Titanium-Mediated [2 + 2 + 2] Coupling of Diynes with Homoallylic Alcohols

Abstract

[reaction: see text] In connection with the known diyne-ene [2 + 2 + 2] cycloaddition reactions mediated by titanium aryloxides, the ability of titanium alkoxides to promote coupling of a titanacyclopentadiene with an alkene has been assessed for the isomerization-free preparation of 1,3-cyclohexadienes. The successful cycloaddition by titanium alkoxides is predicated on the use of homoallylic alcohols as the olefin component. With secondary homoallylic alcohols, high 1,3-diastereoselectivity is observed, which lends itself to enantioselective preparation of functionalized 1,3-cyclohexadienes.

Alkenes in [2+2+2] Cycloadditions

“…In addition, with secondary homoallylic alcohols, high 1,3-diastereoselectivity was observed, paving the way to enantioselective preparation of functionalized 1,3-cyclohexadienes (Scheme 47). [85] Ac ontribution from Oshimaa nd co-workersc oncerned ac hromium-induced[ 2 + +2+ +2] annulation of 1,6-diynes. Chromate reagents are readily availablea nd can be prepared in situ from chromiumc hloride and Grignardr eagents, so that CrCl 3 can be added in catalytic amounts.…”

“…The postulated intermediate with the oxygen coordinated to the metal explained the lack of double bond shifting. In addition, with secondary homoallylic alcohols, high 1,3‐diastereoselectivity was observed, paving the way to enantioselective preparation of functionalized 1,3‐cyclohexadienes (Scheme ) …”

5.34 Early Transition Metal Mediated Reductive Coupling Reactions

“…As illustrated in Scheme 66C, a diyne-alkene [2 þ 2 þ 2] annulation process has also been described that secures an alternative entry to substituted carbocycles. 148 Here, intramolecular alkyne-alkyne coupling produces a metallacyclopentadiene that, through ligand exchange, reacts with the homoallylic alcohol to generate a transient species capable of undergoing annulation. Although the symmetry of the diyne substrate renders these coupling reactions a bit simpler than those described in Scheme 66B (as regioselectivity is not an issue here), these coupling processes proceed with exquisite levels of 1,3-stereochemical control.…”

Control over [2+2+2] and Carbonylative [4+2] Cycloaddition by CO Pressure in Co-Catalyzed Cycloaddition between Internal Diynes and Cyclopentadiene

“…5 In particular, use of conjugated dienes as a dienophile in the [2+2+2] cycloaddition has not been reported yet, as far as we are aware.…”