Thioesters and Selenoesters

Lactams, imides, acid anhydrides, carbonates, acylsilanes, thioesters and selenoesters can also be converted to the corresponding alkenes. 

The photochemistry of o/tfto-sulfinyl benzoates has also been investigated [114]. Compound 223 is converted to orf/io-(methylthio)-benzoic acid in 77% yield in benzene. The mechanism of this reaction was not well established, but it probably begins with cleavage of the carbonyl-0 bond, as the same product was obtained from the thioester and selenoester analogs. In these latter cases, tolyl disulfide and tolyl diselenide were also isolated. It was speculated that the 1-photon product of 223 may actually be or//io-(methylsulfinyl)-ben-zaldehyde 161. As discussed in Section V, this compound is known to undergo photochemical intramolecular oxygen transfer. 

Cycloalkanones. co-Bromo thioesters and selenoesters give cyclic ketones on... 

Radical cyclizations. When heated with tin hydride in benzene, thioesters and selenoesters bearing an alkenyl side chain give cyclic products. The side chain participates... 

Because of their previous findings that a,/l-unsaturated thioesters were more reactive than their ester counterparts in Diels-Alder reactions85, Hart and coworkers86 performed a systematic study of the cycloaddition reactions of a,/l-unsaturated thioesters and a,ft-unsaturated selenoesters with several dienes. Thermal reactions were compared with Lewis acid catalyzed reactions at room temperature (equation 28 and Table 2). The results clearly demonstrated that use of a Lewis acid enhanced the regioselectivity (entries 1 vs 2, 3 vs 4, 5 vs 6 and 7 vs 8) as well as the endo (with respect to the thioester or selenoester group) selectivity (entries 5 vs 6 and 7 vs 8). 

The acyl group RCO is a hard Lewis acid, hence its combination with hard bases forms thermodynamically stable molecules, e.g., carboxylic acids RCOOH, esters RCOOR, and amides RCONR2. In contrast, its union with soft bases results in highly reactive or labile species such as thioesters RCOSR, selenoesters RCOSeR, and acyl iodides RCOI. 

Analogous decarbonylation reactions were observed with acyl halides [18], esters [19], acyl cyanides [20], thioesters [21], selenoesters [21b], acylphospho-nates [22], and even ketones (preceding chapters). Acid anhydrides underwent a fragmentation reaction upon treatment with a nickel catalyst to afford alkenes,... 

The a,P-unsaturated esters 12 and 14, spirobislactone 16, and vinylogous lactone 18 are smoothly methylenated by Petasis reagent. Silyl esters 22 and 24 are converted to silyl enol ethers 23 and 25. Carbonate 20 can be methylenated to give ketene acetal 21. Amide 26 and lactams can be methylenated, however the reaction is generally sluggish and the complete separation of Ti species is usually difficult. In a similar manner, thioester 28 and selenoester 30 are converted to alkenyl sulphide 29 and alkenyl selenide 31, respectively. Additionally it has been demonstrated that acyl silanes can be converted to the corresponding alkenyl silanes. 

Insertion of carbon monoxide into Csp2—Zr bonds occurs readily at ambient temperatures or below to produce a,(5-unsaturated, reactive acyl zirconocene derivatives [27—29]. Early work by Schwartz demonstrated the potential of such intermediates in synthesis [5d], as they are highly susceptible to further conversions to a variety of carbonyl compounds depending upon manipulation. More recently, Huang has shown that HC1 converts 16 to an enal, that addition of a diaryl diselenide leads to selenoesters, and that exposure to a sulfenyl chloride gives thioesters (Scheme 4.11) [27,28]. All are obtained with (F)-stereochemistry, indicative of CO insertion with the expected retention of alkene geometry. 

Apart from aryl esters and amides of carboxylic acids, aryl carbonates, and carbamates, other types of organic compound have been reported to undergo PFR. They include oxalates, formiates, sulfonates, sulfonamides, thioesters, selenoesters, and telluroesters. 

In most cases these C,N2 fragments are prepared by reaction of activated carboxylic acid derivatives (C, fragment), such as dithiocarboxylates 2, thioesters 3. nitriles 5, imidates 6, amidines 7, thioamides 8, selenoamides 9, selenoesters 11, orthocarboxylates 14, with hydrazine (N2 fragment) and are transformed to l,4-dihydro-l,2,4,5-tetrazines 17 without isolation. Therefore, these syntheses are treated as a (2 + 1 +2+1) fragment method. 

Most studies have been directed toward the use of selenoesters and thioesters as radical precursors to generate acyl radicals in radical cyclizations [43]. However, selenoesters and thioesters as radical traps have not been well studied [40a]. Since the cyclization step is much slower than direct reduction by n-BusSnH, the use of (n-Bu3Sn)2 is required. Intramolecular cyclization of thioester 17 furnished cyclo-... 

Thioesters, selenoesters and related compounds. - Cobalt (II) chloride 280 281 282 283 284... 

Although less common, some examples of PFR with thioesters, selenoesters, and telluroesters have also been reported [102-105]. Thus, irradiation of S-phenyl thioacetate gives minor amounts of ortho- and para-rearranged products escape of the phenylthio radical out of the cage predominates. 

Thioesters, Selenoesters, and Thioamides.—Commercially available phenyl dichlorophosphate has been reported to be a superior reagent to MV-dimethyl-phosphoramidic dichloride (4, 244) for the preparation of thioesters directly from carboxylic acids and thiols. Carboxylic acid chlorides can be efficiently converted into thioesters by reaction with adducts formed between thiols and l-methylpyridine-3,5-dicarboxylates. Alternatively, copper(i) mercaptides, formed from thiols and CuaO, or thallous thiophenoxide can be used. The latter method can also be used to prepare selenoesters as well as a-phenylthio-and a-phenylseleno-acids and esters from a-halo-acids and a-halo-esters, respectively. 

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