Boron alkynides

The synthesis of unsymmetrical derivatives has been reported, which involves stepwise treatment of an alkenyldialkylborane with lithium alkynide followed by boron trifluoride. Protonolysis of the intermediate affords in a good yield the iso-merically pure unsymmetrical (Z, )-dicnc (Eq. 83) U1). 

Novel nonchelation phenomena are observed with a steroidal a-hydroxy aldehyde. The reaction of a lithium or magnesium alkynide with the aldehyde gives the (20/, 22/ )-diastereomer piedominantiy, the formation of which was explained by Cram s cyclic model. When BF3-OEt2 is added to the lidiium alkynide prior to the addition of the aldehyde, the stereoselectivity is inverted, and the (20) ,225)-isomer is obtained as the principal product. Transformation of a-alkoxy aldehyde to the boron ate complex is suggested. Other l wis acids, such as B(OMe)3, AlCh, etc., are less effective (equation 29). °... 

The fust important test of this methodology came in Hanessian s investigation of the spiroketal portion of avermectin Bu- This highly convergent approach incorporates all the oxidation levels and functionality required for carbons C(15)-C(28), except for the necessity of alkyne to alkene conversion. The lithium alkynide was prepared at -78 C and then mixed with boron trifluoride etherate under the conditions of Yamaguchi (Scheme 19). (Direct condensation of the lithium salt and lactone lead to substantial amounts of a, -unsaturated lactone.) Addition of the lactone in stoichiometric amounts to the solution of the modified alkynide led to the formation of the desired hemiketal in acceptable yield. Further improvements could be obtained by the recycling of starting material. ... 

Yamaguchi and Hirao found that alkyneboron difluorides, which are prepared from the alkynide ion and boron trifluoride etherate, react with oxiranes to give the 2-alkynols (Scheme 15). The same research group also found that ring opening of oxetane could be accomplished, but in this case boron trifluoride had to be added as a Lewis acid (Scheme 15). 

Nucleophilic addition of lithium alkynides to Af-[bis(tri-methylsilyl)methyl] aldimines mediated by boron trifluoride etherate afforded )V-[bis(trimethylsilyl)methyl] propargylamines in moderate to good yields (eq 7). For example, imine (12, R = n-Pr) reacted with lithium phenylethynide to give propar-gylamine (13, R = n-Pr, R = Ph) in 81% yield. 

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