethylocta-3,7-diene (12) (R = CH3). Thus the known 1-acetyl-1-methylcyclopropane (13) (R = H) was converted, by condensation with diethyl carbonate, into the keto ester 13 (R = CO2Et). The enolate anion of this keto ester was alkylated with methallyl chloride to give 14 (R = CH3) which, on treatment with barium hydroxide followed by mineral acid, underwent hydrolysis and decarboxylation to give the unsaturated ketone 15 (R = CH3). Lithium aluminum hydride reduction of this ketone afforded the carbinol 11 (R = CH3), treatment of which with phosphorus tribromide and collidine effected its conversion into a mixture of bromides apparently containing substantial amounts of cyclopropylcarbinyl and cyclobutyl bromides. This mixture of bromides, on treatment at 0° with a suspension of anhydrous zinc bromide in ether, was smoothly isomerized to give a product which was mainly the trans-bromo diene 12 (R = CH3). Only trace amounts of the cis isomer were formed. The over-all yield for the total process was good. The method has also been applied to the preparation of the lower homolog 12 (R = H). © 1968, American Chemical Society. All rights reserved.