Dihydrothiophenes. II. Preparation and properties of some alkylated 2,5-dihydrothiophenes

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The preparation of a series of vinyltriphenylphosphonium salts, all of which bear a substituent on the carbon attached to phosphorus, is described. Yields are generally poor unless a symmetrical secondary allylic halide can be used. The use of these salts in the preparation of substituted 2,5-dihydrothiophenes has been examined. In general, reaction times are increased and product yields decreased relative to the reactions of the unsubstituted salts previously reported. This result is ascribed to steric inhibition of the intramolecular Wittig reaction.

Section: 'supporting

confidence: 76%

“…I).' Application of this technique to heterocyclic synthesis has afforded a wide range of substituted dihydrothiophenes, (5,6), dihydrothiopyrans (7), and products derived therefrom.…”

mentioning

confidence: 99%

Some methods for the preparation of α-alkylated vinylphosphonium salts and their use in 2,5-dihydrothiophene synthesis

McIntosh¹,

Steevensz²

1977

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“…As we have shown, these compoi~nds are readily available from the reaction of vinylphosphonium salts and a-mercaptoketones. The synthetic advantages to this method have already been outlined (2).…”

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confidence: 99%

“…exist as highly insoluble dimers o r polymers (2) were more difficult to prepare than the corresponding ketones but their reactions with the representative vinylphosphonium salts were more rapid. In most cases these reactions were essentially complete in a few hours at 100" but in others a longer reaction time was required for maximum yields (Table 1).…”

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confidence: 99%

Dihydrothiophenes. III. Preparation of Some 3,4-Unsubstituted Examples

McIntosh¹,

Steevensz²

1974

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Using the reaction between four representative vinylphosphonium salts and three a-mercaptoaldehydes, 12 3,4-unsubstituted 2,5-dihydrothiophenes (1) were prepared.The reaction proceeds in excellent yields under mild conditions. The use of these compounds in thesynthesisof dienes is indicated.On a prepare 12 dihydro-2,5 thiophenes non substitues en position 3,4; ces syntheses impliquent la reaction de quatre sels reprtsentatifs d e vinylphosphoniurn avec trois a-mercaptoaldthydes. L a reaction se produit avec d'excellents rendements d a n s des conditions douces. O n montre comment ces composes peuvent &tre utilises pour la synthese de dienes.[Traduit par le journal]Can. J . Chem..52,1934Chem..52, (1974 Recently, we have reported (1, 2) a new route to conjugated dienes (2) using 2,5-dihydrothiophenes (1) as key intermediates (Scheme 1). As we have shown, these compoi~nds are readily available from the reaction of vinylphosphonium salts and a-mercaptoketones. The synthetic advantages to this method have already been outlined (2).The dihydrothiophenes reported thus far have been derived from a-mercaptoketones and are substituted at the 3-position. As the final step in the reaction (Scheme 2) is a Wittig reaction, a process known to be dramatically subject to steric hindrance (3, 4), we had every reason to expect that employing a-mercaptoaldehydes in the reaction should afford excellent yields of 1. exist as highly insoluble dimers o r polymers (2) were more difficult to prepare than the corresponding ketones but their reactions with the representative vinylphosphonium salts were more rapid. In most cases these reactions were essentially complete in a few hours at 100" but in others a longer reaction time was required for maximum yields (Table 1). Although all reactions were run usingequimolar quantities of reactants it has been observed that a slight ( l o x ) excess of the vinyl salt increased the yields dramatically (5).As can be seen from Table 2, with the exception of compound 18, the n.m.r. absorptions for the vinyl hydrogen atoms of the dihydrothiophenes occurred as sharp singlets. The chemical shifts of these appear t o be dependent upon the degree of substitution at the 2,5-positions. We are pursuing this point. It should also be noted that the n.m.r. spectra provided no evidence for the presence of geometric isomers where these were possible. As the further conversion ofthe dihydrothiophenes to dienes occurs by a stereospecific pathway (2, 6) a highly stereoselective synthesis of compounds of type 1 would be of great importance. At present, we are engaged in a study of the reaction stereochemistry which will be reported at a later date. SCIIEME 2 ExperimentalThese expectations were realized (Table I). The Unless otherwise noted, n.m.r. spectra were obtained reactions were run as ~reviouslv described (2) salts by in situ isomerization of their allyl isomers Gas-liquid chromatography analyses were carried out on a n F and M Model 720 gas chromatograph utilizing a n when required. The a-mercaptoaldehydes whic...

“…6. Yields and spectral data on compounds 2a, c-e are incorporated in a manuscript now in preparation.…”

Section: Resultsmentioning

confidence: 99%

A New Thiophene Synthesis

McIntosh¹,

Khalil²

1975

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