Experiment # 2: Piperine – Synthesis and Isolation of a Natural Product INTRODUCTION Piperine is the naturally occurring alkaloid that gives the spice black pepper its characteristic biting taste. The stereoselective synthesis of this natural product, in good yield from inexpensive and readily available starting materials, exemplifies the powerful synthetic utility afforded by the Wittig reaction and its variations such as the Horner-Wadsworth-Emmons reaction. The important impact of the Wittig reaction on modern synthetic organic chemistry was recognized by the 1979 Nobel Prize that was awarded in part to Georg Wittig for his discovery and subsequent development of this reaction. OOON Piperine The original Wittig reaction refers to the reaction of phosphonium ylides with aldehydes or ketones to form alkenes: R R'OR'''R''P(C6H5)3O P(C6H5)3R'R R'''R''+O P(C6H5)3+R''R'R R''' As shown, this reaction mechanism, which has been studied extensively, proceeds through a four membered oxaphosphetane ring intermediate. This intermediate breaks down to yield the corresponding alkene and phosphine oxide. Formation of the stable P=O bond of the phosphine oxide is a major driving force for the reaction. Although the original Wittig has proven to be a highly useful reaction in organic synthesis, there are2 limitations with this reaction that have been largely overcome by the use of phosphonate ester carbanions (see below). Phosphonate esters can be easily made by reaction of an alkyl halide with a trialkylphosphite: R POOCH2CH3OCH2CH3+P(OCH2CH3)3R X+CH3CH2Br The carbanion is then generated by reaction of a base with the parent phosphonate ester: R POOCH2CH3OCH2CH3R POOCH2CH3OCH2CH3Base The phosphonate ester carbanions are significantly more nucleophilic than the phosphonium ylides used in the “classical” Wittig reaction and therefore react readily with a wider variety of aldehydes and ketones. Also, the phosphate side product produced from these reactions is highly water soluble and generally easy to separate from the alkene product in contrast with the phosphine oxide byproduct of the Wittig. In order to undergo the Wittig type reaction with alkenes, the substituent on the α-carbon of the phosphonate ester must be capable of stabilizing the carbanion by resonance. In the synthesis carried out here, the stabilizing group is methyl 2-butenoate: POOEtOEtOOPOOEtOEtOO Reaction of these stabilized phosphonate ester carbanions with aldehydes and ketones to form the corresponding alkenes constitutes the variant of the Wittig reaction known as the Horner-Wadsworth-Emmons reaction shown at the top of the next page.3 R' POOEtOEtR HO+O P(OEt)2R R'(EtO)2PO2RR'+ A remarkable aspect of this reaction is it’s generally high degree of stereoselectivity for the trans alkene when both cis and trans possibilities exist. The preference for the trans product is understandable based on steric effects in the cyclic intermediate. The trans alkene results from the less sterically strained four membered ring in which the largest groups are on opposite sides of the ring: O P(OEt)2R R'(EtO)2PO2RR'+ Such stereoselectivity is very important in natural product syntheses where the biologically important properties of a compound are usually specific to one stereoisomer. In the case of piperine, the unique property of its desirable peppery taste is specific to the (E,E) stereoisomer. In this series of lab experiments, piperine will be obtained from two different sources, one synthetic and one natural. The synthetic route will be a three-step synthesis utilizing the Horner-Wadsworth-Emmons reaction. The crude synthetic product should be sufficiently pure to complete its purification by recrystallization. The natural isolation route will use an extraction procedure to isolate piperine from ground black pepper. The crude piperine from pepper will be purified using flash column chromatography. You will then compare the piperine from the two different sources using spectroscopic characterization techniques (i.e., 1H NMR, 13C NMR).4 SYNTHETIC SCHEME The following scheme summarizes the reactions that will be used to synthesize piperine: POOCH2CH3OCH2CH3OOBrOOP(OCH2CH3)3OOOONaOCH3OOOHNaOCH3NHOOONMethyl 4-bromobutenoateMethyl 4-(diethoxyphosphinyl)-2-butenoateMethyl PiperatePiperine In the first reaction, the phosphonate ester is prepared from methyl 4-bromo-2-butenoate and triethylphosphite. The product, methyl 4-(diethoxyphosphinyl)-2-butenoate, is reacted in the second step with sodium methoxide to generate the phosphonate carbanion in the presence of piperonal. The phosphonate carbanion undergoes a Wittig type reaction with the piperonal to form the trans alkene, methyl piperate ((E,E)-5-(3,4-methylenedioxyphenyl)-2,4-pentadienoate). Finally, reaction of methyl piperate with piperidine in the presence of sodium methoxide in refluxing methanol solution gives piperine.5 EXPERIMENTAL Synthesis of Methyl 4-(diethoxyphosphinyl)-2-butenoate Caution: Because of the toxicity of ethyl bromide and the stench of triethylphosphite, this experiment should be carried out in a fume hood. Methyl 4-bromo-2-butenoate (3.5 mL, 5.3 g, 30 mmol) (see Note 1) is placed in a 25 mL round bottom flask equipped with a magnetic stirrer. To the neck of the flask is attached a Claisen head adapter fitted with a thermometer adaptor allowing the thermometer to extend down into the liquid in the flask (see Figure 1 below). The side arm of the Claisen adapter is connected to a simple distillation apparatus. Triethylphosphite (5.0 mL, 4.8 g, 30 mmol) (see Note 2) is added to the stirred methyl 4-bromo-2-butenoate. The stirred mixture is gently heated. After a brief induction period, an exothermic reaction takes place and ethyl bromide (bp 37-40 oC) distills from the reaction mixture. The mixture is heated to 120-130 oC and maintained in this temperature range for 20 min or longer if necessary until ethyl bromide ceases to distill over. The crude product is characterized by IR and 1H NMR and saved as starting material for