RICERCA ITALIANA     

CYCLOADDITIONS WITH REDUCED ENVIRONMENTAL IMPACT FOR THE SYNTHESIS OF PRODUCTS OF BIOLOGICAL INTEREST

Università degli Studi di Firenze

Abstract

This project is submittet by five research units whose participants are all components of the "Centro Interuniversitario sulle Reazioni Pericicliche" (http://www.unifi.it/cirp/).
It is inserted in one of the top areas of the organic chemistry, since the pericyclic reactions are one of the best ways towards a stereocontrolled synthesis of products of biological interest. This goal is not the only scope of the project. The intent is to set up new synthetic methodologies involving environmental friendly processes, both by acting on the reaction medium (e.g. ionic solvents, melted sugars) and through the development of reaction conditions (e.g. biocatalysis, microwaves, ultrasound, solar energy) able to minimize the waste release. This will be achieved by setting new processes applying the "atom economy" concept in order to exploit enhancing of stereo-selectivities. The common and main target is the preparation of modified carbohydrates interesting in themselves or as synthons for the synthesis pseudonucleosides. The cycloaddition reactions, both homo- or hetero-Diels Alder and 1,3-dipolar cycloaddition, will be the main synthetic tools for the preparation of reagents or or key intermediates. Using these synthetic tools the control of the regiofunctionalization and of the stereochemistry of different stereocenters will be assurded.
Modified Carbohydrates
Different classes of modified carbohydrates will be prepared in enantiopure form through known or original protocols. Sulfinyl sugar, glycosyl heterocyles, iminosugars and pseudo (1-2)-linked imino-C-disaccharides which are of biological interest are going to be obtained:
- thioglycosides, where the six-membered rings are connected by a sulfinyl spacer will be prepared through [4+2] cycloaddition of enantiopure glycosylsulfinyl-dienes and -dienophiles, as reaction partners.
- glycosyl heterocycles will be synthesized through the [4+2] cycloaddition, by dye-sensitized photooxygenation, of singlet oxygen and furans substituted with monosaccharides. The structural elaboration of the cycloadducts will provide the direct access to functionalized residues, that can be used as starting material in the synthesis of potentially bioactive glycosides.
- The stereoselective synthesis of unnatural iminosugars containing indolizinine structure and pseudo (1-2)-linked imino-C-disaccharides by intra – and intermolecular 1,3-dipolar cycloaddition of chiral hydroxypyrroline N-oxides.
Pseudo-nucleosides
Modified nucleosides are of general interest because are characterized by a better metabolic stability. In the preparation of such products the RU are particularly interested in the modification of the sugar unit. Both carbocyclic, thiocarbocyclic and azacarbocyclic sugars characterized by a different substitution pattern will be prepared. According to the nucleoside target, different synthetic strategies will be developed. By way of cycloaddition reactions pseudosugar rings will be prepared. Alternatively, by using a dipolarophile functionalized with the suitable base, the 1,3-dipolar cycloaddition reaction will give directly the modified nucleoside. Moreover, the cycloaddition reactions are well suited to allow the diastereo- and enantioselective formation of the desired products.
The pursuit of reduction of environmental impact in cycloadditions is an essential feature of this project. In particular, some classical processes, such as the synthesis of nitrile oxides from nitroalkanes, will be revisited in eco-friendly reaction conditions. These innovative protocols will be put at all RU's disposal to be applied for the preparation of substrates involved in the synthesis of targets of biological interest. Because of the homogeneous design and complementarity of the expertises, methodologies and contributes, a synergic effect is believed to occur. <<<

Principal Investigator

Francesco DE SARLO Università degli Studi di FIRENZE

Research Objectives

Objective of this project is the development of either PRODUCTS and/or PROCESSES by means of pericyclic reactions. The synthesis of products will be focused on targets appealing for various applications in pharmaceutical or medicinal chemistry. The research program is characterized by, as the main target, the synthesis of modified carbohydrates interesting in themselves or for the synthesis of other classes of compounds such as pseudonucleosides. The cycloaddition reactions (CR), both Diels Alder (DA) and hetero-DA and 1,3-dipolar cycloaddition (DC), will be the main synthetic tools for the preparation of reagents or key intermediates. One of the main feature of these reactions is their stereoselectivity so it will be possible to control the stereochemistry of several stereocenters, plan the formation of different diastereomers, utilize chiral synthons allowing to prepare enantiopure compounds. The whole project will be developed devoting considerable efforts to find out environmental benign processes. With this aim, the PROCESSES adopted include the use of operating methods reducing the environmental impact, such as: reduction of the number of steps in the preparation of precursors and intermediates; use of catalytic reactions, including biocatalysis; reduction or improvement of separation and purification steps, achieved in several ways: i) solid phase synthesis; ii) use of ionic solvents and other reaction media (e.g. melted carbohydrates), iii) use of microwaves, ultrasound, solar energy.
Objective: PRODUCTS
Modified carbohydrates
Different classes of modified carbohydrates will be prepared through known or original protocols in enantiopure form. RU I, II, IV are involved in the preparation of sulfinyl sugar and the corresponding polysaccharides, glycosyl heterocyles, iminosugars and (1-2)-linked pseudo imino-C-disaccharides which are of biological interest. The obtainment of more or less complex thioglycosides, depending on the starting materials, where the six-membered rings are connected by a sulfinyl spacer is planned by RU II. The synthesis requires the preparation of enantiopure glycosylsulfinyl-dienes and -dienophiles, reaction partners in DA reaction. The synthesis of glycosyl heterocycles is the main goal of RU IV. These glycosides are characterized by residues, at the anomeric carbon, which cannot be used as acceptors in the usual coupling reactions. This goal will be achieved through the DA reaction, by dye-sensitized photooxygenation, of singlet oxygen and furans substituted with monosaccharides. Particularly, the elaboration of the cycloadducts will provide the direct access to functionalized residues to be used as starting material for the synthesis of potentially bioactive glycosides. The stereoselective synthesis of “iminosugars” will be the target of UR I. The synthesis of polyhydroxyindolizidines in enantiopure form by intramolecular 1,3-DC of the pyrroline N-oxide will be developed. The condensation of crotonic acid derivatives with the above compound affords proper substrates for the intramolecular 1,3-DC. This approach gives access to products in both the enantiomeric forms but requires the nitrone function to be protected before the esterification reaction. The RU I will study this protection using a solid-phase supported acrylate. After esterification the protected nitrone will be irradiated using microwave (MW) to promote the cycloreversion (acrylate elimination) followed by intramolecular cycloaddition. The synthesis of (1-2)-linked pseudo imino-C-disaccharides, in which the iminosugar is directly linked at C-2 of monosaccharides is also an objective of RU I. Attempts will be made to improve the results of the 1,3-DC of chiral pirroline N-oxides and glycals, by using MW and avoiding excess of reagent. The refined reaction conditions will be applied to the "parallel kinetic resolution" (PKR) of both racemic nitrones and dipolarophiles, employing "quasienantiomers", enantiomers differentiated by anchoring one of them to a solid phase, as resolving agents, using the "phase tag" technique (solid phase linked nitrones or dipolarophiles).
Modified nucleosides
Modified nucleosides are of general interest because they are characterized by a better metabolic stability. RU II, III, V are involved in their preparation exploiting different approaches. The CR are well suited to allow the diastereo- enantioselective formation of the products. Both carbocyclic, thiocarbocyclic and azacarbocyclic sugars characterized by a different substitution pattern will be prepared. The project of RU II is based on the synthesis of dipoles carrying the nucleobases followed by 1,3-DC with selected dipolarophiles for the construction of the glycosyl unit. The synthesis of modified nucleosides as potential antiviral agents will be achieved by RU V through 1,3-DC of nitrile oxides to 2-oxa-3-aza- and 2-aza-norbornene, followed by the elaboration of the above cycloadducts. The use of nitrosocarbonyl compounds and allyl alcohols in hetero-DA reaction will allow to obtain new nucleosides. Other synthetic approaches are planned to replace the ribose ring with a polyfunctionalized carbocyclic ring from readily available and cheap enantiopure sugars. Furthermore, thionucleosides will be synthesised containing the heterocyclic base and the hydroxymethylene group in 3 and 4 positions, respectively. The direct synthesis of the tetrahydrotiophene ring containing the base should be achieved also by cycloaddition of a thiocarbonyl ylide to a vinylnucleobase. RU III is planning the synthesis of pseudonucleosides as synthons for the preparation of carbocyclic peptidylnucleosides with potential antifungal activity. The research will be focused on the preparation of i) known carbocyclic AA using innovatory methodology (melted carbohydrates, see below) ii) carbocyclic AA functionalized with hydroxy and amino groups, mimetics of carbohydrates iii) carbocyclic pseudonucleosides functionalized with an AA group which could be used for the preparation of peptidylnucleosides iv) polyhydroxylated AA functionalized with a non aromatic heterocyclic ring.
Objective: PROCESSES
Some classical processes will be revisited in eco-friendly reaction conditions. These innovative protocols will be putted at all RU’s disposal for the preparation of substrates involved in the synthesis of targets of biological interest. The well known nitrile oxides preparation will be revisited by RU I, in view of a straightforward process: reduced number of steps, milder reagents in catalytic amount starting from different nitrocompounds. The RU III will develop the use in CR of an innovative reaction medium represented by melted sugars. This new reaction medium comes from renewable sources, is chiral, cheap and eco-friendly. This methodology will be tested on the preparation of norbornene amino acids, through DA reaction, and will be associated to the use of MW. In addition the project will be related to the possibility of using microbic cells in order to modify functional groups of cycloadducts favouring the separation of the different stereomers. The preparation of supported dipole will be planned and studied by RU I and V. The solid phase syntheses will be applied to the nucleosidic syntheses through hetero DA reactions by RU V: nitrosocarbonyls will be obtained by photochemical degradation of supported nitrileoxides which will be reacted with allylic alcohol. The photolysis of 1,2,4-oxadiazole-4-oxides on a solid support will be evaluated as a more environmental compatible synthetic route to nitrosocarbonyls. As mentioned before, solid-phase supported reagents will be applied by UR I to PKR and to the protection of a chiral nitrone, using MW irradiation to promote the cycloaddition and cycloreversion steps. <<<

Timescale

24 months

National and international background

Cycloadditions are a powerful tool in synthetic Organic Chemistry leading to heterocyclic or carbocyclic compounds from open-chain precursors in a straightforward process. This method intrinsically offers the maximum of atom economy; often the stereoselectivity can be controlled owing to the concerted nature of this kind of reactions and high regioselectivities are very common. The cycloadduct can then be employed for further elaboration, according to well established protocols. The RU offer a large experience in cycloaddition chemistry and the research program concerns the use of cycloadditions for the synthesis of polyhydroxylated compounds related to modified sugars and nucleosides. This approach not only offers excellent opportunities for stereoselective syntheses, but the project, as indicated in the title, is also focused on the development of processes with reduced environmental impact. The present research project is submitted by five research units whose participants are all components of the "Centro Interuniversitario sulle Reazioni Pericicliche" (http://www.unifi.it/cirp/). This centre was constituted in 1992 among research groups of ten italian Universities in the aim of supporting the scientific exchanges among researchers involved in studying pericyclic reactions and their synthetic applications. Every two years the "Centro Interuniversitario" organises a scientific meeting for discussing the more relevant results recently obtained.The scientific background will be illustrated according to the classification of the CYCLOADDITION REACTIONS employed in the program and the PROCESSES planned for the reduction of environmental impact.
CYCLOADDITION REACTIONS
DIELS-ALDER REACTIONS INVOLVING ENANTIOPURE SULFINYL DIENES AND/OR DIENOPHILES (RU II). RU II recently described a diasteoselective synthesis of glycoconjugates, via the Diels-Alder (DA) reactions of enantiopure sulfinyl dienophiles with suitable dienes.[1] The significant stereo differentiating ability of the sulfinyl function, its easy elaboration or removal, stimulated the development of a method for the introduction of the sulfoxide group into various molecules. The RU developed a method based on the syn-addition of transient sulfenic acids to suitable unsaturations, that gives access to substrates possessing a sulfinyl moiety. Thus, enantiopure sulfinyl dienes are obtained by addition of enantiopure sulfenic acids onto the triple bond of enynes. Their DA cycloadditions with dienophiles furnished cycloadducts with good stereocontrol and high functionalisationsome of them being transformed into compounds with a naturally occuring skeleton.[2] A study on hetero-cycloadditions of enantiopure sulfinyl dienes has been performed with oxa, thia, and azadienophiles.[3] Sulfinyl imines[4] and alpha,beta-unsaturated sulfinyl ketones [5] have been used as heterodienes in normal or inverse electron-demanding DA reactions. From L-cysteine and alpha or beta-D-1-thioglucopyranoses sulfenic acids and chiral sulfoxides were produceded.[1,6]1, The generation of anomeric glycosulfenic acids from suitable precursors, obtainable by green procedures,[7] opens the way towards complex pseudo-sugars, functionalised with sulfoxide. The use of unnatural carbohydrates hass become essential in the study of enzyme-substrate interactions in glycobiology.[8] Thus the obtainment of 1-thio-pseudo-sugars represents a stimulating synthetic problem.
DIELS-ALDER REACTIONS OF ACRYLATES AND CYCLOPENTADIENE (RU III). RU III has a great experience in the field of cycloaddition reactions as a tool for heterocyclic synthesis[9] and, more recently, for the synthesis of carbocyclic amino acids by stereocontrolled ways.[10] Among these, the synthesis of alpha-aminocyclopropane-, alpha-aminocyclohexene- and alpha-aminonorbornencarboxylic acids, functionalised with an SR or OH beta-group as cysteine and serine mimetics, should be mentioned. Going on the interest in this field, the RU III plans to obtain polyhydroxylated carbocyclic and acyclic AA in diastereo- and enantioselective form carrying out cycloadditions for the preparation of starting materials under eco-friendly conditions (see below). The field of carbohydrate mimetic AA is of growing interest due to their high metabolic stability.[10,11]10, The RU plans the synthesis of hydroxylated-cyclopentylglycines, functionalised with an amino group, used as precursors for the synthesis of pseudo-nucleosides functionalized with new pyrimidine residues. The development of a synthetic process aiming to the formation of new carbocyclic nucleosides is of outstanding importance due to their metabolic stability.[12] In addition they could be used for the preparation of carbocyclic peptidylnucleosides, analogues of the well-known antifungal[13] peptidylnucleosides nykkomicin and polyoxin. Polyhydroxylated, non cyclic AA, as interesting biological targets, will be also prepared.[14] Fragments of mono-, di-, tri- or polyhydroxylated AA are present in many natural compounds.[15] The same AA could also be considered as building blocks for the asymmetric synthesis of several molecules such as carbohydrates.[16] By the way of the competence of our RU in this field, new and original processes will be studied.[17]
NITROSOCARBONYLS AND DIENES (RU V).The studies on the generation and reactivity of nitrosocarbonyls have been extensively pursued by RU V towards the syntheses of biological active targets.[18] Nitrosocarbonyls are fleeting intermediates, highly reactive as dienophiles or enophile for the preparation of 2,3-oxazines[19] and unsaturated hydroxylamines.[20] A useful alternative to the known procedure for their generation has been proposed through the mild oxidation with tertiary amine N-oxides of nitrile oxides,[21] whose chemistry has been an everlasting topic in cycloadditions.[22] The nitrosocarbonyls are also obtained in the mildest conditions through the cycloreversion of 1,2,4-oxadiazole-4-oxides.[23,24] These methods found applications in the syntheses of compounds of biological interest.[25] The hetero DA cycloadducts of nitrosocarbonyl intermediates can undergo useful synthetic elaborations.[26] Additions to the C=C double bond and the mild reduction of the N-O bond with amalgams can afford isoxazolinic stereo-defined aminols.[27] Starting from these, a method for the linear construction of carbocyclic nucleosides has been developed, affording either purine and pyrimidine type derivatives. Thus, the protocol shows the way to transform cyclopentadiene into carbocyclic nucleosides characterised by an isoxazoline ring fused to the cyclopentane moiety as well as a secondary hydroxy group as distinctive elements with respect to other similar compounds.[28]
SINGLET OXYGEN AND FURAN DERIVATIVES (RU IV). Singlet Oxygen is a very reactive electrophile which can be used to introduce oxygenated functions into organic substrates.[29] and can be efficiently generated by dye-sensitised photooxygenation. This method which use oxygen, solar light and a natural non tossic dye is inexpensive and eco-friedly. The [4+2] cycloaddition reactions of singlet oxygen to aromatic heterocycles are well-known.[30] and the reaction of functionalised furans has been extensively explored.[31] Indeed, it provides both the access to novel compound classes and a more friendly approach to known products. Moreover, the use of 2-methoxyfurans unsubstituted at C4 in the reaction provides a further and useful means to generate carbonyl oxides, 1,3-dipoles which have been used in trapping reactions [32] and in [3+2] cycloadditions to a variety of unsaturated systems.[33] The straightforward of the process, as well as the high stereoselectivity and the good yields, suggest the application of the procedure for the synthesis of functionalised carbohydrates. In this context, the research is directed to the development of novel approaches to natural glycosides.[34] Particular attention is devoted to natural and synthetic C-glycosides, since they could display biological properties being at the same time more stable than their O- and N-analogues for the presence of a C-C bond at the anomeric center.
CHIRAL NITRONES (RU I) Recently, a versatile synthetic approach to enantiopure polyhydroxylated azaeterocycles has been developed.[35] The method, that employs highly stereoselective inter- and intramolecular 1,3-DC of chiral hydroxypyrroline N-oxides followed by the elaboration of the adducts, is particularly appealing since offers the selective generation of stereochemically distinct products starting from common chiral-pool compounds. With this methodology, several natural iminosugars and non natural analogues have been synthesised.[36] The same strategy afforded (12)-linked pseudo imino-C-disaccharides when glycals were used as dipolarophiles.[37] Anyway, this strategy cannot be applied to poorly reactive dipolarophiles. In these cases the severe reaction conditions cause nitrone decomposition and low yields of adducts are obtained. A promising means to solve the stability and reactivity problems in the 1,3-DC is the use of MW irradiation. Several research groups have proved that MW causes a dramatic acceleration of reaction rates and higher yields and purity of the resulting products when compared with conventional heating.[38] To this end, the scope of applications of MW is very extended and concerns a wide spectrum of organic reactions including cycloadditions. However, up to now, the effect of MW on cycloaddition and cycloreversion of chiral hydroxypyrroline N-oxides has not been reported. INTRAMOLECULAR CYCLOADDITIONS OF NITRONES AND NITRILE OXIDES. CARBONYL YLIDES AND ENONES (RU V).The diastereo- and enantio-selective syntheses of modified N,O-nucleosides have been carried on by privileging the potentialities offered by inter- and intra-molecular 1,3-dipolar cycloadditions.[39] Suitable substrates are polihydroxylated enantiomerically pure cyclopentanes[40,41] , which can be easily obtained from monosaccharides by opening of the sugar ring according to the Vasella method.[42] Aldehydes are smoothly converted into the corresponding oximes which are the direct precursors of isoxazolidinic or isoxazolinic bicyclic systems. The insertion of the nucleobases[39,43]39, is based on different approaches according to the nucleoside target: the presence of the amino group allows the direct construction of pyrimidine or purine rings.[44] the presence of hydroxy groups allow to insert the nucleobase.[45,46] , The replacement of the ring oxygen atom with the sulfur atom affords thionucleoside analogues, with various and potent biological activities[47-53] , , , , , , and an enhanced stability of carbon-sulfur bond with respect to other known thionucleosides.[54,55] , To perform these processes, the use of microwaves heating and environmentally sustainable approaches such as solid phase syntheses or ionic solvents as well as benign and recyclable catalysts can ensure higher reaction rates, better yields and selectivities.
SYNTHESIS AND REACTIVITY OF NOVEL NITRILE IMINES (RU II). RU II has conducted studies on 1,3-dipolar cycloadditions with new C-Br and C-H nitrilimine dipoles leading to the obtainment of spirobarbiturates, with a complete control of regioselectivity of the process and smooth conditions of reaction.[56] An original procedure has been set up, in absence of any solvent, for the obtainment of pyrrole derivatives, from enol-carbonyl substrates and mesoionic systems, such as 5(4H)-oxazolones. These compounds are useful tools in 1,3-DC for their versatile reactivity strictly connected with the possibility of obtaining structures incorporating an aminoacid function.[57] MW-assisted cycloadditions of mesoionic 1,3-oxazole-5-olates onto acrylic esters have been successfully accomplished, leading to pyrrole derivatives in one step, excellent yields and high regioselectivity.[58]
PROCESSES
NITRILE OXIDES BY DEHYDRATION OF PRIMARY NITROCOMPOUNDS(RU I). Nitrile oxides are key intermediates in the synthesis of isoxazole derivatives and of other heterocycles, which, in turn, can be converted into many targets.[59] Primary nitrocompounds have become a common source of nitrile oxides once their dehydration with phenylisocyanate had first been reported.[60] Several other dehydrating procedures described later include treatment with excess of sulphuric acid [61] or with catalytic PTSA under either prolonged heating,[62,63] , or microwave irradiation. However most methods have in common the O-acylation of the nitrocompound as an intermediate step: thus, chlorides (AcCl,[64] TosCl,[65]65 PhSO2Cl, [65] ClCOOR,[66]66 POCl3,[67] ) anhydrides (Ac2O,[71] Boc2O,[67]) ketene[68] as well as isocyanates[61,69]61, were the reagents of choice. The accepted mechanism [68,70-76] , , ,68, , , , includes the nitronic anhydride as the key intermediate, which then collapses to nitrile oxide. In the presence of a dipolarophile, the expected cycloadduct is obtained.[77] It is possible that in some cases cycloaddition occurs on the nitronate derivative intermediate followed by elimination: the cycloadduct is produced and in this case the nitrile oxide is not a reaction intermediate.[71]71 The above procedures, when applied to nitromethane, gave different results. However the dehydration of nitromethane on treatment with a transition metal salt or complex has been reported to lead to fulminate complexes or isocyanate complexes of the metal employed. Thus, e.g., the complex trans-difulminatobis (triphenylphosphane)platinum(II) was easily isolated by heating the tetrakistriphenylphosphine Pt(0) complex with nitromethane.[78]
SOLID PHASE SYNTHESES (RU I, RU V).The use of solid supported reagents for enantioselective processes was reported above (1,3-DC of chiral nitrones, RU I). (RU V) The use of non-conventional methods, such as the solid-phase syntheses, to realise eco-friendly processes involving the 1,3-DC and the generation of nitrosocarbonyl intermediates is linked to the opportunity to plan processes without by-products, with less hazardous synthetic protocols and non-toxic reagents as well as simplified work-up procedures, minimising the waste. Recent studies on solid phase nitrile oxide 1,3-DC showed a unexpected stability of the supported dipoles, allowing the synthesis of purer isoxaxoline and isoxazolidine in high yields as well as their oxidation into nitrosocarbonil intermediates.[79]
DIELS-ALDER REACTIONS IN MELTED SUGARS (RU III). Organic solvents are tipically used in large excess compared to the reactants and have the tendency to escape in the environment so their elimination is an important target which can be gained by using solvent free methodologies, or benign solvents. The RU plans the use of melted carbohydrates, which are reusable, cheap, eco-friendly and also chiral, as innovative alternative solvents. Ultrasounds and MW will be used to favour and accelerate the cycloaddition reactions studied. In addition, the exploitation of microbial enzymes is to be considered as complementary and/or eco-friendly alternative to conventional methods. The use of new isolated enzymes as catalyst is of outstanding relevance in the project. The sound competence of the RU in this field[80] will allow to gain this goal. Careful re-examination of well known experimental conditions will allow to find out new and unexpected applications: so we intend to come back to some synthetic procedures developed in the past for the preparation of known AA of interest for this project carrying out new synthetic protocols eco-friendly using the binomium melted sugar/enzymes as resolving agents. <<<