A practical improvement of crystallization-induced asymmetric transformation of allene-1,3-dicarboxylates

Enantiomerically pure allene-1,3-dicarboxylates were easily synthesized by using epimerization–crystallization of dissymmetric allene compounds, which were prepared from acetone-1,3-dicarboxylates and naturally abundant chiral alcohols, that is, (−)- and (+)-menthols, borneol, and isoborneol. After scrutinizing the crystallization of several allene-1,3-dicarboxylates in the presence of triethylamine, it was found that allene-1,3-dicarboxylate carrying bornyl groups was the most easily prepared as a single isomer because of it having suitable solubility to be crystallized in hexane at 0 °C to room temperature. Diels–Alder reaction of the enantiomerically pure allene-1,3-dicarboxylates and cyclic dienes, such as N-Boc-pyrrole and cyclopentadiene, afforded endo-adducts having the same configurations at two newly generated stereogenic centers.

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(R)-(−)-Bis[(1S)-(−)-bornyl] 2,3-pentadienedioateC25H36O4[α]D26=-206.8 (c 0.98, CHCl3)

(R)-(−)-Bis[(1R)-(−)-isobornyl] 2,3-pentadienedioateC25H36O4[α]D25=-263.8 (c 1.43, CHCl3)

(1S,2R,3E,4R)-(+)-[(1S)-(−)-Bornyl] 3-[2-(−)-bornyloxy-2-oxoethylidene]-7-tert-butoxycarbonyl-7-azabicyclo[2.2.1]hept-5-ene-2-carboxylateC34H49NO6[α]D31=+19.3 (c 0.64, CHCl3)

(1S,2R,3E,4R)-(−)-[(1R)-(−)-Isobornyl] 3-[2-(−)-isobornyloxy-2-oxoethylid-ene]-7-tert-butoxycarbonyl-7-azabicyclo[2.2.1]hept-5-ene-2-carboxylateC34H49NO6[α]D27=-16.7 (c 2.46, CHCl3)

(1S,2R,3E,4R)-(−)-[(1S)-(−)-Bornyl] 3-[2-(−)-bornyloxy-2-oxoethylidene]bicyclo[2.2.1]hept-5-ene-2-carboxylateC30H42O4[α]D26=-31.5 (c 0.64, CHCl3)