Lithium Borotritide LiB

Early applications of LiB H4 were commonly in the carbohydrate field, where it was used to selectively reduce aldonic lactones to give [l- H]aldoses. An example of such a reaction is the conversion of227to228 the reaction requires pyridine as solvent, in order to minimize further reaction to the corresponding [l- H2]alditol. 

As mentioned above, LiB H4 has been used very successfully for reduction of a tertiary amide to an amine. Whereas treatment in trial reactions of238 with LiBHLt in THF solution at 65 °C gave the benzyl alcohol, the outcome in methylene chloride as solvent was different. Namely, reduction of 238 with LiB H in CH2CI2 at 40 °C gave 239, the mucolytic agent bromhexine, in 43% yield with a specific activity of 54Ci/mmol . 

LiEtaB H at nearly 100% abundance tritium is readily produced from Li H (as prepared in Section 4.3.4) and EtaB (THF r.t., lmin) ° . Earlier methods using less reactive crystalline Li H at lower specific activities had to be more drastic (r.t., 2 days or reflux, 2h) and are therefore less appropriate for the preparation of a highly tritiated reagent. Despite its excellent synthetic potential only a few applications have been published so far, aside from test reactions on model substrates. Among the simpler but elegant were tritio-deoxygenation of (/ )- and (5)-[l- H]ethyl and [l- H]butyl tosylates to give (R)- and (S)-[l- H,l- H]ethane and (/ )- and (5)-[l- H,l- H]butane, respectively, for use in 

In a more complex situation, LiEt3B H was employed in the synthesis of (2-hydroxy-[l,l,2,2- H]ethyl)everolimus 249, an immunosuppressant rapamycin derivative. Reduction of methyl 0-tcrt-butyldiphenylsilyl glycolate 247 followed by purification on silica 

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It was in this way that the acetic acid depicted as (i )-acetate in Scheme 13.22 was shown to be (R) and, since reduction of the mesylate with lithium borotritide [LiB( H)4] occurred with inversion of configuration, the original rearrangement must itself have gone with inversion. 

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