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Abstract:
Synthesis of Epimedoicarisoside A
A synthetic procedure towards the total synthesis of Epimedoicarisoside A could successfully be established (see Scheme 49). It involved a total of 16 steps and 12 steps in the longest linear sequence. The crucial cycloisomerization of the ortho-alkynylated biaryl precursor was carried out using a gold(I) complex of a dicationic phosphine ligand as catalyst. To the best of our knowledge, no synthetic procedure for this 9,10-dihydrophenanthrene derivative has been reported before.
However, optimization is necessary in some steps. A screening of different catalysts for the cycloisomerization is in due course. Also, a more efficient procedure for the global deprotection of the sugar moiety is needed, possibly using methanol as solvent or conducting the reaction under elevated hydrogen pressure.
Synthesis of Ester-substituted Alkynes to use in the Gold-catalyzed Cycloisomerization
The envisaged substrates 64 - 68 could be prepared with one exception. The gold-catalyzed cycloisomerization using the dicationic gold complex 19 was carried out and turned out to be selective for the corresponding 5-exo products. Hence, this can be regarded as an access to 9H-fluoren-9-ylidene derivatives.
Investigation of the effect of other (e.g. electron donating) substituents on the alkyne moiety will be an interesting task for the future (see Scheme 50). Moreover, the development of new reaction conditions involving the application of new catalysts seems to be appropriate for the compounds needing long reaction times to convert.
