Synthesis of β-Methyl Alcohols: Influence of Alkyl Chain Length on Diastereoselectivity and New Attractants of Rhynchophorus ferrugineus

Abstract

The diastereoselectivity of adducts in the addition reaction via the Felkin−Anh model is affected significantly by the steric effect of bulky groups. However, the influence of steric alkyl chain length has not been studied for the diastereoselectivity. In this work, we present a new strategy for the racemic synthesis of β-methyl alcohols to obtain various diastereomer ratios using the Felkin−Anh model. The addition of alkyl Grignard reagents to α-methyl aldehydes afforded diastereomer ratios of threo/erythro ≈ 2:1, while the reduction in structurally related ketones using LiAlH 4 afforded ratios of threo/erythro ≈ 1:1. The experimental data showed no effect of alkyl chain length on either side on the stereoselectivity of adducts. All synthesized analogues were evaluated for attractiveness to Rhynchophorus ferrugineus weevils in the field. Five novel derivatives, including two alcohols and three ketones, were found to attract weevils in the field trials. Among them, 3-methyldecan-4-one (5b) and 4-methyldecan-5-ol (11a) were found to be the most attractive to the insects.

Recent advances in the synthesis of insect pheromones: an overview from 2013 to 2022

“…123 Generally, methyl branching is introduced by (asymmetric) alkylation reactions (oen using Evans oxazolidinones) or by using a methyl-branched precursor such as citronellol, a natural product commonly used as a building-block for total synthesis. The enantioselective gram-scale synthesis of (R)-and (S)-13methylheptacosane (139), the sex pheromone of the pear psylla Cacopsylla pyricola was described by Yuan et al (2021) starting from tetradecanoic acid (Scheme 10). 108 The key step was asymmetric methylation using Evans-type oxazolidin-2-one chiral auxiliaries (step b, Scheme 10), followed by Wittig reaction of the chiral methyl-branched alkyl triphenyl phosphonium salt with tridecanal (step e, Scheme 10) to give the target sex pheromone (R)-or (S)-139 in high enantiomeric purity.…”

“…Fig. 7 shows the methyl-branched pheromones of Tenebrio molitor (Col., Tenebrionidae): (R)-156, [124][125][126][127][128] Oryctes rhinoceros (Col., Dynastidae): 157, [128][129][130][131] Kheper nigroaeneus (Col., Scarabaeidae): (R)-158, 126,128,132 Oryctes rhinoceros (Col., Dynastidae): 159, 133 Euproctis pseudoconspersa (Lep., Erebidae): 160, 134,135 Miltochrista calamina (Lep., Erebidae): (5R,7R)-161 136,137 palm weevils (Col., Curculionidae): (4S,5S)-162 and (4S,5S)-163, 138 Rhynchophorus ferrugineus (Col., Dryophthoridae): 164, 165, 133,139 Scolytus multistriatus (Col., Curculionidae): 166, 140 Sitophilus granarius (Col., Dryophthoridae): 167, 141 Diabrotica longicornis (Col., Chrysomelidae): (2S,8R)-168, 142,143 Bicyclus spp. (Lep., Nymphalidae): 169, 144 Cryptothelea variegata (Lep., Psychida): (3R,13R,3 ′ S)-170, 145,146 Eurytoma maslovskii (Hym., Eurytomidae): (2S,10R)-171, (2S,8S)-and (2S,8R)-172, 147 Pellaea stictica (Hem., Pentatomidae): 173, 148,149 Edessa meditabunda (Hem., Pentatomidae): 174, 150 Trichogramma turkestanica (Hym., Trichogrammatidae): 175, 151 Margarodes prieskaensis (Hem., Margarodidae): 176, 177.…”

Host plant recognition by two odorant‐binding proteins in Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

“…The aggregation pheromone secondary component 4‐methyl‐5‐nonanone (ferruginone) has poor attractant power but strongly improves the efficacy of ferrugineol 32 . Recently, two synthesized β‐methylalcohols were found to be more attractive than ferrugineol 33 . The esters mitted by different host plant tissues, such as ethyl acetate, ethyl propionate and ethyl butyrate, elicit strong responses of RPW in electroantennography trials 31,34 .…”

“…32 Recently, two synthesized ⊎-methylalcohols were found to be more attractive than ferrugineol. 33 The esters mitted by different host plant tissues, such as ethyl acetate, ethyl propionate and ethyl butyrate, elicit strong responses of RPW in electroantennography trials. 31,34 Ethyl acetate has been widely used as a strong synergistic lure.…”