Development and applications of remote stereocontrol using allylic organobismuth reagents

Abstract

Reactions of 5-benzyloxy-4-methylpent-2-enyl(tributyl)stannane with aldehydes promoted by bismuth(III) iodide were usefully stereoselective in favour of the (E)-1,5-anti-6-benzyloxy-5-methylalk-3-en-1-ols. Similar stereoselectivity was observed for reactions of analogous 5-benzyloxy-4-methylpent-2-enyl bromides with aldehydes when promoted by a low valency bismuth species prepared by reduction of bismuth(III) triiodide with powdered zinc so providing a "tin-free" procedure. The analogous reactions of 4-benzyloxypent-2-enyl(tributyl)stannane with aldehydes promoted by bismuth(III) iodide were also stereoselective but gave lower yields. Attempted 1,6-stereocontrol using these reactions resulted in only modest stereoselectivities. Aspects of the chemistry of the products were studied in particular their stereoselective conversion into aliphatic compounds with methyl bearing stereogenic centres at 1,5,9,13- and 1,3,5-positions along the aliphatic chain. Mechanistically, allylic organobismuth species may be involved in both sets of reactions but this was not confirmed although the similar stereoselectivities observed for both the bismuth(III) iodide mediated reactions of the pent-2-enylstannanes and the low-valency bismuth promoted reactions of the pent-2-enyl bromides are consistent with participation of similar intermediates.

Systematic Comparison of Sets of 13C NMR Spectra That Are Potentially Identical. Confirmation of the Configuration of a Cuticular Hydrocarbon from the Cane Beetle Antitrogus parvulus

“…The synthesis of the requisite C(1)–C(11) fragment is based on chemistry developed for the stereoselective synthesis of hydrocarbons with methyl substituents disposed at 1,3,5-positions down an aliphatic chain. 9 The key step in this synthesis is the reaction of ( E )-5-benzyloxy-2,4-dimethylpent-2-enyl bromide with aldehyde 9 . Such reactions are selective for the formation of ( E )-1,5- anti -products when mediated by the low valence bismuth species formed by reduction of bismuth(III) iodide with zinc powder.…”

“…Such reactions are selective for the formation of ( E )-1,5- anti -products when mediated by the low valence bismuth species formed by reduction of bismuth(III) iodide with zinc powder. 9,10 …”

“…The bismuth-mediated reaction of aldehyde 9 with the pent-2-enyl bromide 10 (9) gave a major product identified on the basis of precedent, 10 as the (3 E ,6 S )-2,6- anti -2,4,8-trimethylundec-3-en-6-ol 11 . Minor products were detected at the 5% level but could not easily be separated.…”

“…These data were corrected for small calibration errors (see Supporting Information), though again the correction is not crucial to the outcome. 9 …”

Strategies for the Synthesis of Deoxypropionates

“…Thomas developed a Bi-mediated reaction of pent-2-enylbromides with achiral aldehydes to give (E)-alk-3-enals with promising 1,5-stereoselectivity. 73 The low-valency bismuth species was generated from BiI 3 and activated zinc. Catalytic hydrogenation of the (E)-alk-3-enals and repeating this sequence gave deoxypropionate units with three contiguous methyl branchings.…”