Seleno esters

The /V-phenylselenophthalimide (NPSP)-tributylphosphine (Bu3P) system is also a useful reagent for the facile transformation of carboxylic acids into seleno esters, and of alcohols into alkyl phenyl selenides (including some carbohydrate derivatives).102 The utility of iV-phenylselenophthalimide (N-PSP) as a selenylating agent has been reviewed by Nicolaou and co-workers.103... 

Seleno esters are also produced in good yield by reactions of dimethy-laluminim methanoselenolate103 (CH3)2AlSeCH3, with the protected C-ribofuranosyl acetate 269. Treating the selenol ester 270 with either cuprous or mercuric chloride produced the (Gensler) lactone.110 271, a product useful in the synthesis of various C-nucleoside antibiotics.111,112... 

Stabilized radicals are generated. A list of the main type of radical precursors used for selenium atom transfer is depicted in Fig. 1. Malonates [64, 66-73], -ketoesters [74, 75], malononitriles [76-79], haloalkanes [80], diphosphonate [81], glycolate derivatives [82, 83] and dithianes [84]. More substrates such as a-seleno-esters, -nitriles, -ketones and -sulfones can also be used, however, the efficiency of the process is reduced [68]. 

Tri-n-butyltin radicals can also be used to generate radicals from selenium compounds. An example is the formation of acyl radicals from seleno esters. 

Note that seleno esters (ArCOSeAr) were prepared from aryl iodides, CO, PhSeSnBua, and a palladium catalyst. ... 

Acyl halides react with thiols, in the presence of zinc, to give the corresponding thio-ester. " The reaction of acid chlorides or anhydrides (see 16-62) with diphenyldiselenide, in the presence of Sm/CoCl2 or Sm/CrCl3 gave the corresponding seleno ester (PhSeCOMe). 

The situation is clearly different for a-seleno ketones (Schemes 34-36), ° " " a-seleno esters (Schemes 10 and 37),56.74,75.ii2.ii3 selenolactones (Scheme a-seleno acids (Scheme 39)56,ns gjjjj... 

Lithium enolates derived from a-seleno esters have a marked tendency to produce the C-3 adduct, and in fact the C-1 adducts are the most difficult to obtain. Representative results are collected in Schemes 152-154. 

The formal total synthesis of the selective muscarinic receptor antagonist (+)-himbacine was accomplished by M.S. Sherburn and co-workers using an intramoiecuiar Dieis-Aider reaction, a Stiiie cross-coupling, and a 6-exo-trig acyi radical cyclization as the key steps.In order to prepare the selenoate ester precursor for the radical cyclization step, the aldehyde-enyne substrate was converted to the carboxylic acid via the Pinnick oxidation without affecting the delicate enyne moiety. 

Removal of the furan ring (148) was accomplished in two steps in 45% overall yield from carboxylic acid 125 (Scheme 24) [57]. Carboxylic acid 125 was coupled with selenophenol via the mixed anhydride using phenyl dichlorophosphate to give the phenyl seleno ester. Decarbonylation of the phenyl seleno ester was accomplished with tributyltin hydride and AIBN in toluene. 

This approach is based on the rearrangement of the bicyclic seleno ester 67 prepared by pressure-promoted inverse electron demand cycloaddition of carbomethoxypyrone 65 and vinyl selenide 66 (Scheme 7.15). The cycloaddition reaction occurs regioselectively and czzc/u-diastereoselectively. 

Hence, Claisen rearrangement of / -amino ester 1404 and /i-alkoxy ester 2511 were followed by further transformation. For similar reactions of /i-seleno esters see refs 233 and 512. 

Recently, Pattenden, following the same D strategy, has developed the use of a-ketenyl cyclooctanyl radical 114 toward a new and concise formal synthesis of modhephene (Scheme 33). The (x,)5-unsaturated seleno ester 112, treated under usual BusSnH-AIBN conditions, generates the corresponding a,/9-unsaturated acyl radical 113, which transannularly cyclizes via its mesomeric radical counterpart 114. The resulting tertiary radical undergoes a 5-cxo-dig cyclization onto the ketene central carbon giving rise to a final enoxy radical, which is reduced to provide the tricyclic ketone 115 [49]. 

Displacement of activated halides. a-Halo esters, amides, and acetals undergo radical allylation, which is initiated by AIBN or light. A related transformation, such as displacement of a-seleno esters (e.g., that of p-hydroxy-a-phenylseleno esters) requires a Lew is acid (e.g., MejAl) to give a high anh-selectivity. 

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