A-MethylSeleno ketones

Methylphenylpropylphosphine, 387 Methyl phenyl selenoxide, 291 (2-Methyl-2-propenyl)trimethylsilane, 7 Methyl pseudomonate A, 66 Methyl pseudomonate C, 66 N-Methylpyrrolidone, 176 Methyl salicylates, 257 a-Methylseleno ketones, 345 1-Methylseleno-l-lithiocyclopropane, 318 3-Methyl-2-selenoxo-l,3-benzothiazole, 270... 

C, 1-phenylseleno- and 1-methylseleno-l-alkenyllithiums in almost quantitative yields (Scheme 101). The stereochemistry of the reaction is not well defined and the only available results are those concerning the stereochemistry of the compounds resulting from further reaction of these organo-metallics with electrophiles. Both stereoisomers are formed if the methylselenoalkenyllithium is hydrolyzed or reac with an aldehyde or a ketone, whereas only one stereoisomer of an a, -un-saturat carbonyl compound is found from l-methylseleno l-alkenyllithiums and and from the... 

The reactions of a-selenoalkyllithiums with aliphatic and aromatic aldehydes and ketones are not usually stereoselective regardless of the solvent used (ether or THF). Even in the most favorable cases, such as that of l-methylseleno-2,2-dimethylpropyllithium and heptanal, in which well-diffeiendated bulky groups are involved, the stereoisomeric ratio ranges from 1 1 to 3 2 (Scheme 124). - However, in the case shown in Scheme 86, b, in which the lithium can coordinate to the silyloxy group, only one of the two stereoisomeric 3-hydroxyalkyl selenides is formed. 

It is interesting to note that the latter result is exceptional since l,l-bis(phenyl-seleno)cyclopropane is the only selenoacetal derived from ketones to be at least partially cleaved under these conditions and even the homologous cyclobutyl derivative is inert under these conditions. This may be due to the extra stabilization introduced by the cyclopropyl ring. The case of 2-decy 1-1,1-bis(methylseleno)cyclo-propane merits further comment. It is difficult to assess whether the cleavage of the carbon-selenium bond occurs on the methylseleno moiety cis or tram to the alkyl group, since this organometallic leads to a mixture of the two possible stereoisomers on further reaction with electrophiles (Scheme 16). 

Similar yields of p-heterosubstituted alcohols have been observed with deoxybenzoin and 2,2,6-trimethylcyclohexanone for all three heterosubstituted lithio derivatives if the reactions are performed at —78 °C in THF (Scheme 34g, i), whereas the best results are obtained when (methylseleno)cyclopropyllithium is reacted at the same temperature but in ether instead of THF (Scheme 34h, j)). This is probably due to a reduction of the degree of enolisation of the starting ketone when ether is used This is a tendency which proved to be general for other a-selenoalkyl-lithiums... 

A novel synthesis of isoselenazoles (150), using bromoseleno-intermediates in the final cyclization stage, comprises the successive treatment, at -80 C, of selenoethers of type (148), e.g. cis-/3-methylseleno-2-propenal, with bromine and ammonia it generally affords the desired heterocycles in very good yield. The precursors (148) are ultimately derived from the appropriate ketones (146), which are first converted into the chloro-aldehydes (147) by the Vilsmeier-Haack reaction, and then treated with methaneselenol in pyridine-triethylamine. They arise as mixtures of cis- and trans-isomers, separable by spinning-band fractional distillation. The u.v. and n.m.r. spectra of the isoselenazoles were recorded and discussed. ... 

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