| Abstract: | Novel Diterpenoids from Leaf Glands of Plectranthus barbatus (Labiatae). The Absolute Configuration of the 2-Hydroxypropyl Group in Coleon E In addition to the previously reported quinone methides coleon E ( 1a ), coleon F ( 2a ) and the spirocoleon 9 (plectrin), novel rearranged abietanoid dienediones, called (16R)-plectrinon A ( 3a ) and plectrinon B ( 5 ) as well as the allylroyleanone 8a have been isolated from Plectranthus barbatus and their structures elucidated mainly by spectroscopy. Closer investigation of acetylations of 1a and 2a established the hitherto unknown structures 1c , 2c , 3b , 10a , 10b , 11a , 11b , 12a , and 12b . The derivatives 3b , 11a , 11b , 12a , and 12a , and 12b are the products of a nucleophilic attack at the quinone methide system. The analysis of the 13C-NMR spectra led to the full assignment of the signals in 1a , 1b , 2a , 3a , and 8a . Correlations by partial syntheses of 3a , from coleon E( 1a ), after oxidation or acetylation/saponification of the latter, established the (R)-configuration of the 2-hydroxypropyl group in 1a . The biomimetic transformation of plectrin ( 9 ) into (16R)-coleon E ( 1a ) is shown to proceed via the unexpected, highly reactive 2-methylspirocyclopropane-1,2′-(2′H)-phenanthrene]-1′,3′, 6′-trione 13 . The solvolysis of the spiro(methylcyclopropane) moiety takes place with inversion of the configuration at the attacked C-atom, as established in a previous communication. The 1,3,6-trione 13 is supposed to be also the key intermediate in the biosynthesis of the allyl group in coleon F ( 2a ) which proceeds via a homosigmatropic 1,5]-H shift. |
