Tandem [4 + 2]/[3 + 2] Cycloadditions of Nitroalkenes. 13. The Synthesis of (−)-Detoxinine

Recent advances in the total synthesis of polyhydroxylated alkaloids via chiral oxazines

This review summarizes recently established methodologies developed for the enantioselective and diastereoselective synthesis of chiral 1,3-oxazines. These compounds are of interest as advanced synthetic intermediates in the total synthesis of structurally complex and biologically active polyhydroxylated alkaloids such as (+)-1-deoxynojirimycin, (-)-anisomycin, (+)-castanospermine, (+)-casuarine, (-)-conduramine F-1, (-)-sphingofungin B, Neu5Ac methyl ester, and other natural products. The devised synthetic approach aims to offer a direct, efficient, and adaptable method for obtaining both pure enantiomers and pure diastereomers. It revolves around utilizing chiral building blocks like syn,syn-, syn,syn,anti-, syn,anti-, syn,anti,syn-, anti,syn-, anti,syn,syn-, and anti,syn,anti-oxazines. By integrating oxazine chemistry with established and innovative transformations, this approach enabled the synthesis of 30 polyhydroxylated amines across various studies conducted between 2007 and 2022.

[3+3]-Annulation of Cyclic Nitronates with Vinyl Diazoacetates: Diastereoselective Synthesis of Partially Saturated [1,2]Oxazino[2,3-b][1,2]oxazines and Their Base-Promoted Ring Contraction to Pyrrolo[1,2-b][1,2]oxazine Derivatives

A rhodium(II)-catalyzed reaction of cyclic nitronates (5,6-dihydro-4H-1,2-oxazine N-oxides) with vinyl diazoacetates proceeds as a [3+3]-annulation producing bicyclic unsaturated nitroso acetals (4a,5,6,7-tetrahydro-2H-[1,2]oxazino[2,3-b][1,2]oxazines). Optimization of reaction conditions revealed the use of Rh(II) octanoate as the preferred catalyst in THF at room temperature, which allows the preparation of target products in good yields and excellent diastereoselectivity. Under basic conditions, namely, the combined action of DBU and alcohol, these nitroso acetals undergo ring contraction of an unsaturated oxazine ring into the corresponding pyrrole. Both transformations can be performed in a one-pot fashion, thus constituting a quick approach to oxazine-annulated pyrroles from available starting materials, such as nitroalkenes, olefins, and diazo compounds.

Fluoronitroalkenes in tandem [4 + 1]/[3 + 2]-cycloaddition: one-pot three-component assembly of fluorinated bicyclic nitroso acetals

Novel fluorinated bicyclic nitroso acetals were obtained in a highly regio- and diastereo-selective manner via tandem [4 + 1]/[3 + 2]-cycloaddition of fluoronitroalkenes.

Complete stereodivergence in the synthesis of 2-amino-1,3-diols from allenes

Herein, we describe studies to understand how both reagent and substrate control can be effectively employed in the stereodivergent oxidative amination of allenes, with transfer of the axial chirality of an enantioenriched precursor to point chirality in each possible diastereomeric 2-amino-1,3-diol product.

Tandem 6pi-electrocyclization and cycloaddition of nitrodienes to yield multicyclic nitroso acetals

Upon heating, nitrodienes rearrange through 6pi-electrocyclization to form nitronate intermediates, which can be captured through tandem [3 + 2] dipolar cycloadditions to form highly functionalized nitroso acetals. The one-pot, two-step domino process is highly efficient, proceeding with good facial selectivity and exoselectivity. Dipolarophiles featuring electron-rich, -neutral, and -deficient carbon-carbon double bonds are viable substrates for [3 + 2] cycloadditions with the in situ generated nitronates. In addition, the highly functionalized nitroso acetal products can be hydrogenolyzed selectively to form densely functionalized spirocyclic hydroxy amides or hydroxy gamma-amino acids.

The Diels-Alder reaction of 4,6-dinitrobenzofuroxan with 1-trimethylsilyloxybuta-1,3-diene: a case example of a stepwise cycloaddition

The reaction of 4,6-dinitrobenzofuroxan (DNBF) with 1-trimethylsilyloxybuta-1,3-diene (8) is shown to afford a mixture of [2+4] diastereomeric cycloadducts (10, 11) through stepwise addition-cyclization pathways. Zwitterionic intermediate sigma-adduct 9, which is involved in the processes, has been successfully characterized by (1)H and (13)C NMR spectroscopy and UV/visible spectrophotometry in acetonitrile. A kinetic study has been carried out in this solvent that revealed that the rate of formation of 9 nicely fits the three-parameter equation log k=s(E+N) developed by Mayr to describe the feasibility of nucleophile-electrophile combinations. This significantly adds to the NMR spectroscopic evidence that the overall cycloadditions take place through a stepwise mechanism. The reaction has also been studied in dichloromethane and toluene. In these less polar solvents, the stability of 9 is not sufficient to allow direct characterization by spectroscopic methods, but a kinetic investigation supports the view that stepwise processes are still operating. An informative comparison of our reaction with previous interactions firmly identified as prototype stepwise cycloadditions is made on the basis of the global electrophilicity index, omega, defined by Parr within the density functional theory, and highlighted by Domingo et al. as a powerful tool for understanding Diels-Alder reactions.

Superelectrophilicity of the Nitroolefinic Fragment of 4-Nitrobenzodifuroxan in Michael-Type Reactions with Indoles: A Kinetic Study in Acetonitrile

The kinetics of the coupling of 4-nitrobenzodifuroxan (NBDF) with a series of indoles 8 a-e to give the expected Michael-type adducts 9 a-e have been investigated in acetonitrile solution. No significant influence of the nature of the isotopic substitution at C-3 of the indole ring has been found, indicating that the NBDF addition step is the rate limiting step of the SEAr substitution of the indole moiety. This implies that the measured second-order rate constants (k) for the reactions are identical to the second order rate constants (k1NBDF) associated to the C--C coupling step. By using the known N and s parameters characterizing the nucleophilicity of indoles, the k1NBDF rate constants are found to fit nicely to the three parameters equation logk1=s(N+E) introduced by Mayr to describe the feasibility of nucleophilic-electrophilic combinations. Based on this, the electrophilicity parameter E of NBDF could be determined as E=-6.15. This corresponds to a positioning of the reactivity of the nitroactivated double bond of NBDF in the domain of superelectrophilicity previously defined for nitrobenzofuroxans, in accord with the finding that the rates of coupling of 8 a-e with NBDF are only one order of magnitude lower than those for the coupling of these indoles with 4,6-dinitrobenzofuroxan (DNBF). The theoretical scale of electrophilicity introduced by Domingo et al. on the basis of the global electrophilicity index omega defined by Parr is also a very useful tool to discuss the relative reactivities of NBDF, DNBF, and a number of differently activated C==C double bonds.

Tandem double intramolecular [4+2]/[3+2] cycloadditions of nitroalkenes. The fused/bridged mode

A new class of tandem [4+2]/[3+2] cycloadditions of nitroalkenes is described in which both pericyclic processes are intramolecular. Two subclasses of intra [4+2]/intra [3+2] cycloadditions have been explored in which the dipolarophile is tethered at either C(5) or C(6) of the nitronate. For both families of precursors, the cycloadditions occur in good yield and are found to be highly regio- and stereoselective. This method converts linear polyenes to functionalized polycyclic systems bearing up to six stereogenic centers.

Novel rearrangements of 4-silyl-3-buten-2-ones

Two 4-silyl-3-buten-2-ones, 2a and 2c, underwent an interesting rearrangement involving migration of the allyl or phenyl group on the silicon atom to the adjacent enone carbon when treated with various bases.

Tandem double intramolecular [4 + 2]/[3 + 2] cycloadditions of nitroalkenes

[reaction: see text]. A new class of tandem [4 + 2]/[3 + 2] cycloadditions of nitroalkenes is described in which both pericyclic processes are intramolecular. Two subclasses of intra [4 + 2]/intra [3 + 2] cycloadditions have been explored in which the dipolarophile is tethered at either C(5) or C(6) of the nitronate. For both families of precursors, the cycloadditions occur in good yield and are found to be highly regio- and stereoselective. This method converts linear polyenes to functionalized polycyclic systems bearing up to six stereogenic centers.

[4 + 2] cycloadditions of nitroalkenes in water. Highly asymmetric synthesis of functionalized nitronates

The [4 + 2] cycloadditions of (E)-2-aryl-1-cyano-1-nitroalkenes 1 with achiral and enantiopure vinyl ethers 2 and 3 carried out in sole water are reported. These reactions occur in a heterogeneous phase under mild conditions and are fast and highly stereoselective. By using (-)-N,N-dicyclohexyl-(1S)-isoborneol-10-sulfonamide as a chiral auxiliary, the cycloadditions are totally asymmetric. The face selectivity is discussed in terms of the shape of the chiral auxiliary and the reactive conformation of vinyl ether.

Synthesis of (+)-1-epiaustraline

A highly efficient total synthesis of (+)-1-epiaustraline ((+)-1), a tetrahydroxypyrrolizidine alkaloid of the alexine/australine subclass, is described. The key step is a tandem intramolecular [4 + 2]/intermolecular [3 + 2] nitroalkene cycloaddition involving dienylsilyloxy nitroalkene 3 and chiral vinyl ether 4, which establishes four of the five stereocenters present. The final center was installed by a diastereoselective dihydroxylation. Hydrogenolytic unmasking of the nitroso acetal tosylate 17 containing the silyl ether linkage was thwarted by a slow alkylation and an undesired Peterson-type elimination. Prior removal of the silicon moiety by Tamao-Fleming oxidation proceeded in excellent yield and provided a substrate suitable for hydrogenolysis and deprotection. The complete synthesis required only 10 steps to deliver the (+)-1-epiaustraline in 7.0% overall yield.

Dual behavior of 4-aza-6-nitrobenzofuroxan. A powerful electrophile in hydration and sigma-complex formation and a potential dienophile or heterodiene in diels-alder type reactions

In investigating the reactivities of aza analogues of super-electrophile 4,6-dinitrobenzofuroxan (DNBF, 1), we have found that the nitro-substituted pyridofuroxan 2 gives a remarkably stable hydrate 3 in aqueous solution (as evidenced by the requirement of ca. 50% H(2)SO(4) (H(0) approximately -3) for complete recovery of 2). The equilibrium constant K(H)()2(O) for hydration of 2 is estimated to be >/= 100, being comparable only with the K(H)()2(O) values reported for hydration of highly activated neutral polyazaaromatics such as 2- and 6-hydroxypteridines or 7-azapteridine. Interestingly, the NH group of 3 undergoes ionization at rather low pH (p = 5.79), affording an anionic hydroxy sigma-adduct 4 which is thermodynamically 10(8) times more stable than the related sigma-adduct of pteridine. The experimental evidence is that 4 is slightly more stable than the hydroxy sigma-adduct of DNBF, indicating not only that 2 ranks among the most electrophilic heteroaromatics known to date but also than an aza group may in fact be as efficient as a nitro group in promoting sigma-complex formation. 2 is also found to be a versatile Diels-Alder reagent, as a result of the low aromatic character of its six-membered ring. Upon treatment of 2 with cyclopentadiene and 2,3-dimethylbutadiene, various reactivity patterns have been observed. These led to different cycloadducts arising from normal as well as inverse electron-demand condensations involving the pyridine ring as the dienophile or the heterodiene contributor. Altogether, the results reveal major differences between the reactivity of 2 and that of DNBF, with in particular a remarkable tendency of the pyridofuroxan adducts to undergo covalent hydration, resulting in the formation of stable carbinolamines. Also noteworthy is the characterization of a diadduct which results from a Diels-Alder trapping of the o-dinitroso intermediate involved in the exchange of the 1-oxide and 3-oxide tautomers of 2.

Synthesis of (-)-7-epiaustraline and (-)-1-epicastanospermine

Highly efficient and selective syntheses of the title compounds are described. The cornerstone of the synthetic plan is the tandem inter [4 + 2]/inter [3 + 2] cycloaddition process. These syntheses differ from previous applications of this strategy in that they incorporate an alkylation in the hydrogenolysis step to close the second ring of the azabicyclic systems. Notable features of the sequence are (1) the highly regio- and stereoselective [3 + 2] cycloaddition of nitronate 15 with siloxymethyl (Z)-beta-silylvinyl ketone (Z)-22b and (2) the highly selective reduction of the resulting ketone 24a with L-Selectride. A single-crystal X-ray structure analysis of synthetic (-)-7-epiaustraline confirmed that the targeted structure was successfully synthesized. This stimulated a reexamination of the structural assignment of the natural product. (-)-1-Epicastanospermine was synthesized in four steps from the common intermediate 27a. The absolute configuration of (-)-1-epicastanospermine was assured by single-crystal X-ray structure analysis of intermediate (-)-27a. Thus, the sign of the optical rotation had to be revised. The overall efficiency of these syntheses were 9 steps and 23% yield for (-)-7-epiaustraline and 10 steps and 20% yield for (-)-1-epicastanospermine

Tandem Cycloaddition Chemistry of Nitroalkenes: Probing the Remarkable Stereochemical Influence of the Lewis Acid

The influence of several Lewis acids on the stereochemical course of the [4 + 2] cycloaddition of nitroalkene 1 and chiral, nonracemic propenyl ether 8 has been examined. All of the Lewis acids examined favored ul relative diastereoselection ("exo" approach); TiCl(4), TiBr(3)(Oi-Pr), SnCl(4), and ATPh were the most selective. Within the titanium-based Lewis acids, it was found that increasing the halide-to-alkoxide ratio increased the degree of ul (relative) selectivity, as did switching from chloride to bromide. The internal diastereoselectivity was also dependent on the Lewis acid; most titanium isopropoxide-halides (bromide and chloride) and SnCl(4) were highly selective for (1,3-lk) approach, with the selectivity increasing with increasing halide content. Two aluminum-based Lewis acids (MAPh and ATPh) were selective for the opposite sense of internal diastereoselection. The high lk (relative) diastereoselectivity observed only with TiCl(2)(Oi-Pr)(2) is proposed to arise either from Coulombic stabilization of an endo approach or precomplexation of the vinyl ether to the Lewis acid. The switch in internal diastereoselectivity seen in the exo manifold is thought to arise from subtle changes in the steric nature of the Lewis acid-nitroalkene complex.

Tandem Inter [4 + 2]/Intra [3 + 2] Cycloadditions of Nitroalkenes. The Bridged Mode (beta-Tether)

A new variation of the tandem inter [4 + 2]/intra [3 + 2] cycloaddition of nitroalkenes has been developed. This method involves the Lewis acid-promoted [4 + 2] cycloaddition of nitro olefins 12 with 1-alkoxy-1,4-dienes 3. The resulting nitronates 13, bearing a C(5) tethered dipolarophile, undergo thermal, intramolecular [3 + 2] cycloaddition to afford stable tricyclic nitroso acetals 14, which can be subsequently reduced to unveil a new carbocycle 16. Thus, in three steps, highly functionalized aminocyclopentanes can be stereoselectively constructed in high yield.

Tandem Inter [4 + 2]/Intra [3 + 2] Cycloadditions of Nitroalkenes. Asymmetric Synthesis of Highly Functionalized Aminocyclopentanes Using the Bridged Mode (beta-Tether) Process

An asymmetric, tandem inter [4 + 2]/intra [3 + 2] bridged mode (beta-tether) cycloaddition of nitroalkenes has been developed. This new sequence involves the Lewis acid-promoted [4 + 2] cycloaddition of nitro olefins with enantiopure 1-alkoxy-1,4-dienes. The resulting nitronates, bearing a C(5) tethered dipolarophile, undergo thermal, intramolecular [3 + 2] cycloaddition to afford stable tricyclic nitroso acetals, which can be subsequently reduced to provide interesting aminocyclopentanes. Thus, in three steps, highly functionalized, enantiomerically enriched aminocyclopentanes can be constructed with good yield and high ee. Additionally, the Lewis acid was found to impart a remarkable influence on the stereochemical outcome of the [4 + 2] cycloaddition.