Cycloaddition 3 + 2 , , and

Apart from the thoroughly studied aqueous Diels-Alder reaction, a limited number of other transformations have been reported to benefit considerably from the use of water. These include the aldol condensation , the benzoin condensation , the Baylis-Hillman reaction (tertiary-amine catalysed coupling of aldehydes with acrylic acid derivatives) and pericyclic reactions like the 1,3-dipolar cycloaddition and the Qaisen rearrangement (see below). These reactions have one thing in common a negative volume of activation. This observation has tempted many authors to propose hydrophobic effects as primary cause of ftie observed rate enhancements. 

Figure 10 12 shows the interaction between the HOMO of one ethylene molecule and the LUMO of another In particular notice that two of the carbons that are to become ct bonded to each other m the product experience an antibondmg interaction during the cycloaddition process This raises the activation energy for cycloaddition and leads the reaction to be classified as a symmetry forbidden reaction Reaction were it to occur would take place slowly and by a mechanism m which the two new ct bonds are formed m separate steps rather than by way of a concerted process involving a sm gle transition state... 

The chemistry of ketenes is dominated by their high reactivity most of them are not stable under normal conditions, many exist only as transient Species. Nucleophilic attack at the j -carbon, [2 + 2] cycloadditions, and ketene iasertion iato single bonds are the most important and widely used reactions of such compounds. 

Methacryhc acid and its ester derivatives are Ctfjy -unsaturated carbonyl compounds and exhibit the reactivity typical of this class of compounds, ie, Michael and Michael-type conjugate addition reactions and a variety of cycloaddition and related reactions. Although less reactive than the corresponding acrylates as the result of the electron-donating effect and the steric hindrance of the a-methyl group, methacrylates readily undergo a wide variety of reactions and are valuable intermediates in many synthetic procedures. 

Photochemical Reactions. Increased knowledge of the centraUty of quinone chemistry in photosynthesis has stimulated renewed interest in their photochemical behavior. Synthetically interesting work has centered on the 1,4-quinones and the two reaction types most frequentiy observed, ie [2 A 2] cycloaddition and hydrogen abstraction. Excellent reviews of these reactions, along with mechanistic discussion, are available (34,35). 

Solvent has an important influence on the course of this cycloaddition, and in the reaction of 2,5-dimethylthiazole with DMAD in DMF the product analogous to (415) was obtained. However, in DMSO or acetonitrile a thiazolo[3,2-a]azepine was formed in addition to this product, whereas with THF, dichloromethane or nitromethane, only the thiazoloazepine was isolated. 

Scheme 13.1. Some Examples of Photochemical Cycloaddition and Electrocyclic Reactions...

Scheme 13.1 lists some example of photochemical cycloaddition and electrocyclic reactions of the type that are consistent with the predictions of orbital symmetry considerations. We will discuss other examples in Section 13.4. 

Large, rigid molecules with functional heterocyclic units (effectors) located on a carbocyclic framework and taking part in 1,3-dipolar cycloaddition and heterodiene reactions 98SL566. 

Different azanthraquinones 390-392 were prepared from 3-amino-4-imino-4//-pyrido[l,2-a]pyrazines 373 with 1,4-quinones in one pot reactions via [4-1-2] cycloaddition and the subsequent ring transformation (Scheme 9) (97T5455). 

The reaction course of the cycloaddition reaction can also be dependent on the Lewis acid complex used as the catalyst. When the substrate contains an allylic C-H bond, both a cycloaddition and an ene reaction can occur. In the reaction of glyoxylate 4 with 2,3-dimethyl-l,3-butadiene 5 both the cycloaddition product 6... 

Chiral C2-symmetric bisoxazoline-copper(II) complexes [30, 31] were introduced as catalysts for cycloaddition and ene reactions of glyoxylates with dienes [9] leading to intense activity in the use of these catalyst for different cycloaddition reactions. 

Desimoni et al. have found that chiral BOX-manganese(II) complexes can catalyze the intramolecular cycloaddition and ene reactions, with the latter as the major product [37]. 

The photochemical cycloaddition of a carbonyl compound 1 to an alkene 2 to yield an oxetane 3, is called the Patemo-Buchi reaction - This reaction belongs to the more general class of photochemical [2 + 2]-cycloadditions, and is just as these, according to the Woodward-Hofmann rules, photochemically a symmetry-allowed process, and thermally a symmetry-forbidden process. 

A two-step sequence of nitrile oxide-olehn cycloaddition and reduction of the resulting A -isoxazolines offers a unique and attractive alternative to the classical aldol reaction and its many variants (2J). The procedure bypasses traditional problems, including enolate equilibrium and cross condensation (20). 

The following reaction takes place in two steps, one of which is a cycloaddition and the other of which is a reverse cycloaddition. Identify the two peri-cyclic reactions, and show how they occur. 

The synthesis of monocyclic thiepins from thiophene and dimethyl acetylenedicarboxylate is often accompanied by the loss of sulfur. In particular, in cases where room temperature is required for efficient rates of cycloaddition and rearrangement76 (see Section 2.1.3.3.), the desulfurization reaction proceeds rather quickly with the consequence that thiepin formation can be monitored by low temperature HNMR spectroscopy, but the products cannot be isolated.76 - 78 However, in the case of thiepin 1 where R1 = R2 = C02Me and R3 = H, refluxing toluene is necessary for the extrusion of sulfur.78... 

In a sequence of cycloaddition and cycloreversion, 3-phenyl-l, 2,4-triazine 1-oxides react with benzyne, generated from 2-aminobenzoic acid (see Houben-Weyl, Vol. 5/2 b, p 622 ff), to give 2-phenyl-l, 3-benzoxazepines in moderate yield.419... 

The preparation of 5-ACETYL-l,2,3,4,5-PENTAMETHYLCYCLO-PENTADIENE is of value in the synthesis of pentamethyleyclo-pentadiene and many pentamethylcyclopentadienyl metal carbonyl derivatives that are more soluble in organic solvents than those derived from cyclopentadiene. Simple preparations of 5,6-DIHYDRO-2-PYRAN-2-0NE and 2-//-PYRAN-2-ONE make these hitherto rather inaccessible intermediates available for cycloaddition and other reactions. The already broad scope of the Michael reaction has been widened further by including an efficient preparation of ETHYL (E)-3-NITROACRYLATE. Workers in the field of heterocyclic chemistry will find a simplified method for the preparation of 2,3,4,5-TETRA-HYDROPYRIDINE of help. 

All of the butenes, pentenes, and hexenes were examined, taken in pairs. Methyl substituents increase the rates of cycloaddition, and different isoselective relationships... 

An interesting cycloheptatriene (182) synthesis has been described using thiophene 1, 1-dioxides (180) and cyclopropenes 181 (equation 121)ns. Concerted [4 + 2]cycloaddition and subsequent cheletropic extrusion of sulfur dioxide are suggested by the second-order kinetics (first in each reactant), and by the large negative activation entropy. 

Abstract The photoinduced reactions of metal carbene complexes, particularly Group 6 Fischer carbenes, are comprehensively presented in this chapter with a complete listing of published examples. A majority of these processes involve CO insertion to produce species that have ketene-like reactivity. Cyclo addition reactions presented include reaction with imines to form /1-lactams, with alkenes to form cyclobutanones, with aldehydes to form /1-lactones, and with azoarenes to form diazetidinones. Photoinduced benzannulation processes are included. Reactions involving nucleophilic attack to form esters, amino acids, peptides, allenes, acylated arenes, and aza-Cope rearrangement products are detailed. A number of photoinduced reactions of carbenes do not involve CO insertion. These include reactions with sulfur ylides and sulfilimines, cyclopropanation, 1,3-dipolar cycloadditions, and acyl migrations. 

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