Salvia is the largest genus in the economically and medicinally
important Labiatae family. Many species of this widely
distributed genus have been used for centuries as folk medicine
to treat a variety of illnesses. One such species is Salvia
przewalskii Maxim, a traditional medicinal plant used as a
surrogate of S. miltiorrhhiza (Danshen) for treating various
cardiovascular diseases.1,2
In 2007, Zhao et al. isolated przewalskin B (1) from the
acetone extract of S. przewalskii Maxim.2 1 has attracted the
attention of synthetic chemists because it possesses a unique
tetracyclic skeleton that contains a spirocyclic enone system
and an α-hydroxy-β-ketone lactone moiety. Moreover, it shows
modest anti-HIV activity with EC50 = 30.32 μg/mL. In 2011,
She and Tu achieved the total synthesis of (−)- and
(+)-przewalskin B, respectively.3 She used an intramolecular
nucleophilic acyl substitution (INAS) reaction to close the B
ring and determined the absolute configuration of the natural
product. Tu, on the other hand, adopted diastereoselective
organocatalytic aldol cyclization to form the B ring and initiated
Rh2(OAc)4-mediated intramolecular carbene insertion to
construct the five-member spiro ring. In this paper, a concise
method for the total synthesis of (±)-przewalskin B is
described from another perspective.
Scheme 1 outlines our retrosynthetic