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 (Δ15’,16’) and (+)-3 (Δ14’,15’) of the structure (1) (Δ16’,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 C18 and chiral HPLC analysis for pyrinodemin A and the synthetic compounds as well as ESIMS data of oxidative degradation products of pyrinodemin A.

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 ∆ 15',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 2iodobenzoic acid (IBX) [6] in DMSO and THF afforded its aldehyde. Treatment of the aldehyde with NH 2 OH·HCl and NaOAc in MeOH provided oxime which was reduced with NaBH 3 CN in MeOH to afford hydroxylamine 6a [7,8]. Condensation of 6a and optically active aldehyde 7 [8] in CHCl 3 containing Na 2 SO 4 at r.t. gave the nitrone 8a, which was followed by heating to afford ciscyclopent[c]isoxazolidine [9] 9a in 58% yield. Scheme 1. Treatment of 9a with 3N HCl in dioxane gave diol, which was converted into its aldehyde by treatment with NaIO 4 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 CBr 4 and PPh 3 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 ∆ 14',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 C 18 HPLC [Wako sil-II 5C18 RS, Wako Ind., Ltd., 4.6 x 250 mm; flow rate 1.0 mL/min: eluent; MeOH/H 2 O (91:9); UV detection at 263 nm] and found to be separated (1, t R 21.6 mim; 2, t R 17.0 min; 3, t R 15.8 min), while the retention time (t R 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 (∆ 15',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 R 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 OsO 4 and then NaIO 4 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.

Acknowledgments
We thank Professor B. B. Snider (Brandeis University) for generous offer of synthetic samples of (±)-1 and (±)-2. This work was supported in part by grants from the Akiyama Foundation and the Takeda Science Foundation and a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

General
Optical rotations were determined on a JASCO P-1030 polarimeter. Infrared spectra were obtained on a JASCO FT/IR-230 spectrometer. Proton and carbon NMR spectra were recorded on a Bruker 600 MHz spectrometer. Chemical shifts are reported in δ values relative to chloroform (δ 7.26 for proton and δ 77.0 for carbon NMR. EI mass spectra were measured on a JEOL JMS-DX303 spectrometer.