A total synthesis of guanacastepene C

Electrochemically Enabled Total Syntheses of Natural Products

Electrochemical techniques have helped to enable the total synthesis of natural products since the pioneering work of Kolbe in the mid 1800's. The electrochemical toolset grows every day and these new possibilities change the way chemists look at and think about natural products. This review provides a perspective on total syntheses wherein electrochemical techniques enabled the carbon─carbon bond formations in the skeletal assembly of important natural products, discussion of mechanistic details, and representative examples of the bond formations enabled over the last several decades. These bond formations are often distinctly different from those possible with conventional chemistries and allow assemblies complementary to other techniques.

Epoxy-Tethered Diels-Alder Reaction toward the Tricyclic Core of Kalihinols

A chiral-template-driven intramolecular Diels-Alder reaction has been used to build the tricyclic core of kalihinols, a group of antimalarial marine natural products. The key starting materials are commercially available nerol and sulcatone.

The isolation and synthesis of neodolastane diterpenoids

This account summarises the progress in isolation, characterisation and synthesis of diterpenoids sharing a tricyclic neodolastane skeleton as well as their biogenetic origin and diverse biological activities.

Studies of neodolastanes — Synthesis of the tricyclic core of the trichoaurantianolides

Studies toward the synthesis of trichoaurantianolide C (5) are described. Stille cross-coupling reactions of (E)- and (Z)-β-stannyl-α,β-unsaturated esters with allylic acetate 32 provide for the stereocontrolled formation of nonconjugated 2,5-diene-1-ols. Studies of the asymmetric Sharpless epoxidation are utilized to establish diastereofacial selectivity for the preparation of a crucial C2 tertiary allylic alcohol for subsequent esterification and ring-closing metathesis. SmI2 reductive cyclization of the key butenolide precursor 49 led to formation of the central seven-membered ring of the tricyclic core of the natural product.

Cyclohexyne cycloinsertion in the divergent synthesis of guanacastepenes

The guanacastepenes are a family of 15 diterpenes that share a common 5-6-7 tricyclic core, which is decorated with quaternary centers, unsaturation, hydroxyl and carbonyl groups. Some of these natural products show interesting antimicrobial potency. Their collective structural and biological features have stirred up vibrant activity among organic chemists. Herein, we disclose an account of our studies toward the synthesis of a number of guanacastepenes. The synthetic strategy relies on the use of cyclohexyne in a cycloinsertion reaction to rapidly construct the guanacastepene core. Isolation of a cyclobutenol as intermediate in the cyclohexyne cycloinsertion provided us with the possibility to study further the reactivity of this metastable compound, and we uncovered novel rearrangements and ring-opening reactions. Stereoselective, late-stage oxidative diversification of the carbon scaffold allowed the synthesis of guanacastepenes N and O and paved the way for the synthesis of guanacastepenes H and D.

Asymmetric [4 + 3] cycloadditions between vinylcarbenoids and dienes: application to the total synthesis of the natural product (-)-5-epi-vibsanin E

The total synthesis of (-)-5-epi-vibsanin E (2) has been achieved in 18 steps. The synthesis combines the rhodium-catalyzed [4 + 3] cycloaddition between a vinylcarbenoid and a diene to rapidly generate the tricyclic core with an effective end game strategy to introduce the remaining side-chains. The [4 + 3] cycloaddition occurs by a cyclopropanation to form a divinylcyclopropane followed by a Cope rearrangement to form a cycloheptadiene. The quaternary stereogenic center generated in the process can be obtained with high asymmetric induction when the reaction is catalyzed by the chiral dirhodium complex, Rh(2)(S-PTAD)(4).

Modern synthetic efforts toward biologically active terpenes

Terpenes represent one of the largest and most diverse classes of secondary metabolites, with over 55,000 members isolated to date. The terpene cyclase enzymes used in nature convert simple, linear hydrocarbon phosphates into an exotic array of chiral, carbocyclic skeletons. Further oxidation and rearrangement results in an almost endless number of conceivable structures. The enormous structural diversity presented by this class of natural products ensures a broad range of biological properties-ranging from anti-cancer and anti-malarial activities to tumor promotion and ion-channel binding. The marked structural differences of terpenes also largely thwart the development of any truly general strategies for their synthetic construction. This review focuses on synthetic strategies directed toward some of the most complex, biologically relevant terpenes prepared by total synthesis within the past decade. Of crucial importance are both the obstacles that modern synthetic chemists must confront when trying to construct such natural products and the key chemical transformations and strategies that have been developed to meet these challenges.

Domino Metathesis Involving ROM-RCM of Substituted Norbornenes. Rapid Access to Densely Functionalized Tricyclic Bridged and Condensed Ring Systems

Domino metathesis involving ROM-RCM of appropriately constructed norbornene derivatives having multiple alkene chains leads to direct access of highly functionalized bridged tricyclic compounds while that of a compound having two norbornene units tethered through one carbon produces a linearly arrayed condensed tricyclic system.

Synthesis of the guanacastepene A–B hydrazulene ring system through photochemical ring transposition

The A-B hydrazulene ring system of the guanacastepenes has been synthesised using a photochemical ring transposition of a 6-6 bicycle.

Enantioselective total synthesis of guanacastepene N using an uncommon 7-endo Heck cyclization as a pivotal step

A convergent, enantioselective total synthesis of (+)-guanacastepene N was developed that features a 7-endo Heck cyclization as the key step. In the course of this synthesis, short syntheses of the enantiomerically pure cyclopentenone and cyclohexene building blocks 5 and 6, which constitute A and C ring fragments of guanacastepene N, were developed. These fragments were linked by a challenging conjugate addition reaction that also generated the C11 quaternary carbon stereocenter. Regioselective 7-endo Heck cyclization gave rise to a tricyclic intermediate, which was elaborated to complete the first total synthesis of guanacastepene N and the second enantioselective total synthesis of a guanacastepene natural product.

Diterpenoids

This review covers the isolation and structures of diterpenoids, including labdanes, clerodanes,pimaranes, abietanes, kauranes, cembranolides, taxanes and marine diterpenoids. The literature from January to December 2005 is reviewed, and 195 references are cited.