Protosappanins E-1 and E-2, stereoisomeric dibenzoxocins combined with brazilin from sappan lignum.

An inseparable mixture of two new dibenz[b, d]oxocins combined with brazilin, named protosappanins E-1 (1) and E-2 (2), was obtained from the heartwood (Sappan Lignum) of Caesalpinia sappan L. (Leguminosae). They were separated from each other as isomeric hexamethyl ethers C38H38O<11>, [α]23D -80.8°(3) and [α]24D -9.6°(4). The mixture of 1 and 2 afforded brazilin and protosappanin B on reductive cleavage with sodium borohydride in alkaline methanol. On acid treatment, one of the hexamethyl ethers (3) yielded the other hexamethyl ether (4), l-11b-hydroxybrazilin trimethyl ether (13), and the dimethylacetal (11) of the trimethyl ether of protosappanin C. Compounds 1 and 2 were concluded to consist of protosappanin C combined with 11b-hydroxybrazilin through an acetal linkage and to be stereoisomeric in respect of the acetal carbon. The stereochemistries at the acetal carbons of 3 and 4 were determined by means of nuclear magnetic resonance (nuclear Overhauser effect) experiments.