3- tert-Butylthiophene

Nakayama and coworkers reported that 3,4-di-tert-butylthiophene 1-oxide 114 is thermally stable but still an extremely reactive substrate. They reported that the Diels-Alder reactions of 114 with varieties of electron-deficient and electron-rich dienophiles took place exclusively at the syn-n-face of the diene with respect to the S=0 bond (Scheme 53) [62, 63]. 

Scheme 53 Diels-Alder reactions of 3,4-di-tert-butylthiophene 1-oxide 114 with varieties of dienophiles...

When 2,5-fc rf-bu tylthiophene S-oxide (688) is photoirradiated, 2,5-tert-butylfuran (689) is produced almost quantitatively. When 2,4-tert-butylthiophene S-oxide is photoirradiated, 2,4-ferf-butylfuran is isolated. Here, however, also side products can be observed. 

An unusual methylene-transfer reaction along with [4-I-4] cyclodimerization was observed, when 5,A-d5-tert-butylthiophene 1-oxide was refluxed in toluene with 2-methylene-l,3-dimethylimidazolidine <2001GL758>. Initially a Michael adduct is formed by the addition of 104 to 103. This adduct upon intramolecular cyclization gives the cyclopropyl compound with elimination of a carbene. This on further oxygen-transfer reaction gives products 105 and 106. The 1,4-Michael adduct on further Michael reaction with 103 produces another adduct, which on cyclization followed by elimination of 104 gives 107 (Scheme 25). 

Raman spectra have been reported of only three compounds, 2,5-di-tert-butylthiophene, benzo[fr]thiophene, and 3-methylbenzo[b]thiophene 1,1-dioxides [302]. 

The stereochemistry of the Diels-Alder reaction was investigated by the reactions of 3,4-di-tert-butylthiophene 1-oxide with cis- and lrans-3-hexenes. The reactions occur on heating 3,4-di-tert-butylthiophene 1-oxide and an excess of the alkene at 100°C in a sealed tube and demonstrated that Diels-Alder reactions take place exclusively at the sy -jr-face of 3,4-di-tert-butylthiophene 1-oxide (Scheme 20) [33]. 

Thiophene 1-oxides can also react with sulfur compounds through Diels-Alder reactions to afford the corresponding sulfur-containing cycloaddition products. For example, disulfur monoxide (S2O) can undergo a yn-Jt-face-selective Diels-Alder reaction with 3,4-di-tert-butylthiophene 1-oxide, leading to the sulfur-containing product 47 (Scheme 43) [47, 48]. 

Though normally, Diels-Alder reactions of 3,4-di-tert-butylthiophene 1-oxide and related compounds occur exclusively at the syn-n-face, exceptions have also been observed. For example, in its reaction with thiobenzophenone, two diastereo-mers 50 and 51 were obtained. The major diastereomer 50 was isolated in 76% yield, but the minor diastereomer 51 was also produced, in 15% yield, the conformation of which was established by X-ray diffraction analysis (Scheme 46). This example reveals that the reaction takes place predominantly at the syn-jr-face to the S=0 bond, though not exclusively. The jr-face selectivity was explained in terms of the extent of conformational changes of 3,4-di-[Pg.248]

Category F Bis-Adduct Formation. The primary adducts, cyclohexadiene derivatives, which are formed by Diels-Alder cycloaddition of thiophene 1,1-dioxides with dienophiles upon the loss of a SO2, may undergo further Diels-Alder cycloaddition with the dienophile. Thus, the reaction of 3,4-di-tert-butylthiophene 1,1-dioxide with 4-phenyl-l,2,4-triazoline-3,5-dione (PTAD) provides a unique pyridazine synthetic route since the bis-adducts 102 are converted into the corresponding pyridazine 103 in one pot and in good yield by treatment with KOH in methanol (Scheme 98) [120]. 

Scheme 98 Synthesis of 4,5-di-t-butylpyridazine through Diels-Alder cycloaddition of 3,4-di-tert-butylthiophene 1,1-dioxide with 4-phenyl-l,2,4-triazoline-3,5-dione (PTAD) [120]...

Sulfur monoxide, generated in situ by thermolysis of thiirane 5-oxide, reacts with 1,3-di-tert-butylbutadicne to give 2,5-dihydro-2,4-di-fert-butylthiophene 1-oxide (1). Although two diastereomeric forms of 1 are possible, only one diastereomeric sulfoxide was isolated75. 

Silica gel or alumina was used as catalyst for the Diels-Alder reaction of 3,4-di-ferf-butylthiophene 1-oxide with 1,4-benzoquinone to produce 6,7-di-terf-butyl-1,4-naphthoquinone 36 and bis-adduct 2,3,6,7-tetra-tert-butylanthraquinone 37 [40]. The silica gel (alumina)-catalyzed enohzation of 38 to form 39 may play an important role in the formation of 36 (Scheme 26). 

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