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Sulcatol: Synthesis of an Aggre ation Phero Shirley-Ann Black and Keith N. Slessor Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada
Pheromones, the chemical messengers of the insect world, have become a major area of study for the organic chemist in the last few years. Interruption of the insect pest's life cycle by the use of minute amounts of these natural specific attractants offers an alternative means of control to insecticidal spraying. The following experiments introduce the students to the pheromone sulcatol, 6-methyl-5-hepten-2-01, the population aggregation pheromone of Gnathotricus sulcatus. The insect, an ambrosia beetle, is an economically important pest in the coniferous forests of the North Pacific Coast. The adult male releases sulcatol which attracts both male and female to the new site. This natural process is the first step in the recycling of windblown trees and stumps but becomes economic when the attack occurs on harvested timber or milled lumber. Using synthetic phqromones, the beetles in an economically important area can be lured to traps preventing attack on commercial lumber and logs ( I ) . Sulcatol was first identified from the frass or boring dust of 21,000 adult males from which 0.5 mg of the pure pheromone was isolated. Identification of 6-methyl-5-hepten-2-ol followed from infrared, NMR, and mass spectral studies (2). The alcohol was synthesized as desiribed in this experiment and found to be attractive to the natural population. The synthetic sulcatol prepared by our undergraduates is used routinely in the pest control program in various locations throughout British Columbia. ~ s i i from e the current biological interest, this experiment illustrates a number of chemical reactions and laboratory techniques that students are required to learn in an introductory organic chemistry laboratory. As is evident from the illustration, the borohydride reduction of the ketone is the central reaction, but a number of significant points are covered.
e = 2,4-dinitrophenylhydrazine d = pnitruhenzoyl chloride i n pyridine
The purification of a liquid product by a simple vacuum distillation using a water aspirator; the characterization of two liquids as solid crystalline derivatives; the following of the progress of an oxidation by thin layer silica gel microslides; are some of the other techniques utilized. Spectroscopic techniques, especially IR and NMR, are easily applicable to ketonelalcohol conversion illustrating several major functional erouu. absorbances (IR) and a well-spaced, simply-coupled proton system (NMR). The chirality of the alcohol is of special interest since it has been shown that G. sulcatus releases a mixture of 65% S(+) and 35% R(-) (2). The insect responds well to the racemic (+) borohydride product but poorly to S(+) and not at all to pure R(-), an example of chiral synergism ( 3 ) .A closely related species, G. retusus, utilizes only S(+)-sulcatol for aggregation, and its response is inhibited by amounts of R(-) greater than 5% - (41. ,-, The nhvsical urouerties of the sulcatol p-nitrobenzoates (5) ( ~ a b i 1) e empiasize the difference befween chiral ~
The extent and complexity of the experiment can be varied according to the time available and the orranization of the curricu1;m. In Table 2 we present an indication of the time necessary for our students to carry out the procedures described. By having students run two experiments simultaneously, considerable time-saving can be accomplished. The sulcatol experiment is technically not a difficult set of experimental procedures and can be handled easily by firstTable 1. Some Physical Properties of Derivatives of the Sulcatol System (f)-rulcatol pnitiobenroate S(+)-sulcatol pnitrobenroate (5) R(-)-suicatol pnitrobenroate ( 5 ) 2.4dinilrophenylhydrazone of 6-methyl-5-hepten-2-one
36-37' 54-55O 54-55O 83-64'
4 0 +61.4' -61.2
'
* O
Table 2. Class Time Normally Required for Experiments
where a = NaBHI in methanol b = CrOz in acetone
Preparation of: (a) sulcatol (b) pnitrobenroate (c) 2,4-dinitrophenylhydrazone Oxidation/TLC Total
Volume 59
Con~ecutivelv
Sirnuitaneousl~
3-4 hr 2-3 hr 1'12-2 hr 1'I-2 hr upto 11 hr
3X3hr
Number 3
March 1982
2X4hr
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semester organic students. It h a s a biological background m a k i n r i t of more interest t o life science students. Occasionally, we have r u n hioassays o n t h e synthetic sulcatol i n a walking olfactometer ( 6 )as a demonstration. h u t this reauires
t o be u n d u i y toxic a n d b o t h t h e ketone a n d alcohol have pleasant odors. W e have found t h e sulcatol e x ~ e r i m e n ta stimulating a n d very successful experiment in o u r u n d e r graduate laboratory over t h e last six years.
Procedure
Preparation of (*)-Suicatoi Cool a 125-mL Erlenmeyer flask containing 35 mL of 60% aqueous methanol by stirring magnetically in an ice water bath. When the temperature of the aqueous methanol is below 10'C add 0.50 g (0.013 mol) sodium borohydride (Fisher Scientific Company) to the vigorously stirred solution. Meanwhile, position a 50-mL dropping funnel containing 4.2 g (0.033 moll 6-methvl-5-henten-2-one IAldrich Chemical C o m ~ a n- .v ) over the flask. As soon as the sodium bokhydride has dissolved, begin dropwise addition of the 6-methyl-5-hepten-2-onea t such a rate that the temperature does not rise above 25-C. At the completion of the addition (about 5 min), rinse the dropping funnel with a few milliliters of methanol and run the washings directly into the flask. When there is no further increase in temperature remove the ice water bath and allow the solution to stir for a further % hr. During this 'I2hr, completion of reaction may he checked for by TLC using silica gel coated microslides, CHzClzas a developing solvent and iodine vapor for visualization. When the reaction is completed, add 15 mL ofwater to the Erlenmeyer flask, replace the ice water bath and cool the solution to ahout 15°C. Fill the dropping funnel with 5 mL of 6 M HCl and add the acid dropwise and slowly to the rapidly stirred solution until the pH is approximately 2 (by Duotest pH 1.04.3 paper). Transfer the solution to a separatory funnel and extract with methylene chloride (3 X 20 mL). Then wash the combined organic extracts with 30 mL saturated NaCl solution to which has been added 0.2 g NaHCOa. Dry the methylene chloride extract over anhydrous magnesium sulfate the" filter into a l 0 0 ~ m Lr.b. flask and remove methylene chloride by flash evaporation. Set up apparatus (r.b flask containing sulcatol, stillhesd, eondenser, receiver adapter, and preweighed receiver flask) for a vacuum distillation using the water aspirator to obtain the reduced pressure and a heating mantle with a variac for heat source. Place a manometer and s trap between the receiver adapter and the tap. Use boiling sticks t o control bumping. When the pressure has been reduced to 20 mm or less, turn on the heat source. Suleatol distills a t 82-84T and 16 mm. When distillation is complete, dismantle equipment, reweigh receiver flask, and calculate percent yield (67% is an average yield).
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256
Journal of Chemical Education
IR's and NMR's may be done on the starting ketone and the prepared sulcatol. Preparation of the 2,44-initrophenylhydrazone of 6-methyi-5-hepten-2-one( 7 ) Dissolve '12 mL of 6~methyl-5-hepten-2-onein 20 mL 95% EtOH. Add 15 mL 2,4-dinitrophenylhydrazinereagent. Allow the mixture to stand a t room temperature for 15 min. Filter on aBuchner funnel with fiberglass filter paper. Recrystallize derivative from 95%EtOH, mp 83-84T (lit 78"). Anal. calcd. for CldH18NaOa:C 54.89, H 5.92, N 18.29; found C 55.05, H 5.97, N 18.53. Preparation of (%)-6-methyl-5-hepten-2-ol p-nitrobenzoate Add 1.5 g (8.1 mmol) p-nitrobenzoyl chloride (MCB) to a 50-mL Erlenmeyer flask containing 1.0 g (7.8 mmol) (f)-sulcatol and 20mL pyridine. Stir magnetically for 1hr. Pour solution intc 50 mL of water and extract with petroleum ether (30-60') (3 X 25 mL). Wash the combined petroleum ether extracts with 10%aqueous HCl(2 X 15 mL) and then with 15 mL 5% aqueous sodium bicarbonate solution. Dry the petroleum ether extracts aver magnesium sulfate, filter and remove petroleum ether by flash evaporation. Cool flask in ice to promote crystallization. Recrystallize in warm methanol (-15 mL), mp 36-37T. Anal. calcd. for CMHISNO~: C 65.20, H 6.57, N 5.07; found C 65.01, H 6.72, N 5.07. Oxidation of (f)-6-methyl-5-hepten-2-01 Mix 2 mL of a 0.1% solution of (f)-6-methyl~5-hepten-2-01 in diethyl ether with 2 mL of a chromic acid reagent (prepared by slowly adding a suspension of CrOs (10 g) in concentrated HzSOa (10 mL) to 90 mL distilled water) in a test tube. Prepare 10-12 TLC microslides (coated with silica gel) with standards of (5)-6-methyl-5~hepten-2-01 and 6-methyl-5-hepten-2~one (0.1% in diethyl ether). Shake the test tube periodically and monitor the reaction by spotting the ether layer of the reaction an the prepared TLC slides every 10-15 min. The microslides are developed in CH2C12as before and detection is with iodine vapor. Acknowledgment W e wish to t h a n k t h e many organic undergraduate students who contributed t o t h e success of t h i s sulcatol experiment. Literature Cited J. H., J. Ecan. Eni., 72,165 (1979). (2) Byme,K.J..Swigar.A.A.,Siiventein,R.M.,Borden. J. H.,andStokkink.E.. J.Insect. Phy8ial.,20, 1895 (1974). (31 Borden, J. H.,Chong.I..,McLean,J.A.,Slessor,K.N.,andMori,K.,Scirnis. 192,894 (1) McLenn,J.A, sndBurden,
