Cycloaddition with dienes

Concerted Nonpolar Reactions. Maleic anhydride exemplifies the model dienophile for cycloaddition with dienes such as 1,3-butadiene... 

A versatile synthetic route to enantiomeric ally pure Diels-Alder adducts was deduced and found dependent on the application of enantiomerically pure 5-methoxy-174a (R=Me) and 5-(l-menthyloxy)-2(5//)-furanones 174b (R = menthyl), which were expected to undergo tt-face-selective cycloaddition with dienes. The reaction was effected by heating no Lewis acid catalysts were required (Scheme 55) (88JOC1127). 

Other advances include the construction of seven- and nine-membered rings via the analogous [4-1-3] and [6-1-3] cycloadditions with dienes and trienes respectively. Heterocycles, such as tetrahydrofurans and pyrrolidines, are accessible using carbonyl compounds and imines as substrates. The following discussion is organized around these recent discoveries. It serves to illustrate the versatility and the high degree of selectivity which are some of the distinctive features of the Pd-TMM chemistry. 

The tricarbonylchromium complex 6 of methyl 1/f-azepine-l-carboxylate undergoes photoin-duced [6 + 4] 7t-cycloadditions with dienes, e.g. 7, to give, after decomplexation with oxygen, azabicyclo[4.4.1]undecanes, e.g. 8, in high yields.276... 

There has been some investigation of auxiliary-controlled cycloadditions of azir-ines. Thus, camphor-derived azirine esters undergo cycloaddition with dienes, with poor diastereoselectivity [70]. The same azirines were also observed to react unselectively with phenylmagnesium bromide. Better selectivities were obtained when Lewis acids were used in the corresponding cycloaddition reactions of 8-phe-nylmenthyl esters of azirine 2-carboxylates (Scheme 4.48) [71]. The same report also describes the use of asymmetric Lewis acids in similar cycloadditions, but mediocre ees were observed. 

Singlet oxygen undergoes [4 + 2] cycloaddition with dienes. 

Thus BCP seems to follow two competitive pathways in the cycloaddition with dienes (i) a stepwise diradical process giving the [2 + 2] adduct, or (ii) a concerted pathway giving the [4 + 2] adduct. Accordingly, the proportion of the latter increases with the reactivity of diene in Diels-Alder reactions. Conversely, the reaction with 2,3-dicyanobutadiene (529), generated in situ by electrocyclic ring-opening of 1,2-dicyanoeyelobutene [142], furnishes selectively the [2 + 2] cycloadduct 530 (Table 42, entry 4) due to the presence of substituents able to stabilize the diradical intermediate [13b],... 

Diels-Alder reactions between dienes and highly reactive dienophiles such as alkyl vinyl ketones can be carried out using the corresponding acetals in order to prevent polymerization of the dienophile. Activation of the masked dienophiles to undergo ionic [4 + 2] cycloadditions with dienes can be by EGA using the conditions in Scheme 18 [44]. Yields in the range 20 to 85% were obtained depending on the... 

Section 14.2 describes the highly stereoselective cyclopropanation chemistry of the donor/acceptor-carbenoids (Fig. 14.1a) [16]. This section introduces the range of vinyl, aryl, alkynyl, and heteroaryl functionalities that have been used as donor groups in this chemistry. Also, chiral auxiliaries and chiral catalysts that achieve high asymmetric induction in this chemistry are described [25]. The next two sections cover chemistry that is unique to the vinylcarbenoid system, namely [3-t4] cycloaddition with dienes (Fig. 14.1b see also Section 14.3) [13] and [3-1-2] cycloaddition with vinyl... 

The Dewar thiophene (2), possessing a strained double bond substituted with highly electronegative groups, undergoes cycloaddition with dienes (77JCS(P1)2355) and azides... 

A large number of metal-coordinated chalcogenacycles were prepared from the heteroaldehyde and -ketone complexes [M(CO)5 E=C(Ph)R ] (M = Cr, Mo, W E = S, Se, Te R = H, Aryl) and 1,3-dienes. The cycloadditions proceeded rapidly, even at low temperatures.179180,228-233 As dienes 2,3-dimethyl-l,3-butadiene [for an example see Eq. (27)], isoprene, transA, 3-pentadiene, cyclopentadiene, pentamethylcyclopentadiene, and 1,3-cyclohexadiene were used. Decomplexation with pyridine/THF yielded the free heterocycles. Although the tellurobenzaldehyde complex [W(CO)5 Te = C(Ph)H ] (99c) proved too unstable for isolation, it could be generated and employed in subsequent cycloadditions with dienes.180... 

The benzothiazepinone 448 is a source of 2-substituted 3,6-dihydro-2-//-thiopyrans through its conversion to the bridgehead salt 449 by a [4+2+] cycloaddition with dienes and subsequent reduction (Scheme 154)... 

The allyl cation has three atoms but only two electrons so it can take part in cycloadditions with dienes—the total number of electrons is the required six. This is one of the few reactions that works only to produce a seven -me mbered ring. 

Alkenyl complexes can undergo cycloaddition with dienes. Thus the complex 6, prepared from vinyllithium, can be used as an alternative to the less reactive methyl acrylate. The cycloadducts can undergo various useful transformations (Scheme I). The reaction of 6 with cyclopentadiene gives a mixture of the endo- and exo-adducts in the ratio 94 6 in 78% yield. 

R)- and (S)-f-Butyl-5-methylene-l,3-dioxolan-4-one, a Chiral a-Alkoxy Acrylate. It is also possible to introduce an ex-ocyclic double bond onto the dioxolanone ring, as in compounds (9)-(ll), derived from lactic and malic - acids. These a,3-unsaturated carbonyl derivatives are acceptors for radical additions and undergo cycloadditions with dienes and heterodienes. The Diels-Alder adduct (12) of ent-(9) with cyclopentadiene is formed with exo selectivity (96 4) and serves as aprecursor to norbomenone (13). Cycloadduct (14), obtained from methylenedioxolanone (9) and an open-chain triene, is also the result of an exo addition and is used in tetronolide synthesis. ... 

Cychadditioas. Treatment of 2-methoxyallyl bromide with silver trifluoroacetale (1, 1018-1019) under carefully controlled conditions generates the 2-methoxyallyl cation (a), which undergoes cycloaddition with dienes. For example, the bromide in... 

Ditertiary phosphane complexes of nickel were found to be effective in the formation of pyrone 108 by cyclocotrimerization of alkynes with carbon dioxide. The formation of the nickelacyclopentadiene 105 from two moles of alkyne and a nickel complex is followed by CO2 insertion into a nickel-carbon bond to give the oxanickelacycloheptadienone 106, which then eliminates 108 with intramolecular C—O coupling. Another route involving [4 + 2] cycloadditions of 105 with CO2 in a Diels - Alder reaction to give 107 cannot be ruled out but is less probable because CO2 does not undergo [4 + 2] cycloaddition with dienes. Addition of another alkyne to 105 results in the formation of a benzene derivative (Scheme 38). ... 

Cycloaddition reactions of the C(3)=N bond of azirines are common (Scheme 45) <71AHC(13)45, B-83MI 101-03,84CHEC-I(7)47>. Azirines can participate in [4 + 2] cycloadditions with dienes including cyclopentadienones, isobenzofurans, triazines, and tetrazines. They also participate in 1,3-dipolar cycloadditions with azomethine ylides, nitrile oxides, mesoionic compounds, and diazomethane. Cycloadditions with heterocumulenes, benzyne, and carbenes are known. Azirines also participate in other pericyclic reactions, such as ene reactions. 

Pyrrolidone 41, obtained by literature procedures from phenylalanine, was acylated adjacent to the amide carbonyl to give 42, followed by oxidation to the dienophile 43. The ensuing cycloaddition with diene 40 afforded the funtionalized octahydroisoindole 44 in 77% yield from 41. The reaction mixture from the room-temperature Diels-Alder reaction was contaminated with small amounts of an undetermined isomeric by-product (44 by-product >15 1). The additional carbons required for the macrocyclic ring were now appended via a cleverly conceived sulfur-ylide ring expansion. 

In other examples of the use of azo compounds in pyridazine synthesis, dialkyl azodicarboxylates undergo cycloaddition with dienes to give 1,2,3,6-tetrahydropyridazines. Azobis(formamidine) reacts similarly. Diethyl azodicarboxylate also reacts with furans to give an adduct which is transformed into a pyridazine by acid. In a photochemical synthesis,... 

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