1,4-Dimethoxynaphthalene-2-methyl (‘DIMON’), an oxidatively labile protecting group for synthesis of polyunsaturated lipids

Abstract

A new benzyl-type protecting group (1,4-dimethoxynaphthalene-2-methyl, ‘DIMON’) for hydroxyl functions can be selectively removed under oxidative conditions without damaging polyunsaturated fatty acyl groups. Its application is shown by the first synthesis of an ether (plasmanyl) phospholipid containing the docosa-(4Z,7Z,10Z,13Z,16Z,19Z)-hexaenoyl group.

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Among the components of the human lipidome are alkyl-glycerophospholipids, which contain an alkyl group at the sn-1 position and an acyl group at the sn-2 position (so-called ether or plasmanyl phospholipids) in contrast to the familiar diacylglycerols and their corresponding phospholipids (Fig. 1).^1,2 Much interest is centred on those molecules where the fatty acyl moiety is derived from the polyunsaturated arachidonic acid (20 : 4) or docosahexaenoic acid (22 : 6).³ We now report the first scalable synthesis of this type of ether phospholipid, thus making these molecules readily available for biological studies and potential medical applications.

Fig. 1 Examples of diacyl and plasmanyl phospholipids.

A common impediment to the use of benzyl-type protecting groups in organic synthesis, notably for glycerolipids, is that their removal under oxidative, reductive or acidic conditions is incompatible with polyunsaturated fatty acyl groups. For example, although 4-methoxybenzyl (PMB) can be used for protection and deprotection in the presence of an oleoyl group, this group could not be removed oxidatively or by any other means in the presence of acyl groups containing two or more double bonds (e.g. linoleoyl).⁴ Difficulty with oxidatively cleaving PMB in the presence of 1,3- and 1,4-dienes is a widely reported problem.^5–8 3,4-Dimethoxybenzyl (DMB)⁹ can be removed oxidatively more readily than PMB, but was also unsatisfactory for molecules with diene moieties.¹⁰

We have found a novel benzyl-type protecting group (1,4-dimethoxynaphthalene-2-methyl, ‘DIMON’), (Fig. 2) that can be removed under oxidative conditions without damaging polyunsaturated acyl groups. This group was selected after consideration of synthetic accessibility and, above all, redox potentials (E_1/2, V in MeCN solvent) for methoxy-substituted aromatic systems (for a review see ref. 11): methoxybenzene (1.76),¹² 1,2,-dimethoxybenzene (1.45),¹² 1,4-dimethoxybenzene, (1.34),¹² 1,4-dimethoxynaphthalene (1.10),¹³cf. benzene (2.08),¹⁴ naphthalene (1.34).¹⁴ These data were expected to mirror the ease of removal of the corresponding benzyl-type group with a reagent [e.g. 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ)] operating via an electron transfer mechanism and therefore DIMON should be more easily removed than PMB or DMB.

Fig. 2 Structure of novel benzyl-type protecting group DIMON.

The application of DIMON is illustrated by the synthesis of a variety of protected alcohols, phenols, amines and amides, and 1-alkyl-2-acyl glycerols, where the acyl groups are derived from polyunsaturated fatty acids including (4Z,7Z,10Z,13Z,16Z,19Z)-docosa-4,7,10,13,16,19-hexaenoic acid (DHA). It is critically important that the latter acyl moiety survives removal of DIMON without double bond perturbation or acyl migration.

DIMON protection of heteroatoms requires 2-(chloromethyl)-1,4-dimethoxynaphthalene (1, DIMONCl), which has been prepared either by chloromethylation of 1,4-dimethoxynaphthalene^15,16 or by treatment of (1,4-dimethoxynaphthalen-2-yl)methanol (2) with thionyl chloride¹⁷ or methanesulfonyl chloride¹⁸ Compound 2 is available either by reduction of methyl 1,4-dimethoxy-2-naphthoate¹⁸ or from 2-bromo-1,4-dimethoxynaphthalene via the corresponding Grignard reagent, which was reacted with dimethylformamide followed by reduction with sodium borohydride.¹⁹ We have prepared compound 2 from commercially available 2-bromo-1,4-dimethoxynaphthalene 3²⁰ by lithiation followed by treatment with formaldehyde (Scheme 1). DIMONCl 1 was obtained in quantitative yield simply from reaction of 2 in a two-phase system of diethyl ether and 12 M hydrochloric acid. Separation of the organic phase, drying and evaporation gave 2 as an air-stable, crystalline solid.

Scheme 1 Synthesis of 2-(chloromethyl)-1,4-dimethoxynapthalene (DIMONCl 1): (a) (i) n-BuLi, −78 °C, 2 h (ii) paraformaldehyde, 2 h, rt, Et₂O, Ar, 62%; (b) conc. HCl, Et₂O, rt, 1 h, 100%.

DIMON protection of alcohols and amides (for selected examples see Table 1) was efficiently achieved under standard conditions via the corresponding alkoxide/amide anion, which was alkylated using DIMONCl in the presence of catalytic tetrabutylammonium bromide in THF at room temperature or at reflux for the steroidal substrates. With glycidol 4e (entry 5, oxiran-2-ylmethanol), a key intermediate for phospholipid synthesis, DMF solvent was preferred to THF.

Table 1 DIMON installation onto O- and N-nucleophiles

Entry	NuH	Product	Yield (%)	Method
Method A: 1 equiv. NuH, 1.2 equiv. DIMONCl, 10 mol% TBAI, 1.2 equiv. NaH 60% dispersion in mineral oil, dry THF, N2, rt, 16 h.a DMF used as solvent.b Reaction at reflux. B: 1 equiv. NuH, 1.2 equiv. DIMONCl, 1.1 equiv. Cs2CO3, dry MeCN, N2, reflux, 16 h. C: 1 equiv. NuH, 1.2 equiv. DIMONCl, 1.2 equiv. NEt3, dry THF, N2, rt, 16 h.
1	Eicosanol 4a	5a	93	A
2	Octan-1-ol 4b	5b	75	A
3	Oct-2-en-1-ol 4c	5c	79	A
4	Propargyl alcohol 4d	5d	82	A
5	Glycidol 4e	5e	85	A^a
6	4-Methoxybenzyl alcohol 4f	5f	93	A
7	Cholesterol 4g	5g	91	A^b
8	Ergosterol 4h	5h	89	A^a
9	L-Menthol 4i	5i	71	A
10	4-Chlorophenol 4j	5j	91	B
11	Thymol 4k	5k	80	B
12	Naphth-1-ol 4l	5l	89	B
13	Morpholine 4m	5m	68	C
14	N-Methylbenzamide 4n	5n	86	A

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Phenols were reacted with DIMONCl using Cs₂CO₃ as base, whilst triethylamine sufficed for amines. DIMON was efficiently removed from protected alcohols and phenols within one hour at 20 °C by exposure to 2,3-dichloro-4,5-dicyanobenzoquinone (DDQ) in wet DCM (Table 2). After deprotection, the protecting moiety was easily separated chromatographically as 1,4-dimethoxy-2-naphthaldehyde 6 and could be recycled, whilst any residual DDQ was reduced by aqueous ascorbic acid to 2,3-dichloro-5,6-dicyano-1,4-quinol, which was extracted into an aqueous phase. Importantly, the DIMON ether was selectively oxidised over the PMB ether (Table 2, entry 4) in a 9 : 1 ratio by ¹H NMR analysis. This agreed with the redox potentials of DIMON and PMB, and showed that DIMON can be orthogonally removed in the presence of PMB.

Table 2 Oxidative cleavage of DIMON by DDQ

Entry	DIMON-OR	ROH	Yield (%)
Method: 1 equiv. DIMON-Nu, 1.1 equiv. DDQ, 19 : 1 DCM : H2O, rt, 0.5–1 h.
1	5a	Eicosanol 4a	91
2	5c	Oct-2-en-1-ol 4c	85
3	5d	Propargyl alcohol 4d	99
4	5f	Methoxybenzyl alcohol 4f	86
5	5h	Ergosterol 4h	80
6	5j	4-Chlorophenol 4j	88

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DIMON could also be removed under acidic catalysis (TFA in DCM or HCl in MeOH) or by catalytic hydrogenation (Pd/H₂) with substrates derived from alcohols lacking a non-aromatic C=C bond (see Scheme 2 and ESI†).

Scheme 2 Acidic and reductive cleavage of DIMON from DIMON-menthol to give menthol and DIMON-thymol to afford thymol, respectively.

To define conditions suitable for the synthesis of alkyl phospholipids, DIMON-protected rac.-glycidol 5e was reacted with a 3-fold excess of hexadecan-1-ol 9 in the presence of either catalytic ytterbium triflate or boron trifluoride, the latter giving a better yield of compound 10 (Scheme 3). Acylation of 10 was performed with three representative fatty acids giving the corresponding esters 11a-c: from oleic acid (18 : 1) in 93% yield, from linoleic acid (18 : 2) in 79% and docosahexaenoic acid (22 : 6) in 91% yield.

Scheme 3 Oxidative cleavage of DIMON in the presence of unsaturated and polyunsaturated functionality. (a) 1 equiv. (rac.)-DIMON-glycidol 5e, 3 equiv. hexadecanol 9, 5 mol% BF₃·OEt₂, dry toluene, N₂, rt, 16 h, 75%; (b) 1 equiv. alcohol 10, 1.2 equiv. fatty acid ((i) oleic acid, (ii) linoleic acid, (iii) docosahexaenoic acid), 1.5 equiv. DCC, 0.5 equiv. DMAP, dry DCM, rt, 24 h, (i) 93%, (ii) 79%, (iii) 98%; (c) 1 equiv. DIMON-alcohol 11a-c, 1 equiv. DDQ, 19 : 1, rt, 0.5 h, (i) 86%, (ii) 81%, (iii) 62%.

All three esters were smoothly deprotected by DDQ giving the corresponding primary alcohols 12a-c in 86% (oleyl), 81% (linoleyl) and 62% yield (docosahexaenoyl) with no evidence of double bond perturbation or acyl migration.

To illustrate the application of DIMON in total synthesis of polyunsaturated phospholipids, we repeated the synthetic route shown in Scheme 3 starting from (S)-glycidol 13, giving intermediate 14 in 35% overall yield (cf.Scheme 4). Compound 14 was converted into the naturally occurring plasmanyl phospholipid 15 using a known phosphorylation method, reaction with 2-chloro-1,3,2-dioxophospholane 2-oxide (COP) then trimethylamine, to afford a choline derivative.²¹ This is the first reported synthesis of such an ether phospholipid. The corresponding oleoyl derivative was made from 12a in a similar manner (data not shown).

Scheme 4 Synthesis of plasmanyl phospholipid 15via DIMON-protected S-glycidol 16 (a) 1.2 equiv. DIMONCl 1, 1.2 equiv. NaH 60% dispersion in mineral oil, 10 mol% TBAI, dry DMF, N₂, rt, 16 h, 77%; (b) 3 equiv. hexadecanol 9, 5 mol% BF₃·OEt₂, dry toluene, N₂, rt, 16 h, 75%; (c) 1.2 equiv. docosahexaenoic acid, 1.5 equiv. DCC, 0.5 equiv. DMAP, dry DCM, rt, 24 h, 98%; (d) 1 equiv. DDQ, 19 : 1, rt, 0.5 h, 61%; (e) (i) 1.5 equiv. COP, 3 equiv. pyridine, dry TFT, N₂, rt, 1 h, 100%; (ii) 2 M NMe₃ in MeCN, dry TFT, reflux, 16 h, 35%.

Although this paper describes a specific application of DIMON, we believe that this new protecting group will find application in the synthesis of a wide range of natural products.

We thank the European Research and Development Fund (ERDF) and BiBerChem Research (Newcastle upon Tyne, UK) for financial support (to JT), the NMSF at Swansea University for HRMS and Masuma Begum for technical support.

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3cc04292h

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