Reactions with Thiabutadienes

Incorporation of the thiabutadiene moiety into a camphor framework resulted in the reaction with various dienophiles proceeding with complete Jt-facial selectivity and, in some instances, exo-selectivity. The stereochemistry of one product was confirmed by X-ray analysis. The (arylmethylene)thiocamphor compounds 429 were prepared by thionation of the corresponding ketones with Lawesson s reagent and exist as stable monomers (Equation 125) <1999TL8383>. 

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-... 

Asymmetric Diels-Alder reaction of 2-azadienes with acrylates using Cu(OTf)2 as a Lewis acid has been reported (Sch. 53) [99]. The reaction gives the exo product (244) with high enantioselectivity. Copper Lewis acid-mediated Diels-Alder reactions of thiabutadienes with oxazolidinone acrylate 196 have also been reported [100]. 

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]. 

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. 

Thiabutadienes undergo highly enantioselective hDA reactions in the presence of homochiral bis(oxazoline) and bis(imine) complexes with Cu and Ni (Scheme 37) <99CC1001>. Homochiral camphor-based thiabutadienes show good exo selectivity and give rise to bomene ring-fused dihydrothiopyrans (Scheme 38) <99TL8383>. 

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,... 

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>. 

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. ... 

Reactions of Achiral Thiabutadienes with Chiral Dienophiles... 

Reactions of Chiral Thiabutadienes with Achiral Dienophiles... 

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