By A. I. Scott (auth.), Prof. Dr. Atta-ur-Rahman, Prof. Dr. Philip William Le Quesne (eds.)
Natural product chemistry has gone through an explosive development throughout the latter 1/2 the present century. This has been led to through a few elements. this kind of has been the growing to be variety of sub stances from average assets which exhibit fascinating pharmacological actions. those comprise antibiotics, anti-tumor brokers, immunostimu lants, medicines affecting the cardiovascular and vital worried platforms, analgesics and so on. one other issue has been the advancements made within the know-how of isolation methods consisting of the improvement of such options as excessive strain liquid chromatography which has al lowed the swift isolation of gear that have been formerly diffi cult to acquire through classical approaches. an important issue has been the advance of latest spectrosopic concepts that have spread out entire new vistas during this interesting box. popular in those advan ces has been the appearance of strong superconducting magnets with very sturdy magnetic fields, and pulse NMR within which the period, course and stages of pulses could be correctly managed by way of mini desktops. those have heralded the appearance of two-dimensional NMR spec troscopy which has now become mostly hired in unravelling complicated buildings. No less significant, even though a little bit much less dramatic, were the advances made within the box of mass spectroscopy the place new ionization recommendations corresponding to confident and unfavorable quickly atom bom bardment, box desorption, chemical ionization and so on. have allowed the mass spectra of bigger non-volatile components to be recorded. the sector of natural synthesis has noticeable both speedy advances.
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By way of example, Fig. 9 shows an expansion of the delayed COSY spectrum of the tetra-acetate 7. As the assignments within the sugar rings are known from the COSY spectrum, the ring b set of protons is labelled by finding the 3-H which has a correlation with the 1-H of the terminal ring c. Hence the ambiguity in this case as to whether there is a free hydroxyl group on ring a or on ring b is resolved. By these means we arrived at the structures of all five compounds 4, 5, 6, 7, and 8. The sixth compound 9 was a trirhamnoside with four acetates and again with the dodecanYI ether, [M + Naj+ - 815, and we deduced the structure without needing the delayed COSY experiment.
II I . 'HJ" NOESY (mix tima: 3 S) UI"' .. -I! ' U -a l~ 1-. 5 r!..! -. ~: ... a' a , -- ,--~-t-. ' ,,' t. 5 Ft (PPM) ... ""- 1-"'" L--! ' fl' a'a ~a~ 38 Figure 9. 8 39 6. A FURANONAPHTHOQUINONE STRUCTURE THROUGH SELECTIVE INEPT SPECTROSCOPY Finally, we would like to discuss what can be regarded as a classic example of a structure elucidation where the selective INEPT technique provided rapid resolution to an otherwise difficult problem (ref. 38). Wagner and co- workers (ref. 39) isolated a mixture of the furanonaphthoquinones 13 and 14 from Tabebuia avellandeae (Bignoniaceae), and succeeded in separating one of the isomers by HPLC only with great difficulty.
When the 7-CH3 was observed for the 2' - and the 6' - protons, indicating the presence of a 3,4-substituted system in ring C where the 3substituent is a methoxyl group. e. H-2" and H-6"), indicating that ring D has the same substitution as ring C. On this basis the methoxyl group is placed at C-2, the substituted benzyl group is placed at C-6, the methyl group at C-7 and the substituted phenyl moiety at C-S. Larreantin therefore has the structure 3. Figure 3. CSCM lD spectra of Larreantin (3) s" OH H-S " S' H -S S H-S 6" 5" OH 3 130 I 120 110 , 28 Selective INEPT experiments provided evidence for this structure proposal.