Lithium borotritide

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. 

Tor the sake of convenience, tritide reagents designated by their formulas will always be written, as here, as if they contained only tritium. In reality, all reagents contain a mixture of tritium and hydrogen that varies with the specific activity of the reagent. For example, lithium borotritide of specific activity 80 Ci/mmol has the average formula approximately LiB H2.7Hi.3, and probably consists of a mixture of all possible isotopomers LiBH4, LiB HHj, LiB HoHz, LiB HoH and LiB H.. 

Figure 4.65 Use of lithium borotritide for reductions of a sugar lactone and acyl halides...

Aromatic dehalogenation suffers from the disadvantage that only 50% of the tritium is incorporated, the rest appearing as waste. This situation is even more marked for borotritide reductions but the problem can be overcome by using some of the new tritide reagents that have recently become available as a result of the synthesis of carrier-free lithium tritide (Scheme 13.1) [22], Their reactivity can be fine-tuned through the elements (e. g. B, Al, Sn) to which the tritium is attached and by the electronic and steric nature of the substituents at the central atom. 

The trans isomer (I) was then reduced with lithium or sodium borotritide in tetrahydrofuran (THF) at 20°C to afford the tritio alcohol (XXII). Subsequent oxidation with manganese dioxide afforded the 1-tritio aldehyde (Ib) with a favorable retention of isotope. 

Labeling with tritium in position 15 could easily be accomplished by reduction of retinaldehyde with sodium borotritide (Mayer and Isler, 1971) or of retinoic acid esters with lithium aluminum tritide, and, again, oxidation of the tritiated retinol (Kaegi et aL, 1982c) to retinaldehyde-15- H. Because of the favorable isotope effect, most of the isotope should stay attached to the aldehyde group. However, Futterman et aL (1979) found that all-frany-retinaldehyde-15- H is inadequate as a tracer for 11-c/y-retinaldehyde formation in a biological system. 

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