Rh(II)-Catalyzed Enynal Cycloisomerization for the Generation of Vinyl Carbene: Divergent Access to Polycyclic Heterocycles

Described herein is an unprecedented example of generating
the
endocyclic donor–donor vinyl carbene achieved by the dirhodium(II)-catalyzed
enynal cycloisomerization. The endocyclic vinyl carbene intermediate
could be trapped by a diversity of alkenes, including 1,1-disubstituted
alkenes, dienes, and α-methyl styrenes, which could undergo
[2 + 1], [4 + 3] cycloaddition and an unexpected formal allylation,
respectively, giving spiro and fused polycyclic heterocycles. In these
cascade transformations, up to three rings and three stereocenters
were constructed in one step with good selectivities. The enantioselective
versions were also achieved with up to 84% ee. Mechanistic studies
revealed that a concerted intramolecular [2 + 3] cycloaddition was
the key step for the generation of vinyl Rh(II) carbene. Detailed
studies were conducted to elucidate the intriguing mechanism of the
formal allylation.