Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, USA.
J Org Chem. 2010 Feb 19;75(4):1016-39. doi: 10.1021/jo9025572.
Bacteriochlorins are attractive candidates for a wide variety of photochemical studies owing to their strong absorption in the near-infrared spectral region. The prior acid-catalysis conditions [BF(3) x O(Et)(2) in CH(3)CN at room temperature] for self-condensation of a dihydrodipyrrin-acetal (bearing a geminal dimethyl group in the pyrroline ring) typically afforded a mixture of three macrocycles: the expected 5-methoxybacteriochlorin (MeOBC-type), a 5-unsubstituted bacteriochlorin (HBC-type), and a free base B,D-tetradehydrocorrin (TDC-type). Here, a broad survey of >20 acids identified four promising acid catalysis conditions of which TMSOTf/2,6-di-tert-butylpyridine in CH(2)Cl(2) at room temperature was most attractive owing to formation of the 5-methoxybacteriochlorin as the sole macrocycle regardless of the pyrrolic substituents in the dihydrodipyrrin-acetal (electron-withdrawing, electron-donating, or no substituent). Eleven new dihydrodipyrrin-acetals were prepared following standard routes. Application of the new acid catalysis conditions has afforded diverse bacteriochlorins (e.g., bearing alkyl/ester, aryl/ester, diester, and no substituents) in a few days from commercially available starting materials. Consideration of the synthetic steps and yields for formation of the dihydrodipyrrin-acetal and bacteriochlorin underpins evaluation of synthetic plans for early installation of bacteriochlorin substituents via the dihydrodipyrrin-acetal versus late installation via derivatization of beta-bromobacteriochlorins. Treatment of the 5-methoxybacteriochlorins with NBS gave regioselective 15-bromination when no pyrrolic substituents were present or when each pyrrole contained two substituents; on the other hand, the presence of a beta-ethoxycarbonyl group caused loss of regioselectivity. The 15 new bacteriochlorins prepared herein exhibit a long-wavelength absorption band in the range 707-759 nm, providing tunable access to the near-infrared region. Taken together, this study expands the scope of available bacteriochlorins for fundamental studies and diverse applications.
细菌叶绿素由于在近红外光谱区域具有很强的吸收而成为各种光化学研究的有吸引力的候选物。先前的酸催化条件[BF(3)xO(Et)(2)在室温下的 CH(3)CN]用于二氢二吡咯-缩醛的自缩合（在吡咯环中带有偕二甲基）通常得到三种大环的混合物：预期的 5-甲氧基细菌叶绿素（MeOBC 型），未取代的细菌叶绿素（HBC 型）和游离碱基 B，D-十四氢卟啉（TDC 型）。在这里，对 >20 种酸的广泛调查确定了四种有前途的酸催化条件，其中 TMSOTf/2,6-二叔丁基吡啶在室温下的 CH(2)Cl(2)最为吸引人，因为形成 5-甲氧基细菌叶绿素作为唯一的大环，无论二氢二吡咯-缩醛中的吡咯取代基如何（吸电子、供电子或无取代基）。按照标准路线制备了 11 种新的二氢二吡咯-缩醛。新酸催化条件的应用可在几天内从商业上可获得的起始材料中获得多种细菌叶绿素（例如，带有烷基/酯、芳基/酯、二酯和无取代基）。考虑到二氢二吡咯-缩醛的合成步骤和产率以及细菌叶绿素的形成，评估了通过二氢二吡咯-缩醛早期安装细菌叶绿素取代基的合成方案与通过β-溴代细菌叶绿素的后期安装的合成方案。当不存在吡咯取代基或每个吡咯包含两个取代基时，用 NBS 处理 5-甲氧基细菌叶绿素可得到区域选择性的 15-溴化；另一方面，β-乙氧基羰基的存在导致失去区域选择性。本文制备的 15 种新的细菌叶绿素在 707-759nm 范围内表现出长波长吸收带，为近红外区域提供了可调谐的访问。总之，这项研究扩大了基础研究和各种应用中可用的细菌叶绿素的范围。
