Selenides, benzyl metallation

The completion of the synthesis of key intermediate 2 requires only a straightforward sequence of functional group manipulations. In the presence of acetone, cupric sulfate, and camphorsulfonic acid (CSA), the lactol and secondary hydroxyl groups in 10 are simultaneously protected as an acetonide (see intermediate 9). The overall yield of 9 is 55 % from 13. Cleavage of the benzyl ether in 9 with lithium metal in liquid ammonia furnishes a diol (98% yield) which is subsequently converted to selenide 20 according to Grie-co s procedure22 (see Scheme 6a). Oxidation of the selenium atom... 

Benzyl a-naphthyl selenide, (C6H5.CH2)Se(C10H7).2—This selenide is produced when the complex obtained from magnesium a-naphthyl bromide and metallic selenium is treated with benzyl chloride. It forms small colourless prisms from alcohol, M.pt. 68° to 69° C. It yields a picrate, which separates from ether in orange-red needles, M.pt. 118° C. 

The halogen-metal exchange of secondary alkyl iodides leads mainly to hydrocarbons containing Wurtz-type coupling or elimination products [44], except in the case of benzyllithium derivatives which are easily generated from benzyl selenides [45]. 

The 6-exo-trig cyclization of a 6-heptenylmetal to a (cyclohexylmethyl)metal is much slower than the analogous 5-exo-trig carbocyclization. Indeed, as shown in Scheme 7-64, the 6-exo rearrangement of 6-heptenyllithium requires the presence of TMEDA to facilitate the process [44]. However, in the case of 6-alkenyl benzyl selenides, the formation of the six-membered rings is possible by carbocyclization [70] as well as by the use of Ziegler-Natta conditions [71],... 

Benzyl selenides exhibit a similar pattern of leactivi. Thus, although the parent aryl benzyl selenides are successfiilly metallated with LDA at -78 their higher homologs, as well as benzyl mediyl... 

An ethanolic soln. of benzyl bromide and Na-phenylselenide refluxed 2 hrs. -> benzyl phenyl selenide (Y almost 100%) treated with Li-diisopropylamide and ethyl bromide in tetrahydrofuran, the resulting crude mixture treated with 30%-H2O2, and the product isolated after 24 hrs. -> product. Overall Y 80%. F. e. s. R. H. Mitchell, Chem. Commun. 1974, 990 syntheses of ethylene derivs. via a-metalation of selenoxides cf. H. J. Reich and S. K. Shah, Am. Soc. 97, 3250 (1975). 

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