Ruthenium-catalysed multicomponent synthesis of the 1,3-dienyl-6-oxy polyketide motif

Polyketide natural products are an important class of biologically active compounds. Although substantial progress has been made on the synthesis of repetitive polyketide motifs through the iterative application of a single reaction type, synthetic access to more diverse motifs that require more than one type of carbon-carbon bond connection remains a challenge. Here we describe a catalytic, multicomponent method for the synthesis of the privileged polyketide 1,3-dienyl-6-oxy motif. The method allows for the formation of two new carbon-carbon bonds and two stereodefined olefins. It generates products that contain up to three contiguous sp³ stereocentres with a high stereoselectivity in a single operation and can be used to generate chiral products. The successful development of this methodology relies on the remarkable efficiency of the ruthenium-catalysed alkene-alkyne coupling reaction between readily available vinyl boronic acids and alkynes to provide unsymmetrical 3-boryl-1,4-diene reagents. In the presence of carbonyl compounds, these reagents undergo highly diastereoselective allylations to afford the desired 1,3-dienyl-6-oxy motif and enable complex polyketide synthesis in a rapid and asymmetric fashion.

Applications of pentafluorophenyl boron reagents in the synthesis of heterocyclic and aromatic compounds

Recently, main group reagents have attracted a lot of attention in bond-forming reactions in organic synthesis. This article highlights the use of pentafluorophenyl substituted boron reagents in their reactions with C=C and C≡C π-bonds for the synthesis of heterocyclic and aromatic compounds. These cyclisation reactions fall into four general classes although there is some overlap between classes and often combinations of these different types of reactivity are observed in the formation of the final heterocyclic product: (i) 1,2- (and 1,4-) additions of nucleophile and Lewis-acidic boron centre, (ii) 1,1-carboboration, (iii) carbocation rearrangements and (iv) cycloaddition chemistry. In addition, the prospect of using such boron reagents catalytically in the synthesis of aromatic compounds such as oxazoles and dibenzopentalene derivatives is emphasised.

Activation of alkynes with B(C₆F₅)₃--boron allylation reagents derived from propargyl esters

Novel allyl boron compounds are readily synthesized via rearrangement reactions between Lewis acidic B(C6F5)3 and propargyl esters. These reactions proceed through an initial cyclization followed by ring-opening and concurrent C6F5-group migration. In the absence of disubstitution adjacent to the ester oxygen atom, an allyl boron migration rearrangement leads to formal 1,3-carboboration products. These allyl boron compounds act as allylation reagents with aldehydes introducing both a C3-allyl fragment and a C6F5-unit as a single anti-diastereomer. In these reactions, B(C6F5)3 activates the alkynes, prompting the rearrangement processes and enabling installations of C6F5 and R-groups.

Performance of DFT methods and origin of stereoselectivity in bipyridine N,N′-dioxide catalyzed allylation and propargylation reactions

It is shown that many DFT methods correctly predict the stereoselectivity of bipyridine N,N′-dioxide catalyzed alkylation reactions despite predicting the incorrect low-lying transition state structures. A novel explanation of the origin of stereoselectivity in these reactions is also provided.

Allylation reactions of aldehydes with allylboronates in aqueous media: unique reactivity and selectivity that are only observed in the presence of water

Zn(OH)2-catalyzed allylation reactions of aldehydes with allylboronates in aqueous media have been developed. In contrast to conventional allylboration reactions of aldehydes in organic solvents, the α-addition products were obtained exclusively. A catalytic cycle in which the allylzinc species was generated through a B-to-Zn exchange process is proposed and kinetic studies were performed. The key intermediate, an allylzinc species, was detected by HRMS (ESI) analysis and by online continuous MS (ESI) analysis. This analysis revealed that, in aqueous media, the allylzinc species competitively reacted with the aldehydes and water. An investigation of the reactivity and selectivity of the allylzinc species by using several typical allylboronates (6a-6d) clarified several important roles of water in this allylation reaction. The allylation reactions of aldehydes with allylboronic acid 2,2-dimethyl-1,3-propanediol esters proceeded smoothly in the presence of catalytic amounts of Zn(OH)2 and achiral ligand 4d in aqueous media to afford the corresponding syn-adducts in high yields with high diastereoselectivities. In all cases, the α-addition products were obtained and a wide substrate scope was tolerated. Furthermore, this reaction was applied to asymmetric catalysis by using chiral ligand 9. Based on the X-ray structure of the Zn-9 complex, several nonsymmetrical chiral ligands were also found to be effective. This reaction was further applied to catalytic asymmetric alkylallylation, chloroallylation, and alkoxyallylation processes and the synthetic utility of these reactions has been demonstrated.

The new world of organic reactions in water

Different reactivities and selectivities are observed in water compared with those in organic solvents. In this article, such three examples are described. While ammonia was known not to react in metal-catalyzed allylic amination, palladium-catalyzed allylic amination using aqueous ammonia proceeded to afford primary amines in high yields. Second, allylboronates reacted with aldehydes in aqueous media to afford α-addition adducts exclusively in high yields with high diastereo- and enantioselectivities using Zn(OH)2with ligands as catalysts. Finally, it was found that catalytic use of In(0) was effective for the reactions of allylboronates with ketones in water.

Tandem reactions involving 1-silyl-3-boryl-2-alkenes. New access to (Z)-1-fluoro-1-alkenes, allyl fluorides, and diversely α-substituted allylboronates

The reactions of 1-silyl-3-boryl-2-alkenes with various electrophilic reagents (Selectfluor, N-halosuccinimides, benzhydryl, and propargylic alcohols in the presence of a Lewis acid, N-alkoxycarbonyliminium ion) have been investigated as new routes to α-substituted allylboronates. Further functional transformations, including allylboration, Suzuki coupling, protodeboronation, and cycloisomerization, have been carried out to illustrate the synthetic potential of these γ-borylallylsilanes.

(2R,3R)-1,4-Dimethoxy-1,1,4,4-tetraphenylbutane-2,3-diol: Valuable reagent in the asymmetric synthesis of organoboronates

Tartrate-derived (2R,3R)-1,4-dimethoxy-1,1,4,4-tetraphenylbutane-2,3-diol has found diverse applications in asymmetric organic synthesis. Among these, its utilization as a protecting group for boronic acids has been investigated extensively. Besides being extraordinarily stable and thus allowing a plethora of transformations, it enables access to various diastereo- and enantiomerically pure organoboron reagents and versatile intermediates in asymmetric synthesis.

Isotope effects and heavy-atom tunneling in the Roush allylboration of aldehydes

Intermolecular (13)C kinetic isotope effects (KIEs) for the Roush allylboration of p-anisaldehyde were determined using a novel approach. The experimental (13)C KIEs fit qualitatively with the expected rate-limiting cyclic transition state, but they are far higher than theoretical predictions based on conventional transition state theory. This discrepancy is attributed to a substantial contribution of heavy-atom tunneling to the reaction, and this is supported by multidimensional tunneling calculations that reproduce the observed KIEs.

Highly (E)-selective BF(3).Et(2)O-promoted allylboration of chiral nonracemic alpha-substituted allylboronates and analysis of the origin of stereocontrol

Delta-methyl-substituted homoallylic alcohols 2 were prepared in 71-88% yield, E:Z >30:1 and 93% to >95% ee via BF(3).Et(2)O-promoted allylboration with alpha-Me-allylboronate 1. The origin of high (E)-selectivity is proposed.

(E)-α-Substituted γ-Alkoxyallylboronic Esters as New Reagents: Synthesis and Reactivity toward Aldehydes

We have developed a synthesis of new allylboration reagents based on an allylic rearrangement. This approach led to the alpha-substituted gamma-alkoxyallylboronates 2 with a high stereoselectivity in favor of the E-isomer, independent of the organometallic used. We have also studied the reactivity of these reagents toward aldehydes, showing that the allylboration reaction occurs with an excellent diastereoselectivity to give the anti-diol derivatives 5. Moreover, the sequence can be carried out in a "one-pot" procedure avoiding the purification of allylboronates.

α,β-Unsaturated δ-Lactones from Copper-Catalyzed Asymmetric Vinylogous Mukaiyama Reactions of Aldehydes: Scope and Mechanistic Insights

A direct regio-, diastereo-, and enantiocontrolled access to alpha,beta-unsaturated delta-lactones is described, based on the reaction of a silyl dienolate and an aldehyde in the presence of 10 % of Carreira's catalyst. The scope and limitations of this reaction, as well as mechanistic insights concerning the reactivity of an allyl copper species, are discussed.

The origin of diastereofacial control in allylboration reactions using tartrate ester derived allylboronates: attractive interactions between the Lewis acid coordinated aldehyde carbonyl group and an ester carbonyl oxygen

Transition-state structures for the allylboration reaction between the tartrate ester and tartramide modified allylboronates and acetaldehyde are located at the B3LYP/6-31G* level of theory. An attractive interaction between the boron-activated aldehyde and the ester or amide carbonyl oxygen lone pair is found to play a major role in the favored transition states 11a and 13. This attractive interaction appears to be electrostatic in origin. However, an n --> pi* charge-transfer type of interaction has not been ruled out. The distance (2.77 A) between the aldehydic hydrogen and the carbonyl oxygen in transition state 13 is beyond the sum of van der Waals radii. The formyl C-H...O bond angle (109 degrees) in this transition structure deviates far from linearity. Therefore, hydrogen-bonding interactions between the formyl C-H and the amide carbonyl oxygen are considered negligible. The distance (3.81 A) between the aldehydic oxygen and the amide carbonyl oxygen in the diastereomeric, disfavored transition state 14 is also beyond the van der Waals radii, which suggests that n/n electronic repulsion plays a lesser role in stereodifferentiation in the allylboration reaction than originally proposed.

Diastereo- and Enantioselective Synthesis of syn-alpha-Vinylchlorohydrins and cis-Vinylepoxides

A new method to generate chiral syn-vinylchlorohydrins and cis-vinyloxiranes is reported. Reaction of (alpha-haloallyl)lithiums with methoxy-9-BBN or Ipc(2)BOMe followed by treatment with BF(3).OEt(2) leads to (Z)-(gamma-haloallyl)boranes which react with aldehydes to yield cis-vinylepoxides (de >/= 90%) upon oxidative workup. Alternatively, addition of ethanolamine to the allylboration product yields syn-alpha-halohydrins (de >/= 90%) that are also easily cyclized to cis-vinylepoxides. Extension of this protocol using [(Z)-gamma-chloroallyl]BIpc(2) leads to chiral syn-alpha-chlorohydrins and cis-vinylepoxides in high de (>/=90%) and ee (90-99%). Enantioselectivity of reactions of chiral (Z)-(gamma-chloroallyl)boranes with aldehydes are more sensitive to reaction conditions than enantioselectivity of reactions of other alpha-or gamma-substituted allylboranes. The effects of proportion of BF(3).OEt(2) and the relative efficacies of LiNR(2) bases on diastereo- and enantioselectivity of the chloroallylation are reported.