An Efficient Total Synthesis of

DOI: 10.1002/slct.201601890

Full Papers

z Organic & Supramolecular Chemistry

An Efficient Total Synthesis of (–)-(R), (+)-(S)-Lavandulol Pheromones and Their Derivatives through Proline Catalyzed Asymmetric a-Aminooxylation and [3,3] Claisen Rearrangement Viraj A. Bhosale* and Suresh B. Waghmode*[a] An efficient total synthesis of the enantiomerically pure lavandulol pheromones viz. (–)-(R) and (+)-(S)-lavandulol, (– )-(R)-lavandulyl acetate, (–)-(R)-lavandulyl propionate, (+)-(S)lavandulyl 2-methylbutanoate and (+)-(S)-lavandulyl senecioate

is accomplished from nonchiral and chiral materials. The strategy utilizes a sequence of proline catalyzed asymmetric aaminooxylation and [3,3] Claisen rearrangement for stereodivergent synthesis of chiral C-2 unit containing 4-pentenols.

Introduction

both (–)-(R)-lavandulyl acetate 2 and (–)-(R)-lavandulyl propionate 3 as pheromone components.[9] Other two esters lavandulyl 2-methylbutanoate 4 and cyclobutanoid monoterpene 5 are also constitutes of the female sex pheromone of the pink hibiscus mealy bug, while Maconellicoccus hirsutus (Green)[10] is used as an insect pest. (S)-Lavandulyl senecioate 6 and (S)-lavandulyl isovalerate 7 sex pheromones of the vine mealy bug and are very effective pest in vineyards.[5, 11]

Worldwide there are 28 families and more than 7300 species of scale insects and most of them act as notorious pests especially on agriculture plants, fruits trees, ornamental plants in greenhouse and indoor settings. Mealybugs or Pseudococcidae (Hemiptera: Coccoidea) are the second largest family group within the scale insects.[1] The mealybugs are sap sucking insects and causes direct damage to plants and crops through transmission of plant viruses or by excretion of honeydew, which promotes fungal development and causes loss of fruit quality.[2a] The demand for biologically safe healthy food and the current regulatory restrictions in agriculture pest control by Integrated Pest Management (IPM) demands efforts for development of the new methods for biological control of pests in a more eco-friendly manner.[2b] The considerable progress has been made over the past decade in the application of insect sex pheromones in pest control programs.[2c] Pheromones appears to be an emerging cost-effective technical solution to synthetic pesticides for sustainable management of agricultural pests and scale insects worldwide, through mating disruption due to its specificity and low environmental impact.[3, 4] (R)Lavandulol 1 is the key ingredient of lavender oil, an important additive in perfumes as well as sex pheromones of the vine mealy bug.[5] Both (R) and (S) forms of 1 act as a defensive pheromones in vine mealy bug red-lined carrion beetle Necrodes surinamensis[6] and as a sex pheromone in the vine mealybug Planococcus ficus,[7] strawberry blossom weevil and Anthonomus rubi[8] respectively. The Dysmicoccus grassii uses

[a] V. A. Bhosale, S. B. Waghmode Department of Chemistry, Savitribai Phule Pune University (Formerly University of Pune) Ganeshkhind, Pune 411007, India Tel: + 91-202-560-1395 ext. 585 and 586 Fax: + 91-202-569-1728 E-mail: [email protected] [email protected] [email protected]

ChemistrySelect 2017, 2, 1262 – 1266

Figure 1. Lavandulol pheromones and related derivatives.

Commercial applications of lavandulol pheromones are increasing as perfume additive in cosmetic industries and potential markers in pest control, which demands larger quantities of these compounds. Until now very few efforts have been devoted for synthesis lavandulol pheromones in racemic[12] and optically pure[13, 14] forms. Racemic synthesis of these pheromones mainly based on transition metal (Ti, Ce and Sn) or sulfone, mediated coupling of substituted allylic compounds with formaldehyde, while in some cases use of Claisen and Prins rearrangement reactions also reported.[12] Synthesis of enantiomerically pure forms of pheromones involves the use of chiral pool strategy,[13a, j] selective deoxygenation of allylic alcohol,[13b] chemo microbial synthesis,[13c] diastereoselective alkylation of 3-acylimidazolidin-2-ones[13d, i] and conjugate esters,[13g] Grob type fragmentation of a carvone,[13e] diastereoselective protonation of photodienols,[13h] and resolution of 1262

2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers racemic lavandulols.[14] Unfortunately most of these syntheses have involved use of expensive chiral starting material or catalysts as well as low yield of targeted molecule. As part of our research program[15–17,18a–f] into the total synthesis of biologically active natural products using the proline catalyzed a-aminooxylation or a-amination of aldehydes[18a–f] as well as Wittig olefination-Claisen rearrangement of acyclic allyl vinyl ethers,[15–17] we became interested in these volatile terpenoids. Therefore herein we report an efficient enantioselective total synthesis of member’s lavandulol terpenoids by using organocatalytic proline catalyzed a-aminooxylation and Wittig olefination-Claisen rearrangement protocol as key steps. General schemes for proline catalyzed a-aminooxylation and Wittig olefination-Claisen rearrangement protocol are outlined in Figure 2.

propionate 3, (+)-(S)-lavandulyl 2-methylbutanoate 4, and (+)(S)-lavandulyl senecioate 6, from 1 and ent-1 by esterfication with respective acids. While (–)-(R) 1 and (+)-(S)-lavandulol ent-1 both could be derived from common intermediate 23 by performing selective functional group modifications. Common intermediate 23 can be accessible from diastereomer 14 b by performing benzyl ether protection, which in turns could be obtained via performing Wittig olefination-Claisen rearrangement protocol on (R)2,3-O-cyclohexylidene D-glyceraldehyde followed by reduction of aldehyde. (R)-2,3-O-cyclohexylidene D-glyceraldehyde could be accessible either from D-proline catalyzed a-aminooxylation of aldehyde 8[18] or from D-mannitol.[19] On similar way we envisaged the synthesis of 1 from compound 15, which in turns could be obtained from alcohol 14 a. We envisioned the enantioselective synthesis of (R)-11 from nonchiral aldehyde 8[18b] as well as from chiral D-mannitol as a starting material shown in Scheme 2. On treatment of the

Figure 2. General schemes for proline catalysed a-aminooxylation and Wittig olefination-Claisen rearrangement protocol.

Results and Discussion

Scheme 2. Synthesis of Wittig precursor glyceraldehyde intermediate 11 from nonchiral aldehyde 8 and chiral D-mannitol.

As depicted in Scheme 1, we envisioned enantioselective synthesis of (–)-(R)-lavandulyl acetate 2, (–)-(R)-lavandulyl

Scheme 1. Retrosynthesis of lavandulol pheromones.


aldehyde 8 with nitrosobenzene in the presence of D-proline in CH3CN at 20 8C followed by in situ reduction with NaBH4 afforded crude aminooxy alcohol, which was further treated with 30 mol% CuSO4.5H2O in methanol undergoes cleavage of O–N bond, furnished (R)-3-(benzyloxy) propane-1,2-diol 9 in 66 % yield with 95 % ee.[18b] The absolute configuration of the new stereogenic centre of 9 was analogous to our earlier reports.[18b] The diol 9 was protected with cyclohexanone in p-TSA, DMSO followed by benzyl ether deprotection (H2, Pd/C, MeOH) afforded (S)-(1,4dioxaspiro[4.5]decan-2-yl)methanol 10 in 91 % yield over two steps. Swern oxidation of alcohol 10 gave crude (R)-2,3 Ocyclohexylidene-D-glyceraldehyde 11, which could be also obtained by protection of D-mannitol[19] and C–C bond scissoring. The [3,3] sigmatropic Claisen rearrangement is intramolecular process which involves multiple bond cleavages and migrations and allow one to the preparation of functional synthons in perfect enantio and stereo control manner. Various versions of these rearrangements in the applications of total synthesis of bioactives have been recently reviewed.[20a, b] The thermally induced Claisen rearrangement of acyclic allyl vinyl ethers can be highly diastereoselective based on the concerted nature and chair like geometry of the corresponding transition 1263


Full Papers state which results in an a-chiral aldehyde.[20c, d, e, f] However, considering the achiral nature of allyl vinyl ether obtained after performing Wittig olefination, it has been assumed that some chiral inductor is required to control the absolute configuration of the rearrangement product.[21] We proceeded accordingly with assumption but unfortunately, when the conditions developed in our previous study were employed, the desired product was obtained in less diastereomeric excess.[17] For the chirality transfer with good diastereoselectivity and yield, a series of experiments with different modifications were carried out. At first, Wittig olefination was carried out with allyoxy methylene triphenyl phosphine using t-BuOK as base provided the corresponding inseparable allyl vinyl ether 12 as E/Z mixture of isomers in 1:1.96 ratio. Latter on thermally induced [3, 3] sigmatropic Claisen rearrangement of the allyl vinyl ether 12 was investigated next (Table 1).

Table 1. Optimization for chirality transfer of the Claisen rearrangement entry

conditions

dr ratio[a] of 13

yield[b] 14 a + 14 b

1 2 3 4 5 6 7 8 9 10

neat, 180 8C, 10 min neat, MW, 150 8C, 10 min xylene, 140–145 8C, 8 h xylene, MW, 120 8C, 25 min toluene, 110–115 8C, 18 h benzene, 80–85 8C, 24 h benzene, MW, 85 8C, 1 h neat/MW 80–85 8C, 48 h CH2Cl2, 40–45 8C, 48 h CH2ClCH2Cl, MAD,[d] 25 8C, 48 h

1:1.05 89 % 1:1.40 91 % 1:2.27 94 % 1:1.65 92 % 1:3.05 94 % 1:4.16 96 % 1:3.51 92 % No reaction No reaction No reaction

dr ratio[c] 14 a:14 b 1:1.06 1:1.53 1:2.25 1:1.64 1:3.15 1:4.05 1:3.50

[a]

dr ratios of product were calculated using NMR of crude aldehyde. yields of product are referred to isolated yields of alcohol 14 a and 14 b obtained after Claisen followed by reduction. [c] dr ratio of the alcohols 14 a and 14 b obtained from isolated yields. [d] MAD = methyl aluminium bis(2,6-di-tert-butyl-4-methylphenolate). [b]

attempted but did not gave conversion. Among the studied solvents acceptable conversion and chirality transfer with good diastereoselectivity was found only when reaction was carried out in benzene at reflux condition (Table 1; entry 6). In benzene reaction temperature was comparatively less as compare to xylene and toluene, whereas reaction time was more. The increase diastereomeric excess is might be due to a partial stabilization of dipolar transition state of Claisen rearrangement of allyl vinyl ether in the benzene at 80–85 8C.[20f] After evaporation of solvent under reduced pressure, crude diastereomeric mixture of aldehyde was subjected to NaBH4 reduction in aq. MeOH, provided chromatographically separable mix of alcohols 14 a and 14 b in 1:4.05 ratio (entry 6). The stereochemical assignment of these alcohols 14 a and 14 b were established previously by our group by converting them to the known alcohols.[22] With alcohols 14 a and 14 b in hand, we proceeded for the synthesis of (–)-(R), (+)-(S)-lavandulol and related analogues. The synthesis of (–)-(R)-lavandulol commenced with minor alcohol 14 a was obtained by employing conditions in Table 1; entry 1. Moving forward, the alcohol 14 a was converted to its benzyl ether by treating with sodium hydride and benzyl bromide in dry THF at 0 8C provided benzyl ether 15 in 95 % yield. Further, ozonolysis of olefin in DCM at 78 8C provided crude aldehyde 16, which was further subjected to Wittig olefination with isopropyl-triphenyl-phosphonium bromide salt using n-BuLi as base in diethyl ether gave alkene 17 in 85 % yield. Acid catalyzed deketalization of the spiroketal in 17 followed by silica supported periodate cleavage of the vicinal diol in DCM afforded aldehyde 18 in 85 % yield over two steps. The reaction of methyl magnesium iodide with aldehyde 18 in diethyl ether at 0 8C afforded inseparable mixture of diastereomeric alcohol 19 (1: 1.38 ratio) in 94 % yield. Swern oxidation of alcohol 19 provided ketone, which was subsequently subjected for Wittig olefination with methyl triphenyl-phosphonium iodide salt in dry THF at 5 8C using nBuLi as base delivered compound 20 in 83 % yield over two steps.

Scheme 3. Synthesis of alcohols 14 a and 14 b.

Attempts to increase the diastereomeric excess were made by optimization of reaction conditions. Two reaction variables viz. temperature and solvent were found to have influence on the diastereomeric excess. Conventional as well as microwave heating of the reaction mass proved to be inefficient for our substrate to improve diastereoselectivity, whereas yields were unaffected (Table 1). Dramatic solvent effect on the diastereoselectivity is observed in nonpolar solvents (Table 1; entries 3–7). However the chlorinated polar solvents did not gave conversion at reflux conditions (entries 9–10). The Lewis acid catalysed Claisen rearrangement at room temperatures also ChemistrySelect 2017, 2, 1262 – 1266

Scheme 4. Synthesis of (–)-(R)-lavandulol 1 and derivatives 2, 3, 4.

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Full Papers Our next objective was the selective deprotetion of benzyl group in presence of alkene would complete the enantiomeric synthesis of (-)-(R)-lavandulol 1. Which was more challenging in presence of alkene, a number of methods for the selective debenzylation were tried, which includes use of FeCl3[23] and chlorosulfonyl isocyanate (CSI)[24] resulted in very low yield. While attempt with BF3·OEt2-NaCNBH3[25] was also unsuccessful to give desired product. Finally, the condition with lithiumnaphthalenide system[26] was successful in producing (–)-(R)[13a] lavandulol 1 in 82 % yield, ½a25 D = 10.6 (c = 1.20, MeOH) {ref 25 ½aD =-9.6 (c = 1.00, MeOH)}. Spectral data of 1 is in full agreement with the literature data. Further, conversion of (– )-(R)-lavandulol 1 to (+)-lavandulyl 2-methylbutanoate 4 by Yamaguchi esterification with (S)-methylbutanoic acid is reported previously.[9] Conversion of (–)-(R)-lavandulol 1 to (–)-(R)lavandulyl acetate 2 and (–)-(R)-lavandulyl propionate 3 reported by Alfonso and coworkers.[3] The synthesis of ent-1 is shown in Scheme 5. The synthesis commenced with major

Our initial synthesis of (–)-(R)-lavandulol from minor alcohol 14 a (obtained via employing condition in Table 1; entry 1) provided desired pheromones (1-4) in less overall yield compared to (+)-(S)-lavandulol (ent-1). So, we had turned our attention for synthesis of (–)-(R)-lavandulol from major alcohol 14 b. Scheme 6 shows utilization of major alcohol 14 b for the total synthesis of (–)-(R)-lavandulol 1.

Scheme 6. Synthesis of (–)-(R)-lavandulol and from alcohol 14 b.

IBX oxidation of alcohol 27 smoothly furnished crude aldehyde, which was subsequently treated with methyl magnesium iodide to afforded crude alcohol, which without any chromatographic purification subjected to next reaction. The crude alcohol was oxidized with IBX afforded ketone, which on Wittig olefination with methyl triphenyl phosphonium iodide salt gave alkene 28 in 72 % yield over two steps. Acid catalyzed deketalization of 28 and subsequent oxidative cleavage of diol over silica supported NaIO4 provided crude aldehyde, which was reduced in situ with NaBH4 gave desired (–)-(R)-lavandulol 1 in 84 % yield over two steps.

Scheme 5. Synthesis of (+)-(S)-lavandulol ent-1 and its derivative 6.

alcohol 14 b, obtained by employing condition as shown in Table 1; entry 6. Benzyl protection of alcohol 14 b with benzyl bromide and sodium hydride in THF gave benzyl ether 21 in 95 % yield. On ozonolysis of terminal olefin in DCM at 78 8C gave aldehyde 22 in 95 % yield, subsequently Wittig olefination of 22 with isopropyl-triphenyl-phosphonium bromide salt in THF by using n-BuLi as base gave alkene 23 in 84 % yield over two steps. Acid catalyzed deketalization and subsequent oxidative cleavage of diol over silica supported NaIO4 provided aldehyde 24 in 84 % yield over two steps. Grignard reaction on aldehyde 24 with methyl magnesium iodide in dry diethyl ether delivered an inseparable diastereomeric mixture of alcohol 25 (1:1.28 ratio). Further, oxidation of alcohol 25 under Swearn oxidation condition and consequent Wittig olefination with methyl triphenyl-phosphonium iodide salt afforded diene 26 in 82 % yield over two steps. The lithium-naphthalenide mediated deprotetion of benzyl ether 26 provided (+)-(S)-lavandulol (ent[13a] 1) in 82 % yield, ½a25 ½a25 D = + 9.8 (c = 1.10, MeOH) {ref D = + 10.1 (c = 1.13, MeOH)}. The synthesis of (+)-(S)-lavandulyl senecioate 6 from (+)-(S)-lavandulol ent-1 was reported previously.[9]


Conclusions In summary, we have presented stereodivergent total synthesis of lavandulol pheromones from commercially available starting materials. (–)-(R)-Lavandulol 1 and ent-1 were obtained in 15.8 % and 15.7 % overall yields, in 15 and 16 steps respectively from nonchiral aldehyde 8 through 14 b. (–)-(R)-Lavandulol 1 and ent-1 were also obtained in 25.7 % and 24.3 % overall yields respectively in 11 steps each from aldehyde 11 through 14 b. (– )-(R)-Lavandulol was also obtained in 6.3 % overall yield in 11 steps from aldehyde 11 through 14 a. It is worth noting that targeted molecules can be synthesised either from diastereomeric alcohols 14 a or 14 b by changing some reaction sequences. The key intermediate chiral alcohols 14 a and 14 b were successfully obtained through proline catalyzed asymmetric a-aminooxylation and Wittig olefination-Claisen rearrangement protocol. Further work aimed at the completion of the total synthesis of cyclobutanoid monoterpene 5 is currently underway in our laboratory. Supporting information Summary The detailed experimental section, FT-IR, HRMS, 1H-NMR and 13 C-NMR spectra of all compounds are provided in the supporting information.

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Full Papers Acknowledgements We thank the UGC, New Delhi, India for SRF to V. A. B. and for financial support (F.No.41-329/ 2012(SR))

[15]

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Submitted: December 2, 2016 Accepted: January 18, 2017

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