Regioselective, chemoselective and stereoselective reactions. Menthol is a member of the terpene family of natural products. Asymmetric synthesis use of chiral auxiliaries, chiral reagents and catalysts, asymmetric hydrogenation, asymmetric epoxidation and asymmetric dihydroxylation. Sri kaliswari college, sivakasi an autonomous institution. It generally relates to grignard addition to chiral pyruvates made using chiral alcohols fig 1. A rule which enables one to predict the predominant stereoisomer in the product has come to be known as cram s rule cram and a b d elhafez, 1 9 5 2. The grignard reaction is a key synthetic step to produce high diastereomeric compounds via crams rule and. Recapitulation of prochirality, homotopic and heterotopic ligands, stereoselectivity in cyclic compounds, enantioselectivity, diastereoselectivity, stereoselective aldol. In chemistry semester i code course title l t s c cy 1101 physical chemistry 1 4 0 0 4 cy 1102 inorganic chemistry 1 4 0 0 4 cy 1103 organic chemistry 1 4 0 0 4 cy 1104 nuclear, analytical and environmental chemistry 4 0 0 4 cy 1151 basic physical chemistry laboratory 0 0 4 2. A new approach to 1,4oxazines and 1,4oxazepines via base.
Some planar and all linear molecules cannot exist as stereoisomers. An extension of crams idea of reactive conformation to chiral esters of \\alpha\ketoesterspyruvates is the prelogs rule reported in 1953 3. Crams rule major reference works wiley online library. If a, c of highest priority, then syn if b, c of highest. Stereochemistry stereoisomers 3 fall 2011 problem 2. Total synthesis and structural validation of phosdiecin a via. In certain noncatalytic reactions that diastereomer will predominate, which could be formed by the approach of the entering group from the least hindered side when the rotational conformation of the c. However, according to crams rule, the conformation a in the transition state 3. Enantiomers, diastereomers, racemates, racemisation, resolution, prochirality, prostereoisomerism with suitable examples of one and two chiral centers.
Seeds of life on earth may have originated in space watts. Pericyclic reactions, electrocyclic, cycloaddition and sigmatropic reactions, 1,3dipolar. Minor product o m s r l r m s l o l m s r o l s m r o this is the important interaction that must be minimized. Enantiomers have identical chemical and physical properties in an achiral environment. Reactions of sp 3, sp 2 carbon and aromatic systems, crams rule. Assuming that there isnt an unusually electronegative atom on the carbon next to the carbonyl, the largest group and use the a value table for. Provide stereochemical drawings of the reactants and products. Nucleophilic additions to carbonyls stereoselectivity of. An extension of cram s idea of reactive conformation to chiral esters of \\alpha\ketoesterspyruvates is the prelog s rule reported in 1953 3. Syntheses and applications of 5benzotriazole pyrrolidinones. Why would you think this might not provide as important directing influences. It is said that in the nucleophilic addition to carbonyl compounds with an adjacent chiral center without polar groups attached, the optimal conformation is to minimize the interaction between the largest group and the carbonyl group during the entering of the. Geometrical isomerismez nomenclature determination of configuration of geometrical isomers using physical and chemical methods.
Assuming that there isnt an unusually electronegative atom on the carbon next to the carbonyl, the largest group and use the a value table for this measure prefers a conformation where it is. A potentially useful extension of cram s rule is the asymmetric induction provided by a remote ester prelog s rule. Stereochemistry and mechanism of nucleophilic addition to carbonyl group crams rule. Methods of resolution, optical purity, prochirality. The payment to the practical examiners will be made on the basis of sessions.
Readbag users suggest that microsoft word 04 chemistry. Organic reaction mechanism in aromatic compounds 8 lectures. A potentially useful extension of crams rule is the asymmetric induction provided by a remote ester prelogs rule. Enantiomers differ in their configuration r or s at the stereogenic center. Stereochemistry 12 hrs chirality, elements of symmetry, molecules with more than one chiral centre, diastereomerism. The existing asymmetric center would have a small, medium and large group, denoted s,m and l respectively in the reactive conformation, the carbonyl group would orient itself in such a way that it will rest between the small group and the medium group. Ferguson, the modern structural theory of organic chemistry. Indian association for the cultivation of science mastersintegrated masters phd program in school of chemical sciences scs course structure for 2 years masters program notes. L s mo r large group oriented anti to the carbonyl group cram thought that in the absence of a chelating group sterics played the biggest role in limiting the conformation of the. This rule predicts the stereochemistry of the nucleophilic addition to a carbonyl compound with an adjacent chiral center. Unit v 15 hrs conformational analysis configuration and conformation conformation of simple acyclic compounds like 1,2. Stereochemistry of additions to cyclohexanones different considerations are involved if the ketone being attacked is cyclic, since the conformation of the ring system is now of paramount importance. A new approach was developed to synthesize 1,4oxazine and 1,4oxazepine derivatives without solvent and metal.
Explain the following trends observed by midland and coworkers m. Crams rule free download as powerpoint presentation. Stereochemistry of ring opening in acid and base we turned first to an examination of the stereochem istry of the ring opening of cyclopropanols. Stereo selectivity regio selectivity chemo selectivity diastereo selectivity crams rule felkin anh model contents 4102018 2 3. However, he also expanded on these theories by developing his own. The file contains 18 pages and is free to view, download or print. Preparation and properties of benzene, naphthalene, anthracene and phenanthrene. Asymmetric induction is a key element in asymmetric synthesis.
D the crams rule of asymmetric induction developed by donald j. As a substrate, trans2phenyl1methylcyclopropanol was chosens and was prepared in optically active form. The second piece of evidence involved research to further rule out the possibility of terrestrial contamination as a source of these molecules. Crams rule stereochemistry chemical reactions scribd.
Aug 06, 2012 crams rule posted on august 6, 2012 by dr anthony melvin crasto ph. The crams rule of asymmetric induction developed by donald j. Cram, complete optical resolution by differential complexation in solution between a chiral cyclic polyether and an amino acid, j. Each theory paper will include 20% marks as internal assessment as per university rules. Asymmetric induction also enantioinduction in stereochemistry describes the preferential formation in a chemical reaction of one enantiomer or diastereoisomer over the other as a result of the influence of a chiral feature present in thesubstrate, reagent, catalyst or environment. Mscchemistrysyllabuscsspg2010affiliatedcolleges free download as pdf file. Crams rule posted on august 6, 2012 by dr anthony melvin crasto ph. Crams career touched many subjects, but two themes pervade his chemistry. Configuration and conformation of aldols derived from cycloalkanones and aldehydes. Chemistry curriculum semester i code course title l t p c. Cram in 1952 is an early concept relating to the prediction of stereochemistry in certain acyclic systems. Journal of the american chemical society 2001, 123 37, 89398950. Crams rule c x diastereomeric x c, o, n faces stereogenic how does this center control the direction of attack at the trigonal carbon.
A rule which enables one to predict the predominant stereoisomer in the product has come to be known as crams rule cram and a b d elhafez, 1 9 5 2. Introduction to stereochemistry structural constitutional isomers compounds of the same molecular formula with different connectivity structure, constitution conformational isomers compounds of the same structure that differ in rotation around one or more single bonds configurational isomers or stereoisomers compounds of the same structure that differ in one or more. In one case crams rule the evolution of the current theory is detailed from its first tentative 1952 postulate to the current felkinanhheathcock formalism. Cram in 1952, is an early concept relating to the prediction of stereochemistry in certain acyclic systems. Careyachap72ed 3 unfavorable equilibrium for hydration.
Regioselective cyclization occurred to afford exclusively the exodig product, and stereochemistry was studied by circular dichroism and specific optical rotation techniques. In both cases this is reflected by the 3,4configuration note the numbering change for the glyceraldehyde acetal in example 2. Draw all stereoisomers for the following molecules. To begin this study we need to know the low energy conformers about the sidechain cc bond to the ring. The dominant stereoselectivity results again from the tendency of ecrotyl reagents to form anti diastereomers and zreagents syndiastereomers. Aromaticity, huckels rule, nonaromatic, antiaromatic, homoaromatic benzenoid and nonbenzenoid. Configuration is assigned by the cahningoldprelog cip rules see chem 343 notes or textbook. In one case cram s rule the evolution of the current theory is detailed from its first tentative 1952 postulate to the current felkinanhheathcock formalism.
Draw the two molecules in their most stable chair conformers. Key steps of the synthetic approach include a brown allylation for diastereoselective introduction of the c9 stereochemistry, an asymmetric cbs reduction to establish the lactone c5. According to cram s rule, the less hindered side is likely to be most susceptible. Reactive intermediates and rearrangement reactions.
This stereoselectivity can be explained in terms of crams rule for nucleophilic attacks on aldehydes and ketones see fig. Since cyclohexanes are the most well understood ring systems in terms of conformation, we will look at them only. Stereocontrolled synthesis of erythro nprotected aamino. According to crams rule, the less hindered side is likely to be most susceptible. Liaih 4 and nabh 4 reduction, meerweinpondroff verley reduction. This can occur for planar systems with sp2 hybridised atoms, giving e trans, anti or z cis, syn stereoisomers.
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