Associate Editor: I. Marek
Beilstein J. Org. Chem. 2022, 18, 1642–1648. https://doi.org/10.3762/bjoc.18.176
Received 16 Sep 2022, Accepted 18 Nov 2022, Published 02 Dec 2022
A non-peer-reviewed version of this article has been posted as a preprint https://doi.org/10.3762/bxiv.2022.72.v1
A rhodium-catalyzed intramolecular reductive aldol-type cyclization is described to give β-hydroxylactones with high diastereoselectivities. The stereoselectivity of this cyclization is highly solvent dependent and can give syn- or anti-β-hydroxylactones with high diastereoselectivity. This methodology was also applied to the synthesis of a chiral necic acid lactone which is a structural component of the pyrrolizidine alkaloid monocrotaline.
Keywords: β-hydroxylactone; intramolecular reductive aldol cyclization; necic acid lactone; rhodium catalyst
Carbon–carbon bond-forming reactions are among the most important reactions in the synthetic chemistry toolbox and the aldol reaction is one of the most powerful tools to achieve this transformation [1-8]. In particular, the intramolecular aldol condensation is an important approach to the formation of ring systems such as cyclic β-hydroxy carbonyl products or cyclic α,β-unsaturated carbonyl products. Therefore, various types of intramolecular aldol-type reactions have been developed and widely applied to the total synthesis of diverse natural products [9-18]. The reductive aldol-type reaction is another important variation that has been reported using metal catalysts such as Co [19-21], Cu [22-25], and others [26-32] with hydrosilanes (R3Si-H) or hydrogen as the reductant. In this area, rhodium catalysis has received significant attention [33-40], and we have also reported reductive α-acylations, reductive aldol-type reactions, and reductive Mannich-type reactions using RhCl(PPh3)3 with Et2Zn [41-47]. The rhodium-catalyzed reductive aldol reaction of α,β-unsaturated esters with aldehydes or ketones gives aldol-type products in good to excellent yields (Scheme 1) [43,44]. In addition, the reductive aldol-type reaction could also be applied to an asymmetric system, although the diastereoselectivity was poor. On the other hand, reductive Mannich-type reactions were achieved in good to excellent yields with high diastereoselectivity [45,46]. As part of a wider program of C–C bond formation systems, we herein report a rhodium-catalyzed intramolecular reductive aldol-type cyclization and its application for the synthesis of a chiral necic acid lactone.
When applying our previously reported conditions , the intramolecular reductive aldol-type cyclization of 1a proceeded smoothly and gave the desired product 2a in a good yield, but the diastereomeric ratio was not sufficient as shown in entry 1 (Table 1). To improve the diastereoselectivity of the reaction, we optimized the conditions for the reductive aldol-type reaction by intramolecular cyclization of 1, and the results are summarized in Table 1. The use of [RhCl(cod)]2 in dichloromethane gave the best result with high diastereoselectivity, and the stereochemistry of the major product 2a was found to be the syn-form with regard to the CH3 (Cα) and OH (Cβ) moieties (Table 1, entry 7). Interestingly, using the higher coordinating solvents, DMF or DMPU, preferentially gave the opposite diastereomer, i.e., the major product being the anti-form with regard to CH3 (Cα) and OH (Cβ) moieties (anti-2a, Table 1, entries 10 and 11). For stereochemistry assignment, the relative configurations of syn-2a and anti-2a were confirmed by X-ray crystallography. In addition, a NOESY experiment of the product syn-2a showed an nOe correlation between the methine proton on Cα and one of the protons of the benzene ring on Cβ, but not in anti-2a.
Table 1: Optimization of the reaction conditions.
|Entry||Rh cat.||Solvent||Time (h)||Yielda||dr [syn:anti]b,c|
aIsolated yield; bthe stereochemistry between CH3 (Cα) and OH (Cβ) moieties; cdiastereomeric ratio was determined after purification; ddiastereomeric mixture.
Next, various substrates were investigated and the results are summarized in Scheme 2. The synthesis of products 2a–c proceeded smoothly to give the corresponding β-hydroxylactones 2 in moderate to good yields with high diastereoselectivities, although 2d was obtained in low yield. It may suggest that the existence of substituent(s) in γ- and/or δ-position of 2 help the formation of the intermediate structure which works in favor of the intramolecular cyclization. β-Substituted substrates on α,β-unsaturated ester moiety of 1 also gave the products (2g and 2h) in low yields, but the formation of the 7-membered ring (2f) was not achieved. On the other hand, when the previous conditions using the RhCl(PPh3)3 catalyst was applied to the aldol-type cyclization, 5- and 6-membered products were obtained in good yields (see the yields and dr in parentheses in Scheme 2). However, the yields were also greatly affected by the substituents on the β-position of the α,β-unsaturated ester moiety, and all diastereomeric ratios were inferior in the case of the RhCl(PPh3)3 catalyst. The relative configurations of 2b were confirmed by X-ray crystallography, and the relative configurations of 2c, 2g, and 2h were confirmed by NOESY experiments.
The reaction mechanism of the intramolecular cyclization can only be speculative at this stage. We have already reported the generation of a rhodium hydride (Rh–H) complex from RhCl(PPh3)3 and Et2Zn, in which the reaction with tert-butyl acrylate formed the corresponding E-silylenolate via 1,4-reduction at 0 °C , even if the reaction was performed at −45 °C (Scheme 3). Also in relation to this result, Mikami and his group reported a rhodium-catalyzed carboxylation of alkenes or activated alkenes by using CO2 with Et2Zn, and a similar Rh–H complex derived from [RhCl(cod)]2 and Et2Zn played an important role in this reaction . Furthermore, Hopmann et al. detected the Rh–H complex derived from [RhCl(cod)]2 and Et2Zn by 1H NMR, and the detailed mechanism disclosed that the Rh–H complex did not interact with CO2 but with the benzene ring in the substrates through an η6 binding intermediate by DFT calculation .
Although another mechanism could not be denied in which a Z-enolate intermediate changes to an E-enolate under thermodynamic control, we propose the following mechanism on the basis of the above results (Scheme 3). The Rh–H complex 5 from [RhCl(cod)]2 and Et2Zn would generate predominantly the corresponding E-enolate 6 via 1,4-reduction, which is stabilized through η6 binding with benzene ring of the substrate. Subsequent transmetalation with zinc species 4 readily reacts with the carbonyl group to form the intramolecular C−C bonds at the α-position, then providing the product syn-2a with high regioselectivity. On the other hand, the use of higher coordinating solvents such as DMF or DMPU might break the weak η6 binding of rhodium complex to give anti-2a, predominantly.
There are several reports of bioactive natural products that have a 3-hydroxy-2-methyllactone scaffold in the molecular structure. For example, cytospolide K2  containing a 10-membered lactone and feigrisolide  containing a 7-membered lactone are known to exhibit cytotoxicity and antimicrobial activity. Moreover, antiviral activity was also confirmed for aggregatin B  containing a 7-membered lactone ring, in which the β-position hydroxy group was dehydrated (Figure 1). Monocrotaline is a kind of pyrrolizidine alkaloid and was isolated from seeds of Crotalaria spectabilis in 1935 . Monocrotaline is used as compound for pulmonary hypertension model in rats. To date, some groups have reported synthetic methods and its synthetic supply will potentially contribute to hypertension treatment [54-57]. Although there have been a lot of reports of pyrrolizidine scaffolds or necine base, the synthesis of necic acid lactones such as monocrotalic acid is rare (Figure 2). Consequently, we attempted to apply the rhodium-catalyzed intramolecular reductive aldol-type reaction to the synthesis of a chiral necic acid lactone that is a part of structural component of monocrotaline.
According to the literature, a Sharpless dihydroxylation of benzyl tiglate (8) to form a chiral diol 9 was followed by a Parikh–Doering oxidation to give the corresponding product 10 in 62% yield (Scheme 4) [58,59]. Subsequent acryloylation in the presence of DMAP and hydroquinone gave the intramolecular cyclization starting material (S)-1j in 61% yield. The transformation of the compound (S)-1j in the rhodium-catalyzed intramolecular reductive aldol-type cyclization proceeded smoothly and gave the chiral necic acid lactone (2S,3S,4R)-2j in 32% yield (Figure 3).
In conclusion, during the development of a rhodium-catalyzed intramolecular reductive cyclization, we found that using [RhCl(cod)]2 improved the diastereomeric ratio of the products compared with other Rh catalysts. It seems that using [RhCl(cod)]2 leads to milder reaction conditions that lead to highly improved diastereomeric ratios. In addition, we demonstrated a new approach to a necic acid lactone 2j that is a diastereomer of monocrotalic acid, a key intermediate of monocrotalin.
|Supporting Information File 1: General procedures and analytical data, including copies of 1H NMR, 13C NMR, and X-ray crystallography.|
|Format: PDF||Size: 4.1 MB||Download|