Communication A New Type of NADH Model Compound: Synthesis and Enantioselective Reduction of Benzoylformates to the Corresponding Mandelates

A new type of NADH model compound with good reactivity and enantioselectivity has been synthesized in good yields by an efficient and convenient synthetic method. The structures of these model compounds were confirmed by 1H and 13C-NMR and MS.


Introduction
Since Ohno and coworkers [1] reported the first NADH model compound, a large number of such mimics have been developed [2].In particular, chiral NADH model compounds have been extensively studied with the aim of developing new enantioselective reducing agents [2].Generally, highly stereoselective chiral NADH models have been designed by incorporating remote stericallydemanding side chains [3] or a substituent at the reaction centre: the C4 position of the dihydropyridine ring [4].However, the former approach implies a significant modification of the dihydropyridine ring and the latter one resulted in a loss of chirality at the C4 position during the course of the model reaction.Recently, several NADH models with specific conformations which could steroselectively reduce pyruvate mimics were reported [5], and a few such models have been studied so far.Herein we report a novel chiral NADH model compounds 1a,b, possessing a specific C 2 -symmetric conformation.In our model compounds 1a,b four chiral carbon centers were introduced by the presence of (1R,2R)-diaminocyclohexane [6].Two identical pyridine-3,5-dicarbonyl units were connected by two identical (1R,2R)-diaminocyclohexane units into a large ring.The C 2 -symmetric structure has the following advantages: (i) we can take advantage of an efficient bi-directional synthetic method to prepare 1a,b; (ii) it simplified the analysis of the reduction reactions using our model compounds 1a,b.

Results and Discussion
Generally, the single-protection method was used to synthesize macrocyclic compounds such as our models 1a,b (Scheme 1).One of amino groups of (1R,2R)-diaminocyclohexane was protected and the resulting compound 6 was reacted with pyridine-3,5-dicarbonyl dichloride to obtain compound 7.After deprotection of 7, the resulting amide 8 was converted to macrocyclic compound 4 by treatment with another portion of pyridine-3,5-dicarbonyl dichloride.However, this traditional method suffered from low yields and a long synthetic route.Herein, we would like to report a novel practical and efficient method as outlined in Scheme 2.
Although the pentafluorophenoxy is a good leaving group, bis(pentafluorophenyl)pyridine-3,5dicarboxylate (3) is not so active as pyridine-3,5-dicarbonyl dichloride in the reaction with (1R,2R)diaminocyclohexane. As a result, the key intermediate 4 was obtained conveniently in only one step from (1R,2R)-diaminocyclohexane, avoiding the complicated single-protected method.When an appropriate halide was added to a DMF solution of 4 and the resulting reaction mixture was heated at 80 º C ~90 º C for 12 h, then 5a,b were obtained.Crude 5a,b, used directly without further purification, was reduced by sodium dithionite to afford crude compounds 1a,b.After separation on Sephadex LH-20 with methanol as the eluent, pure model compounds 1a,b were obtained.The C 2 -symmetric NADH models 1a,b could enantioselectivly reduce the pyruvate mimic methyl benzoylformate in acetonitrile in the presence of magnesium perchlorate (Scheme 3).The results summarized in Table 1 shows that NADH models 1a and 1b are similar as reducing agents.The resulting reduction products were the same enantiomer (R-mandelate) and they dispalyed similar enantiomeric excess values, so we believe that the reductibility and the enantioselectivity of model compounds 1a,b is related to their specific C 2 -symmetric conformation, rather than the substituent at the N1 position of the dihydropyridine rings.

Conclusions
In summary, a new type of NADH model compound with good reactivity and enantioselectivity has been synthesized in good yields by an efficient and convenient synthetic method.

General
All starting materials were commercially available.DMF was dried with CaSO 4 and distilled under reduced pressure.THF was distilled from CaH 2 .Melting points were measured on an X-4 digital microscope melting point apparatus (Beijing Tech Instrument Co., Ltd.).IR spectra (KBr disks) were recorded on a Perkin-Elmer Nicol FT-50X spectrometer. 1 H-NMR and 13 C-NMR were recorded on Bruker AC-300 FT or AV-400 FT instruments.The chemical shifts are reported in parts per million (ppm) downfield from internal tetramethylsilane (TMS).Electron impact MS spectra were obtained on a JEOL JMS-HX 100 instrument.High resolution mass spectra (HRMS) were measured in negative ion mode by Electrospray (ESI) on APEX-Qe 94 instrument.Chiralcel OD-H columns were purchased from Daicel Chemical Industries.Column chromatography was carried with silica gel (200-300 mesh), and HF254 silica gel for TLC was obtained from Qingdao Marine Chemistry Co. Ltd., Qingdao, China.Sephadex LH-20 (18-110 µm) was provided by H&E Co., Ltd.

General Procedure for the asymmetric reduction of methyl benzoylformate with NADH model compounds 1a-b [2c]
The NADH model 1a-b (1 mmol), methyl benzoylformate (2 mmol) and magnesium perchlorate (2 mmol) were dissolved in acetonitrile (5 mL).The resulting solution was stirred in the dark under nitrogen at room temperature for 3 days.Water (0.5 mL) was then added and the product extracted with ether.The organic phase was dried and the solvent evaporated.The crude methyl mandelate was purified by chromatography, using 1:5 ethyl acetate/petroleum ether as eluent, to give a white solid.Product identity and enantiomeric excess were determined by HPLC analysis using a Chiracel OD-H column (0.46 cm I.D. x 25 cm L).Chromatographic conditions: injection: 10 µL; eluent: n-hexane/2propanol = 85:15; flow rate: 1.0 mL/min; temperature: 35 °C.For 1a (Figure 2

Scheme 1 .
Scheme 1. Single-protection route to the key intermediate 4.

Table 1
Asymmetric reduction of methyl benzoylformate with 1.
aDetermined by HPLC, see Experimental; b isolated yields.