Synthesis of tert-butoxycarbonyl (Boc)-protected purines
Synthesis of tert-butoxycarbonyl (Boc)-protected purines
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DOI：
10.1021/jo000983i
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发表时间：
2000-11-03
影响因子：
3.6
通讯作者：
Garner, P
中科院分区：
文献类型：
--
作者：
Dey, S;Garner, P
对核酸及其类似物以及核苷抗生素的化学合成的兴趣一直是嘌呤和嘧啶研究的驱动力。一个问题涉及如何掩蔽或保护可能存在于核碱基中的胺官能团。在典型的固相寡核苷酸合成中，例如，腺嘌呤、胞嘧啶和鸟嘌呤的环外胺基团用酰基保护基团封闭。在合成结束时，在碱性条件下的整体脱保护得到完全去封闭的核酸。随着Nielsen肽核酸（PNA）1和相关酰胺连接的寡核苷酸替代物2的出现，需要可以在酸性或中性条件下去除并且与Fmoc介导的固相合成方案相容的保护基团。这样的保护基也可用于DNA-肽缀合物的合成。3不幸的是，应用现有的保护基策略来释放核碱基，特别是纯度，往往还有很多需要改进的地方。虽然有充分记载的例子，核碱基与酸不稳定的保护基团，4最常见的酸不稳定的保护基团的胺，叔丁氧羰基（Boc），据我们所知，尚未成功地扩展到母体嘌呤核碱基。5 Boc保护基具有另外的优点，因为它也可以在中性条件下除去。6我们还需要一个酸不稳定基团来保护含嘌呤的R-螺旋肽核酸（RPNAs）中的环外胺。7 Boc保护被设想为高度有吸引力的策略，因为该保护基团与我们的基于Fmoc的SPPS方案正交，并且与已知的酸不稳定的单甲氧基三苯甲基（Mmt）保护基团相反，可以维持温和的酸性条件。因此，Boc保护的RPNA可以通过温和的酸解和进一步的合成化学从树脂上裂解，所述合成化学涉及缀合、片段缩合等，可以执行。通过适当选择树脂/连接体，也可以进行树脂上脱保护和特定胺基的改性。例如，可以将正交保护的Lys脱保护，然后将荧光团连接到RPNA。我们现在报告的实际合成Boc保护的腺嘌呤，6-氯-2-氨基嘌呤（6Cl 2AP），并纳入我们的RPNA单体合成鸟嘌呤。克数量的这些Boc-保护的嘌呤现在可以从廉价的起始材料合成，而不需要复杂的纯化步骤。我们最初尝试通过用Boc 2 O和催化量的DMAP处理腺嘌呤来制备Boc保护的腺嘌呤，但不是很成功。使用极性溶剂如DMSO和DMF（以溶解腺嘌呤）得到单-、双-和三-Boc保护的腺嘌呤，沿着大量的游离腺嘌呤。值得注意的是，这些产物的比例随时间保持恒定，并且加热反应混合物导致更复杂的反应混合物沿着高度着色的物质的发展。在评价不同的反应条件时，观察到使用过量（4.5当量）Boc 2 O、催化量的DMAP和THF作为溶剂得到单一产物三-Boc-保护的腺嘌呤3（方案1）。通过硅胶简单过滤容易地进行纯化，以90%产率得到3。Tris-Boc腺嘌呤3可以通过用NaHCO 3水溶液处理几乎定量地转化为双-Boc腺嘌呤5，后者可以通过在室温下用NaOH处理3天以非常好的产率转化为所需的单-Boc衍生物7。在此期间必须小心…
Interest in the chemical synthesis of nucleic acids and their analogues as well as nucleoside antibiotics has been the driving force behind research on purines and pyrimidines. One issue involves how to mask or protect amine functionality that may be present in the nucleobases. In a typical solid-phase oligonucleotide synthesis, for example, the exocyclic amine groups of adenine, cytosine, and guanine are blocked with acyl protecting groups. At the end of synthesis, global deprotection under basic conditions gives the fully deblocked nucleic acids. With the advent of Nielsen’s peptide nucleic acids (PNAs) 1 and related amide-linked oligonucleotide surrogates, 2 there is a need for protecting groups that can be removed under acidic or neutral conditions and are compatible with Fmoc-mediated solid-phase synthesis protocols. Such protecting groups would also be useful for the synthesis of DNA-peptide conjugates. 3 Unfortunately, the application of existing protecting group strategies to free nucleobasessespecilly the purinessoften leaves much to be desired. Although there are well-documented examples of nucleobases with acid-labile protecting groups, 4 the most common acid-labile protecting group for amines, the tert-butoxycarbonyl group (Boc), has, to our knowledge, not been successfully extended to the parent purine nucleobases. 5 The Boc protecting group has the additional virtue in that it can also be removed under neutral conditions. 6We also required an acid-labile group to protect the exocyclic amines in purine-containing R-helical peptide nucleic acids (RPNAs). 7 Boc protection was envisaged as a highly attractive strategy since this protecting group is orthogonal to our Fmoc-based SPPS protocol and, in contrast to the known acid labile monomethoxytrityl (Mmt) protecting group, can sustain mildly acidic conditions. Thus, Boc-protected RPNAs can be cleaved from the resin via mild acidolysis, and further synthetic chemistry involving conjugation, fragment condensation, etc., can be carried out. With the appropriate choice of resin/linker, on-resin deprotection and modification of a specific amine group would also be possible. One could, for example, deprotect an orthogonally protected Lys and then attach a fluorophore to the RPNA. We now report practical syntheses of Boc-protected adenine, 6-chloro-2-aminopurine (6Cl2AP), and guanine for incorporation into our RPNA monomer synthesis. Gram quantities of these Boc-protected purines can now be synthesized from inexpensive starting materials without the need for elaborate purification steps. Our initial attempts to make the Boc-protected adenine by treating adenine with Boc2O and a catalytic amount of DMAP were not very successful. The use of polar solvents such as DMSO and DMF (to solubilize adenine) gave mono-, bis-, and tris-Boc protected adenines, along with a major amount of free adenine. Significantly, the ratio of these products remained constant over time, and warming the reaction mixture led to a more complicated reaction mixture along with the development of highly colored species. While evaluating different reaction conditions, it was observed that use of excess (4.5 equiv) Boc2O, a catalytic amount of DMAP, and THF as solvent gave a single productsthe tris-Boc-protected adenine 3 (Scheme 1). Purification was easily effected by simple filtration through silica gel to give 3 in 90% yield. Tris-Boc adenine 3 can be converted to bis-Boc adenine 5 almost quantitatively by treatment with aq NaHCO3, and the latter can be converted to the desired mono-Boc derivative 7 in very good yield by treatment with NaOH for 3 days at room temperature. Care has to be taken during …