Determination of the Absolute Configuration of Anisotome Irregular Diterpenes: Application of CD and NMR Methods JOHN W. VAN KLINK, 1 * SEUNG-HWA BAEK, 2 ANNA J. BARLOW, 3 HIDEKI ISHII, 4 KOJI NAKANISHI, 4 NINA BEROVA, 4 NIGEL B. PERRY, 1 and REX T. WEAVERS 3 1 Crop & Food Research, Department of Chemistry, Otago University, Dunedin, New Zealand 2 Department of Herbal Resources, Professional Graduate School of Oriental Medicine, Wonkwang University, Iksan, Korea 3 Department of Chemistry, Otago University, Dunedin, New Zealand 4 Department of Chemistry, Columbia University, New York, New York ABSTRACT Several Anisotome diterpene derivatives were synthesized in an attempt to obtain a crystalline compound for X-ray analysis. Although we were unable to obtain a suitable crystal, the absolute configuration of the irregular diterpene skeleton was determined using two other techniques: a circular dichroism (CD) protocol based on a tetraarylporphyrin molecular tweezer that allowed prediction of the absolute stereo- chemistry on a microscale level, and a method employing differences in NMR shifts from derivatization of the naturally occurring acid 1 with enantiomers of a phenylglycine methyl ester (PGME) chiral anisotropic reagent. The excellent agreement between the CD and NMR methods led to the assignment of a 2S-absolute configuration for anisotomenoic acid 1. Chirality 16:549 – 558, 2004. A 2004 Wiley-Liss, Inc. KEY WORDS: irregular diterpene; synthesis; NMR; circular dichroism; exciton chirality; porphyrin tweezer; phenylglycine methyl ester; PGME The irregular diterpenes anisotomenoic acid 1 and anisotomenol 2 (Chart 1) have been found only in New Zealand plants of the genus Anisotome. 1,2 The relative configuration of these conformationally flexible com- pounds was assigned by 2D NMR spectra and molecular modeling. 3 A significant structural feature of both aniso- tomanes is the presence of two carbons bearing two methyl groups. This suggests an irregular biosynthesis involving a head-to-head coupling of geranyl units. Such dimerization, while well established for the triterpenes, has only been encountered rarely among the diterpenes and sesquiterpenes. Many of these rare structures have been found in the Apiaceae family. 3 In view of this novelty, and our interest in the biosynthesis of compounds 1 and 2, we sought to determine their absolute stereochemistry. There are several methods available for determining absolute configuration of chiral compounds. X-ray analysis of a crystalline derivative containing a heavy atom has been employed extensively. 4 Since the anomalous dispersion effect of heavy atoms can be measured very accurately, the absolute stereostructure is unambiguous. 5 However, it can be difficult obtaining a suitable crystal for analysis. Recently developed is a microscale chiroptical protocol based on 1:1 complexation between a chiral substrate (guest) and an achiral dimeric zinc tetraarylporphyrin molecular tweezer (host). In this nonempirical method, the absolute configuration of the chiral substrate is generally predicted by the relative steric sizes of substituents at the stereogenic center. 6 The complexes with P-1/P-2 por- phyrin helicity, where the larger group L at the stereogenic center protrudes outside the binding pocket with the medium group M inside, are formed preferentially (Fig. 1). This stereocontrolled complexation leads to characteristic circular dichroism (CD) exciton couplets that are diag- nostic for the resulting preferred porphyrin helicity, and hence for the absolute configurational assignment of the chiral substrate, provided that the steric distinction be- tween large (L) and medium (M) substituent has been made in a clear-cut manner. The method has been success- fully applied to various natural products and chiral syn- thetic compounds, such as secondary alcohols, 7,8 mono amines, 9 and a-chiral carboxylic acids. 10,11 One of the most widely used approaches to determine absolute configuration is centered on the Mosher method, where the 1 H NMR signals of two diastereoisomeric Contract grant sponsors: New Zealand Foundation for Science Research and Technology; Wonkwang University, Korea; Columbia University through an NIH GM grant; Contract grant number: 34509. Present address for A. J. Barlow: School of Biological and Chemical Sciences, University of Exeter, Exeter, United Kingdom EX4 4PS. This article includes Supplementary Material available via the Internet at http://www.interscience.wiley.com/jpages/0899-0042/suppmat *Correspondence to: John W. Van Klink, Crop & Food Research, Department of Chemistry, Otago University, Dunedin, New Zealand. E-mail: vanklinkj@crop.cri.nz Received for publication 30 April 2004; Accepted 24 May 2004 A 2004 Wiley-Liss, Inc. CHIRALITY 16:549–558 (2004) DOI: 10.1002/chir.20074 Published online in Wiley InterScience (www.interscience.wiley.com).