Synthesis of compounds related to the naturally occurring Striga weed germination stimulant, strigol

This thesis describes the synthesis and the bioassay of germination stimulants for the seeds of parasitic weeds, Striga and Orobancher. The germination stimulants are analogues of strigol, a potent naturally occurring parasitic weed germination stimulant, in which the lactony1 C-ring has been replaced by monocarbonyl rings. The preparation of these compounds of ketone-derived strigol analogues, namely, 3( R )-methly-5( R )-(1-0x0-3,4-dihydro-1H-naphthalen-2- ylidenemethoxy)-5H-furan-2-one (13) and 3(S)-methly-5(S)-(1-oxo3,4dihydro-1H-naphthalen-2-ylidenemethoxy)-5Hfuran-2- one (ent.13) followed the literature procedure, whereby the starting materials – the simple ketones, indanone, a-tetralone. cycloheanone and cyclopentanone were treated with ethyl format in the presence of potassium rert-bu toxide (fonnylation reaction) after which each was coupled with 5-chloro-3-methyl-2(5H)-furanone via an enol ether linkage to give the indanone derived strigol analogue (12), tetralone derived strigol analogue (13), cyclohexanone derived strigol analogue (15), cyclopentanone derived strigol analogue (14), respectively (chapter III). The germination stimularory activity of these strigol analogues were assayed using the seeds of Striga hermonthica and Orobanche crenata. The bioassay of the germination stimulatory activity of compounds 12-15 towards the seeds of Striga hermonthica revealed germination percentages between 70 and 6.6 at the concentration range of 0.01 to 1 mg/L. The germination percentages of the reference compound, GR 24, which was assayed with respect to compounds 12-15 ranges between 64.8 and 59. l at the concentration range of 0.01 and 1mg/L. The bioassay of the germination stimulation activity of these compounds (12-15) on the seeds of Orobanche crenata showed the germination percentages between 59.6, 5 _5, and 23.5 as compared with compounds 13 (54.4, 51.8 and 32.1), 14 (3.3, 2.2 and 0.6) and 15 (11.8. 5.6 and 9.5) all at the concentrations of 1, 0.1 and 0.0 l mg/L, The preparation of enantiopure compounds of ketone-derived strigol analogues, namely, 3( R)-methyl -5( R)-( 1- oxo-3,4 -dihydro-1 H -naphthalen- 2 - ylidenemethoxy)-S H -furan-2-one (13) and 3( S)-methyl-5( S)-( 1-oxo-3,4-dihydro-l H ­ naphthalen-2 -ylidenemethoxy )-S H -furan-2 - one (ent. 13) followed the literature procedure. a-Tetralone and cyclohexanone were each treated with ethyl formate in the presence of potassium tert-butoxide (formylation) after which each was then coupled with the latent D-synthon, namely, the exo-5(S)-chlorolactone (9) and exo-5(R)-chlorolactone ( ent. 9) via anenol ether linkage to give 2( R)-niethyl-5( R)-(1-oxo--3,4-dihyd ro-1H ­ naphthalen-2-ylidenemethoxy )-4-oxa-tricyclo[5. 2.J .02,6 J dec-8-en-3-one (20) and 2( S)­ methyl-5( S)-( 1-oxo--3,4-dihydro-1H-naphthalen-2- ylidenemetho :xy)-4-oxa- tricycl o[ 5.2. l .02,6 J dec-8-en-3-one ( ent. 20) and 2( R )-me thy l-5( R )-( 2 - oxo­ cyclohexylid enemethoxy)-4-oxa-tricyclo[ S.2. J .Q2,6 J dec-8-en-3-one (24) and 2( S)­ methyl-5( S)-( 2-oxo-cyclohexylidenemethoxy )-4-oxa-tricyclo[5.2 .J .02.6 J dec-8-en-3-one ( ent. 24). The cycloreversion (deprotection) of 2( R)-methyl-5( R)-( 1-oxo-3,4-d ihydro- 1H-naphthalen-2- ylidenemethoxy )-4-oxa-tricyclo[5 .2.J .02,6 J dec-8-en-3-one (20) and 2( S)-methyl-5( S)-( 1-oxo-3,4-dihyd ro-1H-naphthalen-2-ylidenemethoxy )-4-oxa- tricyclo[ 5.2.l .02,6 ) dec-8-en-3-one ( ent. 20) by solution thermolysis gave 3( R)-methyl - 5( R)-( 1-oxo-3,4-dihydro-1 H-naphthalen-2- ylidenemethoxy )-S H -furan-2-one (13) and 3( S)-methyl-5( S)-( 1-oxo-3,4-dihydro-l -H-naphthalen-2- ylidenemethoxy )-SH-furan-2- one (ent. 13) as enantiopure compounds.