Elaeocarpus alkaloids. I. The structures of (±)-elaeocarpine, (±)-isoelaeocarpine, and (±)-isoelaeocarpicine, three new indolizidine alkaloids from Elaeocarpus polydactylus

Abstract

Three new interrelated alkaloids have been isolated from Elaeocarpus poly-dactylus Schltr. (family Elaeocarpaceae). Two of these alkaloids, (±)-elaeocarpine (I) and (±)-isoelaeocarpine (II), are isomeric and the structure and complete stereochemistry of (±)-elaeocarpine have been established from an X-ray crystal structure analysis of (±)- elaeocarpine hydrobromide.¹ From a detailed analysis of the spectroscopic properties of (±)-elaeocarpine and (±)-isoelaeocarpine, and in particular of their 100-Mc/s n.m.r. spectra, it has been shown that the alkaloids are epimeric at C7 and that (±)-isoelaeocarpine has a cis ring junction at C7,C8, whereas (±)-elaeocarpine is known to have a C7,C8 trans configuration. In methanolic sodium hydroxide solution (±)-elaeocarpine and (±)-isoelaeocarpine are interconvertible. (±)-Isoelaeocarpicine (VIa) is closely related to (±)-elaeocarpine and (±)-iso-elaeocarpine, and has been so named because the configuration at C7,C8 corresponds to that of (±)-isoelaeocarpine. (+)- Isoelaeocarpicine is phenolic, differs from (±)-elaeocarpine and (±)- isoelaeocarpine in molecular composition by the elements of water, and is converted into a mixture of elaeocarpine and isoelaeocarpine by the action of methanolic sodium hydroxide solution. Study of the 100-Mc/s n.m.r. spectrum of (+)-isoelaeocarpicine has established the complete stereochemistry shown in (VIa). The structures indicated for the three alkaloids (I), (II), and (VIa) depict relative stereochemistry only and are not intended to imply a particular absolute configuration. Study of the reaction between (+)-isoelaeocarpicine (VIa) and acetic anhydride shows that an O-acetyl derivative (VIb) is formed rapidly, but that with longer reaction periods the amide (VIIIa) is obtained. The steric requirements of the borohydride reduction of (I) and (II) have been studied, and it has been found that the n.m.r. spectra of solutions of the alcohol (XIIa) in CD₃CO₂D or of its hydro- chloride in CD₃OD indicate an equilibrium between two deuteronated forms of (XIIa), one of which is considered to have a cis and the other a trans ring junction for the indolizidine ring. A minor alkaloid, C₁₇H₂₁N₃, amounts to <1% of the total alkaloids, and a minor non-alkaloidal constituent has been identified as 2- hydroxy-6-methylacetophenone.