Elsevier

Carbohydrate Research

Volume 442, 10 April 2017, Pages 31-40
Carbohydrate Research

A Morita-Baylis-Hillman based route to C-5a-chain-extended 4-epi-isofagomine type glycosidase inhibitors

https://doi.org/10.1016/j.carres.2017.03.003Get rights and content

Highlights

  • Morita-Baylis-Hillman reaction provides novel enantiomerically pure precursors of elaborate iminosugars.

  • The route from D-glyceraldehyde to 5a-C-alkyl-4-epi-isofagomines is reported as example.

  • Final products are potent inhibitors of a panel of β-galactosidases.

  • One compound is a powerful pharmacological chaperone for human lysosomal β-galactosidase.

Abstract

By Morita-Baylis-Hillman reaction of 2,3-O-isopropylidene-D-glyceraldehyde with α,β-unsaturated carbonyl as well as hetero analogous carbonyl compounds such as acrylonitrile, suitable precursors of isofagomine and of 4-epi-isofagomine are available. Elaboration of the structures by amine introduction, followed by intramolecular ring closure and subsequent hydroboration of the double bond provides 4-epi-isofagomine derivatives featuring chain extensions at C-5a which are determined by the structures of the carbonyl compounds employed. As an example, the synthesis of C-(5aR)- and C-(5aS)-5a-C-pentyl-4-epi-isofagomines, powerful inhibitors of β-galactosidases, is outlined. In line with reported data, the (C-5aR) epimer was found a highly potent experimental pharmacological chaperone for GM1-associated human lysosomal β-galactosidase mutant R201C.

Introduction

Iminoalditols and related alkaloids such as 1-deoxynojirimycin (1), its epimer at C-4 (2) and castanospermine (3) are characteristic examples of potent glycoside hydrolase inhibitors, which have been demonstrated valuable tools for enzyme investigation [1], [2], [3], [4], [5]. Due to the essential roles of these enzymes, particular inhibitors may exhibit remarkable biological properties against various pathological processes, including certain forms of hereditary lysosomal disorders [6], [7], [8], [9]. This group of about 50 metabolic diseases arises from mutations in specific genes and leads to deficiencies in enzymes involved in the lysosomal degradation of glycolipids and glycans. Impressive research has been conducted to provide novel therapeutic compounds that may relieve the various symptoms caused by the inability of lysosomal glycosidases to process their respective substrates which, consequently, accumulate in the cells and lead to irreversible damage of nerve tissue, bones and various organs.

Contributions by many researchers have shown that evaluation of iminosugar- as well as carbasugar-based glycomimetics may lead to suitable therapeutic agents that function either by inhibiting upstream enzymes and, thus, by reducing the production of metabolites that cannot be degraded rapidly enough by the respective mutant enzyme (substrate reduction therapy, SRT) or by supporting the folding and trafficking of mutant enzymes to the lysosome (chaperone mediated therapy, CMT), for which it was proposed that sub-inhibitory dosing of active site specific molecules (pharmacological chaperones) could be exploited [6]. Whereas mutant proteins that cannot obtain/retain their functional conformation are recognized as misfolded by the quality control machinery in the endoplasmic reticulum and are eventually degraded, these carba or iminosugars coordinate and stabilize such mutant enzymes including lysosomal β-glucosidase, β-galactosidase or β-N-acetylhexosaminidase in their functional folded conformations thus enabling their exit from the endoplasmic reticulum and their passage to the lysosome where, due to the acidic milieu [10], the chaperone/inhibitor can exit the active site rendering an active glycosidase which now can take up its hydrolytic activity albeit at a lower turnover rate than the corresponding healthy wild-type enzyme.

Important work by several leaders in the area also reported in recent reviews [11], [12], [13] has demonstrated that N-substituted iminoalditols and similarly functionalized inhibitors bind better than their more polar parent compounds through stronger interactions with the aglycon binding site or with lipophilic pockets around the enzymes' active sites.

In this context, N-butyl-1-deoxynojirimycin is certainly one of the best studied iminosugars [9]. Wong [14] as well as Aerts and Overkleeft [15] have noted that large, lipophilic N-alkyl substituents, in particular adamantyl capped spacer arms, dramatically improve the interaction between iminoalditol derivatives and the lysosomal β-glucosidase.

The isoimino sugars invented by Lundt [16] and Bols [17] and their co-workers are “designer molecules”. They were prepared on the hypothesis that substrate-analogous compounds which stabilize positive charge at the (formal) anomeric position may be superior glycosidase inhibitors. Indeed, the first such isoimino sugar, isofagomine (4), was a potent β-glucosidase inhibitor (Fig. 1). C-5a-Chain extended derivatives such as C-nonyl analog 5, introduced by Fan and co-workers [18] were found even more powerful. Other such chain-extended derivatives 6 and 7 were investigated by Withers and collaborators [19].

For lysosomal β-galactosidase related to GM1-gangliosidosis and Morquio B disease, only very few types of β-D-galactosidase inhibitors have been investigated as potential pharmacological chaperones, thus far. The first reported compound, amphiphilic carbasugar NOEV (N-octyl-epi-valienamine, 8) was reported a powerful inhibitor [20] and even at sub-micromolar concentrations a potent pharmacological chaperone for a noteworthy range of lysosomal β-galactosidase mutants [21]. Other active compounds were structurally based on lead structure 2 which by various modifications had been converted into a few typical derivatives (916, in case of several N-substituent-variations in the same publication, only the most active is depicted) with interesting properties as experimental pharmacological chaperones [22], [23], [24], [25], [26], [27], [28], [29].

Derivatives of the isoiminosugar 4-epi-isofagomine [30] (17), for the same purpose have been lacking until very recently. Until 2014, only 5a-C-methyl analog 18 has been prepared in different context [31].

Motivated by the early encouraging results with our 1-deoxy-D-galacto-nojirimycin derivatives [24], [25], [32] we became interested in extending our collection of experimental chaperone candidates for GM1-gangliosidosis and have recently targeted C-5a-modifications of parent compound 17 such as 19 and 20 (Fig. 2) [33], [34]. Independently, Martin and collaborators have reported (5aR)-chain extended 4-epi-isofagomine derivative 21 and its impressive biological properties [35].

With a view to a broader and faster access to the piperidine system under consideration and for improved variability of the C-5a chain (which introduces the pronounced biological activities found thus far), we have recently explored a competitive protocol based on a Morita-Baylis-Hillman (MBH) [36], [37], [38], [39] key step (Scheme 1).

Section snippets

Synthesis

The synthetic approach relies on the MBH reaction of 2,3-O-isopropylidene-D-glyceraldehyde (22) with α,β-unsaturated ketones such as 23 (Scheme 2), which are commercially available or easily accessible in two steps by Grignard reaction of (commercial) aldehydes with vinylmagnesium halide.

Due to the chiral properties and the structural bias of 2,3-O-isopropylidene-D-glyceraldehyde (22), (S)-configuration of the newly formed secondary alcohol was found favored in the MBH reaction, thus producing

Conclusions

Following a simple protocol, novel derivatives of 4-epi-isofagomine, bearing chain extensions at C-5a are available by a Morita-Baylis-Hillman based de novo approach. In a typical example, recently reported compound 21 was prepared by this new, alternative route. As a potent inhibitor of a panel of D-galactosidases including human lysosomal β-galactosidase (Table 1), it was confirmed a powerful experimental chaperone with enzyme mutant R201C. Clearly, by variation of the α,β-unsaturated ketones

General methods

Optical rotations were measured at 20 °C on a Perkin Elmer 341 polarimeter at a wavelength of 589 nm and a path length of 10 cm. NMR spectra were recorded on a Varian INOVA 500 operating at 499.82 MHz (1H), and at 125.894 MHz (13C) or on a Bruker Ultrashield spectrometer at 300.36 and 75.53 MHz, respectively. CDCl3 was employed for protected compounds and CD3OD for unprotected inhibitors. Chemical shifts are listed in delta employing residual, non-deuterated solvent as the internal standard.

Acknowledgments

Financial support by the Austrian Fonds zur Förderung der Wissenschaftlichen Forschung, Vienna, (Project P 24815-B21) is gratefully acknowledged. SGW thanks GlycoNet, the Canadian Network of Centres of Excellence in glycoscience, for financial support. Anna Migglautsch (Institute of Organic Chemistry, Graz University of Technology) is thanked for competent support with the HPLC-MS measurements.

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