Diels-Alder reaction with cyclic dienes

Knunyants showed that such perfluoroalkenes, under forcing conditions, undergo Diels-Alder reactions with cyclic dienes such as cyclohexadiene [72] (equation 65) or furan [7J] (equation 66). 

Z)-4-[(5)-2,2-Dimethyl-l,3-dioxolan -ylmethylene]-2-phenyl-5(47/)-oxazolone 632 can react as a dienophile in diastereoselective Diels-Alder reactions. Thus, 632 undergoes a thermally induced Diels-Alder reaction with cyclic dienes, for example, cyclopentadiene and cyclohexadiene, to afford a mixture of the four... 

Bicyclic diazenes, in which the diazcnc group is not activated by acyl substituents, can act as dienophiles in the Diels-Alder reaction with cyclic dienes only in the presence of a stoichiometric amount of tetrafluoroboric acid to give. V,/V"-bicyclic hydrazines 111 . When two diastereomers can be produced, due to the nonsymmetric dienophile (i.e., when n 2), a single diastereomer is obtained ( H-NMR studies). 

The 5-methylene-2(5//)-furanone 216 was found to be a good dienophile in Diels-Alder reactions with acyclic dienes (R = H, 2-Me, 2,3-di-Me, 1-Me, 1,3-di-Me). The reaction took place specifically at the cxo-cyclic double bond to give the corresponding spiro adducts 217 in good yields (Scheme 59) (90JOC3060). 

Diels-Alder reactions with unsymmetrical dienes and/or dienophiles can lead to the formation of different isomers. One of the most interesting aspects in these systems is stereoselectivity, observed in reactions involving cyclic dienes. In these cases, two different stereoisomers can be formed endo and exo. 

An extensive review of the hetero-Diels-Alder reactions of 1-oxabuta-1,3-dienes has been published. Ab initio calculations of the Diels-Alder reactions of prop-2-enethial with a number of dienophiles show that the transition states of all the reactions are similar and synchronous.Thio- and seleno-carbonyl compounds behave as superdienophiles in Diels-Alder reactions with cyclic and aryl-, methyl-, or methoxy-substituted open-chain buta-1,3-dienes.The intramolecular hetero-Diels-Alder reactions of 4-benzylidine-3-oxo[l,3]oxathiolan-5-ones (100) produce cycloadducts (101) and (102) in high yield and excellent endo/exo-selectivity (Scheme 39). A density functional theoretical study of the hetero-Diels-Alder reaction between butadiene and acrolein indicates that the endo s-cis is the most stable transition structure in both catalysed and uncatalysed reactions.The formation and use of amino acid-derived chiral acylnitroso hetero-Diels-Alder reactions in organic synthesis has been reviewed. The 4 + 2-cycloadditions of A-acylthioformamides as dienophiles have been reviewed. ... 

Diels-Alder reactions involving cyclic dienes give rise to bridged adducts. Any substituent X found at the bridge may adopt a syn or anti position with respect to the double bond ... 

Diels-Alder reactions with unsymmetrical dienes and/or dienophiles can lead to the formation of different isomers. One of the most interesting aspects in these systems is stereoselectivity, as observed in reactions involving cyclic dienes. In these cases, two different stereoisomers can be formed endo and exo (Scheme 2). Experimental observations show that in most cases the endo product is predominant over the exo one. Theoretical calculations do not always agree with the experimentally observed endo exo selectivity. Jorgensen et al. have shown that the computed endo/exo selectivity is dependent on the level of calculation for the reactions of acrylonitrile (5) and methyl vinyl ketone (6) with cyclopentadiene. 

To date, the stereocontrolled synthesis of the tricyclic vinigrol core was only achieved through the use ofpericyclic reactions. In 1993, Hanna et al. disclosed the first synthesis of the tricyclic core of vinigrol 92 (Scheme 8.13) [35]. Their approach features an intermolecular Diels-Alder reaction between cyclic diene 87 and 1,4-benzoquinone 88 to give tricycle 89, which upon treatment with NaBH afforded 90 in 62% over two steps. After functional group manipulations, compound 91 underwent an anionic oxy-Cope rearrangement to deliver 92 in 82% yield. 

Strained pertrifluoromethyl-substituted valence tautomers of aromatic systems, such as tetrakis(trifiuoromethyl)Dewar thiophene [87] and hexalas(tnfluorQ-methyl)benzvalene [Diels-Alder reactions with various cyclic and acyclic dienes (equations 76 and 77). 

Cyclic 1,3-diacetoxy-l,3-dienes can be generated in situ from cyclic 1,3-diketones under the influence of isopropenyl acetate. The dienes then undergo Diels-Alder reactions with maleic anhydride giving as products 1-acetoxybicycloalkane dicarboxylic anhydride derivatives (10). The procedure is also successful with cyclic 1,2- and 1,4-diketones as well as cyclic a,j3-unsaturated ketones. The products, after hydrolysis to... 

The hybrid cyclic pentayne 181 underwent Diels-Alder reaction with the electron-deficient diene tetrachlorothiophene 1,1-dioxide 192, but only at one of the two triple bonds of the 1,3-diyne moiety. This was followed by loss of SO2 to give the tetrachlorobenzannelated cyclotetradecenetetrayne 193 (Scheme 37) [18]. 

The substrate for the synthesis of cyclobutanes by elimination of molecular nitrogen is the 3,4,5,6-tetrahydropyridazine skeleton 1. The most important methods for obtaining these compounds are the hetero Diels-Alder reaction of suitable dienes with azo dienophiles followed by oxidative hydrolysis of the adducts, and the oxidation of six-membered cyclic hydrazines (see Houben-Weyl, Vol. E16d, pp 1-111 and ref 1). 

Mukaiyama reported that 4—10 mol% of Ph2Sn=S and AgC104 catalyzed the Diels-Alder reaction between but-3-ene-2-one (183) or (E)-1 -phenylbut-2-en-1 -one (182) and unactivated dienes 184-186 to afford the products 187-192 in 57-95% yield (Scheme 2.48).80 The reaction with cyclic dienes 185 and 186 was highly endo-selective (dr 99 1). Naphthalene-1,4-dione also participated in the reaction with similar yields. 

A plethora of different acyclic and cyclic diaza dienes has been employed in aza Diels-Alder reactions. With regard to acyclic dienes, the main interest has focused on the cycloadditions of 1,3-diaza-1,3-butadienes. A current example of these transformations is the preparation of highly substituted pyrimidine derivatives such as 3-65 by cycloaddition of diaza-1,3-butadienes e.g. 3-64 with electron-deficient acetylenes (Fig. 3-20) [299]. 

Chiral (Z)-l-(alkylsulfinyl)-2-nitroalkenes underwent Lewis acid-promoted Diels-Alder reactions, with complete control of the diastereoselectivity as well as endo selectivity, to yield single cycloadducts.114 Enantiopure (-)-trans-benzo[d ]dithi-ine-S.S -dioxide underwent Diels-Alder cycloadditions with a series of cyclic dienes, affording adducts with diastereoselectivities ranging from fair to high.113... 

Hetero-Diels-Alder Reactions of Aldehydes. Cyclic conjugated dienes, such as 1,3-cyclohexadiene, are excellent substrates for the hetero-Diels-Alder reaction with ethyl glyoxylate catalyzed by Cu[(5, 5 )-t-Bu-box] (OTf)2 (eq 7). The rate of this reaction is dependent on the counterion and the solvent. To obtain... 

Simple, neutral electron-rich imines are generally not useful dienophiles unless q>propriately reactive dienes are used. Thus it is possible to effect cycloadditions of neutral imines with some electron-deficient dienes. - In a series of p rs, Langlois and coworkers have described Diels-Alder reactions of cyclic imines and methyl 2,4-pentadienoate to afford adducts used in alkaloid synthesis. For example, the cycloadditicMi shown in equation (IS) was used in a total synthesis of the indole alkaloid vincamine (45). 

The reactions of p-nitrostyrene (81a) with both acyclic and cyclic enol-ethers have been studied. In general, when electron-rich alkenes interact at 1.5 GPa with p-nitrostyrene (81a), mixtures of bicyclic or tricyclic regioisomers are obtained. For example, the reaction of 81a with enol ether 86 (Scheme 7.21) led to a 7 3 mixture of compounds 87 and 88. p-Nitrostyrene (81a) first reacts as an electron-poor diene in an inverse electron demand Diels-Alder reaction with the enol ether, and then as an electron-poor dipolarophile with the formed monoadduct in a 1,3-dipolar cycloaddition. 

It is remarlmble that 0.1 mol equiv. of this chiral acyloxyborane complex induced fast (-78 C) and highly enantioselective Diels-Alder reactions of cyclic or acyclic 1,3-dienes with simple acrylic acid (entry 1), " or a,3-unsaturated aldehydes (entries 2-9)."" Table 32 reveals a striking stereomrecting... 

In a Diels-Alder reaction, a conjugated diene reacts with a dienophile to form a cyclic compound in this concerted [4 + 2] cycloaddition reaction, two new a bonds are formed at the expense of two tt bonds. The conjugated diene must be in an s-cis conformation. The reactivity of the dienophile is increased by electron-withdrawing groups attached to the sp carbons. The HOMO of one reactant and the LUMO of the other are used to show the transfer of electrons between... 

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