Ring size configuration effect and the transannular intrinsic rates in bislactam macrocycles
作者:Thanee Guedez、Andrés Núñez、Evis Tineo、Oswaldo Núñez
DOI:10.1039/b207233e
日期:——
We have synthesized compounds: N-(2-aminoacetyl)-2-pyrrolidone (1) and N-(2-aminoacetyl)-2-piperidone (2). When these compounds are dissolved in aprotic or protic solvents a fast equilibrium ca. 1:1 between the cyclol form (tetrahedral intermediate) and the bislactam macrocycle is established. The same result has been reported previously for N-(2-aminoacetyl)-2-caprolactam (3). For compounds 2 and 3, dynamic 1H-NMR (using the methylene signals α to the carbonyl and to the amino group) through spectrum simulation has been used to evaluate the exchange between the two mentioned forms at different pH. However, for compound 1 the exchange was evaluated using magnetization transfer technique. The more stable bislactam configuration of the macrocycle form in compounds 2 and 3, is the trans–cis
(one lactam with the cyclic alkyl chains trans oriented and the other cis oriented). However, the same form for compound 1 has a more stable cis–cis bislactam configuration. This difference in configuration induces substantial changes in the appearance of the methylene 1H-NMR signals that precludes the use of line-shape analysis to evaluate the rates. The rate law for the proposed mechanism of exchange between the cyclol form and the macrocycle is: K
=
[macrocycle]/[cyclol]
=
kobs.f/kobs.r
=
Kak2[H2O]/[H+]/k−2Kw/[H+]
=
Kak2[H2O]/k−2Kw; where Ka is the acidity equilibrium constant of the cyclol form, Kw
= 10−14 M2 and k2 and k−2 are the second order rate constants for the specific exchange catalysis. Therefore, both, the macrocycle formation (kobs.f) and the cyclol formation (kobs.r) are specific base catalyzed; however the equilibrium constant is independent of pH. Since K is ca. 1, the ΔG≠ associated with the measured rate constants represent the intrinsic barrier for this non-identical thermoneutral transformation where a cleavage of a tetrahedral intermediate is involved. The activation energies associated with the reverse rate constants then correspond to the intrinsic barrier for transannular cyclolization.
我们合成了化合物:N-(2-氨基乙酰基)-2-吡咯烷酮(1)和N-(2-氨基乙酰基)-2-哌啶酮(2)。当这些化合物溶解在非质子或质子溶剂中时,会快速达到平衡。环醇形式(四面体中间体)和双内酰胺大环之间的比例为1:1。之前对于 N-(2-氨基乙酰基)-2-己内酰胺 (3) 也报告了相同的结果。对于化合物 2 和 3,通过光谱模拟动态 1H-NMR(使用羰基和氨基的亚甲基信号 α)已用于评估不同 pH 下两种上述形式之间的交换。然而,对于化合物 1,交换是使用磁化转移技术进行评估的。化合物 2 和 3 中大环形式的更稳定的双内酰胺构型是反式-顺式
(一种内酰胺的环状烷基链为反式取向,另一种为顺式取向)。然而,化合物 1 的相同形式具有更稳定的顺式-顺式双内酰胺构型。这种构型差异导致亚甲基 1H-NMR 信号出现显着变化,从而无法使用线形分析来评估速率。所提出的环醇形式和大环之间的交换机制的速率定律为: K
=
[大环]/[环醇]
=
kobs.f/kobs.r
=
Kak2[H2O]/[H+]/k−2Kw/[H+]
=
Kak2[H2O]/k−2Kw;其中 Ka 是 cyclol 形式的酸度平衡常数,Kw
= 10−14 M2 和 k2 和 k−2 是特定交换催化的二阶速率常数。因此,大环形成(kobs.f)和环醇形成(kobs.r)都是特定碱催化的;然而,平衡常数与 pH 值无关。由于 K 约为。如图 1 所示,与测量的速率常数相关的 ΔG≠ 代表了这种不同的热中性转变的内在势垒,其中涉及四面体中间体的裂解。与反向速率常数相关的活化能则对应于跨环环化的内在势垒。