Dimethyl cycloaddition with

Benzo[6]thiophene, 1,2-dimethyl-cycloaddition reactions with dichloroethene, 4, 793 Benzo[6]thiophene, 2,3-dimethyl-nitration, 4, 763... 

Heterocyclic enamines often undergo two-step 1,3 cycloaddition with methyl vinyl ketone. This involves electrophilic attacks by an olefinic carbon and by a carbonyl carbon (24,25). For example, 1,2-dimethyl-Zl -pyrroline (14), when treated with methyl vinyl ketone, produces 1,6-dimethyl-2,3,4,5-tetrahydroindole (15) (24). The requirement which must be met so that this type of cyclization reaction can take place is that the a position of the heterocyclic enamine be carbon substituted. This provides... 

Dimethyl acetylenedicarboxylate (80) undergoes initial 1,2 cycloaddition with acyclic enamines to form cyclobutene intermediates which immediately decompose into acyclic dienaminodiesters (94,95). When an acyclic n/c-enediamine is used instead of a simple acyclic enamine, a dienediamino-diester is produced via a cyclobutene intermediate (95a). A cyclization reaction of dimethyl acetylenedicarboxylate with an acyclic enaminoketone... 

Dihydropyrans have been produced by the 1,3 cycloaddition of methyl vinyl ketone (77) or acrolein (29-J7) with enamines (see Section II.A.2). S-Lactones have been formed as a side product in the reaction of dimethyl ketene with enamines (77), and as the primary products in the reaction of excess ketene with enamines derived from ketones (75) (see Section II.A.4). 

UV irradiation. Indeed, thermal reaction of 1-phenyl-3,4-dimethylphosphole with (C5HloNH)Mo(CO)4 leads to 155 (M = Mo) and not to 154 (M = Mo, R = Ph). Complex 155 (M = Mo) converts into 154 (M = Mo, R = Ph) under UV irradiation. This route was confirmed by a photochemical reaction between 3,4-dimethyl-l-phenylphosphole and Mo(CO)6 when both 146 (M = Mo, R = Ph, R = R = H, R = R" = Me) and 155 (M = Mo) resulted (89IC4536). In excess phosphole, the product was 156. A similar chromium complex is known [82JCS(CC)667]. Complex 146 (M = Mo, R = Ph, r2 = R = H, R = R = Me) enters [4 -H 2] Diels-Alder cycloaddition with diphenylvinylphosphine to give 157. However, from the viewpoint of Woodward-Hoffmann rules and on the basis of the study of UV irradiation of 1,2,5-trimethylphosphole, it is highly probable that [2 - - 2] dimers are the initial products of dimerization, and [4 - - 2] dimers are the final results of thermally allowed intramolecular rearrangement of [2 - - 2] dimers. This hypothesis was confirmed by the data obtained from the reaction of 1-phenylphosphole with molybdenum hexacarbonyl under UV irradiation the head-to-tail structure of the complex 158. 

In sharp contrast to the uncomplexed l//-azepine, which yields a C4 — C5 adduct, the tricarbonyliron complex of ethyl 1 W-azepine-l-carboxylate with dimethyl l,2,4,5-tetrazine-3,6-dicar-boxylate furnishes the C2 —C3 adduct 4 in excellent yield.273 Likewise, cycloaddition with 2,3,4,5-tetrachlorothiophene 1,1-dioxide yields adduct S.131... 

It is well-known that cycloaddition of dimethyl acetylenedicarboxylate with six-membered en-... 

Thiazolotriazines 636 (R = CO,Me) were prepared [84JCS(P1)2707] by cycloaddition of dimethyl acetylenedicarboxylate with triazine derivative 632. Derivatives of thiazolo[3,2-b][l,2,4]triazin-3,7-diones 637 have been formed (74JPR163) on reaction with aromatic aldehydes and diazonium salts to give 636 (R = Ar) and 638, respectively. Regioselective catalyzed... 

Engberts [3e, 9] has extensively investigated the Diels Alder reaction in aqueous medium. Recently Engberts and colleagues reported [9c] a kinetic study of a Diels Alder reaction of N-alkyl maleimides with cyclopentadiene, 2,3-dimethyl-1,3-butadiene and 1,3-cyclohexadiene in different solvents. The reaction rates of the cycloadditions with the open-chain diene relative to w-hexane are reported in Table 6.3. The aqueous medium greatly accelerates the Diels Alder reaction and the acceleration increases as the hydrophobic character of the alkyl group of the dienophile increases. These and other kinetic data [3e, 9], along with the observation that the intramolecular Diels-Alder reaction is also accelerated in... 

The sterically unbiased dienes, 5,5-diarylcyclopentadienes 90, wherein one of the aryl groups is substituted with NO, Cl and NCCHj), were designed and synthesized by Halterman et al. [163] Diels-Alder cycloaddition with dimethyl acetylenedicarbo-xylate at reflux (81 °C) was studied syn addition (with respect to the substituted benzene) was favored in the case of the nitro group (90a, X = NO ) (syrr.anti = 68 32), whereas anti addition (with respect to the substituted benzene) is favored in the case of dimethylamino group (90b, X = N(CH3)2) (syn anti = 38 62). The facial preference is consistent with those observed in the hydride reduction of the relevant 2,2-diaryl-cyclopentanones 8 with sodium borohydride, and in dihydroxylation of 3,3-diarylcy-clopentenes 43 with osmium trioxide. In the present system, the interaction of the diene n orbital with the o bonds at the (3 positions (at the 5 position) is symmetry-forbidden. Thus, the major product results from approach of the dienophile from the face opposite the better n electron donor at the (3 positions, in a similar manner to spiro conjugation. Unsymmetrization of the diene % orbitals is inherent in 90, and this is consistent with the observed facial selectivities (91 for 90a 92 for 90b). 

The combination of two successive [4+2] cycloadditions has already been described by Diels and Alder [la] for the reaction of dimethyl acetylenedicarboxylate with an excess of furan. A beautiful, more modern, example is the synthesis of pagodane (4-5) by Prinzbach [2], in which an intermolecular Diels-Alder reaction of 4-1 and 4-2 to give 4-3 is followed by an intramolecular cycloaddition. The obtained 4-4 is then transformed into 4-5 (Scheme 4.1). 

Di- and trisubstituted nitroalkenes tethered to dipolarophiles (unsaturated esters, nitriles) undergo tandem [4+2]/[3+2] cycloadditions with 2,3-dimethyl-2-butene or butyl vinyl ether in the presence of Lewis acids (Eq. 8.112). For the dimethylene tether, the E-configuration of the dipolarophile is preferred, and the products arise selectively from a syn-endo pathway.177... 

Recently, synthetic routes to longer ladderanes with C-C double bonds in the terminal rungs have been improved the [3]-, [5]-, and [7]ladderane derivatives were synthesized by the repeated cycloaddition of cyclobutadiene derivatives,30 and the [w]ladderane derivatives (w = 3,4,5,6,7,9) were synthesized by the successive and alternate cycloaddition with cyclobutadiene and dimethyl acetylenedicarboxylate.31 X-ray crystallographic analysis of the [5]ladderane derivative31 shows a corrugated backbone without twist, in contrast with saturated and unsubstituted ladderanes.22,23... 

A combination of an intramolecular [5+21-cycloaddition of the pyrone 110 followed by an intermolecular [4 + 2]-cycloaddition with dimethyl-butadiene leads to the fused 6,7,5-tricyclocarboxylic system 112 via 111 as described by Rodriguez et al (scheme 22).1621 The prepared skeleton is found in nature in several terpenes. The starting material for this domino reaction is obtained from maltol 109... 

Seitz and colleagues108 made 10-ethylcolchicide (138), a colchicine derivative, react with several dienophiles. The reaction of 138 with dimethyl acetylenedicarboxylate (57) afforded a single Diels-Alder adduct (139) which underwent a consecutive [3 + 2] cycloaddition with another equivalent of dimethyl acetylenedicarboxylate to give 140. The formal elimination of C2H6 afforded 141, whereas fragmentation led to 142 (equation 41). 

Like their stable heavy ketone congeners, the germanetellones Tbt(R)Ge = Te underwent [2 + 3] and [2 + 4] cycloadditions with mesitonitrile oxide and 2,3-dimethyl-l, 3-butadiene, respectively144 (Scheme 39). 

The 3 + 2-cycloaddition of 1,2-dithiophthalides with nitrilimines yields benzo[c]thio-phenespirothiadiazoles regioselectively. The azomethineimines isoquinolinium-iV-aryllimide and A-(2-pyridyl)imide readily undergo 1,3-dipolar cycloaddition with electron-deficient dipolarophiles, dimethyl fumarate and dimethyl maleate, to yield tetrahydropyrazolo[5,l-a]isoquinolines in high yield. ° The 1,3-dipolar cycloadditions of electron-poor 1,3-dipoles, bicyclic azomethine ylides (27), with ( )-l-A,A-dimethylaminopropene to yield cycloadducts (28) and (29) are examples of non-stereospecific cycloadductions (Scheme 9). The synthesis of protected... 

Although dimethyl acetylenedicarboxylate was reported earlier not to undergo cycloaddition with the acridizinium ion, it was later... 

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