In the yesteryear the usage of nitrones in the synthesis of organic compounds and their applications in the synthesis of big assortment of heterocyclic compounds and natural merchandises is proved to be really useful21. With a scope of dipolarophiles the nitrones undergo cycloaddition reactions to acquire monocyclic and polycyclic pealing systems. Nitrones can be used for the stbilization of the free groups in the reaction systems. in many diaries and literatures already many methods have been mentioned for the synthesis of acyclic and monocyclic nitrones and still a uninterrupted research is traveling on for the innovation of new man-made methods23,27.from the past literature it is apparent that utilizing oxidative and non-oxidative methods a great reasearch has been done for the innovation of many new improved methods for the synthesis of acyclic and monocyclic nitrones21,23,27. But it is besides apparent that there are merely few methods are at that place for the synthesis of bicyclic nitrones27. The chief purpose of this survey is to look into the many new methods for the synthesis of a scope of bicyclic nitrones and besides for the probe of methods for the synthesis of powerful natural merchandise, dioxyerthratidinone which is a powerful acetylcholine adversary.
1.1 Cycloaddition reactions
The reaction in which two or more unsaturated molecules combine to organize a cyclic compound with a net decrease in the bond multiplicity is called a cycloaddition reaction29. In normal reactions the add-on normally occurs across dual or ternary bonds. But in cycloaddition reactions the cyclic merchandise is formed by the debut of two or more new I?-bonds. These cycloaddition reactions are really utile in the synthesis of organic compounds with glandular fever or heterocyclic pealing systems. The most of import cycloaddition reactions which are present are ( 1 ) Diels-Alder reaction and ( 2 ) 1,3-dipolar cycloaddition reaction. In Diels-Alder reactions normally a six membered cabocyclic ring is produced from a conjugated diene and dienophile30,31, while in 1,3-dipolar cycloaddition reactions a five membered ring is produced utilizing 1,3-dipoles and dipolarophiles. After this a few old ages subsequently Gresham and Steadman stated that a methanal can be used as a dienophile for the synthesis of six membered heterocyclic ring system, this study gave rise to a construct called Hetero-Diels-Alder reaction. The categorization of cycloaddition reactions can be done harmonizing to the formation of the ring size this representation can be done as [ a+b ] where a and B are the atoms of part by the reactant species towards the formation of the ring system. Since in Diels-alder reactions the part of atoms by the reactant species is to synthesize a six membered ring system, hence they are classified as [ 4+2 ] cycloaddition reactions. and likewise in 1,3-dipolar cycloaddition reactions the part of atoms by the reactant species is to synthesize a five membered ring system hence they are classified as [ 3+2 ] cycloaddition reactions. The another manner of categorization of cycloaddition reactions is done by the construct the figure of reactant negatrons tha take portion in the formation of the ring system. In this manner both the Diels-alder and 1,3-dipolar cycloaddition reactions are considered to be as [ Iˆ4s + Iˆ2s ] or [ 4+2 ] cycloadditions. Therefore in 1,3-dipolar cycloaddition reactions 4Iˆ negatrons from dipoles and 2Iˆ negatrons from dipolarophiles are involved in the formation of the ring system.
Examples of cycloaddition reactions:
2-Diisopropylborylaminopyridine undergoes cycloaddition with diphenylcarbodiimide and malononitrile, while 2-dibutylborylaminopyridine undergoes the correspondent reaction with phenyl isocyanide.
[ 1- ( N2N-Diphenylamidino ) pyridon-2-iminato ] diisopropylboron ( IV ) undergoes thermic isomerisation to [ N1- ( pyrid-2-yl ) -N2, N3-diphenylguanidinato ] diisopropylboron ( V ) .
In IUPAC literature the definition of the alkylation is given as permutation of an alkyl group with the H of another molecule or it may be defined as the transportation of the alkyl group from one molecule to the another molecule1,2. And the transferred or the substituted alkyl group may be a free radicle, a carbocation, a carbanion or a carbene.
The agents which are used for the alkylation are called alkylating agents2. The categorization of the alkylating agents is done on the bases of the alkyl group they are replacing or reassigning to execute a reaction3. They are classified as ( a ) Nucleophilic alkylating agents
( B ) Electrophilic alkylating agents
( degree Celsius ) carbene alkylating agents
( a ) Nucleophilic alkylating agents: They provide or present alkyl anions known as carbanions. These compounds can be easy added to a carbonyl group which is electron deficient. These compounds can be used for the supplanting of halides substituents on the C atom3. The illustrations of most normally used nucleophilic alkylating agents are organosodium, organolithium, organocopper and organomagnesium ( grignards reagent ) 3.
( B ) Electrophilic alkylating agents: They provide or present alkyl cations to execute reactions. Alkyl halides are the best illustration of electrophilic alkylating agents.
( degree Celsius ) Carbene alkylating agents: They provide or present carbenes for alkylation. The extremely reactive alkyl groups are carbenes. They are used for assailing the unactivated C-H bonds2,3.
In malignant neoplastic disease chemotherapy alkylation of DNA of malignant neoplastic disease cells is done to damage the Deoxyribonucleic acid of malignant neoplastic disease cells. Alkylation is of chief usage for polishing the oil in the petrolium industry.
Michael add-ons or Michael reactions
Originally these reactions were discovered and defined by Michael Arthur in the twelvemonth 18873. Harmonizing to the Michael it is that to an I± , I?-unsaturated carbonyl compound an enolate of a ketone or an aldehyde is added at the place of the I?-carbon12,13. Later on kohler defined this reaction in a different manner that a double stabilised C nucleophile is added the to an I± , I?-unsaturated carbonyl compound at the place of I? C in a 1,4-addition way5. Beta-ketoesters, malonates, and beta-cyanoesters are some of the best illustrations of the nucleophiles nowadays.
Some of the best illustrations of the Michael add-on reactions are the reactions between diethyl malonate and diethyl fumarate5,
2-nitropropane and methyl acrylate3,
Nitropropane and methyl vinyl ketone5,
Ethyl phenyl cyano ethanoate and acrylonitrile4,
Mesityl oxide and diethyl malonate3,
Diethylmalonate and methyl crotonate3.
Typical reaction mechanism of Michael add-on
In the above conventional diagram the compound 1 is a nucleophile in which the R is a alkoxy residue. Deprotonation of the compound 1 leads to the formation of a carbanion which is the compound 2, negatron withdrawing groups are bracing this compoun. 2a, 2b and 2c are the three resonance constructions of the compound 2. Out of which two of them have enolate ions. In a conjugate add-on reaction this nucleophile reacts with an electrophilic olefine 3 to organize a compound 4. Proton is abstracted from the protonated base by an enolate 4 to organize a compound 5 is the concluding measure of tthis reaction.
Asymmetric Michael add-on
This method consists of chiral stage transportation contact action, consists of chiral quaternate ammonium salts which causes activation of enamine or iminium with chiral secondary aminoalkanes, which are the derived functions of proline.
The diagrram below represents the reaction between the cyclohexanone and nitrostyrene in which the base proline is derivatized which in concurrence plants with p-Toulenesulfonic acid the syn and anti add-on is favoured with 99 % enatiomeric surplus.
A good illustration of Michael reaction is the synthesis of Coumadin from 4-hydroxycoumarin and Benzylideneacetone. Several asymmetric versions of this reaction exist by the use of chiral accelerators. 18,19
In the mukaiyama michael-addition reaction the accelerator used is titanium tetrachloride and the nucleophile is the silyl ether19.
The construct of 1,3-dipole and dipolarophile cycloaddition reactions
1,3-dipolar cycloaddition reactions are besides known as Huisgen cycloadditions or huisgen reactions33. It is the name given after the German chemist Rolf huisgen for his parts in chemical science. Huisgen reaction is considered to be the member of big category of cycloadditions33,34,36. It is the chief way for the organic synthesis of heterocyclic rings. Huisgen reaction can be considered as the reaction between the 1,3-dipole and dipolarophile for the formation of a five membered ring. In organic chemical science 1,3-dipolar cycloaddition reaction is considered to be the most of import for the building of five-membered heterocyclic ring37,38. 1,3-dipolar reaction is a conjunct reaction which means both the bond devising and the bond breakage occurs at the same time. Stereospecific creative activity of new chiral Centres in organic molecules is benefitted by the 1,3-dipolar cycloadditions. Depending upon the construction of the dipole in a individual measure upto four uninterrupted chiral Centres can be formed in 1,3-dipole cycloaddition. When 1,2-disubstituted olefine is involved in cocerted reaction with 1,3-cycloaddition, two chiral Centres are formed on the olefine due to the syn onslaught on the dual bond. This is good represented by the 1,3-dipole reaction types such as allyl anion type reactions and allenyl anion or propargyl type reactions.
1,3-dipoles are the three atoms Iˆ electron systems in which the four negatrons are delocalised over three atoms.these consists of the elements from the 4, 5 and 6 groups of the periodic tabular array. The cardinal atom in the 1,3-dipoles is either the N or O. Some of the illustrations of the 1,3-dipoles are azides, ozone, diazo compounds, nitro compounds and some oxides like nitrones, azotic oxide, nitrile oxides, carbonyl oxides and azoxide compounds. Some imines like carbonyl imines, nitrilimines, and azomithine imines. Some ylides like carbonyl ylide, nitrile ylide and azomethine ylides.
1,3-dipolar cycloaddition reactions have merely been explored for five types of dipoles.A Nitrones are the chief topic of survey in this field. Nitrones can be readily obtained from the compounds such as aldehydes, aminoalkanes, imines and oximes and this is the chief cause of this ground. Cyclic nitrones are less stable when compared to acyclic nitrones which can be stored easy under ambient conditions.The readying of Azomethine ylides is done by in situ since they are really unstable. Many methods are in usage for the synthesis of Azomethine ylides, they areabstracting proton from imine derived functions of alpha amino acids, aziridines subjected to photolysis or thermolysis and imonium salts subjected to dehydrohalogenation. Azomethine ylides are found to be of big usage for the synthesis of nitrones. among the 1,3-dipoles Carbonyl ylides are less common for their usage in synthesis. though the entree to carbonyl ylides via Rh carbenes made a tremendous development in this country. Since past three old ages metal catalysed asymmetric reactions are appearing.for the synthesis of five membered heterocyclic rings Nitrile oxides are in close competition with nitrones. since they are readily available from aldoximes or primary nitro compounds, due to high responsiveness and rapid dimerization most nitrile oxides must be prepared in situ43,44. the catalytic control of this reaction may be one of ground for the high responsiveness of nitrile oxides.
It is considered to be the unsaturated system in this the 2nd reactant that undergoes cycloaddition reaction with 1,3-dipoles. Alkenes, acetylenes and their tremendous derived functions may respond as dipolarophiles44.
Stuart Warren ( 1987 ) . Organic synthesis The Disconnection Approach. Great Britan: John Wiley & A ; Sons. 93-102
The mechanism chosen or elected to execute a reaction either to bring forth a more stable merchandise or to follow a most encouraging kinetic tract which will be of less energy ingestion is called a stereoselective reaction and the term used to depict this phenomenon is called as stereoselectivity47.
The reaction which is used to bring forth a specific stereochemical compound is called a stereospecific reaction47, and the term used to depict this reaction is called stereospecificity. This explains the stableness of the attendant merchandise and this is performed by sing that each stereoisomer of the merchandise will be different from that of the stereoisomer of the get downing stuff. There may be enantiomorphs or diastereomers of the attendant products47.
An illustration of stereoselectivity:
The formation of two Commonwealth of Independent Statess and trans intoxicants by the decrease of the ketone.
Regioselectivity is the phenomenon where a reaction takes topographic point by breakage of bond or by the formation of bond other than the normal possible manner. And the reaction is called regioselective reaction.
Based upon the selectivity of the reaction to continue the compounds can be termed as high or low regioselective. If the favoritism is 100 % the reaction is said to be wholly regioselective and it is said to be partial if the merchandise obtained at different responding sites is different44.
Choosing an unsymmetrical ketone for Alkylation and a nucleophiles is added to an unsaturated carbonyl compounds to acquire either Michel add-on or way add-on.