Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

2//-Thiopyrans, Diels-Alder reactions

The combination of thionation by Lawesson s reagent [98] of oxoenamino-ketones 96 with normal electron-demand Diels-Alder reaction of conjugated aldehydes allows a variety of thiopyrans 97 to be synthesized by a regio-selective and chemoselective one-pot methodology [99] (Equation 2.28). Thionation occurred at the more electrophilic ketonic carbonyl group. O O... [Pg.69]

Normal Diels-Alder Reactions. Synthesis of Pyrones and Thiopyrans... [Pg.122]

Keywords chiral thiopyrans, intramolecular Diels-Alder reaction, diastereos-electivity, carbohydrates... [Pg.321]

The method has then been efficiently used to remove selectively the exocycHc methylsulfanyl group from phosphorylated thiopyranyl derivatives 85 resulting from hetero Diels-Alder reaction of a phosphono-dithioformate or dithioacetate (see Sects. 2.1.2 and 2.2.3). FimctionaHzed thiopyrans 86 [17,18,27a] are thus obtained (Scheme 25). Owing to this selective desulfanylation, phospho-nodithioesters can be used as heterodienophiles in place of the corresponding phosphonothioaldehyde, not described so far and probably very unstable. [Pg.179]

Introduction of trimethylsilyl substituents attached directly to the ot-carbon atom of a-(benzotriazol-l-yl)alkyl thioethers provide new opportunities. Thus, treatment of lithiated monosubstituted a-(benzotriazol-l-yl)alkyl thioethers with chlorotrimethylsilane produces a-(trimethylsilyl)alkyl thioethers 837. In reactions with hexamethyl-disilathiane and cobalt dichloride, thioethers 837 are converted to thioacylsilanes 838 that can be trapped in a Diels-Alder reaction with 2,3-dimethylbutadiene to form 2-alkyl-4,5-dimethyl-2-trimethylsilyl-3,6-dihydro-27/-thiopyrans 839 (Scheme 133) <2000JOC9206>. [Pg.94]

Several examples of ring transformation can be found in CHEC-II(1996) <1996CHEC-II(7)921 > and some are referred to earlier in this chapter (see Sections 10.23.9.1.1 and 10.23.9.2.1). The benzobisthietes 114 and 115 can be ring-expanded by Diels-Alder reactions with dimethyl fumarate to afford the corresponding thiopyrans in moderate yield, due to competing oligomerization (Equations 59 and 60) <1998TL9639>. [Pg.1260]

The use of isopropylidine acetals (112) as tethers in the intramolecular Diels-Alder reactions of dienes with alkenes facilitates the formation of civ-fused cycloadducts (113) from an endo transition state (Scheme 41).218 The intramolecular Diels-Alder reaction of 4-[tris-(2-mcthylcthyl)silyl]oxy-2//-thiopyran derivatives with potential dienophiles tethered at C(2), C(3), C(5), and C(6) positions yielded cycloadducts when the dienophiles were activated with a carbomethoxy group.219 By the substitution of a phenylsulfonyl group on the dienophile of 2-benzopyran-3-ones, it is possible to enhance exo addition during intramolecular Diels-Alder cyclizations to yield a predominance of trans-fused hexaphenanthrenes related to natural products.220 The intramolecular Diels-Alder reaction of 2-furfuryl fumarates has been investigated by molecular mechanics (SIBFA)/continuum reaction field computations.221 The intramolecular 4 + 2-photo-cycloaddition of A-benzylcinnamamides (114) in the presence of C(,H6 gives 3-azatricyclo[5.2.2.01,5]undeca-8,10-dien-4-ones (115) with high stereoselectivity (Scheme 42).222... [Pg.454]

The intermolecular Diels-Alder reactions of 1,3-dithioles (163) with DMAD and DEAD produced the first 7//-thieno[2,3-c]thiopyran-7-thiones (164) and 47/-thieno [3.2-c]thiopyran-4-thiones (165), the structures of which were confirmed by X-ray diffraction. Further work on the mechanism of the cycloaddition and subsequent rearrangement is in progress (Scheme 46).191... [Pg.382]

A further example of the use of 2//-thiopyrans as surrogates for m-substituted dienes involves the use of the protected 3,4-dihydio-3-(3-oxobutyl)A//-thiopyranA-onc, 3-[2-(2-methyl-l,3-dioxolan-2-yl)ethyl]-4-[tris(l-methy-lethyl)silyl)oxy-2//-thiopyran 328 as an equivalent of l-ethenyl-2-methylcyclohexene in Diels-Alder reactions. The thiopyran reacted with various maleimides to yield the endo cycloadducts and with methyl propenoate to give the exo adduct under either thermal or Lewis-acid-catalyzed conditions. In the latter case concomitant release of the protected ketone functions occurs, acid-catalyzed cyclization of which generates a fused cyclohexenone ring (Scheme 67). Desulfurization, preferably before the aldol cyclization, leads to derivatives of 2,3,4,4a,5,6,7,8-octahy-dro-4a-methylnaphthalenes < 1997CJC681 >. [Pg.832]

When chiral thiocarbonyl S-oxides derived from (S)-proline, prepared from the appropriate l-(trimethylsilylalkylsulfonyl)-2-(alkoxymethyl)pyrrolidine, are subjected to Diels-Alder reaction with 2,3-dimcthyl-l,3-butadiene, the 2-substituted 2-[(2-alkoxymethyl-l-pyrrolidinyl)sul-fonyl]-3,6-dihydro-4.5-dimethyl-2/f-thiopyran 1-oxides 10A and 10B are formed as a mixture of diastereomers. Their ratio depends on the nature of the substituents R1 and R2 as well as on the reaction temperature. The diastereomeric excess values, determined by H NMR in the presence of Yb(tfc)3 or by HPLC, vary from 0 to 41 % 84. [Pg.563]

Thiocarbonyl compounds of all kinds are excellent heterodienophiles. There has not been much systematic study of this class of reaction but it seems that such cycloadditions are generally regio-selective. One difference between carbonyl and thiocarbonyl Diels-Alder reactions is that the dihydro-thiopyran adducts from the latter dienophiles often undergo cycloreversion. The slow development of this field is peifaaps mainly due more to lack of methods for generating certain thiocarbonyl compounds than with difficulties in effecting cycloadditions. [Pg.435]

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-... [Pg.469]

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. ... [Pg.470]

The phosphonodithioformate (336) has been used as an interesting hetero-dienophile in the Diels-Alder reactions with different dienes. A selective radical desulfanylation of the cycloadducts (337) using BusSnH leads to new (3,6-dihydro-2H-thiopyran-2-yl) phosphonates (338)(Scheme 90). ... [Pg.167]

A Diels-Alder reaction occurs between the thioxo function of a thiocarbox-amide and a neighbouring ring-carbon (as a heterodiene) and the triple bond of DMAD to give a high yield of a fused thiopyran. The usefulness of heterodienes in this type of reaction has been reviewed [2455, 3067a, 3880]. [Pg.415]

The formal 47t participation of the monomeric 2-arylmethylene-l-tetra-linthiones in Diels-Alder reactions with isoprene and (E,E)-, 4-diphenyl-1,3-butadiene provides 3,4-dihydro-2//-thiopyran derivatives [Eq. (5)].7d... [Pg.294]

Acid-catalyzed condensation of a-phosphonyl dithioate 67 with aromatic bis-morpholino aminal 68 yielded a-phosphonyl a,p-unsaturated dithioate 69 in high yield, which underwent hetero-Diels-Alder reactions with alkenes to give the 3,4-dihydro-2ff-thiopyrans 70 in high yields (Scheme 17) [54]. [Pg.198]

Chiral thiopyran derivatives have been obtained in Diels-Alder reactions from monosaccharide O-thioformates and dithiooxalates. Intramolecular Diels-Alder cyclization of 1,6,8-nonatrienes and 1,7,9-decatrienes prepared from pentoses or D-glucose afforded multichiral hexahydroindene and octahydronaphthalene derivatives with very high diastereoselectivity. [Pg.112]


See other pages where 2//-Thiopyrans, Diels-Alder reactions is mentioned: [Pg.302]    [Pg.469]    [Pg.637]    [Pg.452]    [Pg.454]    [Pg.452]    [Pg.292]    [Pg.438]    [Pg.872]    [Pg.876]    [Pg.884]    [Pg.74]    [Pg.78]    [Pg.637]    [Pg.452]    [Pg.454]    [Pg.575]    [Pg.463]    [Pg.289]    [Pg.113]   
See also in sourсe #XX -- [ Pg.324 ]




SEARCH



27/-Thiopyrane

4- //-Thiopyran

Normal Diels-Alder Reactions. Synthesis of Pyrones and Thiopyrans

Thiopyrans

© 2024 chempedia.info