9-Borabicyclononane (9-BBN) has been utilized to protect functionalized amino acids for potential chemoselective side chain manipulation. The 9-BBN group imparts organic solubility to otherwise hydrophilic molecules and is tolerant of a wide range of reaction conditions. The high degree of solubility of these molecules in THF is particularly noteworthy. It is cleaved with either aqueous HCl or by exchange with ethylenediamine in methanol. [reaction: see text]
Some years ago we reported results2 on the biosynthesis of streptonigrin (1), a potent anticancer antibiotic obtained from Streptomyces flocculus, which indicated that the 4-phenylpicolinic acid moiety was derived by cleavage of a /3-carboline intermediate. Two mechanisms were presented, illustrative of chemically rational possibilities, that involved oxidation of the D ring. In one the C-8' phenolic oxygen was derived from 02 while in the other it was derived from H20. Subsequently, we characterized lavendamycin, 2,3 from S. lavendulae, clearly a member of the streptonigrin family, and containing the putative /3-carboline moiety. Recently, Rickards et al.4,5 reported the structure of streptonigrone (3) and proposed a third mechanism for the /3-carboline cleavage that was formally nonoxidative and involved a covalent hydration6 at C-8' followed by a 1,6-elimination of the original indole nitrogen. By this mechanism the C-8' phenolic oxygen would be derived from H20. This paper prompts us to report data that clearly indicate the oxygen in question is derived from molecular oxygen rather than from water.A seed culture of S. flocculus was prepared7 and used to inoculate four production broths7 (250 mL in 1-L Erlenmeyer flasks). These were connected in series via two sterile filters to a closed system containing a burette which was refillable with 1802 4 gas (50% enriched, obtained from Cambridge Isotopes, Inc.), a small aquarium pump, and a C02 trap (aqueous KOH). Air was
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