A pentacyclic triterpene from Maytenus imbricata: structure elucidation by X-ray crystallography - Pimenta Junior, Alécio Antonio; Silva, Sílvia Ribeiro de Souza e; Silva, Gracia Divina de Fátima; Barbosa, Luiz Cláudio de Almeida; Ellena, Javier; Doriguetto, Antonio Carlos (Structural Chemistry, 2006)