Synthesis of (R)-and(S)-10,16-dihydroxyhexadecanoic acid: cutin stereochemistry and fungal activation Aqeel Ahmed a , Terry Crawford a , Stephanie Gould a ,Y.S.Ha b , Monica Hollrah a , Farhana Noor-E-Ain a , Martin B. Dickman b , Patrick H. Dussault a, * a Department of Chemistry, University of Nebraska-Lincoln, Lincoln, NE 68588, USA b Department of Plant Pathology, University of Nebraska-Lincoln, Lincoln, NE 68588, USA Received 2 August 2002; received in revised form 18 December 2002 Abstract The first asymmetric syntheses of the cutin monomers (R)- and (S)-10,16-dihydroxyhexadecanoic acid (10,16-DHPA) and con- firmation of (S)(+)-absolute configuration for 10,16-DHPA derived from tomato are reported. The individual DHPA stereo- isomers display differences in their ability to activate the fungal pathogen Colletotrichum trifolii. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Tomato; Lycopersicon esculentum; Solonaceae; Cutin monomers; DHPA; Colletotrichum trifolii; Dihydroxyhexadecanoic acid; Asym- metric synthesis 1. Introduction Cutin, the outer coating on all aerial surfaces of plants, is a polyester primarily composed of poly- hydroxylated fatty acids (Kolattukudy, 2001). Upon attack by some pathogenic fungi, the cutin layer is cleaved by cutinolytic enzymes, and the presence of cutin monomers released has been found to induce expression of the fungal gene coding for these same cutinases(Kolattukudyetal.,1995a,b).Consequently,it is possible that the interaction of cutinase with cutin and cutin derived monomers serves as a signal for fungal gene expression associated with pathogenic development. Oneofthemostabundantclassesofcutinmonomersis the dihydroxypalmitic acids (DHPAs). Surprisingly, no asymmetric synthesis of a DHPA has been reported (Tulloch, 1980) and the single report assigning the absolute stereochemistry of cutin-derived DHPA is only based on ORD (Espelie and Kolattukudy, 1978; Ble´e and Schuber, 1993). We now describe the enantio- selective synthesis of (R)- and (S)-10,16-dihydroxyhex- adecanoic acid (10,16-DHPA) and the stereochemical assignment of 10,16-DHPA from tomato. The relative ability of the individual enantiomers to elicit transcrip- tion of fungal genes in Colletotrichum trifolii, the causal agent of alfalfa anthracnose (Dickman, 2000) is also reported. 2. Results and discussion Our synthetic approach (Scheme 1) was based on the use of a common precursor of both (R)- and (S)-10,16- DHPA (1). The initial portion of the synthesis, illu- strated in Scheme 2,beganwithadditionofthelithiated alkyne to 10-undecenal to furnish racemic enynol 2 (Frantz et al., 2000). The derived alkynone 3 underwent reduction with (R)- or (S)-Alpine borane to furnish enantiomerically enriched propargyl alcohols (R)-2 or (S)-2, respectively, which were carried individually through the remainder of the synthesis (Singh, 1992). Analysis of derived diastereomeric esters by 1 H and 19 F NMR (Scheme 3) indicated that (R)-2 and (S)-2 were each formed in at least 88% enantiomeric excess (Dale and Mosher, 1973). Completion of the synthesis is illustrated in Scheme 4. Ozonolysis under standard conditions (CH 2 Cl 2 /MeOH, followed by Me 2 S reduction) led mainly to the dimethyl acetal.However,ozonolysisinmethanolcleanlyfurnished 0031-9422/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. PII: S0031-9422(03)00003-7 Phytochemistry 63 (2003) 47–52 www.elsevier.com/locate/phytochem * Corresponding author. Tel.: +1-402-472-2732; fax: +1-402-472- 9402. E-mail address: pdussault@unl.edu (P. H. Dussault).