Approaches to Cyclopropazulenes: Transannular Aldol Reactions of Some Derivatives of 1,10-Dibromobicyclo[8.1.0]undecane-3,8-dione

9-Step synthesis of (-)-larikaempferic acid methyl ester enabled by skeletal rearrangement

We report here a concise synthesis of the anti-tumor-promoting (-)-larikaempferic acid methyl ester, a novel and rearranged abietane-type diterpene natural product containing a unique tetracyclic skeleton with a trans-hydrindane, an oxabicyclo[3.2.1]octane, and six stereogenic centers. Our synthesis starts with the cheap and abundant abietic acid and features an oxidative C-C bond cleavage followed by a transannular aldol reaction to skeletally rearrange the 6-6-6 tricyclic carbon skeleton of abietic acid to the desired 6-5-7 tricyclic carbon skeleton and an intramolecular oxa-Michael addition to form the oxa bridge. This skeletal rearrangement strategy enabled us to synthesize (-)-larikaempferic acid methyl ester in 9 steps.

Rearrangement of the Tetra- and Tricyclic Skeletons of Pepluanol B to Access the Core Structures of Tigliane- and Myrsinane-Type Euphorbia Diterpenes

Pepluanol B is a new Euphorbia diterpene with an unprecedented tetracyclic backbone. However, its biogenetic relationship with known Euphorbia diterpenes is unclear. We report herein that its β-hydroxyl ketone motif could undergo a base-promoted retro-aldol/aldol process in two pathways and afford the skeletons of tigliane- and myrsinane-type Euphorbia diterpenes through the formation of the C8-C14 and C7-C13 bonds, respectively. The retro-aldol/aldol cascade indicates that pepluanol B is possibly a biosynthetic precursor of lathyranes and other relevant dipterpenes.

Intramolecular Diels-Alder Reaction of Cyclopropenyl Vinylarenes: Access to Benzonorcarane Derivatives

Intramolecular Diels-Alder vinylarene reaction (IMDAV) is a [4 + 2] cycloaddition that employs styrene derivatives as conjugated dienes, whose poor reactivity arises from the required loss of aromaticity, which is recovered by a subsequent [1,3]-H shift. Herein, we describe the use of cyclopropene as a dienophile, harnessing its strain energy to drive the IMDAV reaction. Benzonorcarane scaffolds form in good yields, excellent stereoselectivity, and broad functional tolerance. Theoretical calculations and NMR studies have revealed significant mechanistic insights.

Reaction of cyclopropenes with a trichloromethyl radical: unprecedented ring-opening reaction of cyclopropanes with migration

The triethylborane-mediated direct addition of chloroform to cyclopropenes afforded trichloromethylcyclopropanes. In contrast, the use of dimethylzinc gave the unconjugated diene via ring-opening reactions.

Vinylogous Nicholas reactions in the synthesis of bi- and tricyclic cycloheptynedicobalt complexes

The Lewis acid mediated intramolecular Nicholas reactions of allylic acetate enyne-Co₂(CO)₆ complexes afford bicyclic- and tricyclic cycloheptenyne-Co₂(CO)₆ complexes.

Direct access to 6/5/7/5- and 6/7/5/5-fused tetracyclic triterpenoids via divergent transannular aldol reaction of lanosterol-derived diketone

In an effort to access biologically relevant chemical space, a complex natural product derived nonsymmetrical diketone was prepared as a substrate for divergent transannular aldol reactions. The use of common aldol conditions resulted in predominant syn-addition via pathway a, while the use of alumina provided access to the anti-adduct. Screening of a range of Lewis acids of varying strength unexpectedly resulted in the formation of aldol products with 6/7/5/5-fused molecular skeleton via pathway b.

Chiral cyclopropenyl ketones: reactive and selective Diels-Alder dienophiles

The synthesis and Diels-Alder reactions of cyclopropenyl ketones are described. Cyclopropenyl ketones are highly reactive dienophiles that can engage a range of cyclic dienes and 2,3-dimethylbutadiene. The strategy of using cyclopropenyl ketones to facilitate Diels-Alder reactions is not limited to products that contain three-membered rings, as reductive opening by SmI2 can be used to produce a product that lacks a cyclopropane but retains a quaternary stereogenic center.

Molecular library synthesis using complex substrates: expanding the framework of triterpenoids

The remodeling of a natural product core framework by means of diversity-oriented synthesis (DOS) is a valuable approach to access diverse/biologically relevant chemical space and to overcome the limitations of combinatorial-type compounds. Here we provide proof of principle and a thorough conformational analysis for a general strategy whereby the inherent complexity of a starting material is used to define the regio- and stereochemical outcomes of reactions in chemical library construction. This is in contrast to the traditional DOS logic employing reaction development and catalysis to drive library diversity.

Recent developments of cyclopropene chemistry

This critical review discusses recent developments in the field of cyclopropene chemistry. Although several excellent reviews that mainly focused on the thermolysis and pyrolysis as well as metal-mediated reactions of cyclopropenes have been published, significant new developments have also been achieved in recent years. This brand new review provides an overview of the progress from 2007 to 2011 on the syntheses and transformations of cyclopropenes as well as their related mechanistic studies (238 references).