Structure Elucidation of Coumarinolignans

With the advent of modem spectroscopic methods, stmcture determination of natural products largely depends upon careful interpretation of various spectroscopic data of the compounds and of their appropriate derivatives, and the coumarinolignans are no exceptions. Different spectroscopic methods used in the stmcture clarification of coumarinolignans are discussed in the following paragraphs. 

In their NMR spectra, the regioisomeric coumarinolignans show almost similar chemical shifts with slight differences in the shielding of C-7, C-8, and C-9, since the mode of fusion of the two CeCa units differs among these compounds. The carbon resonances of some coumarinolignans are presented in Tables 5 and 6. 

An intriguing problem in the structure elaboration of coumarinolignans is the determination of the exact mode of linkage of the phenylpropanoid unit to the coumarin nucleus or, in other words, to decide which of two possible regioisomers the compound in question may be. This difficulty arises because no significant differences in conventional H- and NMR spectroscopic data are expected between two isomers under consideration and hydrogenolysis of the benzyl ether linkage has so far been unsuccessful. One of the earliest NMR methods used to 

Irradiation at a certain hydrogen frequency has been observed to simplify the carbon signal at its p-position due to decoupling. Thus, in isomer A, the same carbon signal is simplified by irradiation of the aromatic hydrogen as well as of the benzylic hydrogen attached to the dioxane bridge. In isomer B, however, two 

Carbon(s) Cleomiscosin A (9) Cleomiscosin B (10) Daphneticin (12) Propacin (31) Venkatasin (33)  

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Lin L-J, Cordell GA (1986) Application of the SEMEPT Pulse Programme in the Structure Elucidation of Coumarinolignans. J Chem Soc Chem Commun 5 377... 

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