Simple dienes

Both conjugated and isolated dienes are usually accessible by extension of the methods suitable for mono-olefins. Allylic functions for ehmination may be produced by double bond introduction a to a functional group or by allylic substitution of an olefin. Both reduction of allylic systems to mono-olefins and elimination to give dienes, may involve rearrangement. 

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Several highly enantioselective Diels-Alder reactions are known for which the di-enophile does not fit any of the above classes. Corey and coworkers applied the chiral aluminum reagent 36 with a C2-symmetric stilbenediamine moiety (videsu-pra) to the Diels-Alder reaction of maleimides as dienophiles [54] (Scheme 1.68). In most asymmetric Diels-Alder reactions the reactants are usually relatively simple dienes such as cyclopentadiene or monosubstituted butadienes, and unsym-... 

Examples Compound <35) is a double diene, capable of Diels-Alder reactions on the simple diene and on the furan ring and it was required to try out a route to polycyclic compounds using both these reactions, Wittig disconnection direct to available aldehyde (36) and easily made (37) is possible, but the alternative Wittig disconnection to (38) takes advantage of the known simple and high yielding condensation of acetone with (36). 

Recently the photoaddition of anthracene and simple dienes was discovered ... 

Simple dienes are not reactive enough toward chiral l-(alkylsulfinyl)-2-nitroalkenes. To resolve this problem, the reaction of optically active l-(alkylsulfinyl)-2-nitroalkenes with simple... 

The azodiamidine (109) also gives Diels-Alder adducts with simple dienes. The initial adducts readily undergo ring closure to give 1,2,4-triazolo-pyridazines.170... 

Benter, Th., Schindler, R.N. (1988) Absolute rate coefficients for the reaction of NOs radicals with simple dienes. Chem. Phys. Lett. 145, 67-70. 

Even simple dienes and polyenes are difficult to classify in comparison with alkenes. Whereas bromination, oxidation and reaction with tetranitromethane (TNM) can identify the number of double bonds and their location in the molecular structure, conjugated double bonds produce very complex mixtures. Furthermore, many of the tests based on 7r-complexation can also apply for aromatic moieties. An example is the TNM 7r-complex which is yellow with benzene and orange with naphthalene and the tests are therefore non-specific. 

The nickel-catalyzed [4 + 4]-cycloaddition of butadiene to form cyclooctadiene was first reported by Reed in 1954.90 Pioneering mechanistic and synthetic studies largely derived from the Wilke group advanced this process to an industrially important route to cyclodimers, trimers, and other molecules of interest.91-94,943 95,96 While successful with simple dienes, this process is not useful thus far with substitutionally complex dienes as needed in complex molecule synthesis. In 1986, Wender and Ihle reported the first intramolecular nickel-catalyzed [4 + 4]-reaction of... 

In a synchronous Diels-Alder reaction, the following facts have come to light about dienes 1. Simple dienes react readily with good dienophiles and they must adopt a cisoid geometry about the bond (s-cis). 

In this review, chemical shifts and coupling constants of simple dienes will first be summarized, and the theory of chemical shift for dienes and polyenes will then be reviewed. Finally, the recent applications of NMR spectroscopy to a variety of polyenes and dienes and specific systems (allenes, solitons and fullerenes) will be reviewed. 

The asymmetric induction by catalyst 379 was extensively studied in the cycloadditions of simple dienes with substituted a,/)-unsaturated aldehydes. It proved that a-substitution on the dienophile increased the enantioselectivity, whereas -substitution dramatically decreased it. In the case of substrates having both a- and -substituents, high enantiose-lectivities were observed240. 

Engler and colleagues256 demonstrated that the way in which catalyst 406 is prepared has a strong effect on the regioselectivity and enantioselectivity of quinone Diels-Alder reactions. The most effective catalyst was prepared from a 1 1 1 mixture of titanium tetrachloride, titanium tetraisopropoxide and chiral diol 416. The cycloadditions of 2-methoxy-l,4-benzoquinones such as 414 with simple dienes to give adducts like 415 proceeded with high yields and enantioselectivities of up to 80% ee using this catalytic system (equation 123). 

In recent years, much work has been done on catalyzed and asymmetric cycloaddition reactions. In the presence of 5 mol% bismuth trichloride, the simple dienes 10 (R1 = R" = H R1 = H, R2 = Me or R1 = Me, R2 = H) react with diethyl mesoxalate to afford mixtures of the cycloadducts 11 and the products 12 of an ene-reaction (equation 13)11 12. 1,3-Cyclohexadiene and ethyl glyoxylate give solely the endo adduct 13 in 50% yield (equation 14)12. 

The strong dependence of the reaction rate on the catalyst concentration relative to control experiments in which the amino-hydrogen atoms of 7 were substituted by methyl groups demonstrate that hydrogen bonding represents the major interaction responsible for the observed accelerations. Diels-Alder reactions are also accelerated by hydrogen-bond donors. It was shown that a biphenylenediol 9 is able to catalyse [4 + 2]-cycloadditions of cyclopentadiene, 2,3-dimethylbutadiene and other simple dienes with various a,fi-unsaturated carbonyl compounds (Table 14)175. 

Our initial studies focused on the transition metal-catalyzed [4+4] cycloaddition reactions of bis-dienes. These reactions are thermally forbidden, but occur photochemically in some specific, constrained systems. While the transition metal-catalyzed intermole-cular [4+4] cycloaddition of simple dienes is industrially important [7], this process generally does not work well with more complex substituted dienes and had not been explored intramolecularly. In the first studies on the intramolecular metal-catalyzed [4+4] cycloaddition, the reaction was found to proceed with high regio-, stereo-, and facial selectivity. The synthesis of (+)-asteriscanoHde (12) (Scheme 13.4a) [8] is illustrative of the utihty and step economy of this reaction. Recognition of the broader utiHty of adding dienes across rc-systems (not just across other dienes) led to further studies on the use of transition metal catalysts to facilitate otherwise difficult Diels-Alder reactions [9]. For example, the attempted thermal cycloaddition of diene-yne 15 leads only... 

Simple dienes react readily with good dienophiles in Diels-Alder reactions. Functionalized dienes are also important in organic synthesis. One example which illustrates the versatility of such reagents is l-methoxy-3-trimethylsilyloxy-1,3-butadiene (.Danishefsky s diene) 1 Its Diels-Alder adducts are trimethylsilyl enol ethers which can be readily hydrolyzed to ketones. The /j-mcthoxy group is often eliminated during hydrolysis. 

Scheme 8.20 Ring closing metathesis of simple dienes with ruthenium-indenylidenes IX, XXI-XXIV.

A low amount (1 mol%) of triflic acid is sufficient to carry out the hetero Diels-Alder reaction of aromatic aldehydes with simple dienes to furnish 3,6-dihyro-2/7-pyran derivatives in moderate to good yields692 [Eq. (5.248)]. The strongly deactivated para-methoxybenzaldehyde and pentanal gave the products in very low yields. 

Tetrahydro-2-pyridones are also produced by a [4+2] cycloaddition of l-aza-2-siloxydiene with simple dienes in a Lewis acid-catalyzed reaction (Equation 142) <2004S2222>. In an analogous manner, hydrazone derivatives have been in aza-[4+2] cycloadditions with reactive dienophiles which can be trapped with aldehydes to form highly functionalized piperidines (Scheme 88) <2004AGE2001, 2004JOC8429>. 

The Diels-Alder reaction is like a nucleophile-electrophile reaction. The diene is electron-rich, and the dienophile is electron-poor. Simple dienes such as buta-1,3-diene are sufficiently electron-rich to be effective dienes for the Diels-Alder reaction. The presence of electron-donating (— D) groups, such as alkyl groups or alkoxy (—OR) groups, may further enhance the reactivity of the diene. 

The reaction is most easily understood when a simple diene reacts with a very small amount (in this case 2 mole per cent) of the catalyst. A cyclization reaction occurs and the product is also an alkene. It contains no atoms from the catalyst indeed, it has lost two carbon atoms, which are given off as ethylene. 

Most asymmetric syntheses require rather more than one or two steps from chiral pool constituents. Male bark beetles of the genus Ips produce a pheromone that is a mixture of several enantiomerically pure compounds. One is a simple diene alcohol (S)-(-)-ipsenol. Japanese chemists in the 1970s noted the similarity of part of the structure of ipsenol (in black) to the widely available amino acid (S)-leucine and decided to exploit this in a chiral pool synthesis, using the stereogenic centre (green ring) of leucine to provide the stereogenic centre of ipsenol. 

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