Synthesis of the Tetracyclic Framework of the Erythrina Alkaloids Using a [4 + 2]-Cycloaddition/Rh(I)-Catalyzed Cascade of 2-Imidofurans
2006-09-15T00:00:00Z (GMT) by
Several 2-imido substituted furans were found to undergo a rapid intramolecular [4 + 2]-cycloaddition to deliver oxabicyclo adducts in good to excellent yields. By using a Rh(I)-catalyzed ring opening of the resulting oxabicyclic adduct, it was possible to prepare several highly functionalized tetrahydro-1<i>H</i>-indol-2(3<i>H</i>)-one derivatives which were then used to prepare several erythrina alkaloids. By taking advantage of the Rh(I)-catalyzed reaction, it was possible to convert <i>tert</i>-butyl 3-oxo-5-carbomethoxy-10-oxa-2-azatricyclo[220.127.116.11<sup>1,5</sup>]dec-8-ene-2-carboxylate into the ring opened boronate by reaction with phenylboronic acid. Treatment of the boronate with pinacol/acetic acid afforded the corresponding diol which was used in a successful synthesis of racemic 3-demethoxyerythratidinone. During the course of these studies, several novel rearrangement reactions were encountered while attempting to induce an acid-initiated Pictet Spengler cyclization of a key lactam intermediate. The IMDAF/Rh(I)-catalyzed ring opening cascade sequence was also applied to the total synthesis of (±)-erysotramidine as well as the lycorine type alkaloid (±)-<i>epi</i>-zephyranthine.