Short Enantioselective Total Syntheses of trans-Clerodane Diterpenoids: Convergent Fragment Coupling Using a trans-Decalin Tertiary Radical Generated from a Tertiary Alcohol Precursor.

The evolution of a convergent fragment-coupling strategy for the enantioselective total synthesis of trans-clerodane diterpenoids is described. The key bond construction is accomplished by 1,6-addition of a trans-decalin tertiary radical with 4-vinylfuran-2-one. The tertiary radical is optimally generated from the hemioxalate salt of the corresponding tertiary alcohol upon activation by visible light and an Ir(III) photoredox catalyst. The enantioselective total synthesis of trans-clerodane diterpenoid 1 reported here was accomplished in seven steps from 3-methyl-2-cyclohexenone. The synthetic strategy described in this report allows a number of trans-clerodane diterpenoids to be synthesized in enantioselective fashion by synthetic sequences of 10 steps or less. This study illustrates a powerful tactic in organic synthesis in which a structurally complex target structure is disconnected at a quaternary carbon stereocenter to fragments of comparable complexity, which are united in the synthetic pathway by conjugate addition of a nucleophilic tertiary radical to a fragment harboring an electron-deficient C-C double bond.

Constructing quaternary stereogenic centers using tertiary organocuprates and tertiary radicals. Total synthesis of trans-clerodane natural products.

A new concise construction of trans-clerodane diterpenoids is reported in which oxacyclic and trans-hydronaphthalene fragments are coupled, and the critical C9-quaternary carbon stereocenter formed stereoselectively, by 1,6-addition of a tertiary cuprate or a tertiary carbon radical to ß-vinylbutenolide. This strategy is specifically illustrated by total syntheses of (-)-solidagolactone (4), (-)-16-hydroxycleroda-3,13-dien-15,16-olide (5, PL3), and (-)-annonene (6).

Divergent synthesis and chemical reactivity of bicyclic lactone fragments of complex rearranged spongian diterpenes.

The synthesis and direct comparison of the chemical reactivity of the two highly oxidized bicyclic lactone fragments found in rearranged spongian diterpenes (8-substituted 6-acetoxy-2,7-dioxabicyclo[3.2.1]octan-3-one and 6-substituted 7-acetoxy-2,8-dioxabicyclo[3.3.0]octan-3-one) are reported. Details of the first synthesis of the 6-acetoxy-2,7-dioxabicyclo[3.2.1]octan-3-one ring system, including an examination of several possibilities for the key bridging cyclization reaction, are described. In addition, the first synthesis of 7-acetoxy-2,8-dioxabicyclo[3.3.0]octanones containing quaternary carbon substituents at C6 is disclosed. Aspects of the chemical reactivity and Golgi-modifying properties of these bicyclic lactone analogs of rearranged spongian diterpenes are also reported. Under both acidic and basic conditions, 8-substituted 2,7-dioxabicyclo[3.2.1]octanones are converted to 6-substituted-2,8-dioxabicyclo[3.3.0]octanones. Moreover, these dioxabicyclic lactones react with primary amines and lysine side chains of lysozyme to form substituted pyrroles, a conjugation that could be responsible for the unique biological properties of these compounds. These studies demonstrate that acetoxylation adjacent to the lactone carbonyl group, in either the bridged or fused series, is required to produce fragmented Golgi membranes in the pericentriolar region that is characteristic of macfarlandin E.