Mid Sweden University

The basic chemical structure of the sex pheromone of the pine sawfly Microdiprion pallipes (Fallen) has earlier been identified as the propionate ester of (2S,3S,7 R/S,11 R/S)/(2R,3R,7 R/S,11 R/S)-3,7,11-trimethyl-2-tridecanol. We now report the results from further investigations on the male response to individual stereoisomers and to blends of stereoisomers, both in electroantennographic (EAG) recordings and in field trapping experiments. We also present our attempts to determine the stereochemistry of the compounds present in females of M. pallipes. By comparing gas chromatograms and mass spectra obtained from natural extracts with those from synthetic compounds it was found that the females contain one or more of the four (2S,3S,7 R/S,11 R/S)-3,7,11-trimethyl-2-tridecanol isomers (SS++-1). The active pheromone component is the corresponding propionate ester 2. In EAG experiments, males responded most strongly to five propionate ester samples, namely two four-isomer blends: SS++-2 and SR++-2, and three individual stereoisomers: SSSR-, SRRR- and SRSR-2. In a series of field trapping experiments it was found that males were attracted to the SR++-2 four-isomer blend and to the individual isomer SSSR-2. Based on the EAG-recordings and field responses of males and the stereoisomers found in the females, we suggest that the propionate ester of (2S,3S,7S,11R)-3,7,11-trimethyl-2-tridecanol (SSSR-2) is used as a main component of the sex pheromone in M. pallipes. Apparently the males react to other stereoisomers in addition to that or those produced by the females.

Several racemic methyl decanoic acids have been synthesised and successfully resolved in esterification with 1-hexadecanol at aw=0.8 in cyclohexane using immobilised Candida rugosa lipase (CRL) as the catalyst. The enantiomeric ratios (E=2.8-68) obtained were surprisingly high even when the methyl group was as remotely located as in 8-methyldecanoic acid (E=25). Interestingly, the lipase shows enantiopreference for the S-enantiomer when the methyl group is located on even numbered carbons i.e. for the 2-,4-,6- and 8-methyldecanoic acids and to the R-enantiomer when the methyl group is located on uneven numbered carbons i.e. for the 3-,5- and 7-methyldecanoic acids.

All sixteen stereoisomers of 3,7,11-trimethyl-2-tridecanol were synthesised in high stereoisomerical purities (> 95%), for use in the identification of the stereoisomers present in females of the pine sawfly Microdiprion pallipes (Fallen) (Hymenoptera: Diprionidae) as the precursor of the actual sex pheromone (which is the propionate), and also for investigation of the biological activities of the esters. The key step in the syntheses was the coupling of each of the enantiomers of cis-3,4-dimethyl-gamma -butyrolactone with each of the four pure stereoisomers of 1-lithio-2,6-dimethyloctanes. The four corresponding alcohols were obtained by lipase-catalysed (Amano PS) kinetic separation, based on selective acylation of either (2R/S,6S)- or (2R/S,GR)-2,6-dimethyl-1-octanol (obtained from the optically pure enantiomers of citronellal). Additionally, a mixture of the 16 possible stereoisomers of 3,7,11-trimethyl-2-tridecanol was also prepared.

The enantioselectivities of lipases from Pseudomonas cepacia (PFL, Amano PS, etc.) towards a series of primary 2-methyl-substituted alcohols using vinyl acetate as the acyl donor in transesterifications in organic solvents were studied. In terms of enantioselectivity, the best results were found for 3-aryl-2-methylpropan-1-ols with enantiomeric ratios (E-values) over 100 in most cases, whereas other 3-substituted primary 2-methylpropan-1-ols generally displayed lower enantioselectivities: 3-cycloalkyl-2-methylpropan-1-ols (E ≈ 20) and 2-methylalkan-1-ols (E ≈ 10). Moving the aryl group closer or further away from the chiral centre resulted in low enantioselectivities: 2-arylpropan-1-ols (E < 10), 2-methyl-4-(2-thienyl)butan-l-ol (E = 12), 2-methyl-5-(2-thienyl)pentan-l-ol (E=3.2) and 2-methyl-6-(2-thienyl)hexan-l-ol (E=3.8).