Carbonates, diene

Lewis-acid-catalyzed cycloadditions of dienophiles, such as a,/l-unsaturated carbonyl compounds, with open-chain carbon-dienes, are generally highly ortho-para regioselective because the oxygen complexation increases the difference of LUMO coefficients of the alkene moiety. 

This reaction has an accompanying endothermicity of ca 4 kJmol-1. Said differently, methylation of ethylene is some 4 kJmol-1 more exothermic than of allene. How general is this greater exothermicity of alkylation of monoolefins over that of related allenes Proceeding to the three cumulated 5-carbon dienes, we may consider the reactions... 

Another synthesis of P-D-lactone based on a resolved starting material is shown in Scheme 13.29. The stereocenter in the starting material is destined to become C-4 in the final product. Steps A and B serve to extend the chain to provide a seven-carbon diene. The configuration of two of the three remaining stereocenters is controlled by the hydrobora-tion step. Hydroboration is a stereospecific syn addition (Section 4.9.1). In 1,5-dienes of this type, an intramolecular hydroboration occurs and establishes the configuration of the two newly formed C—B and C—H bonds. 

Rees and co-workers in their study of the reactions of trithiazyl trichloride in the preparations of heterocyclic compounds have noted that the isothiazolo[5,4-, isothiazole compound 140 was produced in low yield on reaction with conjugated dienes, along with the other heterocyclic systems 142-145 in much higher yields (Equation 28). Since it is known that trithiazyl trichloride is in thermal equilibrium with its monomer NSCl (Equation 29), the authors propose the so-called criss-cross cycloaddition reaction (Equation 30) which has been reported for azabu-tadienes, but this represents the first example of such a criss-cross cycloaddition to an all-carbon diene <1998CC1207>. 

In the first attempt the tetracyclic intermediate (33) was prepared from 3,4-methylenedioxy-ta-nitrostyrene upon Diels-Alder addition of butadiene followed by zinc and hydrochloric acid reduction to an amine eventually converted into 33 by formaldehyde and hydrochloric acid. All other experiments, designed to anticipate the addition of a C6 diene (in order to introduce at once also the C2 carbon unit for ring D formation) or of a four-carbon diene with different functionality, failed. The structure of 33 is based on spectroscopic data, on considerations on the accepted stereochemical courses of this type of... 

Electron-releasing substituents on the oxazoles increase the rate of reaction 5-alkoxy-oxazoles are comparable in reactivity to typical all-carbon dienes. 4-(Trimethylsilyloxy)-oxazoles react smoothly to produce furans at modest temperatures. ... 

On rare occasion, Ketene carbonyl groups can act as heterodienophiles with all-carbon dienes. Other cycloaddition pathways, such as [2 + 2] addition, are usually preferred, and this Diels-Alder reaction is certainly not of general utility.48-49 Equations (17) and (18) indicate how relatively small structural changes in reactants can affect the nature of the cycloaddition products.48... 

Figure 6. Ester, urethane, poly-THF, and carbonate diene comonomers for ADMET copolymerization.

The rate of Diels-Alder reactions of oxazoles with electron-deficient olefins and acetylenes depends on the electron-donating ability of the oxazole substituent Rates decrease in the order OR > NR1R2 > alkyl > 4-phenyl > COCH3 > CO2CH3 3> 2-,5-phenyl 5-aIkoxy oxazoles have reactivity roughly equivalent to an all-carbon diene. 

Nitro-2-phenyloxazole 86 functions as a dienophile with a variety of unactivated dienes. Thus 86 reacted with a fivefold excess of 2,3-dimethyl-1,3-butadiene, 87, in a sealed tube at 110°C to give a 71% yield of the bicyclic oxazole 89 and 13% of 5,6-dimethyl-2-phenylbenzoxazole 90 (Fig. 3.24). Although the intermediate cycloadduct 88 could not be isolated in this case, cyclohexadiene and 86 gave a separable mixture of the endo- and ejco-2-oxazolines in 33 and 30% yield, respectively. In addition to all-carbon dienes, 86 reacted with l-(dimethylamino)-3-methyl-l-azabuta-1,3-diene 91 at 55°C in chloroform to provide, via loss of nitrous acid and dimethylamine from intermediate adduct 92, a 30% yield of the oxazolo[4,5-h]pyridine 93. ... 

Considerable attention has been paid to the reactions of oxazoles with typical Diels-Alder alkene dienophiles much of the early work is relatively inaccessible, being described in Russian and patent literature. The adducts are transformed into pyridines by different routes (section 5.15.1.4). Electronreleasing substituents on the oxazoles increase the rate of reaction 5-alkoxyoxazoles are comparable in reactivity to typical all-carbon dienes. Particularly useful dienophiles are the N-acyl-oxazolones these are synthons for cis-1,2-amino-alcohols. ... 

The Diels-Alder-based retrosynthesis of a cyclohexene begins with the identification within the six-membered ring of the four carbons contributed by the diene and the two carbons from the dienophile.The double bond in the cyclohexene target molecule marks the nuddle two carbons (C2 and Cj) of the four-carbon diene. Two disconnections are made in the ring to separate the four-carbon and two-carbon units. It is helpful to show the mechanism for the retro Diels-Alder, since that will not only achieve the required disconnection, but it will also properly locate all of the necessary pi bonds in the starting diene and dienophile. 

The terms endo and exo are used for bicyclic Diels-Alder adducts to describe the orientation of substituents of the dienophile in relation to the two-carbon diene-derived bridge. Exo (Greek, outside) substituents are on the opposite side from the diene-derived bridge endo (Greek, within) substituents are on the same side. 

Earlier we used curved arrows to show the flow of electrons that takes place in the process of bond breaking and bond forming in the Diels-Alder reaction. As discussed, these reactions involve a four-carbon diene and a two-carbon dienophile and are termed [4 + 2] cycloadditions. We can write similar electron-pushing mechanisms for the dimerization of ethylene by a [2 + 2] cycloaddition to form cyclobutane and for the dimerization of butadiene by a [4 + 4] cycloaddition to form 1,5-cyclooctadiene. 

Penta refers to the five-carbon chain that includes the carboxylic acid carbon dien indicates the presence of two unsaturations (as in diene) at the locations 2 and 4 oic is one of the preferred nomenclature terms for organic acid and from the point of view of searching is redundant with the term, acid. From just this portion of the name we know the following structural features there is a five-carbon chain terminating with a carboxylic acid and the chain has two double bonds at positions 2 and 4 relative to the carboxylic acid carbon. 

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