In
four steps from the tricyclic core, palodesangren B trimethyl
ether and palodesangren D dimethyl ether could be synthesized in 29
and 18% overall yields, respectively. A reaction sequence comprising
the regioselective MgCl2-mediated Casnati–Skattebøl ortho-formylation of phenol, Wittig methylenation, acryloylation,
and Ru(II)-catalyzed ring-closing metathesis (RCM) led to the formation
of the final 2H-pyran-2-one ring of the desired tetracyclic
core.
ortho-Quinone methides (o-QMs) generated from the corresponding benzyl acetate precursors chemoselectively underwent the formal [4+2]-cycloadditions with the olefin of styrene, stilbene, or cinnamate derivatives by using different transition metal salts...
ortho-Quinone methides
(o-QMs)
underwent formal [4 + 2]-cycloaddition reactions with arylallenes
regioselectively at the styrenyl olefin to furnish the corresponding
3-methylene-2-arylchromans in moderate to good yields (up to 88%).
When R ≠ H, the reactions also proceeded with moderate stereoselectivity
(up to 5:1) which was governed by the nature of the R group. The 3-methylene-2-arylchromans
could serve as common intermediates for further functionalization
including epoxidation, oxidative cleavage/Baeyer–Villiger oxidation,
Riley oxidation, acid-catalyzed rearrangement, and Pd-catalyzed cross-coupling
reactions to furnish the corresponding derivatives in moderate to
good yields.
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