Carbonyl groups conjugated

Formation and Elimination of Multiple Bond Functionalities. Reactions that involve the formation and elimination of multiple bond functional groups may significantly effect the color of residual lignin in bleached and unbleached pulps. The ethylenic and carbonyl groups conjugated with phenoHc or quinoid stmctures are possible components of chromophore or leucochromophore systems that contribute to the color of lignin. 

Alcohol (34) is needed and we shall want to use a photocycloaddition to make some derivative of it (ether or ester). One partner in the cycloadd ition will be nucleophilic (CH2=CH0R) so we need a carbonyl group conjugated with the cyclohexene. FGA is required. 

In acyclic systems, a typical example of a carbonyl group conjugated with an adjacent double bond is acrolein, which exists almost exclusively (66JCP104) in the s-trans planar form and intercoverts (66JCP104) into the s-cis form through an energy barrier of 29.3 kJ mol. ... 

The hydrolysis of such esters is not always possible, especially if they are sterically hindered, or if the carboxylic acid involved is aromatic, and the carbonyl group conjugated, Esters of benzoic acid, for example are very difficult to hydrolyse. 

Most compounds studied have a carbonyl group conjugated with the benzene ring and nearly all reactions are intramolecular cycloadditions. In most cases, the alkenyl moiety is connected to the arene ring via an oxygen atom, ortho or para with respect to the carbonyl group. 

In the next example there is a carbonyl group conjugated with the dienophile. Now the less stable cis ring junction is formed because the molecule can fold so that the carbonyl group can enjoy a bonding overlap with the back of the diene. This time the linking chain has to adopt a bo at-like conformation. 

The third case is in many ways the most interesting. We have seen that the alkaline hydrolysis of ethyl esters of benzoic acids (ArCC Et) has a p value of +2.6 and that this is a reasonable value for a reaction involving nucleophilic attack on a carbonyl group conjugated with the aromatic ring. The hydrolysis of the same esters in acid solution, which also involves nucleophilic attack on the same carbonyl group, has a p value of+0.1. In other words, all these esters hydrolyse at the same rate in acid solution. Neither of the previous explanations will do. We need to see the full mechanism to explain this remarkable result. 

The chloride must add in a conjugate fashion and the ethyl Grignard in a direct fashion that removes the carbonyl group. Conjugate addition can happen only if the carbonyl group is intact so must happen first and the lower suggestion will work. 

The Hammett p value shows a modest gain of electrons in the transition state. We must not ti-. the pre-equilibrium into account in this as ArCHO is not involved in that step. In fact a Hamn c p value of -f 2.5 is typical of nucleophilic attack on a carbonyl group conjugated with the arorr.i ring (see p. 1095 for an example). 

If the reaction mixture, or compound 124 alone, is heated to >250 °C, the more stable anti isomer 129 is the only product. Though there are two carbonyl groups conjugated with the enophile only the amide carbonyl can interact with the ir-bond of the ene so that the endo transition state 127 leads to the syn product 126. The exo transition state 128 leads to the anti product 129 and equilibration is presumably by reverse ene reaction. 

Compounds with a carbonyl group conjugated to the double bond are conventionally called Michael acceptors. The Michael reaction is a route to a carbon-carbon bond. A nucleophile adds to a Michael acceptor. Together with nucleophilic addition at the carbonyl group, this is the first step of Robinson s cyclization, which is an excellent method for the preparation of medium-sized (and especially six-membered) rings. 

Kupchan et al. 88a) derived the structure of A-benzoylcycloxo-buxidine-F from the following observations. The UV spectrum showed that 195 is a secondary benzamide having the carbonyl group conjugated with the cyclopropane ring. The IR spectrum revealed its close relationship to A-benzoylcycloxobuxine-F (197) (see p. 50). The PMR spectrum showed the presence of aromatic protons, one amido proton, and other substitution pattern practically identical with that mentioned above 88a). 

You have seen that alcohols are dehydrated when they are heated with acid (Section 12.5). The )8-hydroxyaldehyde and j8-hydroxyketone products of aldol addition reactions are easier to dehydrate than many other alcohols because the double bond formed as the result of dehydration is conjugated with a carbonyl group. Conjugation increases the stability of the product (Section 8.3) and, therefore, makes it easier to form. If the product of an aldol addition is dehydrated, the overall reaction is called an aldol condensation. A condensation reaction is a reaction that combines two molecules while removing a small molecule (usually water or an alcohol). 

From the leaves of P. lyalli a new glycoalkaloid pauridianthoside C27H28N2O10 mp 208-210° [a]D—264° (MeOH), has been isolated. The UV spectrum [Amax(loge) 218 (4.60), 286 (4.20), and 384 (3.82) nm] indicated the presence of a carbonyl group conjugated with the harman nucleus. The mass spectrum showed the expected molecular ion at m/e 540,... 

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