Synthetic Studies on Erythromycin Derivatives: Reactivity of

The reactivity of the C12-21 alkene of some erythromycin A derivatives was studied. This double bond was easily oxidized to the corresponding epoxide with excellent stereoselectivity. A single crystal X-ray structure showed that the epoxide moiety was on the same side as the acetonide. When an erythromycin derivative containing a C12-21 alkene was treated with diazomethane a [3+2] cycloaddition affording a pyrazoline occurred. In the case of 6-O-allylated erythromycin derivatives the C12-21 alkene was selectively epoxidized in the presence of the 6-O-allyl moiety. These results show that the C12-21 alkene is an active reaction site, which can be used for useful further modification of erythromycin derivatives.


Introduction
Erythromycin A (1, Figure 1) is one of the most successful antibiotics ever discovered, and has been used for the treatment of bacterial infections over the past 50 years.Despite its popularity, however, erythromycin suffers from several problems associated with its use.First, erythromycin is not effective against most aerobic enteric gram-negative bacilli.Secondly, erythromycin is metabolized by liver enzymes, competing with other drugs such as theophylline and common hay fever remedies, leading to overdoses of these drugs.Finally, erythromycin is not very stable below pH 6.9, making formulation somewhat difficult.To overcome these problems and drug resistance, many erythromycin derivatives have been synthesized [1,2].Among the modifications examined is the elimination of the 12-OH group, resulting in new analogues with a C12-21 double bond such as 2 [2,3].The C12-21 double bond has been osmylated to provide a diol, hydroborated to a primary alcohol or epoxidized.The epoxide was opened with an amine [4] or other compounds, thereby introducing basic and polar functionalities at the C-21 position.We have previously reported erythromycin derivatives with modifications at the C12-21 alkene [5].We report herein the design and synthesis of new erythromycin intermediates with modifications at the C12-21 alkene.These compounds can be used as starting materials to make a variety of new erythromycin derivatives.

The epoxidation of (9S)-9,11-O-isopropylidene-2',4''-O-bis(benzoyl)-12,21-anhydro-9-dihydroerythromycin A (3).
It is known that the alkene moiety is easily oxidized to generate an epoxide, which can then be converted into other useful structures, such as a diol.For this reason, compound 3, prepared as previously reported [5], was oxidized by treatment with m-chloroperbenzoic acid (m-CPBA, Scheme 1).As expected, not only the C12-21 double bond but also the tertiary amine was oxidized, affording compound 4. The N-oxide 4 was then reduced by treatment with sodium sulfite to afford epoxide 5 in good yield (69 %).The stereospecificity of the C12-21 epoxidation reaction was confirmed by a single crystal X-ray structure study.The epoxide was found on the upper side of the molecule and with C-12R conformation (Figure 2).It should be very interesting to find out whether compounds with different configurations at this position have different activity.The diazo moiety is another versatile chemical moiety that can be easily converted into other groups.Thus, compound 3 was treated with diazomethane, affording the 1,3-dipolar cycloaddition product 6 (Scheme 2).NMR spectral analysis of compound 6 demonstrated that the regiochemical preference of the electron-withdrawing substitutent is toward to the N-portion of the dipole [6,7], affording a C-12-∆ 1 -pyrazoline derivative.Since pyrazoline derivatives have been demonstrated to possess many different biologically activities [8,9], compound 6 might be an intermediate from which active antibiotics could be made.

Epoxidation of erythromycin derivatives containing two alkene groups.
To obtain more versatile erythromycin intermediates, two double bonds were introduced into compound 3.An allylic group was added to the 6-OH of compound 3 by treatment with allyl methyl carbonate in the presence of palladium acetate (Scheme 3) [10], affording compound 7.The latter was then treated with m-CPBA.Surprisingly, the 6-allylic group was not affected during the reaction and only the C12-21 double was oxidized, affording compound 8. Another side product, compound 9, was also isolated.The epoxidation of olefins by peroxy acids is a widely used reaction for synthetic purposes and mechanistic studies [11].The bimolecularity and stereospecificity of the process have been generally rationalized by the Bartlett "butterfly" transition structure [12][13] as shown in structure 10 (Figure 3).An alternative transition structure, which resembles a 1,3-dipolar cycloaddition, has been suggested and is shown in structure 11.The formation of compound 9 agrees with the latter.Since m-CPBA is a weak acid, it is difficult for it to attack the epoxide to form compound 9.

Conclusions
The C12-21 alkene is a reactive site of erythromycin A derivatives, which can be oxidized to an epoxide by organic peroxyacids, even in the presence of a 6-O-allyl moiety in the same molecule.Furthermore, it can react with diazomethane to give a [3+2] cycloaddition product, a C-12-∆ 1pyrazoline derivative.These compounds, with their versatile epoxide and pyrazoline moieties can be used to synthesize potentially useful new erythromycin A derivatives.

General
All reagents and solvents were reagent grade.Further purification and drying by standard methods were employed when necessary.Erythromycin A and its derivatives were pre-dried azeotropically from benzene.THF was distilled from sodium benzophenone ketyl.CH 2 Cl 2 and EtOAc were distilled from CaH 2 .DMF, DMSO and HMPA were redistilled and stored in screw-cap vials with molecular sieve (4A).All organic solvents were evaporated under reduced pressure with a rotary evaporator.The plates used for thin-layer chromatography (TLC) were E. Merck silica gel 60F 254 (0.1 mm thickness) precoated on aluminum plates, and they were visualized under both long (365 nm) and short (254 nm) UV light.Compounds on TLC plates were visualized with a spray of 5% dodocamolybdophosphoric acid in ethanol and subsequent heating.Column chromatography was performed using E. Merck silica gel (230-400 mesh).Melting points were measured using Electrothermal IA9100 digital melting point apparatus and are uncorrected.NMR spectra were recorded at 300K in deuterated chloroform solution on a Bruker DPX-300 spectrometer (operating at 300.13 MHz for 1 H and 75.47 MHz for 13 C).Chemical shifts are reported as parts per million (ppm) in the δ scale relative to the resonance of CDCl 3 (7.26ppm in the 1 H, 77.00 ppm for the central line of the triplet in the 13 C modes, respectively).Coupling constants (J) are reported in Hz.Splitting patterns are described by using the following abbreviations: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet. 1H-NMR data is reported in this order: chemical shift; multiplicity; coupling constant(s), number of protons.Mass spectra (ERMS and HRMS) were obtained with a Thermo Finnigan MAT95XL spectrometer or a API 2000 LC/MS/MS system.Relevant data are tabulated as m/z.Elemental analyses were performed at the Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, China.

Figure 1 .
Figure 1.Structures of erythromycin A and its C12-21 alkene analogue

Scheme 2 .
Scheme 2. Conversion of the C12-21 double bond to a pyrazoline moiety