Napelline-type alkaloids

Table 2 Tyrosinase inhibitory activities of the diterpenoid and napelline type alkaloids and their derivatives [49]...

Sultankhodzhaev et aL reported the tyrosinase inhibitory pattern of six napelline-type alkaloids [49], and only two compounds exhibited some inhibition against tyrosinase. NapelUne (47) exhibited moderate inhibition (ICso = 167.66 p,M) of the enzyme. The presence of a benzoyloxy group at C-l, as in compound 48 (1-0-benzoylnapelline), maybe potentiated the inhibitory activity (IC50 = 33.10 (xM) against tyrosinase [49]. It is interesting to note that free hydroxyl groups at C-l2 and C-l5 were crucial for better potency. Acylation at C-12 and C-15 of other compounds (not shown here), or replacement of the hydroxy fimction (not shown here), results in total inactivation against tyrosinase [49]. The structures of the compounds are shown Fig. 8. 

Fig. 8 Structures of the two tyrosinase inhibitory napelline-type alkaloids (napelline, 47, and l-O-benzoylnapeUine, 48) [49], The structures of the inactive compounds are not shown...

Fig. 5 Molecular structures of tyrosinase-active diterpene (8-10) and napelline (11, 12) type alkaloids [49]...

A New Synthesis of Napelline.—Workers at New Brunswick have reported an excellent stereospecific synthesis of napelline (70), based on their fourth generation methods that were developed in the synthesis of several delphinine-type alkaloids.59 These methods had previously been explored with model compounds, and employ the rearrangement of a denudatine system to a napelline system.60 The starting material (123) for this synthesis was prepared by the route analogous to that used for the preparation of 13-deoxydeIphonine.60,61... 

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