Selenoacetals synthesis

Enol ethers 28 are easily converted to selenoacetals 29 by treatment with benzeneselenol in the presence of boron trifluoride etherate (Scheme 31) [53]. When cyclic enol ethers are employed, the corresponding open-chain products are formed. The selenoacetals obtained are considered to be important intermediates in organic synthesis [54]. 

As discussed in Sect. 2, a-selanylalkyllithiums, generated from selenoacetals, can react with various electrophilic reagents, i. e. chloromethyl isopropyl ether for the synthesis of la-hydroxy vitamin D analogues [25] and with propargylic chloride derivatives for the preparation of alkynols [26]. A synthesis of vinyl-cyclopropane derivatives from l,4-dichloro-but-2-ene was achieved with trans stereoselectivity (>93%) in 68-89% yield. This one-pot cyclization, via an intramolecular allylic substitution, required the presence of two equivalents of u-BuLi [26] (Scheme 23). 

For the synthesis of a-selenoalkyllithiums, the selenium-lithium exchange reaction is a good alternative to the almost impossible metallation of unactivated selenides.Thus it has been found that a large variety of selenoacetals, often readily available from carbonyl compounds and selenols, react with butyllithiums to provide a-selenoalkyllithiums - in very high yields (Scheme 2 see also Section 2.6.2.3). 

Yet another important difference of reactivity towards n-butyllithium has been found between phenyl selenoacetals derived from aldehydes and ketones. This distinction allowed, when the reaction was performed in ether-hexane, the selective synthesis of an a-selenoalkyllithium derived from the phenyl selenoacetal of an aldehyde in the presence of the phenyl selenoacetal of a methyl ketone, which remained untouched (Scheme 11). ... 

The reaction of selenoacetals with butyllithiums allows the synthesis of a large variety of a-seleno-alkyllithiums whose carbanionic centers are unsubstituted, monoalkyl substituted or dialkyl substituted. 

The reaction of l-metallo-l,l-bis(seleno)alkanes with electrophiles is another interesting approach to selenoacetals (Schemes 76 and 77), which inter alia allows the synthesis of l,l-bis(seleno)cyclo-propanes (Scheme 26, a, b and d Scheme 77, c) and also permits the synthesis of functionalized selenoacetals by concomitant C—C bond formation (Scheme 79). Other methods listed above are of limited use. The substitution of dihaloalkanes by selenolates is the method of choice for the synthesis of bis(seleno)methanes (Scheme 72, and can be applied to the synthesis of the relatively unhin-... 

The synthesis of a-selenoalkyl metals reported above is not limited to those compounds which possess alkyl or aryl groups on an sp carbanionic center. For example, a-selenoalkyllithiums bearing a vinyl (Scheme 94), furfuryl (Scheme 95), silyl (Scheme 96), - methoxy (Scheme 97) or seleno moiety (Schemes 77 and 98) 7t have been obtained in alinost quantitative yield from the corresponding fimctionalized selenoacetals - or orthoesters and Bu Li. The Se-Li exchange is in all these cases, except that of tris(phenylseleno)methane, exclusively observed even with those compounds which possess a hydrogen susceptible to metallation. The 6,6-bis(phenylseleno)-p-lactam shown... 

The halogen-lithium exchange is, however, far less general than the selenium-Hnetal exchange from selenoacetals for the synthesis of a-selenoalkyllithiums. It does not provide a-phenylselenoalkyllithiums from a-chloroalkyl phenyl selenides and does not allow the synthesis of methylselenoalkyllithiums from a-haloalkyl methyl selenides. It also suffers from the instability of the a-bromoalkyl phenyl se-lenides, especitdly those which produce the a-selenoalkyllithiums where the carbanionic center is dialkyl substituted. Furthermore the butyl bromide produced concomitantly with the a-selenoalkyllithium... 

Such a propensity of the carbon-selenium bond to be transformed into a carbon-lithium bond on reaction with butyllithiums has in fact been used successfully for the synthesis of various a-selenoalkylmetals from phenyl and methyl selenoacetals. It has inter alias been used for the synthesis of those a-selenoalkylmetals which bear two alkyl groups on the carbanionie center and which are expected to be the less stabilized ones 3 9,11 I2,. It also permits the selective synthesis of a-lithioseleno-acetals from selenoorthoesters8 9t 12). 

Synthesis of 1-Seleno eyclopropyllithiums by Selenium-Metal Exchange from Selenoacetals of Cyclopropanones... 

Allylic alcohols. Two laboratories have reported a novel synthesis of allyUc alcohols from two carbonyl compounds. One carbonyl compound (1) is treated with benzeneselenol (2 eq.) under acid catalysis to give a selenoacetal (2). This product can be cleaved to the carbanion (a) by n-butyllithium in THE at -78. The carbanion reacts with a second aldehyde or ketone to form a /3-hydroxyselenide (3). The final step involves oxidation and selenoxide fragmentation to the aUyUc alcohol (4). j3-Hydroxyselenides have been obtained by... 

Epoxides via methyl selenoacetals. Krief et al. have reported the synthesis of epoxides from two carbonyl compounds. The first step involves preparation of a dimethyl selenoacetal (1), followed by conversion to an a-methyl seleno-carbanion (a). These highly reactive carbanions react with even hindered carbonyl compounds to give 0-hydroxy methyl selenides (2), which are converted into selenonium salts by reaction with methyl iodide or dimethyl sulfate. 

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