Preparation and Reactivity of Acyclic Chiral Allylzinc Species by a Zinc-Brook Rearrangement.

The zinc-Brook rearrangement of enantiomerically enriched α-hydroxy allylsilane produces a chiral allylzinc intermediate, which reacts with retention of configuration in the presence of an electrophile. Two remarkable features of this transformation are the stereochemical outcome during the formation of the allylzinc species and the complete stereocontrol in the organized six-membered transition state, which leads to an overall and complete transfer of chirality within the reaction sequence.

Oligoene-Based π-Helicenes or Dispiranes? Winding up Oligoyne Chains by a Multiple Carbopalladation/Stille/(Electrocyclization) Cascade.

A domino approach consisting of up to five consecutive steps to access either highly substituted dispiranes or π-helicenes from oligoyne chains is reported. The domino sequence consists of several carbopalladation reactions, a Stille cross-coupling to obtain the helicenes, and, depending on the steric demands of the helicene, a final 6π-electrocyclization to afford the dispiranes. Formally, the latter transformation contravenes the Woodward-Hoffmann rules, as revealed by X-ray crystallography of the dispirane. Additionally, the racemization barrier of the (Z,Z,Z)-triene-based helicene has been determined by a kinetic analysis and compared with results from density functional theory calculations. Characteristic points on the reaction coordinate were further analyzed according to their relaxed force constants (compliance constants).

Selective carbon-carbon bond cleavage for the stereoselective synthesis of acyclic systems.

Most of the efforts of organic chemists have been directed to the development of creative strategies to build carbon-carbon and carbon-heteroatom bonds in a predictable and efficient manner. In this Review, we show an alternative approach where challenging molecular skeletons could be prepared through selective cleavage of carbon-carbon bonds. We demonstrate that it has the potential to be a general principle in organic synthesis for the regio-, diastereo-, and even enantioselective preparation of adducts despite the fact that C-C single bonds are among the least reactive functional groups. The development of such strategies may have an impact on synthesis design and can ultimately lead to new selective and efficient processes for the utilization of simple hydrocarbons.

π-Helicenes truncated to a minimum: access through a domino approach involving multiple carbopalladations and a Stille coupling.

A novel type of π-helicenes is reported, in which the π-system is truncated to an all-s-cis all-Z oligoene chain. A domino sequence was developed, consisting of up to four consecutive carbopalladation reactions and a terminal Stille cross-coupling, to generate these entities in one step from the respective linear oligoynes. Despite the minimal π-system, very high optical rotation values were encountered for the single enantiomers. X-ray crystallography confirmed their screw-shaped structure.

Flexible synthesis of anthracycline aglycone mimics via domino carbopalladation reactions.

A synthesis of anthracycline aglycone derivatives is described. The key step utilizes a powerful domino carbopalladation approach and subsequent ring closure. During this process two of the four rings of the anthracycline scaffold are formed. Differently substituted carbohydrates and dialkyne chains serve as versatile and simple starting materials for the reaction sequence. Diverse building blocks lead to a variety of different products and a broad range of structural diversity.

Winding up alkynes: a Pd-catalyzed tandem-domino reaction to chiral biphenyls.

Wrap it up! An intramolecular Pd-catalyzed tandem-domino process leading to highly sterically encumbered biphenyls is reported. The chirality of the newly formed axis is induced by the configuration of the carbohydrate backbone.

A ridge walk between reaction modes: an unprecedented Pd-catalyzed domino sequence of diynyl-substituted bromoarenes.

Diynyl-substituted bromoarenes underwent a novel Pd-catalyzed domino reaction to provide benzofurans, pyridinofurans, isochromenes, and indole derivatives. Slight changes of the substrate push the reaction in another direction resulting in benzene annulation.

A Pd-catalyzed approach to (1-->6)-linked C-glycosides.

A flexible and robust method for the assembly of (1-->6)-linked C-glycosidic disaccharides is presented. The key reaction is a Pd-catalyzed coupling of 1-iodo- or 1-triflato-glycals with alkynyl glycosides. Reinstallation of the native hydroxyl group pattern is achieved after selective hydrogenation of the triple bond using Raney-nickel. Epoxidation with DMDO and reductive epoxide opening gives access to either the alpha- or the beta-derivative, depending on the hydride source.

Hybrids of sugars and aromatics: A Pd-catalyzed modular approach to chromans and isochromans.

Herein we describe the synthesis of highly substituted chromans and isochromans using carbohydrates as starting materials. The key step of our synthetic approach is the annelation of the benzene moiety via a highly efficient Pd-catalyzed domino reaction. This powerful approach led to a small library of highly substituted chromans and isochromans by making use of a variety of different diynes and bromoglycals. We investigated several Pd-catalysts in order to improve the yields and to enlarge the scope of the domino reaction. Furthermore, we elucidated the mechanistic picture of the reaction with isotope-labelling experiments. Most probably the reaction proceeds via an oxidative addition followed by two carbopalladation steps and a final cyclization reaction.

Domino access to highly substituted chromans and isochromans from carbohydrates.

Herein we describe the synthesis of highly substituted chromans and isochromans using carbohydrates as starting materials. Our approach makes use of a Pd-catalyzed domino reaction consisting of oxidative addition, followed by two carbopalladation steps and completed by a cyclization to annelate the benzene moiety. The versatility of this route has been demonstrated by a small library of highly substituted chromans and isochromans.