Thiabutadienes, Cycloadditions

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. 

The first example of an enantioselective thiadiene cycloaddition involved the reaction of 2,-4-diphenyl- 1-thiabuta-1,3-diene with l-propenoyl-l,3-oxazolidin-2-one. Stoichiometric quantities of a copper triflate bis-imine complex catalyst 428 and 4 A molecular sieves are necessary to achieve the highest enantioselectivity and the best endojexo ratio. The absolute configuration of the major endo isomer was determined by reduction of the acyloxazolidine side chain to the known (3/ ,4/ )-5-hydroxymethyl derivative (Scheme 137) <1997J(P1)2957>. The process is improved using a homochiral Cu triflate or Ni perchlorate bis(oxazoline) complex when catalytic amounts are adequate for a range of thiabutadienes <1999CC1001>. 

In an extension of the above hDA reaction, the cycloaddition of the thiabutadiene with maleic anhydride gives a 2-amido-277-thiopyran by way of elimination and addition of Me2NH to the anhydride unit (Equation 128) <2001M947>. 

The introduction of a bulky, optically active amino function at C-2 of 4-phenyl-l-thiabutadiene and activation by the addition of AcCl prior to cycloaddition with Wphenylmaleimide, known to exhibit a high endo preference, results in the formation of two diastereomers. However, the exo product is the major component and indeed this diastereo-mer is the sole adduct when cyclopentene is used as the dienophile. Calculations point to a preference for exo addition from the same face of the molecule as the naphthyl moiety and this is supported by the observed R stereochemistry at the three chiral centers in the cyclopenta[A]dihydrothiopyran 434 (Scheme 140) <1996TL123>. 

In the same field Guingant and Pradere have developed an asymmetric cycloaddition of 3-aza-l-thiabutadienes. The chiral auxiliary was brought here by the dienophile which is an unsaturated Evans amide [232]. It is remarkable that both enantiomers were selectively obtained by proper choice of the conditions catalysis of MgBr2 at 0°C or a pressure of 10 kbar at 20 °C (Table 4, entry 8). 

In contrast to the thorough studies of the Diels-Alder reactions of oxa- and aza-butadienes, the [4 + 2] cycloaddition reactions of 1-thia-1,3-butadienes have not been studied extensively.Although the general participation of thiabutadienes in LUMOdiene-controlled [4 + 2] cycloaddition reactions has been recognized and experimentally verified, most investigations have detailed their 4rr participation in HOMOdiene-controlled reactions with typical electron-deficient dienophiles. In such instances, the complementary C-2 or C-4 addition of electron-donating substituents to the 1-thia-1,3-butadiene increases the rate and regioselectivity of its Ait participation in HOMOdiene-controlled Diels-Alder reactions. With notable exceptions, the LUMOdiene-controlled Diels-Alder reactions provide the expected 2-substituted 3,4-dihydro-2//-thiopyrans and, unlike simple 1-oxa-1,3-butadienes, the HOMOdiene-controlled Diels-... 

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 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... 

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