Thioenol ethers

Also thioalkyl-substituted olefins undergo 1,2-cycloaddition with singlet oxygen producing the corresponding dioxetanes. Thus, thioenol 

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The striking effect of the catalyst is exemplified by the reaction of pregna-4, 16-diene-3,20-dione (10) with benzyl mercaptan. In the presence of piperidine only conjugate addition occurs to give (11) whereas with pyridine hydrochloride only the 3-benzyl thioenol ether (12) is formed. In the presence of p-toluenesulphonic acid both reactions take place to yield (13). 

Dimethyl ketals and enol ethers are stable to the conditions of oxime formation (hydroxylamine acetate or hydroxylamine hydrochloride-pyridine). Thioketals and hemithioketals are cleaved to the parent ketones by cadmium carbonate and mercuric chloride. Desulfurization of thioketals with Raney nickel leads to the corresponding methylene compounds, while thioenol ethers give the corresponding olefin. In contrast, desulfurization of hemithioketals regenerates the parent ketone. ... 

The A" -3-keto group can be selectively protected as a thioenol ether in the presence of a 17- or 20-ketone. High yields are obtained with benzyl mercaptan and pyridine hydrochloride as a catalyst. 

A rather special procedure for the preparation of 21-hydroxy-20-ketopreg-nanes starts with the 17a-ethoxyethynyl-17 -hydroxy steroids described earlier. Free radical addition of ethanethiol to the triple bond, followed by acid-catalyzed hydrolysis and dehydration gives the 20-thioenol ether 21-aldehyde. This can be reduced with lithium aluminum hydride to the C-21 alcohol and then hydrolyzed to the C-20 ketone in the presence of mercuric chloride. The overall yield, without isolation of intermediates, is in the order of 50% ... 

Some more complex examples of this reaction type from the field of natural product synthesis, using a ketone, a thioenol ether, or a phenyl function as an internal nucleophile, are found in references 171-173. 

SR (enol and thioenol ethers), cyclopropyl,or certain aryl... 

Caubere et al. [63, 64] treated 32a with sodium amide-sodium tert-butoxide (NaNH2-NaOtBu) in tetrahydrofuran (THF) in the presence of secondary amines and obtained enamines. Analogously, the corresponding thioenol ethers were formed from 32a and sodium amide-sodium thiolate in the presence or absence of NaOtBu. It was shown, however, that cyclohexyne rather than 6 is the decisive intermediate en route to the enamines as well as the thioenol ethers [63b, 64], As already mentioned above, the enol ether 41 arises inter alia from 32b and KOtBu in DMSO. The best yield (47%) was obtained in refluxing THF (Scheme 6.11) [60],... 

Thus, reaction of (4-3) with A, A -dimethylthioethanolamine leads to the incorporation of the basic side chain in (4-4) via a thioenol ether linkage. One of the methyl groups on nitrogen is then removed by reaction with phenyl chloroformate in the modern version of the von Braun reaction to afford fluradoline (4-5) [5]. 

Thioamide formation benzodiazepinone, 505 heteiodiazepinone, 621 phosphorus pentasulf ide, 323, 600 Thioazole formation, nitrile addition, 301 Thiocarbamate formation, 588 phenol, 95 rearrangement, 517 Thioenol ether formation, 185, 517 addition-elimination, 554 Thioester formation, mixed anhydride, 184 Thioether formation, 241, 300, 413, 416 alkylation, 586, 588 aromatic displacement, 416 Thiohydantoin formation, 293 Thiol interchange, benzothiazole formation, 422... 

The cycloaddition of A Af-dimethylethenamine with triethyl ethenetricarboxylate proceeds even faster but the corresponding cyclobutane is not stable. The less nucleophilic enol ethers and thioenol ethers are less reactive and require higher temperatures and longer reaction times although the cycloaddition products are obtained in respectable yields.30... 

One of the most reactive electrophilic alkenes is l,l-dicyano-2,2-bis(trifluoromethyl)ethene which undergoes cycloadditions with enol ethers, thioenol ethers, ketene acetals and thioacetals even at temperatures as low as — 78 °C. The cyclobutancs are formed as the sole products of the reaction.37-38 The reactions arc regiospecific and highly stereoselective even though evidence for zwitterionic intermediates have been obtained. 

Thioenol ethers can undergo cycloadditions with a,/ -unsaturated ketones in the presence of aluminum trichloride to give cyclobutyl ketones under mild conditions.13 In contrast, thermal [2 + 2] cycloadditions to enones have never been observed. 

Bamford-Stevens decomposition of tosylhy-drazones, 351 p-Benzoquinone, 308 Benzyl ether hydrogenolysis, 139 Benzyl thioenol ethers, 87 Birch reduction, 11, 49, 50 Birch reduction of estrone methyl ether diethyl ketal, 51... 

Disubstituted furans have been obtained from ketones using the reaction sequence shown in Scheme 24 (73JA250). The ketone is converted to the thioenol ether (117) which on reaction with dimethylsulfonium methylide yields the oxirane (118). Rearrangement leads to the dihydrofuran and thence to the 3,4-disubstituted furan (119). The terpenoid furans perillene (122) and dendrolasin (123) have been synthesized by treatment of the n-butylthiomethylene derivatives of the ketones (120) and (121) respectively with dimethylsulfonium methylide (Scheme 25). 

Polarization of double bonds due to the electron releasing effect of thioalkyl groups in thioenol ethers is much weaker than that reported for enol ethers (Table 4.26), as can be verified for the (E) and (Z) isomers of methyl propenyl sulfide in Table 4.40 [326, 327]. 

Enol and thioenol ethers have been used as starting materials in a Diels-Alder reaction used as a key step for the synthesis of carbazo-mycins, which are newer carbazole alkaloids with antibiotic activity (87H325, 87TL3079 89CPB1999 92C441). 

It is interesting that with the rather mild catalyst pyridine hydrochloride, benzene-thiol does not react with saturated ketones, but reacts with a,p-unsaturated ketones to give thioenol ethers (with increased conjugation) or 1,4-additions.35... 

After mercury(II)-assisted hydrolysis of the thioenol ether, aldehyde 3 was obtained. This was then subjected to the critical vinylogous aldol reaction needed to complete the carbon backbone of the natural product. The latter process furnished a 3.5 1 mixture of the y to ot addition products. The stereoselectivity observed in the installation of the C(5)-hydroxyl (natural product numbering) was only 2 1. Fortunately, the predominant isomer was the desired product 2. In retrospect, it can be seen that the level of selectivity attained conformed to the predictions of the Still model.4... 

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