Dienes particularly reactive

Allylic A" -3-hydroxyls are particularly reactive, although some difficulty arises because this system is prone to acid-catalysed dehydration to the 3,5-diene. A" -3-Methyl ethers are readily prepared by direct, p-toluenesulfonic acid-catalysed reaction with methanol. 

Pyrones are useful dienes, although they are not particularly reactive. The adducts have the potential for elimination of carbon dioxide, resulting in the formation of an aromatic ring. Pyrones react best with electron-rich dienophiles. Vinyl ethers are frequently used as dienophiles with pyrones. The regiochemical preference places the dienophile donor ortho to the pyrone carbonyl. 

The reactivity of a particular diene depends on the concentration of the s-cis conformation in the equilibrium mixture. Factors that increase the concentration of this conformation make the diene more reactive. As an example of this effect, consider 2,3-dimethyl-1,3-butadiene ... 

When the ketones are substituted with a second electron-withdrawing substituent (CFj or COjEt) they are particularly reactive towards various dienes, even butadiene, and provide the expected dihydropyrans, " e.g. formation of and 8. in good yield. ... 

Support-bound C-C-C-C fragments serve as components for cyclization in Diels-Alder and hetero-Diels-Alder reactions (HD). In normal Diels-Alder reactions these dienes should preferably be electron rich. The simplest way to introduce the diene is to couple a commercial diene or a diene synthesized by solution phase methodology to a support-bound group. This method, though efficient, restricts the user to a rather limited set of support-bound dienes. In several examples, 2,4-pentadiene-l-carboxylic acid and some of its derivatives were coupled to support-bound amines to give the support-bound diene, which is not particularly reactive in [2 -I- 4]-cycloadditions [301, 302]. 

Considerable attention has been paid to the reactions of oxazoles with typical Diels-Alder alkene dienophiles. The adducts can be transformed into pyridines by different routes (section 5.15.1.4). Electron-releasing substituents on the oxazoles increase the rate of reaction 5-alkoxyoxazoles are comparable in reactivity to typical all-carbon dienes. Particularly useful dienophiles are AT-acyl-oxazolones - synthons for c/j-l,2-amino-alcohols. ... 

Allylidene cyclopropanes proved to be particularly reactive dienes especially if used as one of the partners in the Diels-Alder reaction They differ in that... 

Actually 1 is a particularly reactive Diels-Alder diene it is more reactive than... 

Note here the use of an electron-releasing group (OSiMes) on the diene to compensate for the deactivating effect of C=N in the diene. Also, with its two carbonyl groups, maleic anhydride is a particularly reactive dienophile and is widely used. 

The diene must be in the s-cis conformation to react, and dienes such as cyclopentadiene that are constrained to be in this conformation are particularly reactive. 

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

It has long been known that the Diels-Alder reaction is particularly efficient and rapid when the dienophile contains one or more electron-attracting groups. These substituent effects are illustrated by the data in Table 11.3. In the case of the diene, reactivity is increased by electron-releasing substituents. Some illustrative data are given in Table 11.4. 

The [ 2 + 4]-cycloaddition reaction of aldehydes and ketones with 1,3-dienes is a well-established synthetic procedure for the preparation of dihydropyrans which are attractive substrates for the synthesis of carbohydrates and other natural products [2]. Carbonyl compounds are usually of limited reactivity in cycloaddition reactions with dienes, because only electron-deficient carbonyl groups, as in glyoxy-lates, chloral, ketomalonate, 1,2,3-triketones, and related compounds, react with dienes which have electron-donating groups. The use of Lewis acids as catalysts for cycloaddition reactions of carbonyl compounds has, however, led to a new era for this class of reactions in synthetic organic chemistry. In particular, the application of chiral Lewis acid catalysts has provided new opportunities for enantioselec-tive cycloadditions of carbonyl compounds. 

For a discussion of the mechanistic course of the reaction, many aspects have to be taken into account. The cisoid conformation of the diene 1, which is in equilibrium with the thermodynamically more favored transoid conformation, is a prerequisite for the cycloaddition step. Favored by a fixed cisoid geometry are those substrates where the diene is fitted into a ring, e.g. cyclopentadiene 5. This particular compound is so reactive that it dimerizes easily at room temperature by undergoing a Diels-Alder reaction ... 

The double bonds in a conjugated diene are hydroborated separately, that is, there is no 1,4 addition. However, it is not easy to hydroborate just one of a conjugated system, since conjugated double bonds are less reactive than isolated ones. Thexylborane °(48) is particularly useful for achieving the cyclic hydroboration of dienes, conjugated or nonconjugated, as in the formation of 53." ... 

In order to smdy the effect of perturbation arising from spiro-conjugation on the chemical reactivities, in particular the facial selectivities, sterically unbiased dienes (96 and 97) based on fluorenes in spiro geometry have been synthesized [165]. These dienes react as Diels-Alder dienes with several dienophiles (maleic anhydride (MA), A-phenylmaleimide (PMI), A-phenyl-l,3,5-triazoUne-2,4-dione (PTD) and iV-methyl-l,3,5-triazoline-2,4-dione (MTD)). 

The choice of the acyl substituent X for Diels-Alder reactions of l-N-acylamino-l,3-butadicnes depends on the particular synthetic problem. The acyl substituent has a moderate effect on the cycloaddition reactivity of these dienes, and also determines what amine unmasking procedures are required. As a result of their stability and the variety of amine deprotection procedures available, " the diene carbamates are the components of choice in most cases. A particularly attractive aspect of the diene synthesis detailed here is the ability to tailor the amino-protecting group... 

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