2,3-Dimethylbuta-1,3-diene cycloaddition with

Thiopyrylium ions have been shown to behave as effective dienophiles. Shimizu and co-workers demonstrated that 2-benzothiopyrylium salts undergo [4+2+]-type cationic polar cycloadditions with various 1,3-dienes yield benzo-fused bicyclic sulfonium salts <1994J(P1)3129> (see Section 8.32.2.1.2 of CHEC-II(1996)). However, in this study the authors observed that cycloaddition of 2-benzothiopyrylium salt 40 with 2,3-dimethylbuta-l,3-diene 41a with prolonged reaction times caused a decrease in yield of the cycloadduct 22a (see entries 1 and 2 in Table 5 and Equation 10). In fact, stirring of the reaction mixture for 30 h did not afford the cycloadduct at all, see entry 3, but gave an undetermined complex mixture. 

Benzothiopyrylium salts have also been shown to undergo [4+2+]-polar cycloadditions with 1,3-dienes in dry 1,2-dichloroethane at room temperature <1996J(P1)2227>. Treatment of 43 with 2,3-dimethylbuta-l,3-diene 41a afforded benzo-fused bicyclic sulfonium salts 22, as shown in Equation (12) and Table 6. The 4-cyano-substituted benzothiopyrylium salt 43d proved to be the most reactive of these substrates (see entry 4). 

Recently, much attention has been paid to hetero Diels-Alder reactions as powerful tools for the construction of heterocyclic compounds. For example, cycloaddition of 2,3-dimethylbuta-l,3-diene 41a with 1,2-thiazinylium salt 95, in acetonitrile at room temperature, resulted in the exclusive formation of product 76a resulting from cycloaddition across the C—S1 bond (see entry 1 in Table 15 and Equation 26) <1999TL1505>. Similarly, isoprene 41b and... 

Hetero-Diels-Alder Cycloaddition with Dienes. Under mild conditions, l,l -thiocarbonyldibenzotriazole readily reacts with cyclopentadiene to afford the hetero-Diels-Alder cycloadduct (4) (eq 2) in 98% yield. Analogous reaction with 2,3-dimethylbuta-diene, carried out under harsher conditions, was accompanied with elimination of a benzotriazole molecule to give benzotriazolyl-substituted thiopyran (5) (eq 3). ... 

Trithiocarbonate 55 -dioxides ArS02CSaAr undergo [4 + 2] cycloaddition with 1,3-dienes to afford the unstable A -dihydrothiopyrans (1), which readily lose ArSOaH to give the corresponding 2J -thiopyrans. Addition of ArSOaH to these 2/f-thiopyrans gives the 4-arylsulphinyl-2-arylthio-A -dihydrothiopyrans isomeric with (1). Evidence for the existence of the unstable 4-thioxotetrahydro-furan-3-ones was obtained by trapping them with 2,3-dimethylbuta-l,3-diene, which yielded the spiro-compounds (2) these underwent unusual mass spectral... 

The Diels-Alder adduct isolated from the reaction with 2,3-dimethylbuta-l, 3-diene at elevated temperatures [200] is in fact the product of a two step reaction a 1,3-cycloaddition followed by a 3,2-sigmatropic rearrangement [199] (equaPon 49)... 

The cycloaddition reactions of the anhydrides (88a-88e) with acyclic dienes (butadiene and 1,4-dimethylbuta-l,3-diene) were conducted at 23 °C in dichlo-... 

Table 5 Polar cycloadditions of 2-benzothiopyrylium salt 40 with dimethylbuta-1,3-diene 41a (Equation 10)...

The cycloaddition of dienes to imines to form tetrahydropyridines (equation 46) has been investigated extensively40. Ordinary imines are not sufficiently reactive to add to dienes they have to be activated by the presence of electron-withdrawing substituents. Thus the triester 70 adds to cyclopentadiene under atmospheric pressure to form 71 (equation 47). The reactions with other dienes (cyclohexadiene, isoprene or 2,3-dimethylbuta-l,3-diene) require high pressures41. 

Amino-5-aryl-l,3,4-thiadiazole was treated with thionyl chloride in dry benzene to yield the N-sulfinylamine (93) an unstable compound, characterized by NMR and further derivatization. The sulfinylamine moiety caused an upheld shift on C(2) (4 6 ppm) comparable to a carbonyl or sulfonyl group, indicating the double-bond character of the N—S bond. Reaction of (93) with 2,3-dimethylbuta-1,3-diene yielded (94) via thermal cycloaddition <89JCS(P2)i855>. 

Cycloaddition and ene reactions. Dienes >C=C—C=C< such as buta-1,3-diene, isoprene, 2,3-dimethylbuta-l,3-diene, fraws-piperylene, cyclopentadiene or anthracene react with 92 in Diels-Alder fashion to give [2 + 4] cycloadducts 410 (equation 128)62. Ene products 411 are formed additionally when the relative reaction rates for the [2 + 4] cycloaddition reaction and the ene reaction are comparable (e.g. for isoprene and 2,3-dimethyl-l,3-butadiene) Alkenes with allylic hydrogen (propene, 2-butene, isobutene) give ene products see equation 129. 

The [4 + 2] cycloaddition of a-phenylselenopropenoyl trimethylsilane (75) with 2,3-dimethylbuta-1,3-diene is unusual in that a significant portion of product mixture consists of the hetero-Diels-Alder dihydropyran adduct 76. The phenylselenenyl substituent appears to be responsible for this unusual pattern of reactivity, since propenoyl trimethylsilane gives only the expected regioisomer (77, X = H) (Scheme 116)14. a-Selenenyl substituted a,/l-unsaturated acyl silanes such as 75 were used to prepare a series of substituted dienes in excellent yields through the addition of a-sulphinyl carbanions, Brook rearrangement and expulsion of sulphinate, in a reaction pathway recognisably more typical of acyl silanes (Scheme 117). 

Rhodium (Il)-catalysed 3 + 2- and 3 + 4-cycloaddition of diazocarbonyl compounds (7) with conjugated dienes (e.g. 2,3-dimethylbuta-l,3-diene) provides a simple and rapid route to dihydrofurans (8) and dihydrooxepins (9) in high yields. A stepwise mechanism involving delocalized zwitterions has been proposed for the formation of the cycloadducts (Scheme 2).3... 

Cycloaddition reactions of 4,5-dicyanopyridazine with 2,3-dimethylbuta-l,3-diene have been studied (94T(50)9189. Employment of ortho-directed metalation and cross-coupling reactions has been shown to provide a new route to the antidepressant minaprine 29 [93BSF( 130)488). 

Pyrans.— High-pressure [4 + 2] cycloaddition of i -(-)-menthyl glyoxylate and 2,3-dimethylbuta-l,3-diene has been studied, and although it was found that optical yields varied with pressure, the adduct had the (2f )-configuration regardless of pressure (Scheme 36). The [4 + 2] cycloaddition of unsaturated carbonyl compounds to vinyl ethers gives substituted dihydropyrans, a reaction also favoured by increased pressure (Scheme 37). ... 

Preparation of 7V-Sulfonylimines. The title sulfonamide has been used to generate iV-sulfinyl-2-(trimethylsilyl)ethanesulfo-namide (SES-NSO), which reacts with aldehydes in the presence of BF3 Et20 to form, after loss of SO2, A-sulfonylimine derivatives. These can be trapped with 2,3-dimethylbuta-1,3-diene in Diels-Alder cycloaddition reactions (eq 2) The SES group can then be removed using a fluoride ion source. ... 

The formation of dodecamethyl[6]radialene (72) by a Ni(0)-mediated [2+2+2] cycloaddition of 2,5-dimethylbuta-2,3,4-triene (70) has already been mentioned (see Section 4.2.2). Much better yields could be obtained, when 3,4-dibromo(or diiodo)-2,5-dimethylhexa-2,4-diene was exposed to a Ni(0) complex. Here, the [3]cumulene 70 is a likely reaction intermediate, and the product distribution ([4]radialene 71, [6]radialene 72, and its isomer 73 see Scheme 4.14) depends strongly on the solvent, with donor solvents favoring the formation of 72. With DMF as the solvent and in s/fw-generated Ni(PPh3)4 as a mediator, a yield of 63 and 50% could be achieved for 72 from the dibromo- and the diiododiene, respectively [65, 66]. 

---