Asymmetric synthesis of double bond isomers of the structure proposed for pyrinodemin A and indication of its structural revision.

Asymmetric synthesis of double bond isomers (+)-2 (delta(15',16')) and (+)-3 (delta(14',15')) of the structure (1) (delta(16',17')) proposed for pyrinodemin A, a cytotoxic bis-pyridine alkaloid with a unique cis-cyclopent[c]isoxazolidine moiety from a marine sponge, has been accomplished. Pyrinodemin A was indicated to be a 1:1 racemic mixture of 2 from comparison of C(18 )and chiral HPLC analysis for pyrinodemin A and the synthetic compounds as well as ESIMS data of oxidative degradation products of pyrinodemin A.

Pyrinodemin A, a cytotoxic bis-pyridine alkaloid with a unique cis-cyclopent[c]isoxazolidine moiety, has been isolated from a marine sponge Amphimedon sp., and its relative stereostructure was proposed as 1 (Δ16’,17’) on the basis of spectral data [1]. The unique structure of pyrinodemin A has prompted synthetic chemists to its total synthesis of 1 as well as syntheses of the double bond isomers 2 (Δ15’,16’) and 3 (Δ14’,15’) [2,3,4] followed by different proposals of the structural revision of pyrinodemin A to be 2 [2] or 3 [3,4].

In order to examine the correct structure of pyrinodemin A, we have synthesized (+)-2 and (+)-3, the double bond isomers of 1, as an optically active form, and compared HPLC profiles of the synthetic compounds and pyrinodemin A. In addition, oxidative degradation experiments were performed for a remaining small amount of pyrinodemin A to determine the position of a double bond. In this paper, we describe asymmetric synthesis of (+)-2 and (+)-3, and indication of the structure of pyrinodemin A to be (±)-2.

The Δ15’,16’ double bond isomer (+)-2 was synthesized as follows (Scheme 1). The synthesis of hydroxylamine 6a commenced with known pivaloate 5a [5]. Oxidation of alcohol 5a with 2-iodobenzoic acid (IBX) [6] in DMSO and THF afforded its aldehyde. Treatment of the aldehyde with NH2OH·HCl and NaOAc in MeOH provided oxime which was reduced with NaBH3CN in MeOH to afford hydroxylamine 6a [7,8]. Condensation of 6a and optically active aldehyde 7 [8] in CHCl3 containing Na2SO4 at r.t. gave the nitrone 8a, which was followed by heating to afford cis-cyclopent[c]isoxazolidine [9] 9a in 58% yield.

Scheme 1

Reagents and conditions: (a) IBX, DMSO, THF (69%); (b) H2NOH·HCl, AcONa, MeOH (96%); (c) NaBH3CN, MeOH, pH 3, 0 °C; (d) Na2SO4, 6, CHCl3, r.t.~reflux (58% for 2 steps); (e) 3N HCl, dioxane (80%); (f) NaIO4, MeCN, H2O, 0 °C; (g) Br-[Ph3+(CH2)5CH2OH], n-BuLi, THF, 0 °C (51% for 2 steps); (h) TIPSCl, imidazole, CH2Cl2 (75%); (i) H2, Pd-C, MeOH (93%); (j) DIBAL, CH2Cl2, -78 °C (75%); (k) IBX, DMSO (80%); (l) Br-[Ph3P+(CH2)7CH2OH], n-BuLi, THF, 0 °C (81%); (m) 46% HF, MeCN (55%); (n) CBr4, Ph3P (80%); (o) 3-methylpyridine, LDA, DMPU, -40 °C (64%)

Treatment of 9a with 3N HCl in dioxane gave diol, which was converted into its aldehyde by treatment with NaIO4 and then into alcohol 10a by Wittig reaction [10]. Protection of alcohol 10a as its TIPS ether followed by reduction with Pd-C gave its saturated TIPS ether, which was converted into alcohol 11a with DIBAL. IBX oxidation of 11a followed by Wittig reaction [10] afforded its unsaturated alcohol, which was subjected to deprotection with HF to give diol 12a in 55 %. Treatment of diol 12a with CBr4 and PPh3 provided its dibromide, which was coupled with 3-methypyridine using LDA and DMPU [11] in THF to furnish optically active compound (+)-2. This is the first synthesis of optical active form of 2, although its racemic form ((±)-2) has been synthesized [2,3,4]. The Δ14’,15’ double bond isomer (+)-3 was prepared from pivaloate 5b by almost same procedure as described for synthesis of (+)-2 (Scheme 1).

The position of a double bond and the stereochemistry of pyrinodemin A were examined as follows. Compounds (±)-1 [2], (±)-2 [2], and (+)-3 were subjected to C18 HPLC [Wako sil-II 5C18 RS, Wako Ind., Ltd., 4.6 x 250 mm; flow rate 1.0 mL/min: eluent; MeOH/H2O (91:9); UV detection at 263 nm] and found to be separated (1, t 21.6 mim; 2, t 17.0 min; 3, t 15.8 min), while the retention time (t 17.0 min) of pyrinodemin A was identical with that of 2 under the same condition, indicating that the position of a double bond of pyrinodemin A corresponded to that (Δ15’,16’) of 2. To elucidate the stereochemistry of pyrinodemin A, compound (±)-2 was subjected to chiral HPLC [CHIRALCELL OD-H, Daicel Co., Ltd., 4.6 x 250 mm; flow rate 1.0 mL/min: eluent: hexanes/i-PrOH (95:5); UV detection at 263 nm] and found to be separated (t 44 and 47 min), while the retention time of (+)-2 was 47 min (Figure 1). On the other hand, pyrinodemin A gave the two peaks corresponding to those of (±)-2 in a ratio of 1:1 under the same conditions, indicating that pyrinodemin A is a 1:1 racemic mixture of 2. Furthermore, pyrinodemin A was treated with OsO4 and then NaIO4 to give degradation products, one of which showed an ESIMS fragment ion peak at m/z 242 (M+Na)+, corresponding to an aldehyde (13) of C-7’~C-15’ segment connected to a pyridine ring (Scheme 2). From the results described above, it was indicated that the olefin position of pyrinodemin A was C-15’ and C-16’ (2), as proposed by Snider’s group [2], and that pyrinodemin A was a 1:1 racemic mixture of 2.

Figure 1

Chiral HPLC profiles of (a) synthetic compounds (±)-2, (b) (+)-2, and (c) pyrinodemin A

Scheme 2