Mass spectra of 1,19-epoxycardenolides
The mass spectra of six 18,19-epoxycardenolides (ECs) with various substituents in the 3β position — OH, OAc, rhamnosyloxy have been studied. In all the spectra the contribution of the fragments characteristic for cardenolides formed by the cleavage of the bonds of rings C and D were lowered. The strongest peaks were those of ions with m/z 259, 272, and 285 arising on the splitting out of the elements of rings A and B. It was established by the metastable defocusing (MD) method that in the case of the 3β-ols these fragments are formed from M+ and in the case of acetyl derivatives, in addition, they may arise from the (M — AcOH)+ ions. The spectra of the epoxycardenolides were compared with the structure of 3β,5β,14β,19-tetrahydroxycardenolide (strophanthidol) and its 3β-monoacetate and 3β,19-diacetate. The MD spectra and elementary compositions of the ions showed that other mass-spectrometric conditions strophanthidol monoacetate decomposes partially in the form of the 8,19-epoxycardenolide.
Mass spectrometry in cardenolide chemistry—II
Six partial acetates of gitoxin have been used to establish the value of mass spectrometry, when used in conjunction with deuteroacetylation, in cardenolide structural studies. This technique permits the unambiguous location of acetyl groups upon any of the digitoxose residues, and in some cases confirms acetylation of C16 of the steroid ring. The method is suitable for metabolic studies. The spectra discussed show that neither acetyl exchange nor acetyl migration reactions had taken place during the deuteroacetylation step.
Inferring Learning Strategies from Students ’ Errors
Student models in an Intelligent Tutoring System (ITS) aim at representing both correct and incorrect knowledge of the student in a particular domain of learning. This data serves as the basis upon which the Tutor can dispense remedial feedback. This paper presents a framework in which learners’ errors are diagnosed, classified and linked to cognitive processes associated with learning. The cognitive processes we identify are those of generalization, misattribution and ignorance. Our system applies to the domain of Second Language Acquisition and our approach uses Chomsky’s linguistic theory of Principles and Parameters. We particularly concentrate on the acquisition of the Phrase Structure component.