Conspectus
Three-dimensional cage-like natural products
represent astounding
and long-term challenges in the research endeavors of total synthesis.
A central issue that synthetic chemists need to address lies in how
to efficiently construct the polycyclic frameworks as well as to install
the requisite substituent groups. The diterpenoid alkaloids that biogenetically
originate from amination of diterpenes and diversify through late-stage
skeletal reorganization belong to such a natural product category.
As the characteristic components of the Aconitum and Delphinium species, these molecules display a rich array
of biological activities, some of which are used as clinical drugs.
More strikingly, their intricate and beautiful architectures have
rendered the diterpenoid alkaloids elusive targets in the synthetic
community. The successful preparation of these intriguing compounds
relies on the development of innovative synthetic strategies.
Our laboratory has explored the total synthesis of a variety of
diterpenoid alkaloids and their biogenetically related diterpenes
over the past decade. In doing so, we have accessed 6 different types
of skeletons (atisine-, denudatine-, arcutane-, arcutine-, napelline-,
and hetidine-type) and achieved the total synthesis of 6 natural products
(isoazitine, dihydroajaconine, gymnandine, atropurpuran, arcutinine,
and liangshanone). Strategically, an oxidative dearomatization/Diels–Alder
(OD/DA) cycloaddition sequence was widely employed in our synthesis
to form the ubiquitous [2.2.2]-bicyclic ring unit and its related
ring-distorted derivatives in these complex target molecules. This
protocol, in combination with additional bond-forming key steps, allowed
us to prepare the corresponding polycyclic alkaloids and a biogenetically
associated diterpene. For example, bioinspired C–H activation, aza-pinacol, and aza-Prins cyclizations
were used toward a unified approach to the atisine-, denudatine-,
and hetidine-type alkaloids via ajaconine intermediates in our first
work. To pursue the synthesis of atropurpuran and related arcutine
alkaloids, we harnessed a ketyl-olefin radical cyclization to assemble
the carbocycle and an aza-Wacker cyclization to construct
the unusual pyrrolidine ring. Furthermore, a one-pot alkene cleavage/Mannich
cyclization tactic, sequential Robinson annulation, and intramolecular
aldol addition were developed, which facilitated the formation of
the napelline alkaloid scaffold and the first total synthesis of liangshanone.
Finally, the utility of the Mannich cyclization and enyne cycloisomerization
reactions allowed for access to the highly functionalized A/E and
C/D ring fragments of aconitine (regarded as the “Holy Grail”
of diterpenoid alkaloids). This Account provides insight into our
synthetic designs and approaches used toward the synthesis of diterpenoid
alkaloids and relevant diterpenes. These endeavors lay a foundation
for uncovering the biological profiles of associated molecules and
also serve as a reference for preparing other three-dimensionally
fascinat...