Spongian pentacyclic diterpenes. Stereoselective synthesis of aplyroseol-1, aplyroseol-2 and deacetylaplyroseol-2

Abstract

Natural spongian pentacyclic diterpenes aplyroseol-1 19, aplyroseol-2 18 and deacetylaplyroseol-2 17 have been synthesized in enantiomerically pure form from (+)-podacarp-8(14)-en-13-one 3. Key intermediate in these syntheses is a suitably substituted acid-dialdehyde 2, which is prepared from enone 3 by a sequence of transformations involving stereoselective introduction of a C-7 oxygen functionality, photoaddition of acetylene to the C(8)–C(14) double bond to form a cyclobutene ring system, reductive cyanation of the C-13 carbonyl group, hydrolysis of the resulting nitrile group, and ozonolysis of the cyclobutene moiety. An intramolecular participation of the 7α-hydroxy group in the hydrolysis of the 13α-nitrile 8 is set in focus.