The first and highly enantioselective crotylation of aldehydes via an allyl-transfer reaction from a chiral crotyl-donor

Catalyst Controlled Site- and Stereoselective Rhodium(II) Carbene C(sp3)-H Functionalization of Allyl Boronates

Rhodium(II) catalyst-controlled site- and stereoselective carbene insertion into the distal allylic C(sp³)-H bond of allyl boronates is reported. The optimum chiral catalyst for this reaction is Rh₂(S-TPPTTL)₄. The fidelity and asymmetric induction of this catalytic transformation allows for a highly diastereoselective and enantioselective C-C bond formation without interference from the allyl boronate functionality. The resulting functionalized allyl boronates are susceptible to stereoselective allylations, generating products with control of stereochemistry at four contiguous stereogenic centers.

Synthetic efforts on the road to marine natural products bearing 4-O-2,3,4,6-tetrasubstituted THPs: an update

Scientific literature is inundated with secondary metabolites from marine sources. In this ocean of natural products, the presence of recurring patterns has traditionally led scientists to unravel the biosynthetic mechanisms that naturally yield these products, as well as to imitate Nature to prepare them in the laboratory, especially when promising bioactivities and stimulating molecular architectures are involucrate. For instance, natural products containing multisubstituted oxygenated rings and macrocyclic lactones are recurrently selected as targets for developing total syntheses. Thus, in the last decades a noteworthy number of synthetic works regarding miyakolide, madeirolide A and representative compounds of polycavernosides, lasonolides and clavosolides have come to fruition. Up to now, these families of macrolides are the only marine natural products bearing a tetrasubstituted tetrahydropyran ring with carbon substituents at positions 2, 3 and 6, as well as an oxygen at position 4. Their splendid structures have received the attention of the synthetic community, up to the point of starring in dozens of articles, and even some reviews. This work covers all the synthetic studies towards miyakolide and madeirolide A, as well as the synthetic efforts performed after the previous specialised reviews about lasonolide A, polycavernoside A and clavosolides, published in 2006, 2007 and 2016, respectively. In total, this review summarises 22 articles in which these marine natural products with 4-O-2,3,4,6-tetrasubstituted tetrahydropyrans have the leading role.

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

Unlocking the Drug Potential of the Bryostatin Family: Recent Advances in Product Synthesis and Biomedical Applications

Bryostatins are a class of naturally occurring macrocyclic lactones with a unique fast developing portfolio of clinical applications, including treatment of AIDS, Alzheimer's disease, and cancer. This comprehensive account summarizes the recent progress (2014-present) in the development of bryostatins, including their total synthesis and biomedical applications. An emphasis is placed on the discussion of bryostatin 1, the most-studied analogue to date. This review highlights the synthetic and biological challenges of bryostatins and provides an outlook on their future development.

Total Synthesis, Stereochemical Revision, and Biological Assessment of Iriomoteolide-2a

Iriomoteolide-2a is a marine macrolide metabolite isolated from a cultured broth of the benthic dinoflagellate Amphidinium sp. HYA024 strain. This naturally occurring substance was reported to show remarkable cytotoxic activity against human cancer cell lines HeLa and DG-75 and in vivo antitumor activity against murine leukemia P388 cell line. Herein, the total synthesis, stereochemical revision, and biological assessment of iriomoteolide-2a are reported in detail. Total synthesis of the proposed structure 1 of iriomoteolide-2a featured a late-stage convergent assembly of three components by a Suzuki-Miyaura coupling, an esterification, and a ring-closing metathesis. However, the NMR data of synthetic 1 were not identical to those of the natural product. Careful analysis of the NMR data of the authentic material and synthesis/NMR analysis of appropriately designed model compounds led to consideration of four possible stereoisomers 2-5 as candidates for the correct structure. Accordingly, total syntheses of 2-5 were achieved by taking advantage of the convergent strategy, and comparison of the NMR spectra of synthetic 2-5 with those of the natural product led to the conclusion that 5 shows the correct relative configuration of iriomoteolide-2a. The absolute configuration of this natural product was finally established through chiral HPLC analysis of synthetic 5/ent-5 with the authentic sample. The antiproliferative activity of the synthetic compounds was assessed against HeLa and A549 cells to show that, in contrast to expectation, synthetic 5 and ent-5 were only marginally active in these cell lines. This work clearly underscores the vital role of total synthesis in the establishment of the structure and biological activity of natural products.

Asymmetric Bisvinylogous Aldolation of Aldehydes via 2-Oxonia-Cope Rearrangement Enabling Total Stereochemical Control

Highly stereoselective 2-oxonia-Cope rearrangement reactions between newly designed bisvinylogous aldolation synthons and aldehydes, which can provide ε-hydroxy-α,β,γ,δ-unsaturated esters with excellent enantioselectivities, as well as with unprecedented E- and Z-selectivities without regioselectivity issues, are described.

Rationally Designed Chiral Synthons Enabling Asymmetric Z- and E-Selective Vinylogous Aldol Reactions of Aldehydes

In a conceptually different approach, highly stereoselective 2-oxonia-Cope rearrangement reactions between rationally designed nonracemic vinylogous aldolation synthons and aldehydes are described to provide δ-hydroxy-α,β-unsaturated esters with excellent enantioselectivities and, for the first time, unprecedented Z- and E-selectivities without the regioselectivity issue.

Highly Stereoselective 2-Oxonia-Cope Rearrangement: A Platform Enabling At-Will Control of Regio-, Enantio-, and Diastereoselectivity in the Vinylogous Aldol Reactions of Aldehydes

A distinctly different approach for the vinylogous aldolation of aldehydes is described, which exploits 2-oxonia-Cope rearrangement reactions between two readily available partners, a set of rationally designed chiral homoallylic alcohol synthons and aldehydes, under simple conditions. In these processes, chirality transfer from the former to the latter is nearly perfect, giving rise to excellent enantio- and diastereoselectivity without the regioselectivity issue associated with traditional vinylogous aldol reactions.

Scalable synthesis of bryostatin 1 and analogs, adjuvant leads against latent HIV

Bryostatin 1 is an exceedingly scarce marine-derived natural product that is in clinical development directed at HIV/AIDS eradication, cancer immunotherapy, and the treatment of Alzheimer's disease. Despite this unique portfolio of indications, its availability has been limited and variable, thus impeding research and clinical studies. Here, we report a total synthesis of bryostatin 1 that proceeds in 29 total steps (19 in the longest linear sequence, >80% average yield per step), collectively produces grams of material, and can be scaled to meet clinical needs (~20 grams per year). This practical solution to the bryostatin supply problem also opens broad, facile, and efficient access to derivatives and potentially superior analogs.

Advances in the synthesis of glycosidic macrolides: clavosolides A-D and cyanolide A

Covering: 2005 to 2016Clavosolides A-D and cyanolide A are glycosidic macrolides and represent a new family of marine natural products. They possess a number of unusual structural features and have attracted considerable interest from the synthetic community. This review presents a comprehensive survey of all aspects of the clavosolides A-D and cyanolide A. Specific topics include isolation, structure determination, biological activity, and synthetic approaches.

syn-Selective crotylation of aldehydes using bismuth-crotyl bromide-(1-butyl-3-methylimidazolium bromide) combination: some synthetic applications

The Bi-[bmim][Br] combination has been found to offer high syn-selectivity in the crotylation of aldehydes with crotyl bromide using practically stoichiometric amounts of the reagents. The room temperature ionic liquid (RTIL), [bmim][Br], activated Bi metal in the presence of oxygen to produce crotylbismuthdibromide, which reacted with the aldehydes at room temperature. The major anti-syn diastereomeric product obtained from the crotylation of (R)-cyclohexylideneglyceraldehyde was utilized for the synthesis of dictyostatin and cryptophycin segments, and (+)-cis-aerangis lactone, using standard synthetic protocols.

Stereoselective synthesis of protected l- and d-dideoxysugars and analogues via Prins cyclisations

A de novo approach for the rapid construction of orthogonally protected l- and d-deoxysugars and analogues is described. A novel and robust silicon-acetal undergoes Prins cyclisations with a series of homoallylic alcohols in high yield and excellent stereocontrol. Modified Tamao-Fleming oxidation of the resulting silyltetrahydropyrans gives direct access to deoxyglycoside analogues and the approach was showcased in the synthesis of protected l-oliose, a component of the anticancer agent aclacinomycin A.

Mechanistic insights into the ring-opening of biomass derived lactones

Density functional theory calculations suggest the formation of an oxocarbenium ion intermediate in acid catalyzed ring-opening reactions of biomass derived lactones, which may play an important role in determining it's reactivity.

A Novel Allyl Transfer Coupled with a Grob Fragmentation

A novel acid-promoted rearrangement is disclosed. In the previously unknown transformation, an allyl group migrated to an in situ formed carbocation stabilized by an electron-rich aryl or heteroaryl group, resulting in a stereoselective intramolecular Grob fragmentation. The outcome of the rearrangement observed with an array of substrates can be satisfactorily rationalized using a working hypothesis with the aid of a six-membered transition state similar to those proposed for the anionic oxy-Cope or oxonia-Cope rearrangements, but involving only one instead of two double bonds.

Forming spirocyclohexadienone-oxocarbenium cation species in the biomimetic synthesis of amomols

The oxidation of appropriate 2-(4-hydroxyphenyl)ethyl ketones gives direct access to amomols by means of the formation of a transient spirocyclohexadienone-oxocarbenium ion that is intermolecularly intercepted by an alcohol. Furthermore, homochiral amomols and other new analogues were synthesized for the first time and were biologically evaluated on Plasmodium falciparum.

Polycavernoside A: the Prins macrocyclization approach

An intramolecular Prins macrocyclization reaction was successfully employed in the synthesis of polycavernoside A.

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.

Symmetric macrocycles by a Prins dimerization and macrocyclization strategy

A tandem dimerization/macrocyclization reaction utilizing the Prins cyclization has been developed. This reaction develops molecular complexity through the formation of highly substituted dimeric tetrahydropyran macrocycles. Mild conditions utilizing rhenium(VII) catalysts were explored for aromatic substrates, while harsher Lewis acidic conditions were used for aliphatic substrates. Both aldehydes and acetals are shown to be viable substrates for this reaction.

Synthesis of natural product inspired compound collections

Natural products, their derivatives, and their analogues are among the most important sources for new drug candidates and tools for chemical biology and medicinal chemistry research. Therefore, there is a need for the development of efficient synthesis methods which give access to natural product derived and inspired compound collections. To meet this challenge, the requirements of multistep stereoselective syntheses, and the logic and methodology of natural product total synthesis need to be translated and adapted to the methods and formats for the synthesis of compound collections. Recent developments in the synthesis of natural product inspired compound collections having carbocyclic and heterocyclic scaffolds highlight the fact that this goal can be successfully attained. The progress made has paved the way for the integration of natural product inspired compound collections into medicinal chemistry and chemical biology research.

Rhenium(VII) catalysis of Prins cyclization reactions

The rhenium(VII) complex O3ReOSiPh3 is a particularly effective catalyst for Prins cyclizations using aromatic and alpha,beta-unsaturated aldehydes. The reaction conditions are mild, and the highly substituted 4-hydroxytetrahydropyran products are formed stereoselectively. Rhenium(VII) complexes appear to spontaneously form esters with alcohols and to directly activate electron-rich alcohols for solvolysis. Re2O7 and perrhenic acid are equally effective in catalyzing these cyclizations.

Asymmetric synthesis of naturally occurring spiroketals

Spiroketals are widely found as substructures of many naturally occurring compounds from diverse sources including plants, animals as well as microbes. Naturally occurring spiroketals are biologically active and most of them are chiral molecules. This article aims at reviewing the asymmetric synthesis of biologically active spiroketals for last 10 years (1998-2007).

Stereoselective synthesis of allylic sulfones via the oxonia-Cope rearrangement of homoallylic alcohols containing a homoallylic sulfone moiety

The homoallylic alcohols 3 that can be prepared by the indium-mediated addition of haloallylic sulfones 1 to aldehydes 2 undergo the oxonia-Cope rearrangement with aldehydes 2 to give rise to the allylic sulfones 4 containing a conjugated diene moiety in a highly stereoselective manner. Electron-rich aldehydes preferentially participate in this oxonia-Cope rearrangement with the homoallylic alcohols 3. Excellent correlations of the stereochemistry (anti-3 to trans-allylic sulfone 4 and syn-3 to cis-allylic sulfone 4) have been observed in the oxonia-Cope rearrangement.

Diastereodivergent strategies for the synthesis of homochiral aculeatins

We report concise and stereocontrolled syntheses of aculeatins (-)-A, (+)-B, (+)-D, and (+)-6-epi-D. Diastereodivergent 1,3-inductions in Mukaiyama aldol coupling contribute to reduce steps and to increase flexibility with reactants having sterically restricted proximal substituents (i.e., CH2), involving either a good anti or a moderate syn 1,3-induction, depending on the nature of protecting group (P). In addition, the 3,5-syn-diol-ketone resulting from concomitant deprotection of the beta-alkoxy (Tr = trityl) group proves to be remarkably stable whereas the 3,5-anti diastereoisomer cyclizes spontaneously to the corresponding tetrahydropyran hemiketal, thus enabling a useful and facile separation. The second part of our study is devoted to improving the yield and the diastereoselectivity of the final phenolic oxidation reaction leading to aculeatins.

A highly enantio- and diastereoselective 1,3-dimethylallylation of aldehydes

A highly enantio- and diastereoselective pentenylation of aldehydes is described. The homoallylic alcohol derived from 1,3-dimethylallylation of (-)-menthone undergoes an efficient allyl-transfer reaction with a wide range of aliphatic aldehydes in the presence of an acid catalyst to give rise to the corresponding 4-methyl-2(E)-penten-4-yl-5-ol products in good yields with high enantio- and 4,5-syn-selectivities.