Single Stereodifferentiation Associated with Carbon Atom Insertion during the Oxonium Ion-Initiated Pinacol Rearrangement of Dihydrofuranyl and Dihydropyranyl Carbinols

Concise Large-Scale Synthesis of Tomatidine, A Potent Antibiotic Natural Product

Tomatidine has recently generated a lot of interest amongst the pharmacology, medicine, and biology fields of study, especially for its newfound activity as an antibiotic agent capable of targeting multiple strains of bacteria. In the light of its low natural abundance and high cost, an efficient and scalable multi-gram synthesis of tomatidine has been developed. This synthesis uses a Suzuki-Miyaura-type coupling reaction as a key step to graft an enantiopure F-ring side chain to the steroidal scaffold of the natural product, which was accessible from low-cost and commercially available diosgenin. A Lewis acid-mediated spiroketal opening followed by an azide substitution and reduction sequence is employed to generate the spiroaminoketal motif of the natural product. Overall, this synthesis produced 5.2 g in a single pass in 15 total steps and 15.2% yield using a methodology that is atom economical, scalable, and requires no flash chromatography purifications.

Total Synthesis of Fawcettimine-Type Alkaloid, Lycojaponicumin A

The efficient total synthesis of lycojaponicumin A (1) has been accomplished for the first time. The remarkable features of this novel strategy include the following: (1) rapid construction of tricyclic intermediate 4 through a regio- and stereoselective semipinacol ring expansion, which simplified the construction of rings A and B of 1; (2) the subsequent regio- and stereoselective formation of the highly strained rings C-E of 1 through a tandem oxa-hetero [3 + 2] cycloaddition/N-cycloalkylation.

Synthesis of α-oxygenated β,γ-unsaturated ketones by a catalytic rearrangement strategy

A straightforward two-step entry to α-oxgenated β,γ-unsaturated ketones from readily available α,β-unsaturated ketones is disclosed. It was found that bis(allylic) alcohols undergo a skeletal rearrangement in the presence of 1 mol% of cheap and non-corrosive p-toluenesulfonic acid. Computational studies were conducted to support the mechanism and to rationalise the influence of the catalyst acidity on the product selectivity.

Construction of quaternary stereocentres on carbohydrate scaffolds

This review describes a glimpse of the various strategies for constructing stereo-defined quaternary centres in densely functionalised carbohydrates moiety of structurally intriguing and biologically potent natural products and building blocks.

An unusually stable chlorophosphite: What makes BIFOP-Cl so robust against hydrolysis?

Two chlorophosphites, the biphenyl-based BIFOP-Cl and the diphenyl ether-based O-BIFOP-Cl, exhibit striking differences regarding their reaction with water. While BIFOP-Cl is nearly completely unreactive, its oxo-derivative O-BIFOP-Cl reacts instantly with water, yielding a tricyclic hydrocarbon unit after rearrangement. The analysis of the crystal structure of O-BIFOP-Cl and BIFOP-Cl revealed that the large steric demand of encapsulating fenchane units renders the phosphorus atom nearly inaccessible by nucleophilic reagents, but only for BIFOP-Cl. In addition to the steric effect, a hypervalent P(III)-O interaction as well as an electronic conjugation effect causes the high reactivity of O-BIFOP-Cl. A DFT study of the hydrolysis in BIFOP-Cl verifies a higher repulsive interaction to water and a decreased leaving tendency of the chloride nucleofuge, which is caused by the fenchane units. This high stability of BIFOP-Cl against nucleophiles supports its application as a chiral ligand, for example, in Pd catalysts.

A metal-free dyotropic-like rearrangement of 2-oxa allylic alcohols in the presence of organoboronic acids

The first example of a dyotropic-like rearrangement of 2-oxa allylic alcohols in the presence of catalytic amounts of Selectfluor and DABCO was reported, which provides a facile access to organoboronates. This reaction represents an unprecedented dyotropic rearrangement consisting of cleavage of two vicinal bonds (one C-C bond and one C=C bond).

Stereocontrolled synthesis and functionalization of cyclobutanes and cyclobutanones

In the last decade a certain number of new cyclobutane and cyclobutanone synthesis and functionalization protocols have been published. Organo- and biocatalyzed eco-friendly approaches to cyclobutane-containing molecules have been developed with interesting results. Also, successful new total synthesis of bioactive compounds and drugs have been recently reported where a four membered ring represented the key intermediate. Therefore, the rising interest in this field represents a great point of discussion for the scientific community, disclosing the synthetic potential of strained four membered ring carbocyclic compounds. Herein we report a critical survey on the literature concerning the enantiocontrolled synthesis and functionalization of cyclobutane derivatives, with particular attention to metal-free, low impact methodologies, published during the period 2000-2013.

Direct syntheses of spiro- and fused-hydrofurans by a tunable tandem semipinacol rearrangement/oxa-Michael addition protocol

A highly chemoselective one-pot reaction has been developed involving a tandem semipinacol rearrangement/oxa-Michael addition sequence in which the in situ generated ketol diene intermediate can be transformed specifically to either the spiro- or fused-dihydrofuran products (see scheme). This one-pot tandem reaction represents a general synthetic methodology for the syntheses of the two different kinds of furan derivatives.

Stereoselective synthesis of sugar fused β-disubstituted γ-butyro-lactones: C-spiro-glycosides from 1,2-cyclopropanecarboxylated sugars

A stereoselective methodology was developed for the construction of C-spiro-glycosides in two steps involving bromonium ion activated solvolytic ring opening of sugar derived 1,2-cyclopropanecarboxylates followed by a one-pot dehydrohalogenation, intramolecular hetero-Michael addition (IHMA) and ester hydrolysis. The obtained spirocyclic lactols were further converted to enantiomerically pure spirolactones.

Structural and stereochemical requirements of the spiroketal group of hippuristanol for antiproliferative activity

Hippuristanol is a natural product that has recently been shown to inhibit eukaryotic translation initiation and tumor cell proliferation. To investigate the structure and activity relationship of hippuristanol, we synthesized a series of analogs by expanding the size of its F ring and determined their effects on the proliferation of cancer cell lines. All changes to the F-ring of hippuristanol resulted in 3-fold to >100-fold decrease in activity.

Bromination of deactivated aromatics: a simple and efficient method

Highly deactivated aromatic compounds were smoothly monobrominated by treatment with N-bromosuccinimide (NBS) in concentrated H2SO4 medium affording the corresponding bromo derivatives in good yields. Mild reaction conditions and simple workup provides a practical and commercially viable route for the synthesis of bromo compounds of deactivated aromatics.

Synthesis and Lewis acid induced isomerization of mono-, di-, and tri-spiro α-keto tetrahydro-furans and -pyrans

The stereoselectivity of the acid-catalyzed ring expansion of dihydrofuranyl and dihydropyranyl 1,2-adducts to spirocyclic ketones 8 and 9 has been examined. These substrates have become readily available by the utilization of 1,3-dichloroacetone as a synthetic equivalent of cyclopropanone. The kinetically controlled isomerizations result in ring expansion to dispirocyclopentanones. Third-stage conversion to trispirocyclohexanones was shown to be possible in select examples. On a different front, the syn and anti pairs proved capable of interconversion by heating in the presence of boron trifluoride etherate. A push–pull fragmentation of the ketone ring is proposed to account for these epimerizations. Calculations have been performed to simulate transition state energies in the oxonium ion intermediates and to gain some idea of the preferred ground-state geometries of the final products.Key words: oxonium ion rearrangements, ring expansions, acid-catalyzed epimerizations, cyclopropanone equivalent, spirocyclic ethers.

Asymmetric total synthesis of complex marine natural products

Among nature's ecosystems, the marine environment has been an extremely rich source of structurally complex and biologically active molecules. This review aims to cover the recent developments in the synthesis of marine natural products, also reflecting the trend of their increased use to address biological questions. The examples chosen should be viewed as representative of the different structural motifs on the one hand and the strategies and stimuli for their synthesis on the other.

Synthesis of Functionalized 1-Azaspirocyclic Cyclopentanones Using Bronsted Acid or N-Bromosuccinimide Promoted Ring Expansions

Azaspirocyclic ring systems are present in a variety of alkaloids. Functionalized 1-azaspirocyclopentanones (6, 7, 11, 12) can be efficiently constructed through semipinacol ring expansion reactions of 2-(1-hydroxycyclobutyl)-p-toluenesulfonylenamides (4) promoted by either a Bronsted acid ((S)-(+)-10-camphorsulfonic acid or HCl) or N-bromosuccinimide, an electrophilic bromine source. Reactions promoted by N-bromosuccinimide tend to proceed in higher yields (80-95%) and with greater diastereoselectivity (3:1-1:0) compared to those reactions promoted by a Bronsted acid. In addition, N-bromosuccinimide promoted reactions can produce a complementary stereochemical outcome compared to the reactions using Bronsted acid.

Construction of azaspirocyclic ketones through alpha-hydroxyiminium ion or alpha-siloxy epoxide semipinacol rearrangements

[reaction: see text] Semipinacol-type rearrangements to produce azaspirocyclic ketones are presented. The yields and stereoselectivities of these reactions range from 67-94% yield and 2.8:1 to 1:0 diastereoselectivity, respectively.

1-Oxaspiro[4.4]nonan-6-ones. Synthetic access via oxonium ion technology, optical resolution, and conversion into enantiopure spirocyclic alpha,beta-butenolides

A general approach to the synthesis of enantiomerically pure spirocyclic alpha,beta-butenolides is presented where the fundamental framework is rapidly elaborated by acid- or bromonium ion-induced rearrangement of the carbinol derived by addition of 2-lithio-4,5-dihydrofuran to cyclobutanone. Subsequent resolution of the resulting ketones by either sulfoximine or mandelate acetal technology has been applied effectively. The availability of these building blocks makes possible in turn the acquisition of the enantiomers of dihydrofurans typified by 17, 35, and 38 and lactones such as 25 and 31, as well as the targeted title compounds. Complementary reductions of the early intermediates provide the added advantage that the alpha- and beta-stereoisomeric carbinol series can be obtained on demand. These capabilities have been coordinated to allow the crafting of any member of the series in relatively few steps.

An Enantioselective Ring Expansion Route Leading to Furanose and Pyranose Nucleosides Featuring Spirodiketopiperazines at the Anomeric Position

A study directed at the enantioselective synthesis of spirodiketopiperazine homologues of hydantocidin is described. Furanoid glycals, systems that are amenable to C-5 metalation in the presence of tert-butyllithium, are readily coupled to N-protected 2,3-azetidinediones provided that at least 1 equiv of BF(3).OEt(2) is present to curb enolization. The resulting 1:1 mixtures of carbinols undergo smooth ring expansion to spirocyclic keto amides when heated with pyridinium p-toluenesulfonate in benzene. 1,2-Acyl shifts operate exclusively. Since attempts to engage these products in Beckmann rearrangement proved singularly unsuccessful, recourse was alternatively made to new methodology based upon sequential Baeyer-Villiger oxidation and ammonolysis. The data show that the first of these steps occurs with exclusive migration of the quaternary carbon. Furthermore, nucleophilic attack by NH(3) can be directed regioselectively to the anomeric region. If heating is supplied during acid-promoted cyclization to the spirodiketopiperazines, spiropyranose derivatives are produced in a complementary process. The central issue of this synthesis effort was the utilization of 4-phenylseleno-substituted furanoid glycals so as to ultimately enable introduction of the cis-diol functionality at C-3 and C-4 (hydantocidin numbering).