Hetero-1-thiabutadienes

The 4-oxa-l-thia-l,3-butadienes, the acyl sulfines 25 and acyl sulfenes 26, which have been shown to be capable of 4ir participation in selected Diels-Alder reactions, have been summarized in the preceding section. It 

A wide range of 3-aza-l-thia-l,3-butadiene systems has been extensively investigated as 4ir components of Diels-Alder reactions. The work with each of these systems is summarized in Chapter 9 and includes the 4tt Diels-Alder reactions of appropriately substituted At-thioacylimines (e.g., 29, Chapter 9, Section 3)/ thioacyl isocyanates (e.g., 30, Chapter 9, Section 3), thioacyl isothiocyanates (e.g., 31, Chapter 9, Section 3), and N-thioacyl dithioimidocarbonates (e.g., Examples of 

A growing class of potentially useful polar [4+ + 2] cycloadditions employing in situ generated 2-thieniumbutadiene have been reported. The apparent 4ir participation of arylthienium salts in polar [4+ + 2] cyclizations with alkenes, - alkynes, and nitriles has been detailed [Eqs. (21)-(23)], 

Hoffmann, R., and Hartke, K. (1980). Chem. Ber. 113, 919. The a,/3-unsaturated thionoesters behave similarly. 

Okazaki, R., Ishii, R., Ozawa, Kazumi, Ozawa, Ketiji, and Inamoto, N. (1975). J. Chem. Soc., Perkin Trans. /, 270. 

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The hetero-Diels-Alder reaction of homochiral camphor-derived thiabutadienes has given, for the first time, optically active bornene ring-fused dihydrothiopyrans with high diastereoselectivity (Scheme 53) <2000H(53)1685>. The exo endo- t2X o varied with the nature of substituent, varying from 91 9 (X = CH2, R = R = H) to 1 99 (X = CO,... 

Mechanistic studies were performed for simple systems such as 1-thiabutadiene and ethylene481, although dimerization and cycloaddition of thiochalcones have received more attention, including several calculations482-484. Nevertheless, more synthetic utility possesses the uses of Lewis acids as catalysts in hetero-Diels-Alder reactions (equation 142)485,486 involving 1-thiabutadienes. 

Alder reactions of 1-thia-1,3-butadienes with electron-deficient or conjugated dienophiles provide 3-sub-stituted 3,4-dihydro-2ff-thiopyrans (equation 6). Representative Diels-Alder reactions of 1-thia-1,3-butadienes are presented in Table 7 and hetero-l-thiabutadienes that have been demonstrated to participate in [4 + 2] cycloaddition reactions are summarized in Figure 4. ... 

The chiral Ni(II) complex shown in Sch. 58 catalyzes the enantioselective hetero-Diels-Alder reactions of thiabutadienes with 3-(2-propenoyl)-2-oxazolidinone to afford optically active dihydrothiopyrans [204]. Similar results are obtained with analogous Cu(II) complexes where catalyst loading can be reduced when molecular sieves are added. 

The first catalytic, highly enantio selective hetero Diels-Alder reactions of thiabutadienes with an acyloxazolidinone dienophile using homochiral copper and nickel triflate and perchlorate bis(oxazoline) and bis(imine)complex catalysts to generate dihydrothiopyrans were reported by Saito et al. [155]. 

Saito, T, Takekawa, K, Takahashi, T, The first catalytic, highly enantioselective hetero-Diels-Alder reaction of thiabutadienes, Chem. Commun., 1001-1002, 1999. 

The aza-l-thia-1,3-butadienes are the most widely recognized and thoroughly investigated hetero-l-thiabutadienes capable of 4w Diels-Alder participation. A single report has detailed the [4 + 2] cycloaddition of the selected 4-aza-l-thia-l,3-butadiene 28 as the 47t component of a Diels-Alder reaction [Eq. (20)].55... 

GPa at 20 °C in the presence of t-BuOH an excellent cis/trans selectivity of 8 92 and a yield of 95 % was observed. Again, in this case it must be assumed that the endo-Z-anti transient of the 1-oxa-l,3-butadiene moiety is preferred under high pressure. Also, for this reaction a cooperative pressure and temperature effect seems to be effective. It should be noted that the dihydrothiapyrans 85/86 as well as the dihydropyrans 88/89 can be obtained very efficiendy by a three-component domino-Knoevenagel-hetero-Diels-Alder reaction [1-4] starting from the phos-phonates, a suitable aldehyde and an enol ether without preparing the oxa- and thiabutadienes separately. 

Hetero-Diels-Alder reaction. When catalyzed by nickel perchlorate the condensation of Ai-acrylyloxazolidinone with thiabutadienes gives dihydrothiapyrans. 

Yb(OTf)3 was effective for asymmetric induction in hetero Diels-Alder reaction of thiabutadiene and electron-deficient olefin bearing chiral auxiliary [37]. 10 mol% of catalyst effected the reaction into giving cycloadduct in excellent Tc-facial selectivity, whereas selectivity was reversed with modest degree in the absence of the catalyst (Scheme 13.15). The reaction in which a coordinative solvent such as dimethylsulfoxide (DMSO) or dimethylformamide (DMF) displayed similar selectivity came from that of noncatalyzed reaction, suggesting well-organized coordination of substrates to Yb. 

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